Idles rough but not what you might think

My 1992 Isuzu Rodeo (GM 3.1L V6) won't Idle correctly. It has been like this for over a year now. In that time I've fixed almost everything that could be linked to the problem.

The bad idling started around the time I changed my sparkplugs. The spark plugs which I changed had nothing left of them, destroyed, probably hadn't been changed EVER up till then, Scary thought, but I replaced them with the correct AC Delco plugs which I appropriately gapped. Then the poor idling started. I regapped the plugs to no avail.

Since then I've replaced the wires, EGR, Fuel filter, IAC (idle air control), MAP(manifold absolute pressor) sensor, Air filter. I've cleaned the injectors and throttle body about a million times. The injectors spray like new, the TPS (Throttle Position Sensor is good).

I read something about a leaky intake manifold gasket BUT this engine has over 330,000 Km (about 205,000 mi) on it and it doesn't burn oil. If the intake gasket was leaking I'd be losing at least some oil. I lose about 2 liters (1/2 a gallon) every 5000Km (3100 mi) at which point I change the oil (plus the oil is lost from a leaky drain plug not the intake manifold). It runs great all around except at idle.

The last strange things is that it idles better with the MAP sensor (electric not vacuum) unhooked. of course I couldn't go anywhere with the MAP sensor unhooked or else I'd just drive without it. As I said before it was changed and there was no change. (And the odds are that if the new sensor wasn't working either there would be a different problem (like no acceleration as when it is not hooked up) not the exact same problem.)

The closest dealer is an hour away and they are crooks I probably know more then them (I'll never go to them). I can't get a computer on it to check codes, but the CEL isn't coming on and I don't think it ever has (and it does work).

I know this thread sounds like a murder mystery but I've tried everything, asked every mechanic (which I know lots of), looked on every website and everyone is just stumped. ANY HELP?

PS - does anybody have a power window assembly (drivers door) for this 92 isuzu rodeo. The cable (what kind of company uses a cable driven power window) broke but the motor and switches still work.

as an addendum to above does anyone know if you can swap power window assemblies from front door to back door/driver side to passanger side. (so my driver side window works and my passangers can suffer, lol.)

I personally haven't looked at it but if someone knew and could let me know that would be great. This would provide a quick fix at least til I find money for a new assembly.

It's so good to hear from/about the Isuzu ole timers, i.e. 205K miles!

These things sure are like the energizer bunny. . . going...going...going...going...

I had this same problem with my rodeo when I first got it. I replaced 1st-spark plugs (which I was told by a shop to only put NKG's in and never AC Delco), map sensor, throttle positioning sensor, oxygen sensors and I had the injectors flushed. Still rough idle. So I took it finally to a shop and told them I needed someone to fix it, me and my dad were frustrated with not finding the problem. I went back 2 days later for them to tell me, that I needed to replace 5 of my six injectors, so I did, which wasnt cheap either and now it idles great. They did tell me that for the rough idle, which we all know bad spark plugs can do that, but hell I didnt know a certain brand had to be put in either. Did you ever think to check your spark plug coil packs, might be loose or faulty. I am not an expert, I have learned alot in the 3months I have had mine.
Crystal3477

my injectors are fine. I've thought about changing the coils but I have a feeling I'd be wasting my money. The car runs excellent at any speed over 700 rpm.

I may take another look this weekend, thanks!

I'll always use AC Delco Plugs in a GM motor, just a good idea to mix apples with apples, thats my opinion.

I upped my idle to 800 RPM which I found out is the proper idle speed for it. Still idles poorly.

I might just pull the throttle body again and clean it again just incase I missed something that could cause it.

I read something about a leaky intake manifold gasket BUT this engine has over 330,000 Km (about 205,000 mi) on it and it doesn't burn oil. If the intake gasket was leaking I'd be losing at least some oil. I lose about 2 liters (1/2 a gallon) every 5000Km (3100 mi) at which point I change the oil (plus the oil is lost from a leaky drain plug not the intake manifold). It runs great all around except at idle.

Did you actually do the test for the intake manifold gasket??? I dunno if you would actually see any signs of oil consumption from the gasket, it's not really an oily spot on the engine, at least not like a lower point or valve covers or something. Besides, it's more of a vacuum leak which would keep any oil leaked sucked back into the manifold. Get a can of carb cleaner and meticulously spray around the gasket with the engine running, you are checking for any spikes in the idle since the enriched ether soln burns stronger than gasoline. I would think that a leak would trigger the CEL, but perhaps yours is minor enough as to not lean out the idle so much. It's a cheap diagnostic worth trying out.

Did you replace the plugs? If the problem started after you installed the plugs why not change them. I have bought bad plugs that ran just like the were fouled. Replaced with new and they worked fine. Also the only plugs I have ever had trouble out of are AC Delco.

That's the conclusion I've come to as well. I've been to busy to do it lately though. IF it's not the plugs then its a coil, I've fixed everything else on the car... lol.