When its like 50° and up the truck will start rev up to 1500 rpm then it will fall down to 1100 rpm then pick back up to 1500. Around 40° it will start and rev up to 1500 rpm but it wont fall down until it gets done with the cold start cycle. Ive replaced the IAC valve and the ECT sensor. Any ideas on what my issue may be? All help is greatly appreciated.

 

A rolling idle like you describe can be caused by a vacuum leak. What is the idle RPM when the engine is warmed up? Have you checked for codes?

 

I couldn't find any vacum leaks after using 2 cans of ether. Warm idle 675to rpm. No codes other than 111 KOEO & KOER which is system pass.

 

The warm idle RPM looks to be pretty good. That would lead one to think there are no vacuum leaks. Personally not a fan of the spray method to find vacuum leaks. You can very easily overlook the vacuum reservoir on the passenger side inner fender. The vacuum reservoir for the HVAC on the blower case, if you have a vacuum operated speed control there is another vacuum reservoir behind the driver side headlight. The speed control servo. And then the vacuum motors for the HVAC system.

What brand of IAC did you install? The aftermarket versions tend to have very poor quality control.

A TPS with a noisy signal output can cause idle surging as well without triggering a code. Have you verified the output is linear with a meter?

Did you check the ECT sensor before replacing it? Did you check the new ECT sensor output?

What year, model, engine, transmission does this truck have?

 

The duralast idle bypass motor I just bought is junk for sure.

My old original motor wasn't running, so I thumped it with a screwdriver handle and it started working. Figured it needed replacing, so I went a bought a new one.

The new one made noise, but didn't seem to be working, made no difference when I unplugged it. I took its solenoid off and the valve part was letting air through even with no solenoid, the factory one doesn't.

So I put the new solenoid on the old valve, it was better, but then began to surge until warm.

Put the old solenoid back on the old valve, and both back on the old truck, idles fine again, and the air bypass is quieter too.

 

The truck is a 91 with a 5.0L and a E4OD tranny. I ruled out the vacum can by temporarily bypassing it. The speed control is no longer hooked up the so its ruled out. The IAC valve i put on is a standard brand. When i replaced the ECT sensor i went through three different brands the first was a standard brand the second was a duralast and the third one a Carquest brand had the proper readings. Cand remember off hand what it was when i put it in. The TPS voltage was .98 volts.

 

That TPS voltage at closed throttle is in spec. but you need to verify if the output is linear as the throttle opens. If there is a "dip" or "rise" in the output then the PCM may be chasing the varying signal in an effort to bring down the engine to the RPM it is looking for.

I never had a rolling idle as you described but I did have an intermittent high idle when warm on my old '92 F350. Finally traced it down to a bunch of green corrosion in the TPS connector. It took a little scrubbing and few squirts of contact cleaner to get everything nice and shiny. Applied a little dielectric grease to the contacts, then re-assembled. Issue resolved. Could be worth a shot to inspect the same connector on yours.

 

When i checked the TPS the transition was smooth from from closed to wide open throttle. I'll check the connection to see if there's any corrosion. Also gonna check some grounds to see if that helps.