Idle RPM problem
I did a basic tune up on my truck today. It's a 2001 with the 3.9, 5 speed and about 162,500 miles on it now. I did the tune up because I read it is a good idea to replace your spark plugs after replacing a catalytic convertor, which some of you may know I had that done last week.
Truck has been running and idling fine, even when I still had my old cats in place and the CEL was on. CEL has been out since Friday and I do believe the new cat assembly took care of that problem. Today I put in a new air filter, new plugs - NGK FR6F-11's (stock heat range) and a new cap and rotor. I used my old wires, they are only about 2 years old. I first did the plugs then started the truck and it ran fine. Then I did the cap and rotor, truck ran fine. Then I removed the TB and cleaned the TB and IAC. I have cleaned the IAC a number of times in the past and never had a problem with it before. I put it all back together, went on a test drive, everything was good, truck runs fine.
The problem I have is at idle the RPM fluctuates. It idles at about 700 like it should but every 10 seconds it will increase by about 100 RPM. It increases and then drops back down to 700. It will idle OK for about 10 more seconds and then do the same thing. The real funny thing is it does not do that if I have the AC on. It idles perfectly with the AC on just the low speed.
I put a vacuum gauge on it and at idle it is at about 20. When the RPM surge happens the vacuum goes up by 2 or 3 then when it idles back down the vacuum drops back to 20. I checked the TPS and MAP sensors they are both within specs. I even swapped out both the IAC and TPS with 2 sensors I had in my spare parts box. Back when I bought my V8 TB it had the TPS and IAC still on it. There was no change with the spare sensors in place.
I have no idea what is causing this problem. I checked the PCM with my new scanner tool, it reads live data but it did not tell me much. I am still learning how to interpret the info from the PCM and scanner tool. It did have the green check "all is well" light on when I was testing.
My old plugs were gapped at .044. I gapped my new plugs at .040, I was just going by the service manual. NGK has the recommended gap at .044 on their site, maybe that was what I used last time just because it was what NGK said. I have no idea if the change in gap would cause this idle problem. I checked everything for a long time today. There are no disconnected hoses or anything and I can not hear any vacuum leaks.
I am searching now trying to find help with this. I appreciate any help you guys here might offer.