CEL, Rough Running, Help!

Thanks for looking.

'91 Rodeo 3.1 V-6, truck starts right up. Runs O.K. at idle, a few dips and rises in idle ocassionally. CEL light comes on at 2 K on tach, when that happens motor runs rough, misses. Doesnt get much better the more I push it. Too much fuel? Truck will do 55 mph at best, (faster on down hill.)

No codes are stored. CEL is "intermittent".

Here is what I have done/replaced/checked:

Motor swapped last year. 106,000 on this plant.
New Plugs - Densos
New Wires, Cap, Rotor
New field coil
New Ignition module on the distributor.
New EGR
New fuel pressure regulator
Used computer installed, same results as old computer.
Cleaned grounds behind computer, cleaned grounds on engine block, Neg Battery terminals etc.
I can't find any Intake leaks.
89 Octane gas on last fill up.
HAd a plugged cat, stranded me on highway, wife towed me home.
Replaced cat but not after a week of two of backfiring through the throttle body.
I have tried 2 different O2 sensors.

Good vaccum
12-13 pressure on fuel line.
Good compression (This summer)
Battery is 1 year old.

This truck has me and quite a few "home mechanics " stumped.

I am ready to drag it to the dealer. Pep Boyz couldn't get any info off of the computer. (Fortunately, they idn't charge me.) They said Isuzu will need to read it.
Any thoughts or ideas I will try. I have read the "bucking isuzu" and many pages on this forum and I have tried just about everything. This truck still has some useable service left in it. Please help! (I have been at this for about 2 months off and on.) I can't afford to throw any more new parts at it.

Injectors?
Weather?
Bad Prom?

???

Thanks!

Steve

Mass air flow (MAF) sensor maybe? If it's pickup (sensor) is dirty or broken, you'll get the conditions you describe. Wow! you sprung for denso plugs for your GM 3.1L!!! :worshippy AC Delco's would have done just fine.

G/luck
Joel

Joel,

Thanks for reading. No Mass Air-Flow on 3.1. 2.6 yes 3.1 no. Any other ideas?

Update...

Since I have to move a large gift to my parents , the Rodeo must run today. So yesterday, it started right up, was really loud at first then quieted down. After it warmed on its own I goosed the gas a few times and the truck would idle really low, 400-500 rpm it would dip lower than recover but only to about 400-500. I checked the timing 12 btdc. ( I think that is where I left it the last time I set it.)
In my travels this week a friend suggested the computer was bad, i have tried 2 with same results, maybe I should see what it is recieving for power. Maybe tomorow. If not it goes to the dealer.

I really enjoy taking a 10+ year old truck to the dealer. I think I'll just have them diagnose it.

check the idle air control solenoid also, I assume it is a tbi. It Is on the side of the throttle body. They are 54$ brand new.

When is the last time you changed your inline fuel filter?

12 - 13 PSI sounds low to me for a fuel injected vehicle.


Good luck,
Dale

Fuel filter is good. Fuel pressure meets spec.

I have tried the IAC Valve. (I have a complete spare motor.) The second one made the truck worse.

I am leaning towards a bad intake manifold gasket. Truck is really loud on start-up. 2,000 rpm and a loud hissing noise that dies down as truck warms. Am I right in "assuming" that it is an intake leak?

Steve

was the second iac new or used? Plus if you did not use thread lock on it, air can leak around the threads, and did you use a new gasket? Also check to make sure the air intake air passage to the iac motor is not plugged with carbon. Another thing is to check the distance from the end of the valve to the shoulder on the motor body( the shoulder meaning the part of the motor that fits flush to the throttle body) it should be 28mm or 1 1/8 inch. if it is longer push the pintle in until the correct length is obtained. Antother thing to check is the coolant temp sensor. If it is not working correctly it will have a high resistance when coolant is warm. check the specs. Also to check the operation of the iac with the engine warm, jump the a and b terminals of the dlc (data link connector), ther is only three wires so not to worry. The dlc is behind the drivers side kick panel, it is usually cramed up with all the other wires. Can't miss it, it only has three wires going into a big connector. When you jump it the engine rpm should jump to 1000-1500 rpm for about 5 secs and idle down. if nothing happens then the motor, pcm or wires should be checked. The hissing sounds like a vacuum leak at the throttle body, could be the iac. If you have a vacuum pump you can check the vacuum at the throttle body ports Should be steady and around 14 to 18 in. Hg. at idle (800 rpm). Faster the rpm the lower the vacuum, keep that in mind because it is idling so high.

I will say this, no matter how rough the truck runs it is killer in snow. I had no problems in the Northeast last night, 8+ inches, truck even brought home the snowblower.

Thanks for the last reply, I will try it out once the snow melts.

I appreciate the help very much.

-Steve

ttt I am having the same problem I was wondering if you ever got it fixed? Thanks...