Rough idle when engine cold

rhturbo

Member
Hi everyone,
I am new to the turbo world. Just bought an 87 GN with a few mods. I don't know all the details of the mods but I do know it has an upgraded turbo, upgraded cam, Shift kit and stall converter and upgraded inter cooler (sits in front of rad) Car runs great once warmed up. On cold start it starts instantly and when running on fast idle its very smooth. As it warms up (within 1 min) and the idle drops to normal the idle becomes very eratic and surges up and down. Eventually it just stalls unless I feather the peddle. I have to feather the peddle for at least 2 mins until it warms up and then it settles down and will stay running. Car runs great after that but I will say that even after warmed up there is still a slight fluctuation in idle rpms when at a light etc...

If I go for a drive and then come home , shut car off and then an hour later start it back up the same thing happens. There are no other issues. The car accelerates smoothly and runs great.

Not sure where to start. Could it be MAF or Throttle body needs cleaning ?
 
It could be a MAF issue but cleaning it probably won't help. If the MAF has been replaced at some point which is highly likely it may not read correctly. Most of the remanufactured MAF's are crap as far as the calibration goes. A fair portion of them read low which will cause the car to run lean prior to going into closed loop. There are a number of other things that could be the issue from vacuum leaks to an improper chip calibration. More info is needed.

Neal
 
Thanks for your reply. So are you thinking it may be the MAF ? I do have a scan master. Anything I could look for on it that would help ?
thanks
 
What chip is in the car?

What is the programmed idle rpm?

What cam is in the car? Does the chip match the cam and mods.?
 
Thanks for your reply. So are you thinking it may be the MAF ? I do have a scan master. Anything I could look for on it that would help ?
thanks

In a stock car which I understand yours isn't the MAF should read 5-6 grams per second of airflow at idle when hot in Park or Neutral. It stands to reason with the larger cam the idle RPM would be raised by the person who burnt the chip. Based on that the MAF reading should be higher than stock. See what the scanmaster reads when the car is at a hot idle and let us know.

Neal
 
Feel on / replace all your vacuum lines then take a wrench to all the bolts on the intake to make sure there isn't a leak.
 
What chip is in the car?

What is the programmed idle rpm?

What cam is in the car? Does the chip match the cam and mods.?

I have no idea on any of these as there is no documentation with the car and the previous owner did not do the mods. I do have some documentation but its very vague. The place that did the mods just listed generic names for the mods ie. cam, 60 lb injectors, chip .
 
As mentioned once warmed up it runs and idles great. Its just the initial cold start after it starts dropping down from fast idle that it starts sputtering. I would think if it was a vacuum leak it would run rough all the time even when warmed up would it not ?

I will look at scanmaster tonight when the car is warm and at idle and report back for MAF reading
 
I would think if it was a vacuum leak it would run rough all the time even when warmed up would it not ?
Not necessarily. When the engine is cold the air/fuel mixture has to be a lot richer than when warm. If you have a vacuum leak the mixture could be lean when the engine is cold and as the engine warms up and goes into closed loop the computer will make compensation in the fueling for the lean mixture. I'm not saying that is your problem but with the limited information you are giving us it's very difficult to come up with a viable solution. You could also have a bad O2 sensor. That would give you the symptoms you describe. We're kind of shooting in the dark here with no real data to work with. Pull the cover on the computer and post a picture of the chip that's installed. A picture of the engine compartment would be nice also. Maybe we can see something about your setup that will give us more information on what we're dealing with.
 
As mentioned once warmed up it runs and idles great. Its just the initial cold start after it starts dropping down from fast idle that it starts sputtering. I would think if it was a vacuum leak it would run rough all the time even when warmed up would it not ?

I will look at scanmaster tonight when the car is warm and at idle and report back for MAF reading

When the car is running closed loop the ECM has the ability to adjust the fueling based on feedback from the O2 sensor. Based on the feedback it will either add or subtract fuel from the base calibration. These fuel trims will be reflected in your block learn values. Prior to going closed loop the car will not make fueling changes.

Neal
 
I lived through that exact headache. In my case it was loose intake manifold bolts and a crapped out stuck open/egr valve. faily easy fix. The gasket on the egr is weak at best with the one bolt and the vacuum diaphragm in it is most likely gone, but perf chips usually eliminate egr function. check there for leaks.
 
Update: Just took car out for a drive and warmed it up. Here are the readings from my scan master with car warmed up, started and in park
AF=7
tPs=.50
IAC=00
 
Update: Just took car out for a drive and warmed it up. Here are the readings from my scan master with car warmed up, started and in park
AF=7
tPs=.50
IAC=00
Your TPS setting is too high. Should be in the .42-.46 range.
The IAC is too low. Should be in the 20-30 range.
Go through the TPS - IAC setting procedure and get that straightened out first and see how it runs.
 
Thanks everyone for your replies and advice. I will adjust TPS and IAC . Do you think its possibly the IAC needs replacing ? I don't see or hear any obvious vacuum leaks.
 
Thanks everyone for your replies and advice. I will adjust TPS and IAC . Do you think its possibly the IAC needs replacing ? I don't see or hear any obvious vacuum leaks.
I doubt there's anything wrong with the IAC at this point. The high TPS reading could be an indication the throttle blade is open too far at idle causing the IAC to close down trying to compensate for the excessive air coming through to maintain the proper idle rpm.
 
Someone probably adjusted the set screw on the throttle body in an attempt to raise the idle not fully understanding what they were doing as it relates to the TPS and IAC. Just back the screw out a little which will close the throttle blade, hopefully this will drop the TPS into the normal range and you will start to see IAC counts. You may find the idle is too low if the chip wasn't programmed with a bit higher idle to offset the larger cam which was installed. Maybe you can open up the ECM and see what chip is in it.

Neal
 
I just re-read the Scanmaster doc and realized I should be taking the TPS settibgs with car turned off but key on. I juts took them again
TPS idle posiiton 4.6
TPS Full throttle position 4.80

I am going to attempt to post a pic here of the throttle body on passenger side. Note the open connector with nothing plugged into it. Is that correct ?
<iframe width="480" height="360" src="http://s1079.photobucket.com/user/rhturbo/embed/slideshow/"></iframe>
 
The TPS is still a bit high as the voltage output will change when the car is running. The hose connection you posted pics is for the stock coolant lines that circulate coolant thru the throttle body. There is no harm if these are left open.

Neal
 
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