Fast idle even after replacing my IAC

[Brandon_Lutz]

About two weeks ago my 95 GMC (5.7 engine) began to idle fast. At first I thought it was because the weather had changed and was getting cooler. However, my truck would idle forever before it drop down below 1000 RPM. Also the engine would act like it was stalling a couple of times during a warmup idle but eventually even out.

Case in point, I started the truck up last weekend as it was really cold outside as I was loading it up with supplies. The truck idled for about 20 minutes before I gt in it and drove off. For about 15 minutes of that 20 it idled really fast ( around 1500) before it finally came back down to normal idle.

After doing some research it appeared that maybe my IAC needed to be changed. I installed the new one this afternoon along with the new gasket that came with it. (The old one crumbled in my hands) Fired up the truck and it still idles fast. Also while idling, if I go to rev it up, it stumbles at first. Could this be a bad TPS instead? If I let it run for about 15 minutes, the issue goes away until the next cold restart. BTW there are no codes on the dash and otherwise everything is normal.

 

[jkeaton]

Not too certain if there is a "relearn" procedure that applies to the 95's or not. I replaced mine due to harsh 1-2 shifts and never had a problem with the idle after replacing it. You could try disconnecting the battery for a bit and see if it "resets" everything.

 

[Brandon_Lutz]

Not too certain if there is a "relearn" procedure that applies to the 95's or not. I replaced mine due to harsh 1-2 shifts and never had a problem with the idle after replacing it. You could try disconnecting the battery for a bit and see if it "resets" everything.

I'll try that. Thanks

 

[Brandon_Lutz]

Looks like disconnecting the battery did the trick. Let it sit for about 5 mins and then reconnected the negative lead. It idled high for about 3 minutes and then settled down to a normal range. Before doing that it would be a bare minimum of 15 minutes before it would slow down.

I let it idle up to operating temp for about 10 minutes. Then I shut it down waited a minute, fired it back up and let it idle for another 5 and then shut it off. Now she is starting up and going to idle around 800 to 900 RPM like it should.

 

[Brandon_Lutz]

Well evidently that was a false positive on the problem being fixed. The fast idle returned today. Fired up the truck and it instantly shot up to 1500 RPM and idled like that for a long time. After coming up to around 200 degrees water temp it gradually slows down. Let it idle for a while and then followed an IAC reset procedure I found online that requires a certain amount of idle time, then down time then firing back up, etc.. Always appears to work until the next start up. I could leave it off for 30 minutes and start it up with the water temp still fairly warm and it immediately goes to a fast idle again with the oil pressure gauge reading high until the engine slows down.

Looking into the issue more, I purchased a new CTS sensor and replaced the one up top right next to the thermostat housing hoping that would fix it as it was the factory CTS still installed. Same result, the truck still idles fast even after another IAC reset procedure.

At this point I'm not sure where to go next. The thermostat appears to be good because after about 3 to 4 minutes the temp looks to be around 160 and if I turn on the heater it blows pretty good hot air. Takes a while to get up to the 200 mark though, maybe around 15 minutes. (so maybe the thermostat could be whacky?)

Anyone have any other suggestions that I should try next? As far as I know the truck still has the factory TPS, EGR, etc. I'm quite familiar with the trucks history and I know my father never changed those items nor did my grandfather when he had it. This all started happening about 2 weeks ago out of the blue and it idles real fast regardless if the weather is cold or hot.

*Edit* Also when idling fast, if I put it in drive or reverse and put my foot on the brake to hold the truck in position the RPM's drop to normal and the truck will idle normally while I hold it under load. The second I put it back in park or neutral the RPM's will shoot up if that helps.

 

[skylark]

Get a can of carb clean and spray the intake, base of the TBI, vacuum lines. You are looking for changes in rpm, if you have any, you have a vacuum leak.

 

[ram1220]

I had a fast idle situation in my 90 Sierra 5.7L a few years ago. Only time it would throw a code was cruising on the highway. I changed out the EGR as well as O2 sensor and it solved the fast idle as well as the codes at highways speeds. Just something for you to check.

 

[Brandon_Lutz]

Get a can of carb clean and spray the intake, base of the TBI, vacuum lines. You are looking for changes in rpm, if you have any, you have a vacuum leak.

I tried that last night, even sprayed around the base of the manifold and I didn't notice any change in RPM. I think I'll try the EGR suggestion next. Thanks for your input guys.

 

[Brandon_Lutz]

Well turns out it was the PCV valve. After spraying carb cleaner all around the lines, TBI base, manifold and getting nothing in terms of RPM change, I for some reason realized I had not sprayed on the PCV end of the line. Sure enough the truck stumbled. Did it again for good measure, stumbled again and went back up. One 3 dollar PCV later, and the truck now idles at 800 RPM for the first time in almost 3 weeks.

 

[bigfutz]

Had fast idle and hard shift into gear. Installed a TBI gasket kit (serious cleaning involved), fuel line o-rings, PCV hose (cracked boots), vapor canister hose (decayed). Fast idle gone, but heard a rattling sound, honed in on the PCV valve and replaced it. Replaced fuel filter and o-rings while I was at it. Cranks, idles smooth now, shifting into gear is not so hard now. Let’s see how it goes.