1994 start-up idle problem | Page 2 | Ford Thunderbird forum club group 1955-2005 models

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1994 start-up idle problem

  • Thread starter Thread starter Pearl 94
  • Start date Start date
Went out and started the car this morning, and as you asked, the rpm does jump up to 1200-1300 rpm and then drops back to 450-500 without touching the accelerator. (I recall seeing a change on starting rpm after I replaced the IAC. increased rpm jump at start). This morning, other than low rpm it Idled smoothly, no misses or rough running. On previous drives when the engine is warm, it idles about 500 rpm in gear, and about 700 rpm in neutral. Have ordered a new MAF due to be delivered next Wednesday. More then!

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Hi again
Forgot to mention, that if I just leave the car idling, no gas pedal or movement, after about 5-10 minutes the rpm will gradually increase to around 600 rpm and I can put the car in gear and it doesn't die. Have noticed this in some of my many attempts to find some kind of logic for the weird problem.
 
On the maf, if you was still running on the oem one, a brand new cheap one would probably do better than the one your on, and it will help sort things out while going thru all this, I hope you bought the whole assembly, two clamps, a plug-in and your on your way.........
On the cold start idle, mine goes right to about 1200, then slowly slides down and then wants to hold at around 900, all that takes approx. 15 secs., I'd have to wait for a while, long while, for it to drop to any kind of what I would call a normal idle..........On a warm engine, I'm probably 650 in gear and 700 in park, now I say all this because this is how your idle positions should be or real close to it.........Let me throw this in here, have you ever swapped the throttle position sensor ?, I mention this because you said that if you let it sit there and idle for a while, it picks up about 150 rpm's, seems like only the pcm or the tps could do that........
I hate to say it, but if we have a new IAC, a properly working MAF, a properly working TPS, only the pcm can do that to your idle........
Now the only pcm's we've tried so-far is the oem one that we took out and now using the flagship one pcm currently right ?
This is what happened to me, in the last 6 months I've went thru 6 different pcm's, 2 of them worked great, 2 of them idled ok but would not shift right(missing either 1st or 2nd gear), 1 of them had a 500 rpm right after a cold start so I did'nt even go for a test drive on that one, and that's leaves me the last one, the one I bought from flagship one, although it had a different catchcode and they told me all was ok, I run on that pcm for 300 miles, started up, run fine, I thought the up-shifts were a tad firmer than normal but all thru this 300 miles, I put up with a low idle(2 foot bit all the time) about 500 in park and nearly 400 in gear if Iet it......This almost sounds like what your going thru with you idle...........Put that maf on, consider how old is your tps, and sadly better start looking at some used pcm's on ebay because on all these pcm's I've tried on my bird, if it does'nt show a 1100-1200 rpm on a cold start, try another pcm and like I said I've found 2 that run and idle the way it should.........
 
Oh, you never mentioned about a AIT sensor, (air intake temp) it's screwed into the air filter box, don't know if it's any factor in what were dealing with but it's there for a reason, don't cost much...................
Oh, and I really wanted to stress the point about how long does it take to settle down to a say 700 to 900 idle after coming off the 1100-1200 rpm after cold start, please don't tell me that once it starts and runs up to 11-1200 rpm, it quickly drops to this low idle of 4-500 rpm like within say 20 or 30 seconds.........
 
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I'm going to just toss this into the ring, but have you considered your oxygen sensors? Mixture control takes in many factors, and your O2 sensors are a big part of it. It would be helpful if you had a diagnostic tool that could give you live numbers on all of your sensors at cold start and be able to see which ones are off. A failing sensor will not always give a code until it's too far out of spec or completely failed. And not all new parts are good parts. I have dealt with bad sensors right out of the box.
 
mcm51 - yup, when I cold start the car, the rpm jumps up to 1200-1300 rpm when the engine fires and instantly drops to 400, stumbles a bit and then will settle at about 450-500 if I don't touch the accelerator. Intake air sensor, I put a new air filter in, and when changing it, looked at and checked the sensor, and it looks ok. Pulled the connector out and in a couple of times to ensure good contact with the pins.
74 Harley - I haven't checked the 02 sensors, but i don't think they could be a problem as when the engine warms up, it runs good and smooth, and if one or more of those sensors were bad it would show up during regular running. The Innova reader I have has an advanced option, key on engine off, and key on engine run, and there were no codes. If one of the 02 sensors was bad, i would think it should show up with the engine running.
Drove out this morning to our T-bird club breakfast (a bunch of us get together Sunday morning for breakfast) and it was not too bad. Started it, let it settle, put it in gear and idled back out of the garage. My garage has a bit of a down slope so no need to touch the gas pedal. Stopped, put it in D and gently eased into a bit of gas pedal and drove off. After I drove about two blocks, engine ran "normal", good acceleration and smooth. So whatever the problem is, the past replacements/adjustments appear to have made some improvement.
The "final" test will be a new MAF. Next week!
 
@74 Harley, that's a good thought but being Pearl 94 was only going thru a rough/stumble during cold starts, I was'nt leaning toward o2's because the pcm only pays attention to those after warmed up, until then it's running on memory, until then it's simply running on memory, Thanks for the imput.............
@ Pearl 94, if she cranks up and goes to 1200-1300 and does'nt take a few minutes(2-3 mins.) to completely settle down to anywhere near a normal idle, I don't like that at all, that's not right, now I've had my Bird for 30 yrs. and what it goes thru during cold starts has never changed, always on a cold start, right to about 1250, then slowly(guessing here, 15secs.) settle down to 900 and hold there, thats when I drop it in gear and head down the driveway riding the brakes all the way, with tach showing about 750 as I'm holding it back, now once I get to my first stop sign about a 1/4 mile away, now she's down to about a normal idle, even though my water temp is just coming off the cold mark...............If your Bird is not responding in the cold start idle manner that I described, it's gotta be either pcm, iac, tps, maf...........From what I learned swapping out all those used pcm's on mine is, the one's that were either faulty OR in some manner did'nt belong to the car, had either a idle or shifting issue...................
Couple questions, I've mentioned shifting a few times, since you put the flagship one in, has the up-shifts been normal, especially 1-2nd shift ?
Did you buy the whole assembly on the maf ?
Have you ever changed the TPS ?
Why not swap out that intake air temp sensor screwed into the air filter box ?

Must be nice to go have breakfast with fellow T-Bird people on Sundays, I only wish I could say the same, my Bird and I are lone people here in n. fla, your lucky, enjoy it.........................
 
The shifting has always been good, old PCM as well as the new one. Shifts from 1-2 are good, smooth as are 3-4. and accelerator downshifts are good. As for the TPS being a problem, I don'tt think so. I have a good friend (excellent knowlegible retired heavy duty mechanic) who says, as you did, that the starting circuit is on the closed loop, (memory) and when the parameters from all the other inputs come into the PCM it changes over to the open loop for running inputs and the throttle body and TPS take over.
With that in mind, with the starting circuit on the closed loop, the PCM tells the IAC what to do, I don't think the TPS has any input until the loops change over.
I did order a complete MAF. Funny thing is that the complete unit was only about $30 more than just the sensor.
Air intake sensor - cannot see how that might affect the starting and not the general running. Logic says that if it is giving incorrect air temp, it would do so through all the running cycles.
 
Yea, I was wondering all along if you noticed any shifting change with the new pcm.................I was'nt sure about the tps and running on memory, the connection between the two, but it sounds like you and your friend got that figured out.....................Yes and I know there's a big connection between the pcm and the iac, that's why anyone that's having idle issues, I always think of those two first and a lot of times say, it's one or the other................I'm glad ya went with the full assembly on that maf, don't know why anyone would just swap out the sensor, I also noticed that rockauto don't sell motorcraft reman maf's anymore, a person would probably have to go thru ebay now................yea, and on that intake air temp sensor, God for the life of me, I've never figured out where that sensor comes into play, but they must have put it on there for a reason, I went ahead and swapped mine about 5 years ago, I was still able to get a motorcraft back then...........
Keep us up on the status of your '94 4.6, I'm certainly interested, after all, my '95 4.6 is just a carry over from '94........................
Oh, one final question, and I may have already asked this but did you ever see a 600-700 warm idle on the old pcm or did it idle at 400-500 also ?
 
The idling rpm for both cold and warm has not changed with either PCM, so I suspect there was not much wrong with the original. The biggest change from the original cold starting problem was with the new IAC. Before I changed it, the action at start was stumble, a bit of gas pedal to keep it running and settle at 450 - 500 rpm. After I changed the IAC, was when the starting rpm would jump up to 1200-1300 and then drop back to 450-500 and settle as long as I didn't touch the gas pedal. I was looking back through my "records" to see if there was anything significant to cause the low idle problem, and cannot find any.
Aside from the numerous times I had the battery disconnected for variouis reasons, nothing else was obvious. I discounted the battery disconnections because even if that did affect the PCM, replacing and installing a new one would have remedied that. The new MAF is supposed to be delivered today so will see if that makes any difference. I will be surprised if it does. The symptoms always point to something needing to warm up or stabilize before the idle/running goes to normal. Also, from past experience, even with carbureted cars, if the engine stumbles and barfs, it is usually a too much fuel problem, in which case my fuel injected 4.6 would appear to be getting too much fuel and not enough air at idle, which brings me back to the IAC and PCM. ???
I have read in some explanations that poor quality fuel can cause starting/running problems. The car was starting "normal" from the time I bought it in August, and when our gas prices in the lower mainland BC went to $2.00 a litre, I started using fuel from our local Costco because it was much cheaper. I don't know if that had any effect, but in any case since then I have been using premium fuel and there has been no change. So I wait for the next phase of this peculiar problem. Will keep you posted.
 
Gosh, I still want to say for the record, that I still don't have any faith in the old or new pcm, mainly because the way you describe a cold start idle and where those rpm's should be, once again I don't see why yours should'nt have the exact same rpm's as mine during cold starts, straight to 11-1200, then about 10 seconds later it settles down and then holds at 900, then a minute later, down around 750 or under, AND that's in mild florida weather, in that cooler BC weather, the higher idle should be more enhanced, all this is why I'm just not a big believer in your old o.e.m. pcm or the one ya got from flagship, I kinda wish we had another pcm to try on it, that Bird should'nt be down there idling at 4 and 500 rpm and you said it, that starting up and going to idle is all about the IAC and pcm..........I've plugged 6 different pcm's into mine, 2 worked like a champ, the others either did'nt shift OR had a 4-500 rpm idle, just like what your saying........
The maf swap I'm thinking would work on the cold start roughness/stumble, you know to make sure our air/fuel mixture is right, now that's assuming our main brain, the pcm is the right one and working correctly............
Glad you brought up fuel, Florida went to ethanol fuel in 2009, 2 or 3 years later my entire fuel system went down, I had to replace the pump, regulator and injectors to fix what ethanol fuel did to me, I found out later that the federal goverment advised not to put ethanol fuel in any car older than an 2001, for the last 15 or so years, all I've run is non-ethanol either 87 or 93 octane depending on which part of the county I'm in, my Bird felt stronger on 87 non-ethanol than on 92 octane ethanol fuel, not to mention I picked up a few mpg when I got away from ethanol..........I'm not sure if ethanol fuel even pertains to you living in BC, if it does, I just want to give you that info..............
 
The Canadian government determined that in order to protect the environment, everybody had to use ethanol in their fuel, so all grades have "up to 10%" ethanol. But that isn't the problem because the car has been running with that fuel before and after I got it.
After installding the new MAF, as I suspected, no change. Out of ideas, I thought I would do another reset of the PCM. Well, that did make a difference. Still the same problem at first start, but if I do not touch the gas pedal, the idle at 450 rpm other than being low, is good and smooth, no burps, farts or misses. When everything is warm, with the trans in neutral, the idle is up to 750rpm, and in gear about 650 rpm, so that is an improvement. Plus the engine runs perfectly now. Plenty of power, no misses, shifts smoothly. So, the conclusion is that there is something that needs to warm up. I can't see either the IAC or the PCM being faulty, because when the engine is warm everything runs perfect, and if either one of those was faulty, it would show up in the regular running. Still no fault codes so I guess I will look for a shop or Ford dealer who has a new modern system analyzer to try and find the problem.
 
Now you being in BC this time of year I'm sure it's cold out, are you telling me that on a cold start, she goes right to around 1200 rpm and does not hold there for at least 10-15 seconds, before falling to 900 rpm and holding there for a bit, but rather your Bird drops quickly down to like 450-500 rpm like right after ya start up ? there's only two things that control that idle rpm on a cold start, iac and the pcm, if anyone come and told me that when they do a cold start and the car is down on 450 rpm idle in no time, I'd tell them they are looking at an iac or pcm, in any case, lets forget the low idle for a moment, that idle should not be down any where near there for like 5 minutes, not right after you crank up cold, I'm still putting my money on a pcm issue............your Bird is correct when warm to idle about 750 in park and about 650 in gear, I checked those rpm's on mine warmed-up the other day.........
On getting someone to plug it into the scope machine that the dealer has, so many people will not touch a 30 y/o car and if they will it's $130 bucks an hour but that's here in Florida, don't know bout where your at.........
This will be my last post on this until something changes, I'll let another member get there two-cents in, in closing I'll say why I'm still betting on pcm issue, the pcm that flagship one sent me, that I ended up running on for 300 miles, ran fine, shifted fine, it only had one flaw, it wanted to idle around 4-500 rpm cold or warm, and yes, it was a very smooth idle at that.........
I only wish that I've helped you out in some way, shape or manner..............Mitchell
 
Mitchell
Thankyou for your correspondence, help and suggestions. My reasoning for having an analyser check is as follows: When the engine is cold, an input to the PCM is either missing or incorrect as we agree that the PCM tells the IAC what to do, how much air to allow for the idle. After a few minutes of running, the missing/incorrect input is correct, and the PCM is telling the IAC the right mixture and the idle is as it is supposed to be. There isn't and has never been any error codes so there is not a definite fault with any device that would normally give an error code if there was an error. So, because changing the IAC, the PCM, the MAF, thermostat and temp sensor, plugs and wires has not made a significant difference, I don't believe there is a problem with any of them. Yes, my local Ford dealer (where I have had some work done before) charges about $190 an hour, but if I were to order another IAC $59, or a PCM $162 the cost wouold be the same, not to mention the time, effort involved to to change them.
Again, thanks for your interest, and I will update you when things change.
Cheers
 
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