vector006

Working on a vette for a friend and the car is idling poorly until it warms up. The engine hesitates when you crack the throttle during cold starts. Once engine warms up and goes into closed loop the hesitation is gone, but the car hunts between 800 and 1200rpm, in a rythmic manner.

I have done the following:
- Checked ECT using scan tool-> OK
- Checked TPS using scan tool-> OK
- Checked MAF response using scan tool-> OK
- Watched O2 voltage transient using scan tool
- Replaced IAC
- Performed idle reset using the factory procedure
- Checked throttle body (looked clean)
- Checked spark plug and compression on a couple cylinders that were easily accessible (not all)
- Checked injector ohms -> OK
- Checked fuel pressure while idling poorly -> OK
- Applied vacuum to EGR and observed the diaphragm moving ok
- Sprayed quick start over intake, no change in idle speed
- Disconnected MAF (this raised the idle a bit but the stumbling, hesitation and hunting is still there, just a little bit better)

Ive done my homework and read through Corvette forum at people's solutions to similar symptoms, but haven't found the solution to this particular problem.

No check engine light.

Here's a video the engine warm, running in closed loop. I will upload a vid of it during cold start tomorrow. http://www.youtube.com/watch?v=fwQC6...ature=youtu.be


Thanks

1986coupe

Is the IAC set properly? Has the TPS been set properly? Any vacuum leaks?

vector006

I went through the IDLE reset procedure which i believe sets the IAC valve and TPS. TPS voltage is at 0.54 now (was only 0.36 before, which indicates someone was messing around with the idle screw before i got it).

During a cold start, the engine fires right up, but quickly drops to a very unstable idle and will frequently stall. If it ends up idling steady and you crack the throttle it will stumble and nearly stall until the engine warms up.

Is it just me or do these engines warm up pretty fast? The fans seem to kick on after a short period of time.

I am suspecting bad injectors or a vacuum leak that I cant seem to find using conventional methods.

Cliff Harris

Sounds like a bad CTS (coolant temperature sensor) on the front of the intake manifold. What does the scan tool say about the coolant temperature?

The ECM starts off in "fast idle" mode of about 1200 RPM and slowly lowers the idle speed to the 600-700 RPM area as the coolant temperature goes up. If the ECM thinks the coolant temperature is already hot it will lower the idle speed much faster (which sounds like your symptoms).

The ECM resets the IAC every time you shut off and start the car. It's self-adjusting.

vector006

The scan tool shows good ECT readings all the way from ambient (20C ->90C) as it warms up

vector006

Cold start: http://www.youtube.com/watch?v=tEQu773IbHY
Idle drops to about 800 rpm, fluctuates up to 1200rpm periodically.
Will stall at colder ambient temperatures.
Throttle response is very poor, it falls on it's face momentarily when you crack the throttle. It actually backfired on me in this video at 0:25sec (my phone's audio microphone isnt very good, sounds like popcorn)
Once it warms up there is no throttle response issues.


Cold start with MAF disconnected: http://www.youtube.com/watch?v=Soc1colOjAs
RPM is high (above 1000RPM), engine RPM still fluctuates up to 1200rpm.
Throttle response is decent

I checked the MAF reading using my scan tool and the numbers look ok to me, and transient air flow readings look good. Im starting to suspect there might be a manifold leak that throws off the MAF reading in open loop operation. Once it goes closed loop it adjust AFR with O2 sensor. But if this were the case, i would expect a check engine light.

Here is a better video showing the idle hunting during warm-up: http://www.youtube.com/watch?v=24b_6_yjq-I

vector006

I went to check the ignition timing last night but found out the timing mark on the timing chain cover must have fell off. There is a rust mark on the timing cover where it was pinch welded on.

However, when i disconnected the ECT to check base timing, the hunting went away completely. hmm

vector006

Manifold air temp sensor readings look good. I was surprised to see 50C air temperature after 5 min of idling at 20C ambient.

Lilred87

Make sure that the EGR valve is not stuck open. I fought an idle problem for 4 years that turned out to be the EGR valve. Had to take the plenum off and cleaned the EGR and fixed the problem. Just watching the vacuum module move doesn't mean the egr is in working order. When it is stuck open it creates a huge vacuum leak. I occasionally got an SES34 when it started. Hope this helps.

ChasD

I replaced all of my lines and that solved my idle issue on my 1986.

Cliff Harris

On the '86 the MAT sensor only controls when EGR is enabled, nothing else.

The timing seems to be an issue that needs to be resolved.

You might benefit from a minimum idle adjustment:

http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm

vector006

Cliff:
I already performed a minimum idle adjustment, I was able to get the minimum idle set when the engine warmed up a little. Not sure if that excludes a vacuum leak.

Lilred87:
Thanks for the tip n the EGR, i think im going to have to pull the entire plenum apart looking for clues.

Cliff Harris

I had an idle issue recently that turned out to be a ruptured brake booster diaphragm. It was only slightly torn, just enough to leak without affecting the function. Plugging the vacuum line at the booster fixed my idle.

383vett

Sounds like you need to make sure the timing is correct. You can use a timing tape (6 3/4" balancer) and a piston stop to determine tdc. You can buy an adjustable timing pointer that mounts to the timing chain cover plate mounting bolts. Good luck

JrRifleCoach

Possibly a vacuum leak.

I had a similar problem and with the engine at idle started torqing the runners and plenum bolts.
Problem solved.