94 GMC Idles rough Please Help!! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

wj3v · 09-10-2004, 11:02 PM

Ok, I have had this problem for over a year now. When I am stopped, at idle, the truck is really rough. It doesn't shut off but idles really rough. You can feel it slightly at speed but it is hardly noticeable. It first started when I was pulling a hill. It sounded like severe detonation. I had to pull over and almost stop before the detonation went away. It has had this detonation symptom a few more times but not in the last 3 or 4 months. The truck runs fine at speed, and seems to have the same amount of power as always but the idle is horrible! The service engine light came on a few times but always goes out before I can get it somewere. I had a tuneup about 6 months ago in hope to fix the problem. They replaced the oxygen sensor, plug wires, cap, button, plugs. Still no help. Its a 94 GMC 1/2 ton 305 121,000 miles. HELP!!

ChevyTech · 09-11-2004, 02:50 PM

Hi and welcome to the board.

The service engine light does not have to still be on to have a code stored in the computer.
Prior to OBDII systems you could retrieve the code yourself. I believe your 1994 is pre OBDII. The code should be remembered until 50 starts cycles of the engine. Here is a web site that will tell you how to retrieve your code. If your ALDL connector does not look like this, but is more oval looking, then you do have OBDII and need a scan tool to retrieve the codes.

http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

Edit-
Here is a web site that has a better drawing of your diagnostic connecter and some more info.

http://www.troublecodes.net/GM/

wj3v · 09-11-2004, 05:22 PM

Thanks for the reply... The only code it sends is 12....???

ChevyTech · 09-11-2004, 06:03 PM

A code 12 is normal and should be there when you do this test.

When they did the tuneup did the fuel filter get replaced. Some places consider a fuel filter to be a seperate operation not included in your tuneup.

"It sounded like severe detonation" This would worry me if it were my truck.

wj3v · 09-11-2004, 08:09 PM

I replaced it myself. It was the first thing that was done when the problem started. I have probably put around 4,000 miles on it since the problem appeared.... Could it be the camshaft???

ChevyTech · 09-11-2004, 10:44 PM

I don't think the camshaft is your problem. Your camshaft has roller lifters and they hold up very well.

The fact that it had bad pinging and the rest of your description makes me think you should do a compression test on all cylinders. It sound like it could have got hot resulting in a mechanical problem. Much more likely a valve, piston, or ring problem then a camshaft.

knoblock65441 · 09-25-2004, 10:33 PM

Have you checked the EGR Valve? Carbon builds up and won't let the egr valve close like it should and It will run bad at Idle but will run ok when you get up to speed. But an egr valve should set the check engine light If It's not operating right. still worth checking.

09-10-2004, 11:02 PM	#1
wj3v Registered User Join Date: Sep 2004 Posts: 3	94 GMC Idles rough Please Help!! Ok, I have had this problem for over a year now. When I am stopped, at idle, the truck is really rough. It doesn't shut off but idles really rough. You can feel it slightly at speed but it is hardly noticeable. It first started when I was pulling a hill. It sounded like severe detonation. I had to pull over and almost stop before the detonation went away. It has had this detonation symptom a few more times but not in the last 3 or 4 months. The truck runs fine at speed, and seems to have the same amount of power as always but the idle is horrible! The service engine light came on a few times but always goes out before I can get it somewere. I had a tuneup about 6 months ago in hope to fix the problem. They replaced the oxygen sensor, plug wires, cap, button, plugs. Still no help. Its a 94 GMC 1/2 ton 305 121,000 miles. HELP!!

09-11-2004, 02:50 PM	#2
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Hi and welcome to the board. The service engine light does not have to still be on to have a code stored in the computer. Prior to OBDII systems you could retrieve the code yourself. I believe your 1994 is pre OBDII. The code should be remembered until 50 starts cycles of the engine. Here is a web site that will tell you how to retrieve your code. If your ALDL connector does not look like this, but is more oval looking, then you do have OBDII and need a scan tool to retrieve the codes. http://www.cfm-tech.com/gm_tbi_trouble_codes.htm Edit- Here is a web site that has a better drawing of your diagnostic connecter and some more info. http://www.troublecodes.net/GM/ Last edited by ChevyTech; 09-11-2004 at 03:21 PM.

09-25-2004, 10:33 PM	#7
knoblock65441 Registered User Join Date: Sep 2004 Posts: 5	knoblock65441 Have you checked the EGR Valve? Carbon builds up and won't let the egr valve close like it should and It will run bad at Idle but will run ok when you get up to speed. But an egr valve should set the check engine light If It's not operating right. still worth checking.

09-11-2004, 05:22 PM	#3
wj3v Registered User Join Date: Sep 2004 Posts: 3	Thanks for the reply... The only code it sends is 12....???

09-11-2004, 06:03 PM	#4
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	A code 12 is normal and should be there when you do this test. When they did the tuneup did the fuel filter get replaced. Some places consider a fuel filter to be a seperate operation not included in your tuneup. "It sounded like severe detonation" This would worry me if it were my truck.

09-11-2004, 08:09 PM	#5
wj3v Registered User Join Date: Sep 2004 Posts: 3	I replaced it myself. It was the first thing that was done when the problem started. I have probably put around 4,000 miles on it since the problem appeared.... Could it be the camshaft???

09-11-2004, 10:44 PM	#6
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	I don't think the camshaft is your problem. Your camshaft has roller lifters and they hold up very well. The fact that it had bad pinging and the rest of your description makes me think you should do a compression test on all cylinders. It sound like it could have got hot resulting in a mechanical problem. Much more likely a valve, piston, or ring problem then a camshaft.



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