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  #1  
Old March 30th, 2014, 19:11
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blondejoncherokee blondejoncherokee is offline
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Sudden loss of power -engine just goes flat

We have good compression.
fuel pressure is good.
pluygs, cap, rotor, 02 are new

New TPS, tried multiple TPS, same thing.


its a 87 MJ. the thing out of nowhere has no power. I mean no power. It just like hits a wall when you try to accelerate. we haev checked almost everything.

It wont go past 3500 RPMs no matter what, but we cant even get it to go 2500 RPMs on road. Was fine yesterday. Just out of nowhere.

on the plug wires, 3 read 0.5, 2 read 4 , and one read 1.3. On the 20K setting on the digital multimeter


any ideas?

the thing idles fine too.




New air filter, checked all the vacuum lines.
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #2  
Old March 30th, 2014, 19:32
cheapcherokee93 cheapcherokee93 is offline
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Re: Sudden loss of power -engine just goes flat

Plugged cat?
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  #3  
Old March 30th, 2014, 20:09
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

you know it just passed smog like a month ago. That would be hard to belive, but maybe?

we swapped MAP and coil, no changes, new plug wires, no changes.

seems to have tons of fuel pressure, 31 at idle, 40 if we remove the vaccuum going to the regulator. when we apply throttle the pressure goes up to say 38, and starts to drop back down to 31 the longer we hold the throttle.


its like the thing revvs up real good, but just doesnt go anywhere.

manual not an auto. gutless as hell.

hits a certain RPM and acts like it just dead, starving , I dont know. I mean we cant really drive past second gear. its gutless. just overnight pretty much.
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #4  
Old March 30th, 2014, 20:46
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Re: Sudden loss of power -engine just goes flat

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Originally Posted by cheapcherokee93 View Post
Plugged cat?
Sounds like it to me too. Try pulling the O2 out and taking it around the block with the port open. If it makes a significant difference, the cat is plugged. If not, then at least you positively ruled it out.
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  #5  
Old March 30th, 2014, 21:49
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Re: Sudden loss of power -engine just goes flat

Two different motor management systems (actually three). Cold motor, hot motor (operating temperature around 140-160 or above) and full throttle.

If it happens on both a cold motor and a hot motor, that eliminates some senors as being the cause. Sure sounds like a warm motor O2 sensor issue, have you checked the O2 wiring harness where it runs up the front of the motor for burn marks?

Plugged CAT is a possibility. Plugged fuel filter is another or something with the pump and/or pressure. I've driven around with a fuel pressure gauge taped to the windshield wiper just to eliminate fuel as an issue. I had a similar issue once when my ballast resistor was failing.

I had the same situation when I installed my distributor one tooth off. Idled fine, but would run out of timing advance at higher RPM's.

Adjusting the TPS is just the half of it, the ground has to be good and the wiring to the ECU from the TPS has to be good (low ohm). The TPS has alot to do with timing.

Check your CPS and the CPS wiring, it doesn't sound from your description that this is the issue, but I doubt I've seen every possible scenario. I do know the CPS often gets worse as it heats up. Mine ran pretty bad once as the CPS wiring was slowly cooking on my exhaust manifold, it got progressively worse as the wiring melted.

Weak spark can hurt higher RPM performance. It usually shows up as a bad idle and/or a idle with an occasional miss at idle and then the spark breaks down again at higher RPM's. Seems the motor needs a hotter spark at idle and at higher RPM's than it does below peak torque (around 2200 RPM's or so).

Just on a hunch check the vacuum line for your MAP. Run your finger under the vacuum line and feel for rough, rubbed or chaffed spots.

Last edited by 8Mud; March 30th, 2014 at 21:53.
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  #6  
Old March 30th, 2014, 23:06
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

Two different motor management systems (actually three). Cold motor, hot motor (operating temperature around 140-160 or above) and full throttle.


If it happens on both a cold motor and a hot motor, that eliminates some senors as being the cause. Sure sounds like a warm motor O2 sensor issue, have you checked the O2 wiring harness where it runs up the front of the motor for burn marks?


so the plug at the o2 sensor is all melted looking, the harness side. So we should replace that part of the wiring harness. Ill look into the rest.



Plugged CAT is a possibility. Plugged fuel filter is another or something with the pump and/or pressure. I've driven around with a fuel pressure gauge taped to the windshield wiper just to eliminate fuel as an issue. I had a similar issue once when my ballast resistor was failing.

so we can unbolt the exhaust right before the cat and see if it gets better.


went to change fuel filter and fuel was spraying out everywhere right after shutoff. We didn't change the filter, so even if pressure at the rail is good the filter could still be clogged? Ill make sure we change it asap.



I had the same situation when I installed my distributor one tooth off. Idled fine, but would run out of timing advance at higher RPM's.

we haven't touched the dizzy since we got it. Not sure .


Adjusting the TPS is just the half of it, the ground has to be good and the wiring to the ECU from the TPS has to be good (low ohm). The TPS has alot to do with timing.

tried a brand new TPS and multiple spares and it . I checked the ground first thing, no resistance. Ended up putting the brand new TPS back in. output 17% of input voltage every time. grounds are legit. (since its manual)



Check your CPS and the CPS wiring, it doesn't sound from your description that this is the issue, but I doubt I've seen every possible scenario. I do know the CPS often gets worse as it heats up. Mine ran pretty bad once as the CPS wiring was slowly cooking on my exhaust manifold, it got progressively worse as the wiring melted.

CPS seems to have a jumper wires straight to the ECU, hard wired in, no connector. We have a brand new one to go in. need to swap it to rule it out. I will have him check the wiring. ill need to look into that.


Weak spark can hurt higher RPM performance. It usually shows up as a bad idle and/or a idle with an occasional miss at idle and then the spark breaks down again at higher RPM's. Seems the motor needs a hotter spark at idle and at higher RPM's than it does below peak torque (around 2200 RPM's or so).



Just on a hunch check the vacuum line for your MAP. Run your finger under the vacuum line and feel for rough, rubbed or chaffed spots.


first thing I checked a few months ago. that line was cracked. replaced it immediately.
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #7  
Old April 1st, 2014, 10:44
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

Could a knock sensor pickup some lifter or valve noise as a knock? Or maybe an exhaust leak too?
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #8  
Old April 2nd, 2014, 13:04
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

Here is what we plan to do next. Anyting to add? the jeep is 130 miles away otherwise I would have done all of this by now, trying to get my buddy to do it himself.

unplug idle air control valve-possibly replace with a different one

unplug knock sensor

test resistance across TPS wiring

unbolt the cat converter-drive. see if it helps.

replace fuel filter

more acetone in gas tank and 91 octane in the tank. fill it up at least 3/4 to see if
the fuel pump pickup is fubar. if simply filling the tank fixes it, pull the pump and make sure the pick up is all good.

clean ECU connections/ECU connector refresh

replace crankshaft sensor


clean injector plugs really good-make sure they are on firmly

pull 02 sensor plug to see if it helps


C101 connector refresh

sea foam the intake


EGR stuck open? Delete EGR?


clean all electrical connections under the hood


clean grounds at oilstick , and at firewall, and back of block

pull coolant temp sesnor plug and drive it

bad o2 sensor wiring?-check resistance across that plug that is burnt




LAST RESORT-after more testing of these things:

replace egr valve completely
replace fuel pump
replace distributor with cam sensor too
replace alternator
replace fuel injectors with Ford 19lb injectors
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #9  
Old April 3rd, 2014, 16:13
md21722 md21722 is offline
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Re: Sudden loss of power -engine just goes flat

Timing advance or other part of the distributor?

If the timing/ firing is way off I'd think it could show up as an exhaust O2/ cat problem.

I was in the process of diagnosing/ repairing a Range Rover that had a mechanical distributor that would sometimes run flat and other times would run like a scalded cat. It was totaled when someone crossed the center line so I never did get the distributor in.... but the cat/O2 sensors/fuel, accessories were all eliminated as causes.
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  #10  
Old April 3rd, 2014, 16:18
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

Quote:
Originally Posted by md21722 View Post
Timing advance or other part of the distributor?

If the timing/ firing is way off I'd think it could show up as an exhaust O2/ cat problem.

I was in the process of diagnosing/ repairing a Range Rover that had a mechanical distributor that would sometimes run flat and other times would run like a scalded cat. It was totaled when someone crossed the center line so I never did get the distributor in.... but the cat/O2 sensors/fuel, accessories were all eliminated as causes.
So like the camshaft sensor? There isn't a vacuum advance on these guys, not sure what else could be bad?
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #11  
Old April 5th, 2014, 06:31
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Re: Sudden loss of power -engine just goes flat

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Originally Posted by blondejoncherokee View Post
So like the camshaft sensor? There isn't a vacuum advance on these guys, not sure what else could be bad?
The advance is handled by the ECM. Don't go down this rabbit trail.

And, a Renix Jeep will run with a bad sync sensor. You can unplug it and drive and probably not notice a difference.
Knock sensor causes no driveability issues.

Just because you passed smog a while back doesn't mean the cat hasn't recently come apart.....
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  #12  
Old April 5th, 2014, 09:47
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Re: Sudden loss of power -engine just goes flat

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Originally Posted by cruiser54 View Post
The advance is handled by the ECM. Don't go down this rabbit trail.

And, a Renix Jeep will run with a bad sync sensor. You can unplug it and drive and probably not notice a difference.
Knock sensor causes no driveability issues.

Just because you passed smog a while back doesn't mean the cat hasn't recently come apart.....
Ok good to know. Update: the thing just killed the second egr diaphram. Like melted the thing , like a perfect line , like its getting hot exhaust or something. Thanks for all the help. I sent my buddy a very long list of things to test and he has been doing a few a night.
Swapped a few different o2 sensors no change. Pulled knock sensor wiring no change. Replaced fuel filter no change . Also going to unbolt the cat today, clean c101, clea ecu plug, clean grounds, and after that unbolt the cat.
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram

Last edited by blondejoncherokee; April 5th, 2014 at 09:53.
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  #13  
Old April 5th, 2014, 12:01
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Re: Sudden loss of power -engine just goes flat

Good plan.

How are your soldering skills?

Click on the link below and see post 27....C101 elimination.

http://www.cherokeeforum.com/f51/cru...x-tips-153657/
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  #14  
Old April 5th, 2014, 20:01
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blondejoncherokee blondejoncherokee is offline
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Re: Sudden loss of power -engine just goes flat

It was the cat! He didn't believe me. But he pulled it and the whole muffler was clogged full of honey comb. Back to inning hills in 4th gear again. Thanks a ton for the help guys.
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As soon as I crest the top i'ma turn driver and floor it!

1998 4.6 stroker , aw4 XJ on 60/14b , 5.38s, grizzly/detroit, cage, 40s, psc big bore box , assist, 3 link,transgo aw4 reprogram
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  #15  
Old April 5th, 2014, 21:16
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Re: Sudden loss of power -engine just goes flat

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Originally Posted by blondejoncherokee View Post
It was the cat! He didn't believe me. But he pulled it and the whole muffler was clogged full of honey comb. Back to inning hills in 4th gear again. Thanks a ton for the help guys.
Not surprised for some reason.

Wouldn't be a bad idea to eliminate the C101 some day when you have nothing else to do.
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