Car seems a little "cold blooded"

DrScientist · Apr 26, 2011

Hi all. I seem to be having a little trouble out of my 94GT (bone stock) when I first fire it up for the day.

When I start the car when the motor is cold, it seems to idle ok. But then when you go to put it in reverse, or even 1st gear, it starts choking up and trying to die. It seems to get down to about 600 RPM unless I give it some gas and try and keep the RPM's a little higher. Then of course they shoot up past where they need to be for backing up, or starting off in first gear.

And there is some kind of popping noise coming from the mufflers while all this is going on. Some kind of low pitched, semi-quiet, popping noise. The noise is not rapid or anything, its a little random. Its something like "...poomp... poomp poomp... poomp..." Like its struggling or something. But the weird thing is, once the car warms up, this never happens. Its like the problem just disappears, until the motor gets cold again. Any idea what could be causing this?

ProKiller · Apr 27, 2011

clean the MAF and the IAC motor.

DrScientist · Apr 27, 2011

Got a brand new MAF on the car. Where is the IAC motor? Is that the thing on the side of the throttle body?

ProKiller · Apr 27, 2011

yup. they get carbon build up on the gearing and can act choppy. not saying that will cure your problem but it will probably help with the idle.

DrScientist · Apr 27, 2011

Ok this next one is probably a dumb question but is that something that you need to take off to clean? Or is it something that berryman's throttle body cleaner can fix by spraying it in there?

toyman · Apr 28, 2011

ProKiller said:
yup. they get carbon build up on the gearing and can act choppy. not saying that will cure your problem but it will probably help with the idle.

I'm confused There is no gearing on the IAC valve. It's a solenoid activated bellows as far as I can tell. If there is a problem it's with the solenoid and there is nothing that can be cleaned to fix it.

ProKiller · Apr 28, 2011

mine is a screw type plunger and i sprayed some brake clean in there and all kinds of carbon came out. helped my idle smooth out.

BlackVert · Apr 28, 2011

generally the thing about the iac that makes cleaning it problematic is that the shaft is treated with something that comes off when washed with a cleaner, so while cleaning it may help for a little while, the sticking is bound to come back.

the other thing that makes me think that the iac is not really the culprit here is that when mine was sticking (and i replaced it), it would either idle very high, or try to die, but it would not idle fine and then want to die only when put in gear. but when it would stick (either open or closed), it would be like that constantly until it would stick the other way.

to me, it sounds more like a misfiring type of situation when cold. or maybe it is just really rich when cold and leans out once warm. the computer is programmed to make it richer for the first 30 seconds or so of running, sort of like an old style choke would do. so ... what happens if you let it sit and idle and warm up for a few minutes before putting it in gear?

HISSIN50 · Apr 28, 2011

The IAC is PWM on these cars.

Toluene is a concern with cleaning the throttle body (there's a coating to help keep the TB corrosion free and help with de-icing). I've never heard of the IAC internals having a coating.

Have you pulled codes yet? Are you sure there are no vacuum leaks?

DrScientist · Apr 28, 2011

BlackVert said:
What happens if you let it sit and idle and warm up for a few minutes before putting it in gear?

Well, now it does the same thing any time I'm trying to take off from first. The car still idles alright, but you can feel some kind of miss. When you're sitting in the driver's seat, you can see and feel the car slightly jiggle. It's not trying to die at idle, its just that you can feel that miss and sometimes hear that "poomp" popping noise down in the mufflers.

But I guess you could say that the problem has gotten slightly worse. Because when I made this thread, the car would just barely miss at idle. So it was hard for me to determine at that time, if it was really missing, or if it was just the high mileage motor warming up. But now I know its missing. And it still lacks power and tries to die and makes that poomp sound until you get some higher RPM's. You can't really just take off smoothly like you should be able to. You gotta get some higher RPM's first. But, the car does seem to have most (if not all) of its power once you get going. Once your rolling it doesn't seem to be as bad. Its like it struggles all the way up to 1,200 RPM but then the RPM's just shoot up and the car takes off like its supposed to. All of this is slightly lessened once the car gets warm. But its still there.

toyman · Apr 28, 2011

HISSIN50 said:
The IAC is PWM on these cars.

PWM??

toyman · Apr 28, 2011

You can also get a bog if the fuel mix is too rich. Which gets back to what sensor(s) might be acting up such that the signal(s) received by the ECU calls for extra fuel. Since we are talking about a cold start condition (open loop) that should eliminate the O2's. That pretty much just leaves the MAF, TPS and ECT. I wouldn't be surprised if one of those sensors is throwing a code. Of course a vacuum leak or a bad EGR solenoid could also cause a bog/miss.

BlackVert · Apr 28, 2011

PWM .... hmmmm ... pulse width modulated?

that would mean that there is a solenoid that is either open or closed, and to control the amount of air it lets through, the computer opens and closes the solenoid rapidly. i know it does that sort of thing with the torque convertor in the AOD-E, but i did not know it did it with the iac.

come to think of it, the IAC problem i had was on my 1996 car. i guess i assumed it was the same on the 95. maybe it isn't

toyman · Apr 28, 2011

ProKiller said:
mine is a screw type plunger and i sprayed some brake clean in there and all kinds of carbon came out. helped my idle smooth out.

Hmmm. I have one in my hand. CX-1842 (F4ZE-9F715-AA) It's spring loaded and the plunger closes the bypass when disconnected. I think you perceived the bellows as a screw when you looked into the passage. When it's powered the plunger retracts allowing air to flow past the throttle plate.For sure there can be carbon/crude in the chamber but the plunger is sealed by the bellows. This appears to be how the plunger shaft is protected so it doesn't get gummed up.

BlackVert · Apr 28, 2011

DrScientist said:
The car still idles alright, but you can feel some kind of miss. When you're sitting in the driver's seat, you can see and feel the car slightly jiggle.

this jiggle you are talking about here makes me worry a little about the harmonic balancer. the rubber in them is known to dry out eventually and then they start to sort of come apart. when that happens, then the motor is in danger of more bad things happening, so i would say you should check that, just to be sure.

as to a miss, it may be just a loose wire or something simple. i had a miss that i almost killed myself over. finally i figured out it was a loose wire and felt stoopid about all the time and money i wasted checking and replacing everything else.

toyman · Apr 28, 2011

ProKiller said:
mine is a screw type plunger and i sprayed some brake clean in there and all kinds of carbon came out. helped my idle smooth out.

Hmmm. I have one in my hand. CX-1842 (F4ZE-9F715-AA) It's spring loaded and the plunger closes the bypass when disconnected. I think you perceived the bellows as a screw when you looked into the passage. When it's powered the plunger retracts allowing air to flow past the throttle plate.For sure there can be carbon/crude in the chamber but the plunger is sealed by the bellows. This appears to be how the plunger shaft is protected so it doesn't get gummed up.

HISSIN50 · Apr 29, 2011

Correct on the PWM.

DS, run a cylinder balance test when you pull codes. If there's a miss off-idle and under-load, it's probably there when not moving but you don't notice it [because the vehicle is not under load]. The CBT will reveal a miss that even the pants can't feel.

DrScientist · Apr 29, 2011

Well the check engine light is not on. So I can assume that the computer will still store trouble codes even when the engine light isn't on?

HISSIN50 · Apr 29, 2011

DrScientist said:
Well the check engine light is not on. So I can assume that the computer will still store trouble codes even when the engine light isn't on?

Correct.

toyman · Apr 29, 2011

I seem to recall reading somewhere that the CEL only comes on when the fault causes an emissions related issue. So if the issue self corrects the light goes off but the code is stored. In addition, there may be other faults that are logged but because they aren't emission related the CEL doesn't come on.

Car seems a little "cold blooded"

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