I have a 92 5.2 automatic 4x4. I was coming down a steep grade on the highway today and threw the truck in neutral while I was coasting down the hill. I was traveling about 70-75 mph and I noticed on my tac that my RPM's were fluctuating up and down between somewhere between 600-1200 or so.

When driving in gear there is no surging nor is there any surging/fluctuating while idling at a stop in neutral or in drive. This only occurs while coasting.

I recently replaced my TPS and idle air controller for other issues that where resolved. So I can likely rule those out.

About 2 weeks ago the truck was running really rough and stalling. The check engine light came on and threw a code having to do with the MAP sensor (can't remember which one). I unplugged the sensor and battery to reset everything and all has been well since then.

I plan on replacing the MAP soon, I was just wondering if this issue may be related. The other suspect part in mind is the EGR valve which still has a Chrysler sticker on it and looks pretty old, so I'm assuming is the original.

Could either of these parts cause this issue? Anyone have any experience with this?

 

The MAP sensor could be causing some problems, and replacing it might be a good idea.

But for coasting in neutral problem sounds like its just the PCM being confused since its getting a signal from the speed sensor that the truck is moving, and the PCM doesn't know the transmission is in neutral so its trying to keep the engine running at driving engine speeds.
In other words I think you just need to stop putting in neutral while coating, that is what is causing you problem. Nothing is wrong.

 

OK, thanks for the input Crazy. Makes sense. I wasn't so much concerned about the fact that the tuck was surging in neutral as I was that this might have been a symptom of another problem. Coasting in neutral isn't something I normally do, but thought this might be indicative of a fault somewhere. I guess I won't worry about it.

What about the EGR? Is there anyway to test it? If it's the factory original should I just replace it by now? Not that I think an EGR will fix the neutral surging which I guess is a non-issue, I was just wondering how I would know if the EGR is bad?

 

Mine's automatic too, but a '93 3.9L v6 2WD and when I put mine into neutral while coasting (which I do frequently as it puts less stress on the brakes), and while coasting, my RPMs jump to about 1200 RPM and sit there steady. No surging. Mine has a bad EGR as well, so I would assume you could somewhat rule that out (but don't forget that parts fail in many different ways, so don't completely rule that out!), but the MAP sensor sounds about right. I recently did my coolant temp sensor, and I unplugged the MAP sensor just to get the wiring clip out of the way so I didn't break it. When I was finished, I forgot to plug the MAP back in, and when I started it up, it ran like you described. Only difference was it always ran like that, even at idle. So maybe your MAP is just on its way out, or the vacuum line to it is just broken

 

Ya I'm sure its on its way out, I'm just hoping that the MAP has something to do with the neutral surging. I recently did the TPS and man what a difference it made in power. I pretty sure nothing on the throttle body had ever been replaced before I got the truck last year, and if it was that was at least 7-8 years ago when the motor was rebuilt.

How do you know the EGR is bad on yours? Mine is original, 20+ years old, I can't imagine it's in tip top shape. Wondering if replacement is in order for this too.

 

Mine throws a code for the EGR being stuck open. I've been running the truck for 4 and a half years with a bad EGR and haven't had a single problem with runability. Starts, runs and shuts off just fine.

As for the MAP sensor, that is likely the culprit for the neutral surging. Try unplugging it completely and run the truck. Just idle it in park, and see if that surging is any different from the neutral coast surge your experiencing.

 

does it have any lean codes?i would check for a vaccum leak or throttle body gasket blown out

 

vaccum leaks will throw a map code if its big enuf.which should cause a over fueling condition.if map sensor is reading close to baro prussure pcm will think ur under load or heavy on the throttleshould see black smoke if this is the problem.egr valve will also cause this problem if its hung up.should throw a p0401 or p04xx.hope this helps

 

No no codes other than the map that one time which has since went away. Will check for vac leaks before replacing MAP, already fixed several though I doubt there are any more.

 

On the sensor itself, the is an small elbow vacuum line that goes from the sensor to the throttle body. If there is a vacuum leak to the sensor, that's where it will be. Also like I said, try just unplugging the electrical connector from the MAP sensor and see if the way it runs idling in park with it unplugged is any different from when you are coasting in neutral