'95 C1500 head scratching issues

[LeeroyJenkins1981]

My '95 Chevy 1500 has been causing me a lot of headaches for the last few months. What it does is idles rough, revs for no apparent reason at idle and while driving no matter the speed. No check engine or SES lights either. The funny thing about it is it does this much more once the outside air temp gets around 70 degrees and up. Sometimes it doesnt do it at all. I have checked for and repaired vacuum leaks, replaced the IAC, used injector cleaner (something I don't like to do), cleaned the throttle body, changed plugs & wires, new fuel filter, new thermostat (and the sensor near it which escapes my mind at this time), ran higher octane gas and changed the air filter. Im convinced its a fuel delivery issue but wonder if anyone else has had this problem and knows what they did to fix it.

[Blue Bowtie]

Welcome Aboard!

Yes, I too have experienced the mystery idle/revving. You might want to check the PCV valve and hose, and unless the brakes are HydroBoost, check the brake power booster for leaks. A PCV valve can randomly stick and admit more air at idle than it is designed to pass. This will increase RPM like any other vacuum leak. Internal leaks in a brake power booster can do the same thing.

Another random possibility is the air conditioner. If the AC pressure switch is faulty or if the AC compressor is turned on by the PCM there are also IAC steps added to raise the idle to compensate. I have seen vehicles with failed AC systems have a target idle which hunts up and down.

You might also consider the EGR valve as a potential internal vacuum leak.

[LeeroyJenkins1981]

Update:

I changed the cap and rotor and ignition coil today just for the hell of it. Fired it up, let it run, towed my log splitter up from out in the yard (that time of year again), let it run some more and it didnt miss a beat. Big test will be the 30 mile jaunt to work.

To the above poster:

It doesnt have A/C unfortunately and I pulled the PCV, checked and cleaned it. I forgot to mention that in my list.

[tblake]

If it does it again, clamp off the vacuum booster line just to rule it out.

[j cAT]

Originally Posted by LeeroyJenkins1981

Update:

I changed the cap and rotor and ignition coil today just for the hell of it. Fired it up, let it run, towed my log splitter up from out in the yard (that time of year again), let it run some more and it didnt miss a beat. Big test will be the 30 mile jaunt to work.

To the above poster:

It doesnt have A/C unfortunately and I pulled the PCV, checked and cleaned it. I forgot to mention that in my list.

you may have had a bad coil . should have had check engine light.

the PCV does get plugged when your engine is shot. with a good running engine this componet wears out since it is constantly working ..

with normal average operation 30,ooo mi is a good time to replace and always check those hoses to the valve.

In the GM manual its 12,ooomi replacement , but I think that is excessive ,

TOO much crankcase venting from a worn valve can mess up the fuel mixture as well as other issues........