Vapor lock problem on a 1956

doug7740

Active Member
Joined
Dec 29, 2014
Thunderbird Year
1955
A update on my vapor/??? problem. I've done some more testing and here's what I'm finding:
I had a bit of a chat with Chris Ames (the water pump spacer guy) about vapor lock problems and he told me a couple of things that helped him. One was insulating the line from the fuel tank to the frame rail and the other was to wrap the left side exhaust pipe. So I took his advice and insulated the line from the tank to the frame rail with some DEI stuff. I also a installed fuel pressure gauge. On a mid 90 deg day I drove for 1hr and 40 minutes stop and go mixed with 50 mph portions. First thing I noticed was the fuel pressure started out at 6 psi at idle and then as I drove it would drop to about 4 psi at idle. After it got good and warm the fuel would drop to 2 3/4 to 3 psi under hard accel. After about 1 hour of driving I drove home and pulled the car right up to the garage door and left the car in gear and after about 20 minutes the the temp gauge was to the far right. The car still ran fine. So at this point I think I no longer can blame anything in the ignition system. I did make two changes after the two trips when I had my problems. I added the fuel line insulation and I did remove the carb bowl and the jet plate on the carb and blew out all the passages. I'm sure nothing was wrong because the fuel circuit, jets to the venturi are large passages, and if the much smaller air bleeds were plugged that would only make the car run rich.
With no other changes I drove the car for 45 minutes and then back with the same results. Fuel pressure didn't drop below 2 3/4. If I run the electric pump the pressure will stay at around 4 1/2.
Then I added the exhaust wrap on the left pipe from the start of the hump over the rear end to almost the end of the tail pipe. I then repeated this same trip and this time the fuel didn't drop below 3 psi. The air temp was still in the mid 90's. I'm thinking that Chris's suggestions did the trick but I still don't understand why when I first had the problem and then added the electric pump, why turning the pump on made no difference on how the car ran.
I'm wondering if the drop in idle fuel pressure from when I first start the engine from 6 psi down to around 4 said anything about the fuel pump. It will go down to 4 psi within a few blocks, too short of a run to heat the fuel I would think. I tried to buy a new pump from our t-bird suppliers but they don't seem to have any in stock. Wanting to put the problem to bed I ordered a new pump from a O'Reilly auto parts store. The brand name was Precision Fuel Pumps which looks to be a store brand. When I opened the box the first thing I noticed was it was a pump with Philips screws which I heard are the problem pumps. I moved the pump lever back and forth and figured it was defective right out of the box. I went ahead and mounted it anyway and the car would run on the electric pump but not on the mechanical pump. Not wanting to order another of the same fuel pump I had them order a Delphi pump ( to the tune of about $50. extra) hoping that this will be a pump from a different manufacture. I should get the new pump next week.
Does anyone have any idea what the fuel pressure would be during a hard accel? The only spec I see is 4-5 psi at 500 rpms.
CASCO is showing two different rebuilt pumps that they sell. These pumps are rebuilt from cores that are made in USA, Canada, or Mexico. They use hex head screws or fillister head screws (slotted).

(See attached)

They do not rebuild cores made in China. Typically these use phillips head screws and the bottom sediment bowl has a red washer and a phillips head screw.

https://www.classictbird.com/Fuel-Pump-Rebuilt-1-Per-car/productinfo/9350R/

https://www.classictbird.com/Fuel-Pump-AC-Rebuilt-1-Per-car/productinfo/9350RAC/

No one is selling a new fuel pump at this time. Currently I am testing a new fuel pump for CASCO and after 500 miles of driving it looks promising, but it is not yet available.

doug7740
1955 Thunderbird Blue 9350RAC-3.jpg
 

ron56

Active Member
Joined
Mar 17, 2020
Thunderbird Year
1956
Interesting about the Chinese pumps. If you Goggle "where are Precision Fuel Pumps made" it states they are made in the USA. But on the box it states they are made in China and have the Philips screws and the red washer as Doug 7740 stated. As a point of reference one can buy the Delphi/AC pump from Jegs or Summit for about $50. less than the cost of a rebuilt from CASCO. I took my Airtex 4406 pump apart and really couldn't see anything wrong with it. After driving my car over the weekend I'm now more convinced that my problem is a vapor issue. With a temp of around 100 degrees I made a trip down the freeway, this is using only the electric fuel pump and the dead Precision pump. After about 15 minutes of freeway speed I could watch the fuel pressure drop the longer I drove. Interesting it dropped to 1 psi and still pulled up a pretty good hill at 70 mph with seemingly no issues. As I pulled off the freeway and got on surface streets on my way home the pressure still was very low. Just a few blocks from the house it was below 1/2 psi and the pressure went to zero as I drove into my driveway. I let the car idle and it quit. I left the pump running and got my IR gun and the pump body was 130 deg. along with the outside of the frame rail where it is mounted. The bottom of the stainless steel fuel tank was at 100 (same as ambient). I shut the pump off and went back out after about 10 minutes and turned the pump on and the pressure was bouncing at around 1/2 psi. This morning I turned the pump on and the pressure went back up to 4 1/2. I have a couple of thoughts: The line from the tank to the electric pump seems clear because if I disconnect it fuel will flow out by gravity at a good clip. I replaced the flex line from the steel line to the pump when I replaced the Airtex with non working Precision pump. I'm wondering if the station where I buy the ethanol free gas still has winter low vapor grade fuel in it's tanks. I've never seen anyone buy the clear fuel when I'm there with the t-bird or my other cars. It just seems strange to me that all of a sudden with the warm weather this year I'm having a problem, when for the last two years no sign of a issue. I also question if I shot myself in the foot by drilling a vent hole in my gas cap. I can see that the cap will vent under vacuum (it has what looks like a check valve built into the cap) but when I blew with my mouth I couldn't blow hard enough to see if the cap would vent under pressure. That might mean that the tank would maybe build pressure in hot days which should help my problem.
 

biddle

Forum Moderator
Staff member
Joined
Mar 9, 2003
Interesting about the Chinese pumps. If you Goggle "where are Precision Fuel Pumps made" it states they are made in the USA. But on the box it states they are made in China and have the Philips screws and the red washer as Doug 7740 stated.
If you google "Where are Precision Fuel Pumps Made" there is no "it" or single answer, there are multiple sites and numerous references and explanations including:

While other American manufacturers shifted their production to Asia and started selling cheaply built, premium-priced "American made" products, Kruss established long-term partnerships overseas to maintain the high-quality Precise® is known for. All Precise® products are built to the same specifications – by the same OEM factories in Asia, Europe, Mexico, Brazil, and the U.S. – used by automotive manufacturers and are available at a competitive price. Precise fuel pumps are assembled with precision components that include Original Equipment USA-made pressure sensors and pressure regulators wherever possible.
 

Ward 57

Active Member
Joined
Aug 14, 2019
Thunderbird Year
1957
Interesting about the Chinese pumps. If you Goggle "where are Precision Fuel Pumps made" it states they are made in the USA. But on the box it states they are made in China and have the Philips screws and the red washer as Doug 7740 stated. As a point of reference one can buy the Delphi/AC pump from Jegs or Summit for about $50. less than the cost of a rebuilt from CASCO. I took my Airtex 4406 pump apart and really couldn't see anything wrong with it. After driving my car over the weekend I'm now more convinced that my problem is a vapor issue. With a temp of around 100 degrees I made a trip down the freeway, this is using only the electric fuel pump and the dead Precision pump. After about 15 minutes of freeway speed I could watch the fuel pressure drop the longer I drove. Interesting it dropped to 1 psi and still pulled up a pretty good hill at 70 mph with seemingly no issues. As I pulled off the freeway and got on surface streets on my way home the pressure still was very low. Just a few blocks from the house it was below 1/2 psi and the pressure went to zero as I drove into my driveway. I let the car idle and it quit. I left the pump running and got my IR gun and the pump body was 130 deg. along with the outside of the frame rail where it is mounted. The bottom of the stainless steel fuel tank was at 100 (same as ambient). I shut the pump off and went back out after about 10 minutes and turned the pump on and the pressure was bouncing at around 1/2 psi. This morning I turned the pump on and the pressure went back up to 4 1/2. I have a couple of thoughts: The line from the tank to the electric pump seems clear because if I disconnect it fuel will flow out by gravity at a good clip. I replaced the flex line from the steel line to the pump when I replaced the Airtex with non working Precision pump. I'm wondering if the station where I buy the ethanol free gas still has winter low vapor grade fuel in it's tanks. I've never seen anyone buy the clear fuel when I'm there with the t-bird or my other cars. It just seems strange to me that all of a sudden with the warm weather this year I'm having a problem, when for the last two years no sign of a issue. I also question if I shot myself in the foot by drilling a vent hole in my gas cap. I can see that the cap will vent under vacuum (it has what looks like a check valve built into the cap) but when I blew with my mouth I couldn't blow hard enough to see if the cap would vent under pressure. That might mean that the tank would maybe build pressure in hot days which should help my problem.
The venting of the gas cap is not a bad thing. You don't want to have a vacuum issue even though the EPA may not approve. I'm intrigued how you plumbed a pressure gage so you can see the real time pressure. Yes you would think the added pressure in the tank would help. The reason you see nobody else using non ethanol fuel is the cost. Premium+ prices and the new vehicles will run on the cheapest fuel as anti knock technology may degrade performance but they don't notice.
I'll scratch my head as to why the heat would have such an effect on fuel pressure even with an electric back-up pump. The only other thing I can think of is if there is another flexible fuel line in the system that swells when hot restricting the flow.
 

doug7740

Active Member
Joined
Dec 29, 2014
Thunderbird Year
1955
Interesting about the Chinese pumps. If you Goggle "where are Precision Fuel Pumps made" it states they are made in the USA. But on the box it states they are made in China and have the Philips screws and the red washer as Doug 7740 stated. As a point of reference one can buy the Delphi/AC pump from Jegs or Summit for about $50. less than the cost of a rebuilt from CASCO. I took my Airtex 4406 pump apart and really couldn't see anything wrong with it. After driving my car over the weekend I'm now more convinced that my problem is a vapor issue. With a temp of around 100 degrees I made a trip down the freeway, this is using only the electric fuel pump and the dead Precision pump. After about 15 minutes of freeway speed I could watch the fuel pressure drop the longer I drove. Interesting it dropped to 1 psi and still pulled up a pretty good hill at 70 mph with seemingly no issues. As I pulled off the freeway and got on surface streets on my way home the pressure still was very low. Just a few blocks from the house it was below 1/2 psi and the pressure went to zero as I drove into my driveway. I let the car idle and it quit. I left the pump running and got my IR gun and the pump body was 130 deg. along with the outside of the frame rail where it is mounted. The bottom of the stainless steel fuel tank was at 100 (same as ambient). I shut the pump off and went back out after about 10 minutes and turned the pump on and the pressure was bouncing at around 1/2 psi. This morning I turned the pump on and the pressure went back up to 4 1/2. I have a couple of thoughts: The line from the tank to the electric pump seems clear because if I disconnect it fuel will flow out by gravity at a good clip. I replaced the flex line from the steel line to the pump when I replaced the Airtex with non working Precision pump. I'm wondering if the station where I buy the ethanol free gas still has winter low vapor grade fuel in it's tanks. I've never seen anyone buy the clear fuel when I'm there with the t-bird or my other cars. It just seems strange to me that all of a sudden with the warm weather this year I'm having a problem, when for the last two years no sign of a issue. I also question if I shot myself in the foot by drilling a vent hole in my gas cap. I can see that the cap will vent under vacuum (it has what looks like a check valve built into the cap) but when I blew with my mouth I couldn't blow hard enough to see if the cap would vent under pressure. That might mean that the tank would maybe build pressure in hot days which should help my problem.

Bellow is a Tech Tip that I wrote about the fuel tank venting on a 1955/ 1956 Thunderbird.
Gas Cap Tech Tip: The gas cap on your T-Bird must be vented to allow air to enter the tank. If air cannot get into the tank vacuum will be created which can stop the flow of fuel to your engine. Last summer, I had to replace my gas cap and I discovered that when I removed the gas cap after running the engine for a while, I could hear the sound of air rushing into the tank. To vent the fuel tank, the 1955 and 1956 gas caps have a rib stamped into the surface which allows air to enter from the outside edge of the cap to the center. The problem with my gas cap was that the inside diameter of the gasket was so small that it prevented the venting process. The problem was easily fixed by removing the gasket from the gas cap and increasing the inside diameter of the gasket by cutting it with a razor blade. Removing just a little bit of the gasket from the inside diameter corrected my venting problem.

1-stamped-rib.jpg 2-inside-diameter-gasket.jpg
doug7740
1955 Thunderbird Blue
 
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ron56

Active Member
Joined
Mar 17, 2020
Thunderbird Year
1956
In thinking about the transit time for the fuel in the line from the tank to the electric pump I did a little back of the napkin calculation. On my car I get a little over 13 mpg so assume a 5/16 gas line id of .25 and 60 mph and 6 feet of line from the tank to the electric pump it would work out to a flow of 6 inches/sec or 12 seconds to heat the fuel in the line. Which is longer than I would have thought, but sounds like enough time to heat the fuel before the pump. To answer another question: the only rubber hose in the fuel system is after cutting the fuel line to install the electric pump I used 2 short pieces of reinforced fuel line to make the connection. To install the fuel pressure gauge I took IMG_0614.JPG the piece of 5/16 tube from the section that I cut out to install the electric pump and pushed it through one of the cowl wiring grommet unused holes. I did then use 5/16 fuel line to make the connection to the gauge. I don't have the stock setup for the line from the engine pump to the carb. I'm using a later model carb so I have a fuel filter in line at the carb. I removed the filter and installed a tee to tap the fuel pressure at that point. I'm attaching a picture of the gas cap I'm using. You can see the check valve that is build into the cap. It is free flowing as far as venting but I don't know about the pressure relief side. I'm going to close off the vent hole I drilled because I don't see how a little tank pressure will hurt. The only issue would be if it built up enough pressure to blow open the needle and seat in the carb.
 

jimntempe

Active Member
Joined
Apr 2, 2021
Thunderbird Year
1957
In thinking about the transit time for the fuel in the line from the tank to the electric pump I did a little back of the napkin calculation. On my car I get a little over 13 mpg so assume a 5/16 gas line id of .25 and 60 mph and 6 feet of line from the tank to the electric pump it would work out to a flow of 6 inches/sec or 12 seconds to heat the fuel in the line. Which is longer than I would have thought, but sounds like enough time to heat the fuel before the pump. To answer another question: the only rubber hose in the fuel system is after cutting the fuel line to install the electric pump I used 2 short pieces of reinforced fuel line to make the connection. To install the fuel pressure gauge I took View attachment 18170 the piece of 5/16 tube from the section that I cut out to install the electric pump and pushed it through one of the cowl wiring grommet unused holes. I did then use 5/16 fuel line to make the connection to the gauge. I don't have the stock setup for the line from the engine pump to the carb. I'm using a later model carb so I have a fuel filter in line at the carb. I removed the filter and installed a tee to tap the fuel pressure at that point. I'm attaching a picture of the gas cap I'm using. You can see the check valve that is build into the cap. It is free flowing as far as venting but I don't know about the pressure relief side. I'm going to close off the vent hole I drilled because I don't see how a little tank pressure will hurt. The only issue would be if it built up enough pressure to blow open the needle and seat in the carb.
You seem to be assuming that your electric pump is still good. Perhaps it’s overheating and it is the problem.
 

ron56

Active Member
Joined
Mar 17, 2020
Thunderbird Year
1956
Update: I've done a number of things trying to get to the bottom of my problems. I'll try and give some details because it might help other owners. I ordered a Delphi fuel pump and clearly on the box it stated "made in China" I installed the fuel pump but it barely worked. At idle it is supposed to flow 1 pint in 30 seconds but would only do about 1/2 that. The flow wasn't steady but would flow in spurts. I returned the pump to the auto parts store and then found a rebuild kit for my original Airtex 4406 fuel pump. Doing the same 500 rpm flow check I now had almost 2 pints in 30 seconds. But along the way I think I discovered what my original problem was. I had bought a new flex hose from the frame rail to the fuel pump to replace the flex hose I installed 3 years ago because it showed some cracks on the covering. After taking the new flex hose on and off several times I could see that I had a small leak at the hard line connection. Trying to see if my issue was the hard line or the flex line I put my 3 year old line back on and capped off the 1/8 pipe end and turned on the electric pump. Much to my surprise fuel sprayed out of the hose in several places. Now I'm thinking that my 3 year old flex line (only seen ethanal free fuel) after warming up a bit would allow the fuel pump to suck air along with fuel. That is why the car would stay running slowly, but idle OK. Because the flex line is above the tank level no fuel would leak out with the car just sitting or running. Then 2 bad fuel pumps in a row only made the picture cloudy. One would think that the flex line should last more than 3 years. I ordered the first replacement flex line from a different supplier than my original line, but I was concerned that it only had a 1/8 inch hole in both of the fittings. I ordered a second flex line from the original supplier and it has a 3/16 hole in both fittings and a more substantial hose. Plus it is a little longer so it eliminates the need for a extra fitting at the pump. It looks like the original supplier improved on their product over the last 3 years and made it better. I ran a flow test with the 1/8 and the 3/16 hole and could not see any difference in flow at the pump.
One bit of info if one was to replace the line from the tank to the frame rail connection: I wanted to cover that section of the fuel line all the way with the DEI insulation. In order to remove the line I found that removing the body mount bolt (Just to the right when you open the door) and removing the shims between the body mount and the body you can gain enough clearance to pull the line straight back enough to clear the line where it goes through the rear end spring mount structure. I'm not sure how one would replace the line any other way with the body mounted on the frame.
I now have the line from the tank to the electric pump fully insulated along with the muffler and the tail pipe to the rear. After a hard run with the car and then idling for a minute and then accel I can watch the fuel pressure gauge drop down to around 2 psi and then after a few seconds come back up to 4 psi. I guess this tells me that the fuel in the line still has a vapor issue of some sort. Has anyone experienced this problem or is it normal? In a prior post I noted that the car would still accel up a hill at 70 mph with only 1 psi showing on the gauge. I have cross checked the gauge against another gauge.
One thing I did notice was both of the Chinese fuel pumps did not seem to have any tension as I installed them. Not the case with the Airtex pump I could feel some tension. In each case I turned the engine to TDC at #1 firing (lowest point for the fuel pump cam). Makes me think that the China pumps are built to the wrong spec as far as the pump lever goes. I should have checked for any difference between the pumps, but I didn't.
 
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biddle

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Staff member
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Mar 9, 2003
But along the way I think I discovered what my original problem was. I had bought a new flex hose from the frame rail to the fuel pump to replace the flex hose I installed 3 years ago because it showed some cracks on the covering. After taking the new flex hose on and off several times I could see that I had a small leak at the hard line connection. Trying to see if my issue was the hard line or the flex line I put my 3 year old line back on and capped off the 1/8 pipe end and turned on the electric pump. Much to my surprise fuel sprayed out of the hose in several places. Now I'm thinking that my 3 year old flex line (only seen ethanal free fuel) after warming up a bit would allow the fuel pump to suck air along with fuel. That is why the car would stay running slowly, but idle OK.
You talked a lot about Chinese fuel pumps, but in reading your post it appears your problem was your flex hose fuel line having cracks?

Also,what's the part number and/or brand of the fuel pump rebuild kit you mentioned?
 
Joined
Jun 3, 2020
Thunderbird Year
1955
Interesting. I recently changed out the fuel pump on my 55 with a rebuilt one and had an issue with the flex line to hard line fitting. The nut felt tight but the flare was not snug to its seat. I snugged it a bit more than I normally do and the problem went away.
 

Ward 57

Active Member
Joined
Aug 14, 2019
Thunderbird Year
1957
@ron56 , I still don't think you have vapor lock. Very rare especially on these cars. You either have a clogged pick-up in the tank or still have a suction problem. A vapor lock would show up at low flow volume such as at a idle with really hot weather and not at cruising speed or full throttle.
 
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ron56

Active Member
Joined
Mar 17, 2020
Thunderbird Year
1956
The reason that I talked a lot about the Chinese fuel pumps is because I purchased two different fuel pumps made in China. One made no attempt to work and the other one worked (moved fuel) but not enough to run the car. Two different brands Precision and Delphi. As far as the brand of rebuild kit here is what is on the invoice FPA219rebuild I bought it as a rebuild kit for a 4406 double action fuel pump. I would not recommend the kit to anyone because the price is greater than one can buy a Airtex 4406 pump. I paid the extra money because I mistakenly thought because of the extra cost it might be better. The kit has several problems as it relates to my Airtex 4406 pump.

o... The vacuum side only includes the diaphragm so there is no way to reuse the stem because it is riveted.
o... The fuel pump diaphragm is not clocked correctly to fit the Airtex 4406 body.
o... The fuel pump diaphragm spring might be incorrect because the system will supply fuel at 3.75 psi when the factory spec is 4-5 psi.
o... The pulse dampening diaphragm did not come punched with the two vent holes that the original pump had and is also shown in the Ford manual.
Note: I just checked the eBay website for this kit and after I sent them a email about the problems I now see that they added a statement that the kit won't fit the Airtex 4406 pump.
As far as the slight leak at the hard line soft line goes, the problem was in the flare of the hard line but a small o'ring in the bottom of the soft line fitting fixed the problem.
 
Joined
Dec 12, 2020
Thunderbird Year
1956
If you have an original AC pump or any other USA/Canada pump there is a rebuilder, Arthur Gould in Mass. that rebuilds them. There are a couple of other rebuilders , Terrill Machine in Texas and Cowboy John's Auto Parts in Florida.
 
Joined
Jun 3, 2020
Thunderbird Year
1955
Terrill rebuilt 100's of fuel and water pumps for the company I retired from. Spoke with Fels many times. They are very busy because they do great work.
 
Joined
Feb 9, 2019
Thunderbird Year
2002
Just an observation and a PSA, early in the thread it was mentioned that the poster had installed a "booster" electric pump that operates THROUGH the factory mechanical pump to mitigate vapor lock. This works and I did the same on my 63 Corvette, but this setup should be for EMERGENCY use only. Unless the owner wired an oil pressure cutoff switch into the electrical pump it will run as long as there is power (e.g. with engine stopped). Bad news in a crash, and, secondly, if the mechanical pump diaphragm should fail with the booster pump running the engine may not exhibit any symptoms and you will pump raw fuel into the crankcase thinning the oil and possibly damaging the engine.
 

CSPIDY

Active Member
Joined
Jun 25, 2016
Thunderbird Year
1957
Just an observation and a PSA, early in the thread it was mentioned that the poster had installed a "booster" electric pump that operates THROUGH the factory mechanical pump to mitigate vapor lock. This works and I did the same on my 63 Corvette, but this setup should be for EMERGENCY use only. Unless the owner wired an oil pressure cutoff switch into the electrical pump it will run as long as there is power (e.g. with engine stopped). Bad news in a crash, and, secondly, if the mechanical pump diaphragm should fail with the booster pump running the engine may not exhibit any symptoms and you will pump raw fuel into the crankcase thinning the oil and possibly damaging the engine.
Another option is to install a Ron Francis impact switch that if connected to the power source will kill the power to the pump in the event of a crash.
 
Joined
May 10, 2021
Thunderbird Year
1956
One more thing when the car was running poorly I could rev the engine very high without any issues so I'm pretty sure the problem isn't ignition related


Ummm if you're not under load, why would you rule out vacuum, or vacuum advance?
I would still check the points and gap and clean spark plugs and check gap !
 
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