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.

Thanks

Jimmy

 

Jimmy, mine does the same thing if I merely rest my right foot on the pedal. I think it's slightly unmetered air coming past the blades on mine. On yours, it may just be some slight jittering from the IAC or MAP.

 

Thanks. I went back out after dinner, started it again and the idle was OK. It was not fluctuating like before. Took a test drive, went maybe 10 miles up and down hill. Every time I stopped like at a red light the idle was normal. No surge at all, resting right about 700 RPM. Made me think maybe the TPS or MAP got wet with TB cleaner when I was spraying and cleaning the TB, thought maybe it had dried out by now and truck was OK. Nope, I came home, truck was idling in the driveway, the surge started up again. It's just a slight uptick of maybe 75 to 100 RPM every 10 seconds or so. I tapped on the side of the TPS with a small hammer, it made the RPM jump so I swapped it out again with my other one. After that it idled too high, like up at 900, just shy of 1000. It stayed there until I turned on the AC. Then it dropped back to a steady 700.

I don't know what is wrong, but it has to be something I did. I pulled in and out on the IAC shaft when I was cleaning it. When I put it back in I put the shaft back about where it was to start with. I don't know if that had anything to do with this and I still have the spare IAC in there now anyway. Truck was running fine before I got hold of it today. I need more time to figure it out I guess.

Thanks for the help.

Jimmy

 

The TPS is just a potentiometer, so I can definitely see a tap making the sweeper bounce a little.

 

Well I have read a lot of stuff on the net and a lot of Dodge guys had this same RPM problem. A lot of them posted that a new TPS fixed it. I hope to be able to check it out again after work tomorrow. It has to be a problem with the TB or the sensors on the TB. Truck ran perfect after I put in the plugs and it ran perfect after I did the cap and rotor. I did not remove the MAP or TPS when I cleaned the TB, did not even think about that until this problem came up. I wonder if I ruined the MAP or TPS or both by getting them wet with TB cleaner spray?

Thanks

Jimmy

 

TPS shouldn't be effected. I don't think MAP would be. Like i said, mine does it if I rest my foot on the pedal.

 

Drove the truck to work today, idle is still funny. It will idle normally some of the time and idle high like at almost 1000 RPM some of the time. It also appears to be using more gas than usual, just by judging how the gauge seems to be dropping faster than normal. It did not do anything like this before the tune up. I think I ruined the IAC by pushing and pulling on the shaft. I will replace it later today and see what happens.

Jimmy

 

I swear our gas gauges are parabolic. They sure the hell aren't linear.

 

I put a new IAC in tonight, idle seems to be smoother but is still pulsing up and down by about 75 RPM. It is still steady when I turn on the AC too. Doesn't matter to me, I will keep the new IAC in place because of the old one acting so wonky after I had cleaned it and worked the plunger in and out like I did. I read online you are not supposed to do that. The idle did seem to come back to a normal level a lot faster with this new IAC, but it still pulses up and down. I am going to drive it tomorrow in the hopes it may just be that the PCM needs to learn and/or calibrate the new IAC. I may pull the trigger on a new TPS tomorrow, there seems to be kind of a dead spot in my throttle around 60 mph. I can give it gas and it has a delay in response then slowly creeps up. I don't know if that is what is causing the idle surge or not. Some say TPS will cause it, others say it won't. All I know is this problem did not exist until I took it upon myself to do the tune up yesterday. Gotta keep after it until I figure it out I guess. Gas gauge did not seem to drop as quickly coming home form work but that may be an illusion.

Thanks

Jimmy

 

sure you didn't knock a vacuum line loose somewhere in the rats nest under the cowl?