'96 GMC idling high?

[cam.man67]

Got a puzzler here, guys. My dad just bought a '96 GMC z71. It's kinda high mileage (160k), but runs and shifts great, has all the service records, and was completely rust free. He's been using it as his daily for about 5 months.

The problem: high idle after warming up. It was idling at 14-1600 before i took the IAC off and cleaned it. I tested a new IAC as well, but both the cleaned one and the new one are only able to lower it to 1100. Plugs, wires, rotor, and cap were all replaced but haven't affected idle quality one way or the other. I had a shop check for vacuum leaks and they found nothing.

So what now? Is there a way to adjust idle speed I don't know about? Is there another sensor that I should be checking?

[cam.man67]

Also, ill add: fuel economy hasn't been as good as I was hoping for. On long highway trips it's about 14-15, but around town it's only getting 10-11. Is that normal? That's basically what I was getting in my old Scottsdale, so I just kinda surprised.

[speedygonzales]

Was the idle high since he got it or did it change sometime later? If it was like that from the time he got it, look at the throttle plate. See if it's closing all the way. From the factory, the throttle plate has a "wing" for lack of a better term rivoted to it. This wing can come loose and hold the plate open.

Also the previous owner may have screwed the throttle stop screw in to compensate for a problem. This can also be detected by looking at the throttle plate. The plate should be tight against the side of the throttle body when the pedal is not depressed.

[jokerz71]

1st & formost,are you setting any codes?If so,what are they? Idle can only be adjusted thru the PCM.The best thing to do is have the PCM scanned & looked at in real time.There could be several different things that cause this.MAF sensor,temp sensor,vacum leak,a thermostat sticking open,causing engine to run too cool.

[cam.man67]

Originally Posted by jokerz71

1st & formost,are you setting any codes?If so,what are they? Idle can only be adjusted thru the PCM.The best thing to do is have the PCM scanned & looked at in real time.There could be several different things that cause this.MAF sensor,temp sensor,vacum leak,a thermostat sticking open,causing engine to run too cool.

No codes consistently. There was a code that flashed (forget what it was) right before I did the IAC, but it hasn't come back. The thermostat is an interesting idea...where should the truck be running warm? I've always thought it ran a bit cool, but I don't know what the temp is supposed to be.

Originally Posted by speedygonzales

Was the idle high since he got it or did it change sometime later? If it was like that from the time he got it, look at the throttle plate. See if it's closing all the way. From the factory, the throttle plate has a "wing" for lack of a better term rivoted to it. This wing can come loose and hold the plate open.

Also the previous owner may have screwed the throttle stop screw in to compensate for a problem. This can also be detected by looking at the throttle plate. The plate should be tight against the side of the throttle body when the pedal is not depressed.

I will take a look, thanks! Dad had it in the shop right after he bought it for a sticky throttle, and I know they cleaned the TB at that point.

[speedygonzales]

Originally Posted by cam.man67

Dad had it in the shop right after he bought it for a sticky throttle, and I know they cleaned the TB at that point.

So what was the symptom he took it in for and did it come out OK after they worked on it? How long was it ok?

[cam.man67]

Originally Posted by speedygonzales

So what was the symptom he took it in for and did it come out OK after they worked on it? How long was it ok?

The throttle was sticking right off idle. It's still good, there's no hesitation at all in the pedal. I don't recall what the idle situation was before that; I only drive it once or twice before the shop visit.

[speedygonzales]

The throttle does not stick right off idle. What is happening is the common IAC failure that sticks or is slow to return to correct idle as you approach a light for example. The IAC is commanded to hold the idle speed up as you approach a light. Then once the vehicle comes to a stop, the IAC is lowered to the desired idle speed. As the IAC's fail, the IAC is slow to come back to idle.

[jwinga]

I had one with the same symptoms, the thermostat was stuck partially open causing the ecm to "think"the engine was cold resulting in a high idle condition.

[Cape Codder]

I had the same problem with my 98 4.3. New thermostat cured it. Better economy,lower idle and better hear.

[jokerz71]

With the stock tune, your temps should be approx 190* at normal highway driving.Some HP tunes use a lower temp & use a lower temp thermostat.

[crimsontide1966]

Check for any vacuum leaks.

[cam.man67]

Ok, so I'm back…still fighting this truck on the idle (got another transfer case issue that I'll start a separate thread for) and I'm very much getting frustrated by it. I've been to two different shops now to try and diagnose it, and nobody has found anything wrong.

So what's happening is that when I cold start it, it idles at 1500. No big deal, but when it warms up, it doesn't idle down, like, AT ALL. In fact, it'll usually go up to 1700 once the truck is warm. Pain in the butt trying to fight the truck slowing down with a trailer behind it.

The ONLY fix way to get it to idle down is, before it warms up fully, shut the truck off and let it sit for about 15 seconds. Start it back up, it immediately jumps to 1500 then slowly comes down to 800 in about 15 seconds. From then on, it idles fine. Til I shut the truck off again and have to re-do this entire process. Grr! What in the heck would cause this?? I've ruled out IAC (tried an new one in the shop and it didn't behave differently) and vacuum leaks. Could this still be a thermostat issue?

[Cape Codder]

Have you changed the t-Stat? They only cost about $5.00 and takes a half hr.

[OutlawDrifter]

might be a stretch, but how about an exhaust leak before the o2 sensors? how do the donuts look?

could be in the wiring also, possibly maf sensor.

[cam.man67]

Originally Posted by Cape Codder

Have you changed the t-Stat? They only cost about $5.00 and takes a half hr.

I haven't...that has been suggested a lot so I may try that. Is 190 the correct temp?

Originally Posted by OutlawDrifter

might be a stretch, but how about an exhaust leak before the o2 sensors? how do the donuts look?

could be in the wiring also, possibly maf sensor.

It's possible there's an exhaust leak before...I can hear a leak, just haven't found where yet. I'm not familiar with what a donut is? Haha...this is the only fuel Injected truck I've ever had to work on! Carburetors are much more in my comfort zone. what does the maf sensor do? And where is it located?

[OutlawDrifter]

Donuts go between the exhaust manifold and the down pipe to seal it. The mass air flow sensor is in the air intake after the filter box, before the throttle body. It meters how much air is going into the engine.

[Tynee]

I once had a '94 (TBI) truck that I over tightened the connector on the linkage connecting the Cruise Control to the throttle body. I did that because I was trying to eliminate an issue with the cruise control kicking off randomly. The end result was that it idled too high after it warmed up.

I'm not sure if the cruise control and the throttle body work together the same on the tuned port trucks or not, but it might be something to look into a little further.

[cam.man67]

Ok, so short update...just replaced the thermostat, and no change in the idle issues. My FIL says that he had an Olds with the same issues that needed a new TPS. I've noticed my fuel economy is hovering around 10mpg, despite doing a lot of highway driving.

[82ccshorty]

I was having this issue with my 97 yukon idling at 1800rpm. I removed the air box to find my throttle body not closing properly. Take the 2 cables lose from the thottle body and see if it idles down. Mines was due to cruise control cable being in a bind causing throttle body to open up.

[speedygonzales]

Originally Posted by 82ccshorty

I was having this issue with my 97 yukon idling at 1800rpm. I removed the air box to find my throttle body not closing properly. Take the 2 cables lose from the thottle body and see if it idles down. Mines was due to cruise control cable being in a bind causing throttle body to open up.

It was suggested back in January that he check the throttle plate closure.

Mines? Really?

[cam.man67]

Ok, so I think the issue is fixed! I did check the throttle plates per recommendations, and, of course they were fine. I went ahead and put in a new TPS and, voilà! Fixed. I'm not calling it a victory til I get a few hundred miles on it and see how it runs then, but it looks like the TPS was a winner. The truck idles at around 800 in park, around 600 in drive, and no longer wants to fight against the brakes on the highway. Thanks for the suggestions everybody!

[Cape Codder]

Thanks for the update. Good luck with the fix and keep us posted with the final outcome.