1994 start-up idle problem

[Pearl 94]

Recently, my car will not idle when I first start it. Sometimes will stumble, and than idle roughly, most times will die. Step on the gas pedal to stop the rough idle, and it dies. I can override the problem with the two-foot gas/brake method, bring the rpm up to throttle body opening, put it in gear and away I go. After a minute or two, it will idle when stopping at a light or similar.
My bible-size maintenance manual says the idle is controlled by the IAC, idle air control valve. Got a new one of those, put it in and no change. More reading, the power control module controls all the fuel functions, idle, acceleration, etc.,. Read up and tried the PCM reset options. Before I go and find a new/repaired PCM, I am wondering if that is the problem. Other than the starting problem, the rest of the car works well, Acceleration, gear-shifing etc. I do have an older-style OBD reader, but it shows no codes. It seems as though one of the inputs to the PCM needs to "warm up" or stabilize to enable the idle function. When the engine is warmed up, it idles smoothely, and accelerates smoothly, so I am pretty sure it is not a fuel delivery or injector problem.
FYI: I may have caused the problem inadvertently: I had the battery disconnected for 3 or 4 hours when I had the dash out, to replace the small gears in the odometer. Found out later, that when the battery is disconnected for a long period of time, many of the electronic modules, including the PCM lose the stored information, and have to "re-learn" inputs when the battery is re-connected. As i said, I have tried 2 or 3 times to do the recommended reset but didn't help.

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[Alex]

Beste,
Het kan zijn dat met de kou de accu minder spanning geeft dan 12 volt,meet dat eerst eens vóór je de koude start doet,of sluit de acculader aan vóór je start,het kan zijn dat er een cel kapot is,maar dat merk je pas in de winter,als hij een keer loopt komt de dynamo erbij.
Alex

 

[mcm51]

Is it a 4.6L ?, if so, it sounds a lot like what I went thru years ago, rough idle when I crank up cold, then smooths out after a few minutes, ran fine other wise, I first tried a IAC, no change, got a used PCM off ebay, no more problem, runs like new..................

 

[Alex]

Beste,
Het kan zijn dat met de kou de accu minder spanning geeft dan 12 volt,meet dat eerst eens vóór je de koude start doet,of sluit de acculader aan vóór je start,het kan zijn dat er een cel kapot is,maar dat merk je Pas in de winter komt er een keer de dynamo erbij.
Alex

De pcm houdt niet van te lage spanning

 

[Frankie the Fink]

In the old carb days what you describe is exactly what happens when the old choke systems didn't enrich the cold mixture enough on start up. Rough idle, feathering the gas pedal, etc. I think you might have a lean mixture.
Why ? I can't say...

 

[Pearl 94]

Thanks for the replies, guys. As for replacing the PCM, I think that is what it will take. have looked on line, and they are available. As for a lean mixture, i don't kinow about that. 2 or 3 times when I start the car and have the window open, I can smell gas, so i am wondering if it is a too-rich problem rather than too-lean? Obviously, as it is fuel injected, the smell would not come from the engine, but from the exhaust.

 

[Frankie the Fink]

You should have some black smoke with an over rich mixture and prob fouled plugs.

 

[mcm51]

First off the scent of fuel is normal when it is cold, I smell that on mine sometimes, that's just the PCM pouring in a ton of fuel, that's why stone cold you can drop it in gear and take off down the road like it's all warmed up already.........remember the PCM is setting the air/rich mixture........I'd still consider a used PCM, the car is over 30 y/o, mine is a '95 and I started having starting issues and random roughness during some cold starts, never that once it's warmed up.......If you looked into those PCM's on ebay you probably saw between $60. and $150........Now shouting at the PCM, we are assuming that all the sensors are giving the PCM a proper signal and the PCM is passing back incorrect info to motor as to how to adjust......One more thing, a failing PCM always seems to give ya bad fuel mileage, not super bad but you'll think ya should be getting better..............Oh, when ya pull that PCM, make sure ya buy one that has the correct catchcode, mine was A5WO, that code has to match or you'll have a low idle and it will not shift right........hope all this helps.....

 

[mcm51]

Oh, ya never said anything about codes, have ya checked to see what codes ya got if any? now my failing PCM never gave me a PCM code but it was giving codes that was not there........one other thing a mass air sensor could give you the issues that you describe also, consider how long has it been since the mass air sensor been cleaned or replaced...........just a thought.....................

 

[Pearl 94]

In a bit of a quandry at the monent. looked (again) at the Tbird/cougar electrical troubleshooting manual and it shows the PCM connector to be one large 104 pin unit.
All the parts dealers as well as PCMs for sale, show two connectors 8 double rows each side. That's the easy part. I thought I would pull off the right side kick panel (where the module is supposed to be!) and see if there were any part numbers or such, to relate to replacement module. Pulled the kick panel off and the insulating panel behind it, to find no square PC module. Only small block connectors and wires. Just above the kick panel, right under the dash, is a "module" that would appear to be the PCM. 2 screws on a metal plate bolted to the module. It has two plugs, 6 pin connections on one side and 8 pin connections on the other. The 6 pin side is full, the 8 pin side only uses the inner 6 pin slots. No marking on the module other than FORD. So. Have I found the PCM or is it some other module?
Thinking of a possible cause, as I said, it seems when the idle smoothes out, it is after running for at least a minute as though it takes something time to warm up . With that in mind, I see that the PCM has inputs from 4 heated oxygen sensors. If one of those was bad, it would mke a convenient reason for the rough idle.
Re the question of codes, no - I have checked two or three times and there are no codes., but as I said my OBD reader is like me, old!! Might not be able to pick up the more modern in/outputs.
Hadn't thought about the Mass flow sensor, will have a look at it.