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Engine Problem but ran out of solutions

Alex

NAXJA Forum User
Location
West Palm Beach
Well i have a 1987 Comanche with a 4.0. The engine will start, run, drive, open up at full throttle, but has a terrible idle. It almost sounds like a poorly tuned carbed motor at start up. Best explanation for sounds be the engine firing then the rpms drop..... firing, rpms drop... It was at night time and sometimes the rpms got so low that the motor either nearly shut off, and also my lights dimmed out as if the truck was about to shut off.

Things i have checked or replaced:

-New map sensor couple months ago.
-Sprayed soapy water on all vacuum hoses and connections while running and no bubbles.(no leaks)
-checked the diaphram in the EGR valve and it seemed to move in and out with squeezing of two fingers.
-Throttle linkage was fine.
-Fuel Pressure seemed to be ok.

The only problems i could possibly think of is:

-another sensor that was not listed to check in the Haynes manual like an o2.
-Possibly the fuel pressure regulator (because i replaced it a few days ago, maybe i screwed it up(dont think i did) or the vacuum line...
-Maybe water or a little condensation in my distributor cap from pressure washing? That could affect it right?

After a few things this problem started occuring...Changing valve cover gasket, degreasing and pressure washing motor, and changing out fuel pressure regulator.

Let me know what you think if you could...Thanks

-alex
 
Remove the idle air control solenoid from the throttle body and clean it thoroughly. It's probably covered in sludge, causing it to stick and produce the bad idle that you described. Make sure you also clean the "barrel" that it sits in.
 
the best thing to clean the IAC passage out with is throttle body cleaner. Don't use it on any sensors. Cleaning the connections is best done with a small wire brush and some electrical contact cleaner.
 
If you pressure washed the engine you may have got moisture into some of the electrical connections, causing corrosion. I'd get some electrical terminal cleaner and clean the connections for the idle air control valve and throttle position sensor and see if that helps. Also make sure the C101 connecter by the brake booster is tight. The torque spec. for it is 70 inch/lbs.
 
Ok, update on how its running. I did probably 15 on and off miles on the truck today...the result was this:

It did not matter what gear it was in, park , neutral, 1st 2nd 3rd...It was still idling dangerously low and coming up maybe 100 rpms every so often. No AC running or anything.

What else i noticed was very often through the rpms while accelerating, at 1/4 or so throttle, there was a tremendous loss of power, regardless of speed. It happened from a dead stop crossing roads to acceleration from 50-60mph. I will clean out the connections since thats a good thing to do anyways. What do you guys think about that?

-alex
 
You mentioned you thought fuel pressure was okay, have you confirmed it? Throw a pressure guage on the fuel rail and check it with and without vacuum at the fuel pressure regulator. Should be around 31 with vacuum and 40+ without.

I just went through this with my '88 4.0L and had similar symptoms, and there was no change in fuel pressure between vacuum and no vacuum. Different fuel pump solved it.
 
10-4 bounty hunter...That seemed to be on my mind too. This fuel pump has 193,000 miles of pumping on it. I replaced its sock a couple thousand ago...damnit, i have to pull it again.

If i do get between 30 and 40 on the pressure gauge...what should i do? look at the pump or a filter next?

-alex
 
I noticed that my jeep is running really really rich and smoking some at idle and i'll smoke a little bit more when i open it up....This was not happening before the idle was super rou gh and low.

-alex
 
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