Cherokee 4.0L Backfire

[ZCar]

Have a 1990 4.0L Cherokee automatic transmission that is backfiring on acceleration or load (going up a hill). Runs great down hill (of course) and idles good. In order to go up hill I have to floor the accelerator and kick it down in low gear. It seems to work fine if I floor the accelerator peddle but hesitates if I accelerate gradually.


Things I have done:

Checked injectors (they were replaced about 30,000 miles ago.)
New rotor and cap
New fuel filter
Cleaned the O2 sensor (did not replace so it could still be bad)
Cleaned and regapped the plugs (all showed white deposits at gap)
Cleaned and checked all vacuum lines from manifold to various sensors and all seem to be OK.
New air filter but I am still getting some blow back into the filter from the PCV line into the filter element.
Disassembled the throttle body and cleaned it.

Has anyone had any experience in the symptoms of a plugged catalytic converter on a JEEP? Could this be the problem?

What sensors should I check that could cause this problem?

Is there a relation to acceleration and the automatic transmission? I seem recall seeing some information regarding a sensor or switch that is supposed to adjust throttle as the automatic transmission shifts.


Thanks for any input or suggestions

 

[Tug-n-pull]

Check your timeing! Go under the jeep and beat on the cat converter and see if it sounds like it is full of bee bee's. If it does it is more than likely faulty. good luck tug

 

[XJNick]

Hi,

Yes I have experience a catalytic converter go bad on my Jeep, and it does greatly affect things like acceleration and power.

I think it is definately worth checking/replacing.

Also, does it act that way always, or only after the engine has warmed up?
I ask because the 02 sensor won't have any affect on the computer system until the computer switches to open loop mode (after engine is at operating temperature.)

-Nick :!:

 

[ZCar]

Tug and Nick,

Thank you for your response.

Nick, it seems that the problem gets worse as the engine warms up. I will replace the O2 sensor and see if that helps.

Tug, I will check the timing. I do not know why I had not thought of that but for some reason I though that the computer would handle that. Also I will give tha cat a little banging to see it it sounds like gravel kicking around in there.

I will post the results.

Thank again

ZCar

 

[ZCar]

OK Here is the latest update.

Had the injectors serviced and replaced 2 as they were leaking fuel onto the manifold.

Installed a new O2 sensor.

Installed new plugs.

Current symptoms are:

Jeep start good and always runs good at idle and under load when cold.

When warm (about half of the time but not always) it starts to backfire through the throttle body and loses power when under load like climbing a gradual (or steep) hill. This usually starts when warm and after running about 20 minutes. Under full throttle it seems to run OK. And it feels like maybe it drops a cylinder and then picks it up again under full throttle.

When warm it also starts harder at times but not always.

Considering everything else that I have done (e-mail at the start of this thread) does anyone have any suggestions?

Could an intake manafold vacuum leak cause intermittent problems.
Could a bad coil cause intermittent problems?
Could a bad valve cause intermittent problems?
What about timing on a 1990 4.0L. The manual says timing is not adjustable but what then controlls this?

Any or partial suggestions are appreciated

Thanks

 

[Jason01]

I think that you might have the same problem as me. I havent yet figured it out. I think mines a little worse though. Mine will backfire through the intake just like yours, but mine will do it when its cold or warm. Mine also runs mostly better at full throttle. Does yours start and then die? I have to start mine about 3 times before it finally runs right. The worst part is that even the mechanics who work on this thing cant figure it out. I have a question for someone with a 4.0 jeep that runs fine - how much A/C voltage does your Crank position sensor put out @3k RPM? PLEASE some one, do this..... Maybe you can help both of us out!!!!

 

[XJNick]

Could a bad coil cause intermittent problems?

I've been told that it would either work or it wouldn't work. But anythings possible. Also, the timing is set mechanically by the distributor and electronically in the ECU. They say not to mess with it, but I have heard of people adjusting the mechanical timing of the distributor.

how much A/C voltage does your Crank position sensor put out @3k RPM?

The voltage output of the CPS does NOT change with RPM. The sensor will always put out the same amount of power depending on how strong the magnetic field passing by it is. It DOES however change output frequency and "pulse width" (duration) depending on the speed that the flywheel is spinning. If I remember correctly, mine was putting out somewhere around 0.95 volts MAX when I last tested it. I can't be exact as my voltmeter is not very acurate. It is a very small amount of voltage, just enough to trigger a circuit in the ECU.

General question to both of you:

> Have you checked the Throttle Position Sensor (TPS) with a voltmeter through its entire range? You should read 0.8 volts at closed throttle and around 5 volts at wide open throttle (WOT).

Hope that helps :mrgreen:
-Nick :!:

 

[Jason01]

Checked the TPS, had .75 closed, 4.5 fully open, smooth and gradual climb on my scope.

Just for curiosity's sake, I just checked the CPS with the engine running, and at about 4-5k RPM, the thing was making about 11-12 volts A/C. I think it is normal to do that. I think that I can rule out the CPS as a problem source. What all does the Ignition module do(part that the coil sits on)? Would this cause the problem? The reason that I ask is that the timing is very much erratic on mine. I dont know about Zcar's, but mine will change by 20+ deg. while idleing! What causes this? I'm about ready to make the jeep really die!

 

[XJNick]

The ignition module is what receives the data from the ECU, then triggers the ignition coil. It is also where the tachometer signal comes from. I believe in older Cherokees, the ignition module and coil are one piece.

If you are certain your timing is that crazy, then I would wonder about a bad ignition module/coil or ECU.

-Nick :!:

 

[grizzlybear310]

I know I don't know anything, but would the cam postion sensor cause their problem?

 

[XJNick]

Yes, a bad cam position sensor could cause problems...
Come to think of it, it probably is a commonly overlooked part... being that it's in the bottom of the distributor.

-Nick :!:

 

[Autocrossk20]

Same problem

I think that you might have the same problem as me. I havent yet figured it out. I think mines a little worse though. Mine will backfire through the intake just like yours, but mine will do it when its cold or warm. Mine also runs mostly better at full throttle. Does yours start and then die? I have to start mine about 3 times before it finally runs right. The worst part is that even the mechanics who work on this thing cant figure it out. I have a question for someone with a 4.0 jeep that runs fine - how much A/C voltage does your Crank position sensor put out @3k RPM? PLEASE some one, do this..... Maybe you can help both of us out!!!!

brandon Bryant here I have a 88 cherokee 4.0 has the exact same problem the truck was recently sitting for 3 months blown front end and rear end replaced both of them last week drove truck still had that exact same problem ( anywhere from a toe on the gas to almost full gas is popping backfiring whatever has no powers no throttle response what so ever.) no mechanic I know has a clue what the problem was they have me ideas changing the intake manifold to a carbureted one to delete the ecu controlling fuel cause I mean it feels like a fuel problem if any of you with the same problem pull out your plugs there will be white that means your burning rich constantly . I replaced everything trully everything map sensor, the one 02 sensor I have coolant temp sensor got a ignition coil from a junkyard, cps changed today because it have out two times after I started it up after fixing the rolling part of the truck, tps sensor changed fuel filter, never got a new fuel pump maybe that? Idk I'm outta money right now this truck ****es me off

 

[98Chero]

Re: Same problem

Sorry to bring up this old thread but I am having the same problem as these guys. I see the thread was never resolved.I suspect the MAP sensor vacuum line might have been leaking air, would that cause the engine to fail after warm up ?