Uneven Idle Speed

[DocStressor]

My truck has suddenly developed an uneven idle. It goes from 850 to 800 rpm and back again every 3-5 sec. Power and mileage is still fine but the sound is annoying. Does anybody remember seeing a TSB on this problem? I’ve never had a reflash.

 

[ftb87]

Lee,



It could be your fuel. Try using Stanadyne, it should make it run smooth.



Frank

 

[DocStressor]

Frank,



I have PowerService in the tank right now. I generally use it every other tankful. The problem started during a recent trip to Canada, where the is fuel pretty bad and PowerService wasn't available. So far additive hasn't helped the problem.

 

[Todd T]

run the tank as low as you dare and change the filter and fill her back up.



might try some anti gel additive before running it low, it is designed to incapsulate water and get it through the system better.

 

[Archive]

Check your lift pump pressures.

 

[big barney]

Lee



My truck has done this since it was new. When the engine is cold the idle is rock solid. As soon as it starts warming up I get the erratic idle. It is worst since I've added bigger injectors. I've been using Red Line 85+ since the truck was new. I know there are a few other guys that have this same problem. Sure wish someone would come up with a fix.

 

[Steve Campbell]

Lee, this is way out there on a limb but, I think I read somewhere (or maybe I didn't) that a bad ground could create the conditions you are talking about. Evan Beck is a good trouble shooter. Maybe he has run into this. Good luck

 

[DocStressor]

Well I changed out the filter and checked the fuel pressure which is fine. No change in the idle fluctuation. I'm bringing the truck in to Carson Dodge today for an air conditioning problem. I'll have them check out the idle situation and post what they have to say on Monday. Thanks for all your input.

 

[HoleshotHolset]

I had a somewhat similar experience with a Y2K dually 24v with 47RE..... it would surge all over the place as if some gremlin was pushing the throttle up and down rapidly and darn near push through the brakes at a stoplight. It downright stalled a few times too. It was the crank sensor. Warranty covered it. All is better now.

Hope this helps...



Matt

 

[DocStressor]

According to the dealer, my problem turned out to be caused by a leak in the AC hose. The AC compressor tries to cycle on and off which affects the idle. Now if they could just explain to me why the idle is unsteady with the AC turned off, I would be satisfied. The usual "could not duplicate the problem". Is there a way to check if the crank position sensor is on the way out? The problem only comes on when the engine is warm.

 

[HoleshotHolset]

If you can find yourself a good scan tool to hook up to the PCM of your truck, you might be able to see the RPM's fluctuating. The PCM of the truck I mentioned set two codes, both of which were related to the crank sensor..... is your check engine light on?

I used a Snap-On MT-2500 scan tool... . but any old scan tool compatible with OBDII will be a huge help in determining the issue.



Matt - DIESEL FUEL ONLY

 

[DocStressor]

No, I've got no check engine light. The dealer would have picked up the codes when he did the check. My problem is minor relative to the one you described.

 

[Haney]

Originally posted by Lee Weber

My truck has suddenly developed an uneven idle. It goes from 850 to 800 rpm and back again every 3-5 sec. Power and mileage is still fine but the sound is annoying. Does anybody remember seeing a TSB on this problem? I’ve never had a reflash.

Did you ever figure out what was causing this? Mine has done it since new. I have even noticed that mine continues to do this at a steady cruising speed up to 2100 rpms?

 

[zman]

Mine has done it since new and worsened slightly after installing the DD2's. I can actually hear the RPM changes when outside the cab. My fluctuation appears to be anywhere from about 25 to 75 RPMs, but there doesn't seem to be any sort of pattern to the fluctuation. I haven't had the dealer look at it yet - I figure that they would tell me that it's normal...



Steve

 

[Haney]

Sounds like mine zman.....



I had the same feeling about the dealer... .



I wonder if an ECM reflash would help? Is there any reasons I would not want them to reflash my ECM?

 

[zman]

Jamie - I've had my ECM reflashed a couple of different times for various reasons, but it didn't help the fluctuating idle. I'd bet the problem has something to do with the injection pump. As you stated, mine also fluctuates slightly at cruising speeds as well.



Steve

 

[Haney]

Well, scratch the reflash then.....



Anybody else have any suggestions?????

 

[Steve Campbell]

This is just speculation. Lee, you say the problem is more noticeable or only noticeable when the engine is warm. After much reading and thought about a related matter, I feel that you have a small amount of air in the fuel system. (Actually, I think we all do. Long story about manifestation of that problem. )



In the middle of a lift pump upgrade yesterday, I decided to see if I had entrained air in the diesel. I hooked up to the pre- fuel filter port with an adjustable orifice and hose into a quart glass jar. Lots of bubbles in the fuel. I am going to finish making a farmered up version of a small bypass to send some of this filtered out air back to the tank.



If none of that helped, sorry to have wasted your time. If curious, send me an e-mail.

 

[DocStressor]

That's an interesting idea, Steve. I still have the problem. The irregular idle is most noticable right after a long hard haul when I stop a light just off an interstate. So the fuel in the tank should be pretty warm as the result of return flow from the injection pump. Since the lift pump has to pull fuel from the tank, maybe the decrease in pressure causes air to volitalize from the fuel. Unless you have found a source for a persistent leak in the fuel system.



I will e-mail you for details, but since "they all do it" (dealer speak) maybe it would be better to have the conversation on line.

 

[andrewkoperdak]

You did not give to many parameters about your problem. I have had a much larger swing in RPM at idle during very hot conditions, say around 110 F. My RPM shifts more than 300 RPM for about 20 seconds after coming off a loaded condition. Everytime I hit the the accelerater I would again get the erratic idle for about 20 seconds. My condition comes when I drive around 6 hours at 70 MPH and pull off interstate. From the moment I come off of cruise the problem occurs. I don't know how long it last as I shut my engine down after my turbo cools down (3-4 minutes) My conclusion is that my fuel in my tank must have been around 125-140 as the ambient air was 111 and the fuel gets heated from cooling the HP pump and then recirculated back to the tank. As there is less and less fuel to be cooled as I drive, it just gets hotter and hotter. Therefore my conclusion is that ECM can't find an appropriate fuel/air mixture for the extremely high fuel temperature and therefore it idles widely. I have repeated this problem three times last summer. Dodge Service Reps say they have never heard of it. I hope that maybe something here will help one of us figure out what is going on.

Originally posted by Lee Weber

My truck has suddenly developed an uneven idle. It goes from 850 to 800 rpm and back again every 3-5 sec. Power and mileage is still fine but the sound is annoying. Does anybody remember seeing a TSB on this problem? I’ve never had a reflash.