99 Lumina LTZ stalling issues

1999 Lumina LTZ 3.8l 193K miles.

1)Was having stalling issues at idle with COLD engine. No problems with warm engine, runs fine otherwise. Was setting a low MAF sensor voltage code.

2) IAT, MAF sensors replaced, idle improved. Now will cold start and idle fine and run fine, will not stall when at a stop. But, when warm, it will stall at idle when starting. Will eventually idle and run OK if I keep it at 1K RPM or so for a few minutes.

EGR working fine, I cleaned it out as well...no apparent carbon blockage. PCV valve new. There are no codes set now.

Since the idle issues only happen with a warm engine restart, it seems that it is likely to be temperature related. I have an AutoEnginuity enhanced software program and can monitor a number of things, but cannot find any abnormalities. It wasn't showing a MAF sensor output frequecy, but replacing the MAF sensor fixed that, and the frequency is within normals.

It drives and idles great when I come to a stop, but at warm engine restart it will start, rev up a little over 1K RPM, then stall unless I keep the RPM over about 1K. After a couple of minutes of holding the idle speed up, it will idle fine and drive fine.

Check for raw fuel in the vac hose to fuel press regulator.

If that's the case, the fuel pressure regulator is pooched, correct?

But....if it's bad, wouldn't that affect the cold idle too?

No gas in the vacuum line, no gas coming out of the vacuum fitting on the fuel pressure sensor.

Leaky fuel injectors can cause this type of issue. You can put a fuel pressure gauge on the fuel rail, turn the key to "on" and see the fuel pressure come up, then turn the key off and see if the fuel pressure drops off quickly. It should stay up for a while, but if it is down to zero in less than 10 minutes, it could point to leaky fuel injectors. You can warm the car up like you do whenever it gets hard to start, shut it off, but quickly relieve the pressure in the fuel rail. Then wait the amount of time you need to wait to try starting it to reproduce the hard start issue and try starting it. If it starts better, you have leaky injectors.

Let me clarify, it starts right up...every time...but then the idle drops and it stalls if the engine is warm, but not cold.

It seems to me that if leaky fuel injectors were the problem, it wouldn't start well because there would be fuel leaking through the injectors into the cylinder(s), in effect flooding them with gas before startup, yes?

I'll definitely check the fuel pressure and see if that's the problem, but IF I understand what could be happening if there is/are a leaky injector(s), that doesn't seem to explain what I'm experiencing.

Not to seem argumentative, but it's driving me :screwy: trying to figure this out, even though I'm selling it.

Additional information: Driving along the coolant temperature is 194 (ambient temp outside today between 85 and 90). At idle, which is smooth, temp goes up to 214-215. Idling for a while it goes up to 224-225, the electric fans go on, and the idle gets funky, and it may or may not stall. What happens is the idle speed drops, it recovers, and goes up and down a couple times and usually stalls.

Could this be as simple as a stuck thermistat?

I wouldn't expect the idling temps to be that high.

My car does the same thing, normal, but as the fan comes on and loads the engine the pcm bumps up the iac to raise the idle. I would clean the throttle body, also remove and clean the iac and passages.

Did all that already, in fact replaced the IAC. No luck.

Also checked the fuel pressure; normal.

Now what? I'm flummoxed. :runaround:

Make sure the egr is functioning normally or try disconnecting it for test purposes.

EGR is functioning normally, passages cleaned, no evidence of carbon particles inhibiting pintle seating.

All sensors functioning normally, no codes setting.

Ok, did not read thru previous posts, I have the same car in a 98 LTZ and experienced this on occasion. Injector leak down is possible, that would affect a short hot soak restart, but not stalling at idle hot. Mine starts then stumbles, drops down for a second and recovers. Running temps are about the same mileage about the same. A fault in the pcm could make the iac slow to respond to idle control demands, that would not set a code, I never pursued this far enough to resolve it.

Mine does the same but does not recover; it stalls.

I'm wondering if it is a temperature related vacuum leak issue with the plastic intake manifold; a crack or leak somewhere that occurs only when the engine is warm.

But....that wouldn't explain how it "heals" itself when I keep it revved to 1500 RPM or so for a minute or so, then it idles fine, would it?

So you drive the vehicle around for 15 minutes, pull into a parking lot somewhere, put it in park, shut the key off, and then within 10 seconds try starting it and it stalls? Or do you have to wait about a half hour for it to stall when you restart it? If the answer to the first question is 'no', then a temperature sensitive vacuum leak does not sound like the answer. If the answer to the second question is 'yes', then this is what was happening to my wife's 3.4 liter minivan until I determined that her injectors were leaky and replaced them. Never had another problem since. I never heard that you disproved this theory, so, you just never know. The test for leaky injectors can be done in 15 minutes and you would know. Believe me, I could not believe that they would cause her car to act that way, but they did.

So you drive the vehicle around for 15 minutes, pull into a parking lot somewhere, put it in park, shut the key off, and then within 10 seconds try starting it and it stalls? Or do you have to wait about a half hour for it to stall when you restart it? If the answer to the first question is 'no', then a temperature sensitive vacuum leak does not sound like the answer. If the answer to the second question is 'yes', then this is what was happening to my wife's 3.4 liter minivan until I determined that her injectors were leaky and replaced them. Never had another problem since. I never heard that you disproved this theory, so, you just never know. The test for leaky injectors can be done in 15 minutes and you would know. Believe me, I could not believe that they would cause her car to act that way, but they did. On target, not an usual problem on any of GM's 6 cyl. engines. Fuel pressure test at the rail can confirm this.

The fuel pressure at the fuel rail is 43 psi. Goes up to 50 psi when running if I remove the vacuum to the fuel pressure regulator; goes back down to 43 when I put it back on. Goes up by about 5 psi temporarily when you goose the engine.

Pressure bleeds off to 30 psi after 10 minutes of engine/ignition off and seems to hold there.

Yes, if I drive it around and stop for a while, then it stalls on restart unless given some throttle to about 1000-1500 rpm for a minute or two, then will be able to settle down into a smooth idle at about 730 rpm.

It may start right up if I try to start it immediately with no stalling. After half an hour, it definitely will stall on restart.

Problem solved...I sold it yesterday. Would have been interesting to see if the plastic intake plenum was the problem, but not more interesting than making it someone else's concern. :wink:

Thanks for all the help.