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1990 starts and idles OK, but won't rev up

D

Durango-Bob

NAXJA Forum User
Location
Greensburg, PA
My Cherokee has 130k miles on it and was running fine until yesterday. I have had other problems over the past few months, so it has had the several new parts installed. (CPS, plugs, wires, cap, rotor, coil, fuel pump, complete exhaust, 02 sensor) All these parts were installed several months ago and it has been running fine. Now it will start up and idle just fine, but it will not let me accelerate at all. Any attempt to push in the throttle causes the engine to start to run rough and act like it is trying to backfire. The tailpipe starts spewing out black smoke, too. It just will not go over 1000 rpm's. Any ideas on where to start?
 
I was thinking of that myself, but just wasn't sure if that could cause this problem. I unplugged it and tried it again. Same problem. If it were bad, I would suspect that the problem would change with it unplugged, wouldn't it?
 
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Definately the TPS. Start it and have it idle. Reach in and rotate the little arm on the TPS. It should almost immediately die or at least stumble a ton. If not, your TPS is not working. Try replugging the connector.

I guess in theory it could be low fuel pressure and it just doesn't have enough fuel to run above an idle, but that is a longshot.

Have you washed under the hood lately? Power washers are the death of TPS's.
 
The cat is clear with no restriction. Already verified that. Whatever is going on is the result of something that stopped working. It ran fine the last time it was OK.
 
The TPS change did not fix the problem. I will check out the EGR valve tomorrow. How should it look if it is working properly. I have never had to work on one of these before.
 
if you look at the shaft on the egr valve it should have a recessed groove around it. if you can take a screwdriver and try to push it in towards the intake. if it goes in, it was stuck open.
 
Thanks, I went out with a flashlight and checked out the EGR valve. It moves just like I would have expected and is not stuck. Any other thoughts as to what can be wrong? Since this problem is there all the time, is it possible to take it to a shop and let them connect to the computer to diagnose it?
 
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Did you ever rotate the arm on the tps? What happened?
 
Check the CPS. I had very similar problems, and found out my CPS wire was fried. It sits very close to the exhaust manifold, and mine must have touched it a few times. The new one was about a foot shorter than the one I took out, and now all is well, and I don't think this one will ever come in contact with the manifold. I just did this 2 weeks ago.
 
I had checked the TPS with a meter and it had a very nice smooth change in resistance as the arm was rotated. I installed a new one anyway but this did not change anything.

Removing the fuel pressure vac line showed that the regulator seems to be working OK.

The CPS was new not too long ago, but I will look at the wiring today.
 
There was a reason to rotate the arm not just check it with a meter. That way you can see that the computer is responding to inputs from the TPS. You could have a corroded connection at the sensor or at C101 or even at the computer. C101 is the big connector just above the brake master cylinder. There is a TSB on intermittents with this connector. DC even says to cut out the connector and just hard wire the wires together.
 
Thanks for the clairification. I will do the TPS check right away.

As for the plug wires, I am sure that they are right cuz it was running fine a couple of days ago and the wires are still in the same place.
 
I finally got to try the TPS test. I removed the TPS from the throttle body and with it still plugged in, I started the Jeep and then manually rotated the arm on the TPS. The engine RPM's dropped very slightly but it still ran very smooth. Then with it still not on the TB, I gave it some throttle and the rough running and total failure to gain any RPM's was the same.
 
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