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Rough idle on cold weather start


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11-11-02, 04:47 PM   #1  
jeff5295
Rough idle on cold weather start

I have a 91 GMC 2WD 350/TBI with 135,000 miles. When outside temp is below around 50 deg, the engine initially starts up fine, but runs extremely rough and will usually die if I don't bring the RPM's up a little. It bangs and pops and sounds terrible for about a minute or so, then runs fine. I've replaced plugs, and it has recent wires and cap & rotor. Never have the problem when outside temp is up, or when engine is warmed up and restarted. I also have replaced IAC . Old IAC was stuck, so I thought that was the problem, but it didn't help much, if any. I've taken TBI unit apart as much as I could and cleaned with carb cleaner, no results, tried injector cleaner, no results. Thought about replacing coolant temp sensor, but when I unplug it while running, engine tone changes, so it seems like it's doing something. Other problems consist of occasional stumble or dropped cylinder at idle when in gear and frequent pinging under load, may or may not be related. I get no check engine light codes. Also, tach needle is erratic at idle ocasionally, even when idle is smooth.

Could this be something as simple as a coolant temp sensor, or does it sound like I have some sort electrical gremlin, possibly ECM related? Or am I overlooking something with the IAC? The old one was pretty dirty and would not turn at all. Any help you can give will be greatly appreciated, as it's getting colder here everyday and I wonder when it will decide to not run or start at all.

Thanks in advance,

Jeff

 
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11-11-02, 05:16 PM   #2  
Keithar
Hey Jeff, sounds like you have a bad cold start switch.
The Temp coolant sensor will need to be replaced.Its not an ECM problem. Have you tried any code tests to see if any of the sensors have set off a code?
Have a gr8 day

 
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11-11-02, 05:52 PM   #3  
davidf
Clean throttle body which is the part of engine the big air cleaner duct attaches to.You are supposed to do this periodicaly for good starts and engine smoothness .Buy it at auto parts.

 
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11-11-02, 06:22 PM   #4  
jeff5295
Keithar, I've never heard of a cold start switch, is this a seperate component or am I just replacing the coolant temp sensor? I was thinking about going to get one tomorrow anyway, because I'm running out of options.

I'm not getting a check engine light, so I never tried to see if there was any stored codes. I'll check that out tomorrow also.

Thanks!

 
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11-12-02, 04:25 AM   #5  
Keithar
Originally posted by jeff5295
Keithar, I've never heard of a cold start switch, is this a seperate component or am I just replacing the coolant temp sensor? I was thinking about going to get one tomorrow anyway, because I'm running out of options.

I'm not getting a check engine light, so I never tried to see if there was any stored codes. I'll check that out tomorrow also.

Thanks!
Its more commonly known as a cold start valve.I did some checking and the 97's dont have them.
There is one thing that you can look at as well as the Temp sensor and that is the MAP (manifold Absolute pressure) sensor.
Try disconnecting the battery for 30-60 seconds to clear the ECM and try and start.Then run a code check on the ECM. There may be a ghost code in there thats not showing up.
Also, do a feul pressure test from the shrader valve when its cold.
You may have feul bleeding back to the tank if there is a leak in the return line.
I hope this help some.
Keith

 
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11-12-02, 04:56 AM   #6  
Joe_F
There's no cold start switch on a GM truck with TBI.....

Cold start valves are usually on multiport fuel injected applications...some German (Bosch designed systems) have them, and some Delco systems do, but not this one.

Have you checked the EGR valve for carbon deposits? This will cause the jazzing at idle you describe and is very common with many vehicles, not just GM.

 
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11-12-02, 09:27 AM   #7  
jeff5295
I will buy a new coolant temp sensor this afternoon and take off the EGR valve and have a look at it too. I won't know if the sensor helped until tomorrow morning when it's cooled down again, but I'll let you know if that cured it.

Would a bad/dirty EGR valve cause the pinging under load as well as the occasional erractic idle? I've tried better gas and backing off base timing a degree or two with no results. It's especially bad while towing, but will even do it going up a hill without a load.

What are your thoughts on the tach needle occasionally bouncing at idle, even when the engine is idling smooth? It seems like some sort of electrical malfunction.

Thanks,

Jeff

 
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11-12-02, 04:18 PM   #8  
Joe_F
Sure will....

You're also up there in mileage and there is likely some slack in the timing chain. This will cause some pinging as well.

 
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11-12-02, 07:21 PM   #9  
jeff5295
Well, I read the stored codes in the ECM and I did have a 15 which is a low coolant temp reading. I replaced the temp sensor and will know tomorrow morning if it was the problem. I did find my erractic idle problem though. I noticed it doing it again, so I pooped the hood and could hear a spark everytime the idle dropped. I could see a spark jumping from #8 boot to the manifold. I rotated it away and it stopped.

I still have the pinging though and I took off the EGR valve to have a look. It's not carboned up or stuck, so I'm wondering if there is a way to tell if it's not funtioning properly. I thought about making a temporary block off plate to see if the pinging went away when the EGR passage is sealed. I think that should tell me if the pinging is associated with EGR or not. I'm also going to check the knock sensor to see if it's not funtioning, that could be my problem I guess also.

Thanks for the help so far, looks like I'm making some progress

 
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11-13-02, 04:59 AM   #10  
Joe_F
Simply plug the vacuum line and try it. If there's no change, the EGR isn't the problem.

Check the wire boot closely. It might be split or damaged (unless it wasn't clicked on all the way).

 
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11-13-02, 09:57 AM   #11  
jeff5295
Good news, the truck was able to idle on it's own this morning and ran much better. Idle quality is still a little rough when cold, but it's no worse than it had been previous to this incident, so I'll live with it for now.

About the EGR, would just plugging the vacuum line tell me much? I was under the impression that maybe the valve could be leaking exhaust gas into the manifold all of the time, even while closed, which is why I considered blocking the passage to eliminate any potential leak. Maybe I'm not fully understanding the operation or intent of the EGR system though.

I plan on testing the knock sensor this afternoon too, would just tapping on the block with a metal object while watching the timing mark with a timing light to see if it's retarding tell me the whole story?

I'll probably pull the dist cap of and try rotating the crankshaft back and forth while watching the rotor, to see if there's slack in the timing chain, unless you might have a better suggestion.

Thanks!

 
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11-13-02, 01:04 PM   #12  
Joe_F
Generally, you block off the line to take the EGR out of the picture to "test" it.

The proper way to test EGR valves is in the GM service manual----a definite addition you should add to your toolbox and library.

Unless the diaphragm is completely stuck, you shouldn't have to remove it to test without it.

As for the answers to your other questions, check autolibrary.org in my signature file below.

 
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11-13-02, 11:00 PM   #13  
Check to see if your EGR solenoid is allowing vacuum to pass through at an idle. It should not. If it does, put a hand vacuum pump on the manifold side of the solenoid. With the key off, apply vacuum. A good solenoid should hold the vacuum. If it does not hold, replace the solenoid.
If it does hold, leave the same setup with the pump, apply vacuum and turn the key to "on" (do not start the engine). If your vacuum drops, check to see if the gray wire from the soleniod is grounded. It shouldn't be.

I see you already checked the EGR valve, did you replace the gasket?

My GM manual list the following items to check for Spark Knock;

1. Overheating
2. Coolant Sensor (if you bought yours at autozone I'd check this again)
3. Ignition Timing (knock / EST doesn't work well without a good base setting.)
4. Thermac staying closed (if equiped)
5. Low fuel system pressure
6. EGR not opening
7. Proper TCC operation
8. Excessive oil entering combustion chamber


Let us know what you find out. Post again if you get stuck!

 
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