Motor races while coasting on '92 Dakota

Mar 08, 2008 8 Replies
J

Engine is a 2.5 liter four, trans is a five speed (four plus an overdrive) NP2500.



I'm still trying to figure out why the following is happening and having no luck. This is what's happening:



I park the rig facing three blocks of downhill. I always warm the truck up when I start it up and once it is warmed enough, I put it in neutral and take off the brake. It starts coasting downhill and I don't have my foot on the accelerator. Almost immediately, the motor rpm goes up to about 2000 rpm on its own. When I reach the stop sign, I come to a complete stop at which point the motor just as suddenly drops back to a normal idle.



The motor will race like this whether or not I'm coasting forward or backing up.



Candidates for the problem at this point are:


  1. a stuck throttle cable (I've verified that it isn't though)
  2. broken motor mount (I looked and it's not)
  3. SMEC (Single Module Engine Controller) is causing the AIS (Automatic Idle Speed motor) to act weird for some reason. This can be caused by a bad: a. throttle position sensor b. oxygen sensor (doubtful) c. ? d. bad SMEC

Any other ideas? This one even throws the guy at the dealership's service department.



TIA.


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J

Oh yeah, 2-wheel drive too.

By which I mean, coasting backwards.

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P

You realize you really should not do that right?

I'd guess the AIS is causing it, that the ECM thinks the engine is powering the truck, it is getting a closed throttle and then it is confused and trys to comphensate. (That's a WAG, however). Part of the reason is that I don't think the ECM knows about the hill...

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F

I put it in neutral

What happens if, after you warm up the engine, you put it into Drive, release the brake and coast down the hill?

FMB (North Mexico)

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T

Well, as an experiment, I'd disconnect the speed sensor. I think on your truck it's in the tailshaft of the transmission, rather than on the rear differential, but look in both places for a two-wire connector. Disconnect it (your speedometer will stop working), and do your downhill coast again. This will determine whether it's an electrical or mechanical problem you're chasing (because, without the speed sensor, the computer has no way of knowing if you're coasting or sitting still).

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J

Tom, thanks. That makes sense to me. I'll give that a shot and report back. May take a little while for me to get at it though. Right now I'm nursing a whiskey hangover. I didn't drink too much. Really.

Oww.

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J

As Max Dodge pointed out, it's a five speed manual. 80)>

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J

As Tom Lawrence suggested, the speed sensor should be the only way the truck could know it's moving. I'm going to try disconnecting it and see what happens.

Also, the AIS could be broken.

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