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2nd Gen Dakota 1997 - 2004 Dakota's

Idle RPM problem

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Old 11-11-2013, 07:06 PM
01SilverCC 01SilverCC is offline
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Default 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.

Thanks

Jimmy
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Old 11-11-2013, 08:06 PM
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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.
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Old 11-11-2013, 08:38 PM
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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
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Old 11-11-2013, 08:48 PM
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The TPS is just a potentiometer, so I can definitely see a tap making the sweeper bounce a little.
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Old 11-11-2013, 09:09 PM
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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
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Old 11-11-2013, 09:35 PM
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Originally Posted by 01SilverCC View Post
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.
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Old 11-12-2013, 10:12 AM
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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
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Old 11-12-2013, 08:05 PM
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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.
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Old 11-12-2013, 08:38 PM
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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
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Last edited by 01SilverCC; 11-12-2013 at 08:42 PM..
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Old 11-12-2013, 09:19 PM
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sure you didn't knock a vacuum line loose somewhere in the rats nest under the cowl?
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Old 11-12-2013, 09:25 PM
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Not that I can find, Magnet. I spent a long time looking yesterday afternoon. I hope to get home before dark tomorrow, maybe get another look at it. I don't hear or feel any odd vacuum problems under the hood either. Truck is running fine, has a little more power than it did before the tune up. The idle seemed to settle down some this afternoon coming home, was barely goosing up just a bit and then coming back down. Maybe the PCM had to re-learn the old IAC after I had moved the pintle shaft like I did. I am hoping this new one will settle down too.

Jimmy
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Old 11-12-2013, 10:13 PM
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when in doubt, I'd say just leave it.
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Old 11-12-2013, 10:15 PM
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Hey Magnethead, or anyone else who might know...one thing I just thought of. Yesterday when I pulled the plug wires off a couple of my plugs, the wire boot was stuck on there pretty tight. I mean I had to yank on it hard to get it loose. I wonder if those 2 wires might have stretched the wires inside so now they don't make as good a connection? Can an engine still run OK or maybe run with a slight problem like a weird idle fluctuation with a couple plug wires that might be stretched like that?

Thanks

Jimmy
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Old 11-12-2013, 10:32 PM
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Originally Posted by 01SilverCC View Post
Hey Magnethead, or anyone else who might know...one thing I just thought of. Yesterday when I pulled the plug wires off a couple of my plugs, the wire boot was stuck on there pretty tight. I mean I had to yank on it hard to get it loose. I wonder if those 2 wires might have stretched the wires inside so now they don't make as good a connection? Can an engine still run OK or maybe run with a slight problem like a weird idle fluctuation with a couple plug wires that might be stretched like that?

Thanks

Jimmy
I don't think so. Just means there was no dielectric grease on the connections, so they oxidized/welded together. With the right angle boots, it would be hard to pull the wires loose from the connector.
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Old 11-13-2013, 09:52 AM
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Drove it to work today, idle was fine until it warmed up to operating temp. After that when I would stop at a red light the idle was back to surging form 700 to about 850 then back down again. It does it about every 10 seconds. I don't think the plug wires are bad, there is no spitting, hesitation or sputtering, no lack of power. Truck runs fine, just has this weird idle problem that showed up after I did the tune up.

I may try a new TPS. Thanks for your help.

Jimmy
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Old 11-13-2013, 06:37 PM
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I picked up a new TPS at Auto Zone on my way home from work tonight. I installed it a few minutes ago, it *appears* to have solved the problem. When I first put it on, I still had the fluctuation in RPM at idle. I let the truck run a few minutes, held the throttle at 1500 and 2000 RPM for a few seconds each, let it off, it idled back down to 700 and is now idling steady at 700 RPM sitting still in my driveway at normal operating temperature. I hope this is the final piece of the puzzle, that it really is fixed now. I have to drive it to work tomorrow to know for sure.

Thanks for your help Magnethead. I sure hope I am done with this problem for good now.

Jimmy
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Old 11-13-2013, 10:22 PM
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I picked up a new TPS at Auto Zone on my way home from work tonight. I installed it a few minutes ago, it *appears* to have solved the problem. When I first put it on, I still had the fluctuation in RPM at idle. I let the truck run a few minutes, held the throttle at 1500 and 2000 RPM for a few seconds each, let it off, it idled back down to 700 and is now idling steady at 700 RPM sitting still in my driveway at normal operating temperature. I hope this is the final piece of the puzzle, that it really is fixed now. I have to drive it to work tomorrow to know for sure.

Thanks for your help Magnethead. I sure hope I am done with this problem for good now.

Jimmy
I think dodge designed these trucks around the "fun factor". Fun to drive, fun to work on, and fun to chase weird stupid issues that crop up randomly, most predominantly in the electronics systems.
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Old 11-14-2013, 08:20 AM
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Spoke too soon..idle is still pulsing up and down every 10 seconds or so, but is not as bad as it was before. I guess I need to keep looking for a loose or cracked hose or some other vacuum leak. That is the only thing I can think of that would cause this idle fluctuation...I will leave it alone until the weekend, try to see what I can find in the daylight on Saturday. I am pretty tired of it right now.

Jimmy
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Old 11-14-2013, 06:23 PM
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Maybe I spoke too soon again....

My truck ran beautifully coming home from work, I can definitely tell the new TPS made a ton of improvement. The throttle response is much more crisp and precise and the dead zone at 60 mph is long gone. I could not really test that in the morning commute traffic today but I was able to get on it coming home. My truck is a lot more responsive now.

Coming home when I stopped at a red light 4 or 5 miles from work the idle was normal, precisely at 700 RPM and steady. There was no fluctuation at all. Same thing at a couple more lights here in town and the same thing also happened in my driveway when I got home. Steady 700 RPM. The gas needle does not look like it is dropping as fast now either.

I am thinking/guessing the PCM needed more driving time to learn and calibrate the new TPS. I am calling it done and leaving well enough alone unless it starts the wonky idle again.

Thanks again for your help Magnethead.

Jimmy
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Old 11-14-2013, 07:30 PM
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Maybe I spoke too soon again....

My truck ran beautifully coming home from work, I can definitely tell the new TPS made a ton of improvement. The throttle response is much more crisp and precise and the dead zone at 60 mph is long gone. I could not really test that in the morning commute traffic today but I was able to get on it coming home. My truck is a lot more responsive now.

Coming home when I stopped at a red light 4 or 5 miles from work the idle was normal, precisely at 700 RPM and steady. There was no fluctuation at all. Same thing at a couple more lights here in town and the same thing also happened in my driveway when I got home. Steady 700 RPM. The gas needle does not look like it is dropping as fast now either.

I am thinking/guessing the PCM needed more driving time to learn and calibrate the new TPS. I am calling it done and leaving well enough alone unless it starts the wonky idle again.

Thanks again for your help Magnethead.

Jimmy
Did you reset the PCM after changing these sensors? That may have just been the problem after changing the tps. But at this point it's almost learnt the new sensors and re-adjusted itself
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