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Erratic idle, stumbles, and hard start when warm

Old 02-19-2003, 10:39 PM
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Erratic idle, stumbles, and hard start when warm

Hi. New guy here. I recently bought a 91 Camaro RS with 5.0L and 5 speed.

I'm having problems and hoped you guys could help...

The car starts fine when cold. After intitial warm up (1-2 minutes), the idle hunts up and down and sometimes dies.

It stumbles at lower RPM (below 2500) really bad until it comes up to 160 to 180 degrees. After warming up a bit it runs better, but still surges some under load/acceleration.

If you shut it off when warm and let it sit for 20 minutes or so, it is really hard to start.

I've tested the EGR valve, TPS, MAP, checked timing, cleaned the IAC valve and port, and taken it to my local mechanic. Niether of us can figure out what is wrong.

My next step is to take it to Chevy, but I really don't want to shell out a lot of dough in diags to fix it.

Anyone have any ideas? I'm thinking maybe a malfuntioning IAC valve, O2 sensor, Timing chain, or fuel regulator. Does anyone have any recomendations where I should look first?

Any help would be gretly appreciated!!
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Old 02-22-2003, 12:05 AM
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Car: 1990 RS
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had the same probs. mine was a bad o2
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Old 02-24-2003, 09:34 PM
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bubbz89,

Thanks for the reply! I though maybe I stumped you guys.

So, you say your car did this too? O2 sensor was bad? Interesting.

Do you know of anyway a backyard mechanic with a voltmeter can check it? Or do I need profesional help to test it?
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Old 02-25-2003, 04:20 AM
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it almost sounds like a coolant temp. sensor. Mine did that, drove me nuts. No one could figure it out. What happens is that the sensor is telling the computer that the car is still cold, and is basicly flooding the engine with gas.
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Old 02-25-2003, 07:45 AM
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Car: 92 RS - Fully Restored w/Custom Int
Engine: LO3 with some mods
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Same engine, same problem, fault was a bad fuel pump.

Stumped three mechanics who swore the problem was with the computer/sensors. They never figured it out.

The car eventually got so bad it couldn't get out of its own way before it finally threw an SES code (lean condition).

You might save a lot of time/effort/money if you check your fuel pressure first.
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Old 02-25-2003, 10:09 AM
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Re: Erratic idle, stumbles, and hard start when warm

Originally posted by garone91rs
Hi. New guy here. I recently bought a 91 Camaro RS with 5.0L and 5 speed.

I'm having problems and hoped you guys could help...

The car starts fine when cold. After intitial warm up (1-2 minutes), the idle hunts up and down and sometimes dies.

It stumbles at lower RPM (below 2500) really bad until it comes up to 160 to 180 degrees. After warming up a bit it runs better, but still surges some under load/acceleration.

If you shut it off when warm and let it sit for 20 minutes or so, it is really hard to start.

I've tested the EGR valve, TPS, MAP, checked timing, cleaned the IAC valve and port, and taken it to my local mechanic. Niether of us can figure out what is wrong.

My next step is to take it to Chevy, but I really don't want to shell out a lot of dough in diags to fix it.

Anyone have any ideas? I'm thinking maybe a malfuntioning IAC valve, O2 sensor, Timing chain, or fuel regulator. Does anyone have any recomendations where I should look first?

Any help would be gretly appreciated!!
Is this a TPI or TBI fuel system? In either case, a fuel pressure check would be a good step.

You might also want to remove the IAC for cleaning and lubrication (on either system).

Verifying the CTS is as easy as checking the resistance and comparing it at a known temperature to thte table:



The oxygen sensor can be tested, but it needs to be removed to do so. You'll need a digital voltmeter and propane torch to test the relative output. Remove the sensor and connect the voltmeter on a 2.0VDC scale. Hold the sensing element end in the flame of a propane torch while measuring voltage. The sensor should produce a signal of at least 0.75VDC within 60 seconds. Removing the flame should drop the sensor output to less than 0.10VDC. This does not test the quality of the sensor, but will tell you if it is completely "dead" or not. A propane torch flame is nearly devoid of all oxygen and should produce a fairly high signal as a result, just like a very rich mixture. If you don't get the voltage, your sensor may be toast, or contaminated with coolant, carbon, incompatible RTV curing agents, or oil.

If your sensor is good, test fuel pressure. Low fuel pressure can cause rich operation on a TBI (or any speed/density control scheme) due to the very poor atomization of fuel from the injector tips. Liquid fuel can dribble or stream out of the injectors instead of creating a fine spray mist. This liquid fuel can wet the intake, throttle body, and eventually cause a rich mixture as the engine warms up and the fuel evaporates.
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Old 02-25-2003, 03:45 PM
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man i was so angry with my car i was going to put a carb on it, fuel pressure checked out and everything. the all knowing vader told to me check/replace my o2 sensor. next day i changed it, drivability changed 100%. i could not belive that a bad o2 sensor could cause that bad of a drivibility prob. vader saved my tpi, because a carb was coming!! thanks again vader!!:hail: and good luck finding your problem!!
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Old 02-25-2003, 10:52 PM
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Thanks for all the info!

It's a TBI 5.0... sorry forgot to mention it.

I think I will mess with it some more this weekend... if it's not snowing.

I'll let you know what I find.
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Old 03-04-2003, 06:46 AM
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Car: IROC 5,0 TPI '88
Engine: 305 TPI
Transmission: auto
check distributor cap and rotor !!!!!!
I found under tons of some white humid powder ( somthing like ash ) and total burnout electrodes.
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Old 03-20-2003, 10:04 PM
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Pukka,

I think you were right on when you said fuel pump.

Here is why...

I relpaced the O2 sensor since it was cheap and no change.

I then put a fuel pressure gauge in between the tank and the TBI input.

I'm not getting the 9-13 psi I should be getting.

If I turn on the ignition without starting the car I get 6 psi for a couple seconds, then it drops to 0 psi. I think it should hold at 6 psi.

With the car running, I get 7-8 psi. If I romp on the accelerator, the pressure drops to 6 psi, and then rebounds to 8 psi. If I clamp off the return line from the TBI to the tank, I get no change in pressure. I think the pressure should go up. Also, If I clamp off the input line between the gauge and the TBI, the pressure does not go up. Eventually the car dies (lack of fuel) and the pressure then drops to 0 psi again.

From what I told you here, I think it's my fuel pump. I don't think its the regulator since I never achieve the 9-13 psi I should have... even when the return line is clamped off.

What do you all think?
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Old 03-20-2003, 10:34 PM
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Car: 1986 Trans Am
Engine: built 305
Transmission: 700r-4 shift kit
you also might want to check the fuel filters they could be cloged up.
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Old 03-21-2003, 12:07 AM
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Filters?

I replaced the one on the bottom of the car (near the rear). Are there others? I thought there was only the one.

If not let me know where the others are and I'll check them too.
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Old 03-21-2003, 05:45 AM
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Car: 92 RS - Fully Restored w/Custom Int
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There is only one fuel filter on these cars. The fuel pressure should be pretty much constant.

I'd thoroughly check the IAC and coolant temp sensors like mentioned in the previous posts. You can also check the TPS.

Replacing the fuel pump is a pretty big job but in your case probably needs to be done, especially if you don't know when or if it has been done to your car before. Just make sure you rule everything else out before you start tearing into the car.
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Old 03-21-2003, 06:01 AM
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In my particular case the hose that sucks fuel from the tank into the pump had a pressure tear in it.

When the car was "cold" it would start fine since the tear would be small. If the car was "warm" it wouldn't start, the friction and pressure of the gas moving through the hose would cause the tear to expand and not allow gas to get through to the pump. Let it cool by not starting the car for awhile, the tear shrinks, the car starts fine again "cold".

When the mechanic showed me the hose it looked like someone cut it with a knife. Anyway, good luck with your repairs
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Old 03-22-2003, 10:30 AM
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Thanks... I thought there was only one fuel filter. I've checked the TPS and coolant temp sensor and feel confident they're good. The IAC seems to be working properly. I've made it sit at 1000 RPM with the 10K resistor on the ALCL and it idles up fine when cold. I think the hunting idle is from lack of good fuel pressure since I'm not even at 9 psi.

Interesting about the fuel line tear... I'll make sure to check my lines too.

So from your comments, I think your agreeing with me that I definately have a fuel pump problem. I think I'll start shopping around for a pump. If anyone feels I'm off base here, please straighten me out now!! Otherwise, I'll be tearing the back end of my car apart soon.
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Old 03-30-2003, 08:01 PM
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Well folks... I finally got it fixed!! WHoo HOooo!!!

It was the frigging fuel pump!

I put a pressure guage on it the other day and determined the pump was bad since pressure was low and it wouldn't hold any pressure... even if I clamped off the TBI return line.

So, I bought a pump at the local Carquest and my wife and I put it in this weekend. What a pain in the rump that was!!! Even had my wife swearing like a sailor by the time we had that dammed fuel tank back in the car.

Anyway, got the tank back in and applied power to the pump. Low and behold it had 13 psi... and would develop 18 to 20 psi if I clamped off the input to the TBI. So, we finished buttoning up the car... you know, because you have to take the whole frigging rear end of the car apart. We started it up and it was like a new car... much more power when you actually have fuel going in the engine!

So now I'm going back to the mechanic I took it to and giving him a rash of shi#$ since I told him to check the fuel pressure and he didn't... dumb ***.

I really appreciate all your help!! You all gave the confidence to move forward and fix my car without getting raped by another mechanic or the chevy dealer.

Thanks again!!!
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