Topic: 91 GMC Sonoma issues, idles rough when warm, ideas?

[manjisann]

I bought a 91 GMC Sonoma 4 cylinder in February, it passed emissions with flying colors. Up until yesterday it ran fine, shifted fine (it's a manual, but no slip) and was strong. For some reason yesterday after it warmed up it started to idle rough and I don't really have any idea what it is. When i first start it she's fine and she'll run smooth for the first five minutes or so, but after that she idles kind of rough, almost as if she's not firing on one of the cylinders.

Any suggestions? I really dont' want to put too much money into this vehicle as I'm trying to save up for a nicer one for when this one bites the dust (it's not supposed to happen this fast dang it!) Would the EGR valve do this? Also, I pulled the air filter (it's a wee bit dirty) an in the air filter compartment which sits on top of the throttle body, there is what looks like some really thick oil, almost like dirty engine oil.

Right now I'm so tired and frustrated with old vehicles.... well I'm just tired and frustrated with them really.

Brandon

[traveler]

Carburetor or EFI?

~Joe

[manjisann]

EFI

Brandon

[scottly]

Check spark plugs and plug wires.

[manjisann]

Check spark plugs and plug wires.

A coworker suggested this, I'm just wondering why things would be okay until it warmed up and then suddenly it starts acting weird. I pulled one plug and wire and the plug looked good, nice tan deposits, the wire was odd though, the boot separated from what clipped onto the plug. Not arguing with you, just wondering how this would cause it. Also, how would I check this on a truck??

Brandon

[traveler]

timing module is going bad.  As the engine warms up, the timing is supposed to advance appropriately, along with adjusting the fuel/air mixture.  Apparently, it is giving up the ghost.  Gonna end up needing to get another out of the wrecking yard soon.

~Joe

[scottly]

Just went through this with a buddies Tracker. The resistor wires break down inside, start arcing, and sometimes even burn. You can check with an ohm-meter; should be something like 5K ohms per foot, but not open. Sometimes you can cut off the bad end, put in a U-shaped piece of wire, with one leg shoved into the center, and the other leg on the outside, and re-crimp the plug clip back on.

[Steve F]

I had an S-10 V6 for a while, and the EGR valve would get clogged and would stick, causing the idle to go really rough.
I can be removed and cleaned I found out.

[razor02097]

Is it throwing any engine codes?  It is an early year cars and trucks weren't that smart but still it is a good place to start.


If it only acts up after it warms up then it is likely you will be looking at sensors if the truck can't idle right in closed loop mode.  I'm guessing it doesn't have a mass airflow sensor? 

Brandon I would start by purchasing or borrowing a Haynes manual for that truck.

[manjisann]

Brandon I would start by purchasing or borrowing a Haynes manual for that truck.

Already did, just have to track it down, I hate how I lose stuff!

It's begun to run rough all the time now, it's not super bad, my old car vibrated like this the entire time I owned it, but this truck was super smooth before this started and i kind of miss that. For the moment I'm just going to drive it the way it is. I'm hoping that my bike will be rideable and reliable in the next couple days and after that I'll start working on the truck. My mechanic friend still really suspects the plugs and wires, and since I don't really like how they are put together anyhow I'll go ahead and replace them and see what happens. I'll just have to start slowly replacing parts and see what fixes it.

Brandon

[Inigo Montoya]

I had a 94 sonoma with the first year vortec. One time, it suddenly ran like crap. Turns out a piece of carbon came off and clogged the egr. A simple cleaning took care of it.
Another time, I had a problem with idling just like that. It was on a different vehicle but it would start fine and then after a couple minutes, idle like #$%*. I looked and looked and found nothing wrong. It was pre-computer so no codes to check.
One night I opened the hood and saw a lot of arcing that was invisible in daylight. Wire just got a hole in it and was arcing against the exhaust manifold heat shield. Wires were just old. Just after that, on  really dark night out of town, I opened the hood and could literally see the electrical pulses racing through the wires. Yeah they got changed.

[scottly]

Again, change the wires. Been there, done that, too many times to count on my fingers!