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Unread 12-27-2008, 10:58 PM   #1
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Unhappy Runs rough when cold

I have a 90 Chevy 2500 , 5.7. The truck runs fine when it is warm, but when it is cold it runs rough, rich and stalls .

I have changed the idle air valve, the TPS, and the thermostat sensor without any results. Also, I have taken the truck to the dealer for a diagnostic check. The mechanic states that the ECM, PROM and EGR valve, and solenoid need to be replaced. I am skeptical of this. Can the ECM not work when cold ,then work when warm?I don't want to keep throwing money at this problem. Does anybody have any advice.

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Unread 12-27-2008, 11:02 PM   #2
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well if your throwing codes then it might be you need a new ECM. As for why it only runs rough when cold Im not 100% sure on that. The EGR can get clogged after a lot of miles on a car. It might be that there is thick sludge in there and when its cold its not allowing much recirculated air through. But really not sure.
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Unread 12-28-2008, 01:14 AM   #3
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Gdp is probably right on and on a 1990 Chevy it's not hard to damage a ECM but professional opinion I would change all of the tempature sensors. I have seen many cases that a faulty temp sensor will cause the ecm to add tomuch fuel think it's colder then it really is and make it flood. Your sonario sounds different however I think you may believe it's running rich because you smell gas chances are the ecm isn't correctly advancing and retarding the timing for the tempature changes so it's not making full power and kicking alot of gas out of the tailpipe.

There are a few things that could cause this senerio thow I would believe the dealer did a proper diag although they can be wrong and suspect that's why they want the ecm replaced.

The Ecm is working when cold or else your truck wouldn't be running. They should have 3 temp sensors on the motor the one on the t-stat you changed only reports to the gauge on the dash and not the ECM changing the other 2 replacing the EGR and cleaning the EGR passages would be a great place to start before jumping on a ECm. Acouple more things to look at would be is the time set to far advanced or forward if it is reset it to the factory specs, the vehicle could have developed a dependencie on high octane fuel from some one not using regular unleaded see how it runs on premium for the next couple tanks, and check to see what the fuel looks like coming from the injector at the Trottle body if it's dripping this could cause your problem it could only be properly vaporizing fuel after it hits operating tempature rather then when cold with a properly firing injector.

If you can give me more info sound, long cranking before start, smell, feels like missfire, or anything it's to late at night for me to think of more possability's right now but with more info I may be able to help you narrow it down.

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Unread 12-28-2008, 04:21 AM   #4
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Unhappy more info

thank you. The engine will run at a higher rpm initially,but after it has run for short while ,it will ether stall or run really rough and then stall.It runs rich when it is running rough. When this happens the check engine light comes on. The truck does not miss fire and will run fine once warmed up. I checked the spray coming out of the injectors,the spray seems fine at the at first, but when it starts to idle down they start pulsing. One other thing it will do sometimes, idle high after it has warmed up. I hope this has helped and look forward to A response.

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Unread 12-28-2008, 11:07 PM   #5
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Try something for me on the firewall under the hood on the passenger side of the motor is a timing spout it's basicly a connection with a easy release connector it's the input for the distributor from the ecm you have to disconnect this to set the timing if you can find it disconnect it and try to start the truck under the cold condition see if it does the samething. If you can't locate it ask a mechanic he should be a able to show you.

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Unread 12-29-2008, 05:21 PM   #6
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Default no luck yet

I have set the timing to the specs. It was out a little , but not enough to make a differance. I found the conecter on the fiewall and will check it unpluged.

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Unread 12-29-2008, 07:27 PM   #7
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also if you set the timing and that connector wasn't unplugged then you havn't set the timing it moves while the connector is plugged in you need to get the vehicle warm unplug the connector and then set the timing that could also cause a problem. I'm kinda leaning towards the lines of EGR, fuel pump, or sticky valve.

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Unread 12-29-2008, 11:34 PM   #8
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Default Timing OK

AGAIN, THANK YOU FOR YOUR ADVICE!

I set the timing like you said, and also to the manufacturers specifications. I also tried running the truck with the connector unplugged and it still stalled.

Tonight I am going to remove and inspect the EGR valve and replace it if necessary. Also, I am going to check the ECU coolent temp sensor, although I have already replaced that sensor last winter.

Another thing I noticed, is when the truck is running cold is that it almost sounds like there is a hissing-whistle noise coming from the inside of the throttle body. And this happens right before the engine is about to stall.

What sticky valve are you referring to in your last reply?

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Unread 12-30-2008, 12:13 AM   #9
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Have you checked the Coolant Temp Sensor?

If this is out of range the ECM will make the wrong calculation for fuel.

Use this chart to determine if it is within range. The CTS is located on the intake manifold next to the upper rad hose/thermostat. I've had a few vehicles where this has fixed the problem.

cts.jpg
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Unread 12-30-2008, 03:59 PM   #10
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That might also be a fuel pressure switch. We had two problems, both with Pontiac 3800 V6s, doing that when first started up, and replaced it on both, and they run like tops. It's like $60 at NAPA. Take a look. You can pull a certain vacuum line and if gas comes out that's definitely it.
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