When I start the car, (86 Fiero GT, 2.8) and the engine is cold, it starts normally, then rapidly idles way down till it almost dies, the computer senses this I guess, and the engine rev's up. Every now and then, it will idle down too low before the computer catches it, and stall, and I have to restart it. This will repeat itself for a good few minutes till the engine warms up.

After it warms up just a bit, it settles into a nice smooth idle, with no missing ect.

I can not for the life of me find any reason to explain why it is doing this.

Engine history:

- 2.8 Crate engine with 2,000 miles on it.

- All new parts on the engine less than 1 running hour old to include: New A/C compressor, starter, alternator, distributor cap, rotor, plugs, high-end wires, Fuel Pump, module, Intake sensor, O2 sensor, coolant hoses, thermostat, and tons of other stuff not relevant to this....

The engine has been timed, following the "paper clip procedure" and is right on the mark.

Any ideas?

[This message has been edited by JohnWPB (edited 09-18-2010).]

check fuel pressure while this is happening

FWIW, my 88 2.8 does exactly the same thing except the ECM almost always catches the engine before it stalls....except when it's been sitting for more than a week (or two) and then it will stall once or twice before it will maintain a steady idle. Even when it stalls, within a minute or less of operation ( or help from pressing the throttle) it will maintain a steady idle or restart.

With all of the work you've done i would guess you have...but if you have not, clean the throttle body and remove/clean the Idle Air Control motor and where it seats in the throttle body? Do NOT push the IAC pin back in. Just put some brake clean on a rag and wipe the end of it.

quote Originally posted by phonedawgz: check fuel pressure while this is happening

I am not a certified mechanic, but I can usually reason things through... so my thinking on it being the fuel pump. Is this possible? The fuel pump (also new, with less than an hour it on it) primes when I turn the key on. When it starts, and idles down really low, the computer catches it, and revs it back up. It it were the fuel pump, would there still be enough pressure / fuel to "rev it back up" when the computer catches it?

The ECM doesn't adjust the idle speed that fast.

Checking the fuel pressure would be the first thing I would do. $15.99 for a fuel pressure gauge for injected engines at Harbor Freight.

If the fuel pressure were fluctuating like that the engine would follow.

I don't know that the fuel pressure is your problem, just that would be the place I would look at first to either identify it, or eliminate it as the cause of the problem.

[This message has been edited by phonedawgz (edited 09-18-2010).]

intake gasket leak? Just a thought. I had this happen on a DOHC 3.4L and it turned out to be the intake gasket had a leak.

You might also want to check your throttle stop screw. The screw is used to keep the engine running when the IAC isn't in the right place yet. Start and warm the engine first and turn it off. Then jumper A-B and turn the key on. The ECM will fully close the IAC. Turn the key off and remove the jumper. Then unplug the IAC. Then start the car and adjust the throttle stop screw to achive a 500 rpm idle. Then re-attach the IAC and see if it cures your problem at the next cold start.

quote Originally posted by phonedawgz: Then pull your finger off and hear the engine run fast for a while and how long it takes the ECM to adjust the ICM to get the idle back to where it should be. The response time of the ECM/ICM IS slow.

I think phonedawgz meant IAC, not ICM here.

Ok, thanks, I have a few things to test now.... I will post back any findings. Thanks again!

quote Originally posted by JohnWPB: Ok, thanks, I have a few things to test now.... I will post back any findings. Thanks again!

Try swapping out the battery for a known good one. If that's not possible, put a charger on your battery overnight, give it a full charge prior to starting. May wish to have the battery tested too. If you haven't already done so, check the primary positive feed from the alternator up to the main terminal then up to the battery. Double check your grounding strap? How many accessories are plugged into the car overnight? Check for battery drains. See what I'm getting at here?

quote Originally posted by phonedawgz: You might also want to check your throttle stop screw. The screw is used to keep the engine running when the IAC isn't in the right place yet. Start and warm the engine first and turn it off. Then jumper A-B and turn the key on. The ECM will fully close the IAC. Turn the key off and remove the jumper. Then unplug the IAC. Then start the car and adjust the throttle stop screw to achive a 500 rpm idle. Then re-attach the IAC and see if it cures your problem at the next cold start.

DING-DING-DING-DING!!!

As a matter of fact, I just did exactly the same thing to fix exactly the same problem on a customer's car. If the idle set screw has been messed with, this will most likely be your problem.

The battery is brand new, and nothing seems to be draining it overnight. When I turn the key on, I have a good 12v after letting the car sit for a couple of days.

+1 for the throttle screw... that sounds like where I am going to start. Thanks!

I had a similar prob that turned out to be a defective tps sensor. Also thought possible cold start injector prob?

I'm also having this problem. Only when it's cold. Fuel pump builds pressure. Engine turns over, starts, typically dies unless the throttled is tapped once or twice.
No matter what the culprit is or isn't, I'm keeping an eye on this thread.

Edit: Didn't realize the last reply date - I assume the issue was solved...so what was it !

[This message has been edited by Navras (edited 10-17-2010).]

Start a new thread

quote Originally posted by Navras: I'm also having this problem. Only when it's cold. Fuel pump builds pressure. Engine turns over, starts, typically dies unless the throttled is tapped once or twice. No matter what the culprit is or isn't, I'm keeping an eye on this thread. Edit: Didn't realize the last reply date - I assume the issue was solved...so what was it !

phonedawgz, PM sent