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bucking Jeep

A

aspera

NAXJA Forum User
Location
KC
I have a 91 XJ six cylinder auto that bucks and runs rough. The more throttle, the worse it gets. I daren't (yeah, I use pirate words) :scottm: go full throttle or the engine will fall out.

But. When I leave it parked for a day or so, it runs just fine for about the first 30 seconds. It even runs strong at WOT...for the first block.

So, what kind of problems happen after the Jeep starts to heat up? Fuel pump? Coil? Some sensor?
 
Fuel Pressure Regulator. On the fuel rail in the front of the motor. Check for leaks in vaccuum as well. One or the other most likely.
 
Rocketman said:
Fuel Pressure Regulator. On the fuel rail in the front of the motor. Check for leaks in vaccuum as well. One or the other most likely.
Click to expand...

Thanks. I'll check that out next.

Q: Is the 95 the same as a 91? I have a 95 donor Jeep.
 
also the 02sensor could be bad i my xj had a problem with something along those lines ..i replaced everything i could think of besides the o2sensor ..finaly replaced it cured my problems..
 
Jason-RJR said:
symptom definitely sounds fuel delivery related, most of the vehicles ive had this happen to needed a fuel filter.
Click to expand...

I already put in a new O2 sensor and fuel filter. No difference.
 
To recap:
new FPR
new O2 sensor
new fuel filter
new cap and rotor
pretty new plugs and wires (wires are still clean! heh heh)
I swapped the whole throttlebody assembly off of my 'good' 95XJ, including TPS and IAC.

I think I'll swap the coils next, since it looks easy.
 
Check the air intake manifold tempurature sensor!

also, have you tried checking the fuel injectors themselves?

Try pulling the electrical connection on each injector and see if the engine performance changes any. If it does not when disconnecting an injector, there is something wrong with it.

Did you change the rotor, cap and wires since the problem started or before?

When you swap the HV coil also try swaping the ICM (Ignition Control Module directly under the HV coil).
 
lilredwagn said:
does the 91 have a ballast resistor?
Click to expand...

I replaced that with a really cool heavy duty red switch that I found at Ace Hardware. I need to get another one because the toggle switch looks like it will fit into one of the rectangular slots in my WRX. Then I can label it "rocket assist" or something.
 
Ecomike said:
Check the air intake manifold tempurature sensor!

also, have you tried checking the fuel injectors themselves?

Try pulling the electrical connection on each injector and see if the engine performance changes any. If it does not when disconnecting an injector, there is something wrong with it.

Did you change the rotor, cap and wires since the problem started or before?

When you swap the HV coil also try swaping the ICM (Ignition Control Module directly under the HV coil).
Click to expand...

Where exactly is the air intake manifold temp sensor on a 91?

What does the ICM look like?

I changed the cap an rotor since the problem started, but the old ones really weren't that old.
 
The Air temp sensor is on the top of the intake manifold behind the throttle body assy. It has a brass head with a 2 wire connector at the end. Look between the brake master cylinder and the throttle bottom and the firewall, it is threaded into the top of the the air intake manifold.

The ICM is directly underneath the high voltage coil. It has a one wire and 3 wire connector visably attached. The high voltage coil is attached to it, and gets its electrical signals from the ICM, but the electrical connections are hiden under the HV coil!!!! They can get loose and dirty too!
 
I pretty much had the exact same thing with my 90 XJ. I eventually found I had less then 20 psi fuel pressure. No matter what i did I couldn't get it above 20. Changed out fuel pump Idles at 31 psi. Changing my fuel pump cured not only my power loss issue but my wandering idle and hard starting.

Just a note. I checked my fuel pressure on the fuel rail by pushing in the schrader valve and it had good spray but obviously once i got a proper gauge the proper was there.
 
Blaine B. said:
What is the sensor that is screwed down into the intake?
Click to expand...

Are you asking about the Air Temperature Sensor in the intake manifold itself? If so it works with the MAP sensor giving the ECU the complete data it needs to determine the actual MASS air flow rate (not just volume, but the mass of air) into the engine. If it is disconnected, bad electrical, or giving bad data the ECU can not properly control the air to fuel ratio.
 
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