OK, In summary, Here are the things I need to check, and in the approx order:

Catalytic Converter. Probably will temporarily remove it and run open exhaust for the test.

Fuel Pressure

Compression test

Coil--How can I check the coil?

 

Should be able to ohm it out. Basically just a length of wire in a coil.

 

id skip the cat for now and think simpler.....fuel pressure first.....
Spark
condition of plugs/wire/cap and rotor

just a FYI i am a active FULL time diesel mechanic..

I installed a set up plugs in my jeep...had maybe 10k on them...jeep started running horrible missing..acting up...wouldnt rev..i checked eveything...it turned out to be my new plugs had some carbon build up causing spark jump.....so just think simple....

 

I'm with you on the do the simple stuff first. I'm a long-time member on a Mercedes board ( a diesel, by the way), and a Ford diesel board. It amazes me the way some kid will buy an old diesel (whether ford or MB) and want to really tear into it. We had a kid with a no start on a ford diesel and the old hands told him to check the wiring from the batteries to the starter. He wanted to change the starter, pull the Injection Pump --all kinds of things.
Now there is another kid on the MB diesel site that has an old 240D that smokes. Again, the old hands tiold him that based on the history he gave, it was likely gummed up oil control rings--soak it w/ MMO for a few days when its warm, and see if that helps. He wants to pull the head, the injection pump, pull the engine and re-ring it. He has no idea what MB engine parts cost!

I am a great fan of the KISS philosophy.

When I first saw the Cherokee, I thought "plugs". The ones that were in it had a "gap" ( if one could call it) approaching 0.100inch.. New plugs made no difference.

 

UPDATE.

I finally got it to my garage.
Battery is DEAD, so I will charge it overnight.
With a good, hot temporary battery it would try to start but immediately stall out. I will put some fuel in the tank to rule out THAT possibility.

Is there a link to checking fuel pressure on this 4.0 liter 6 cyl?
Should the fuel pressure be present when the key is "on", or do I need to crank it?

Today we got it started well enough to drive onto the tow dolly, but nothing since.

 

base fuel pressure should be present with the key on...around 40psi it is i believe......

 

Here is some real data:

Fuel Pressure with key ON--20 PSI
" " cranking the engine 48-50 PSI
" " running--all conditions 48-50 PSI

COMPRESSION TEST, WARM ENGINE
CYL DRY WET
1 135 150
2 125 135
3 160 165
4 170 170
5 120 140
6 165 165

Exhaust pipe temperature right out of the manifold was around 800 degrees F within 5 minutes of the start of the engine.
I will check this again tomorrow after I put a fresh battery in the IR thermometer.

When it starts, it will run right up to 3500 RPM. Above that speed, if I go WOT, I hear what I can describe for the rest of you old guys as the sound a small block V8 makes at low rpm when the secondaries of the 4 bbl carb open up but before the engine picks up enough speed to use that extra venturi area. Do you know what I mean?

Does this suggest anything to you?

 

Good luck finding an engine out of a Cherokee at a JY. Thanks to the brilliant cash for clunkers the government seized up most of the engines with liquid glass.

 

I would remove the Pre cat o2 sensor, and then try to rev it up. That will show whether or not the cat is plugged.

 

I'm thinking 'Fump'!

Just by the sound that it's making alone, "opening" it up and just getting air and no fuel... although i'm a little confused with your fuel pressure test results, (Running- All Conditions 48-50psi). Does "All" include when it's actually acting up?

Honestly, i'm not at the least bit familiar with problems with CAT's, so i'm taking the back seat on that one!




Very interested on what you find out!!!

 

sounds like a clogged cat to me. check the temp erature after the cat and see what it is. it should be within 100* or so. also, hit with your hand and see if it rattles. if so the "element" in side could have broken and turned sideways, blocking air flow. as was suggested, pull the cat or an O2 sensor and see what kind of difference it makes.

zedpapa

 

On the Fuel Pressure, YES- The needle never wavered from that spot with the engine running. I will verify it again, later today.

 

OK, someone will have to educate me on the meaning of "fump".
I know "fubar", but not "fump"

Today I ran it for a few minutes and recorded theses temperatures. After it cools off a while I will remove the O2 sensor, or crack the manifold to exhaust pipe bolts.
It ran right to 4000 rpm right after starting ( after allowing a little time for the oil to get up to pressure ( 40+). The a minute later the max is 3000 rpm just like before. Fuel pressure stayed right at 48-50 psi during all this, including when it ran poorly.

Here are the temps ( everything at 37 degrees F at the start.)
All temps are in degrees F

time after start twin exhaust pipe, single pipe front cat
30 seconds 300 200 60
1 minute 380 230 75
1:30 530 280 125
2 minutes 660 300 170
2:30 750 360 200
3 minutes 770 460 250
3:30 780 470 300
4 minutes 810 480 350

Any thoughts on TPS sending wrong data to the computer?

 

Lets close this thread.

I disconnected the ex. pipe at the manifold, and started the engine.
IT RAN GREAT!. Right up to and above ( briefly) 4000 rpm. I let it run for 15 minutes, and it still ran up to and over 4000 rpm.

Don't you just HATE "professional" mechanics who pronounce an engine "toast" but won't provide details as to what broke? I guess he would have replaced the engine, and when the new one ran just like the old one, he would have figured out the problem, and replaced the exhaust system and never said anything about it being the real cause of the problem.

Thanks to all here who provided help. Eliminating the fuel system as a problem was critical to solving this.

 

that would have been weird..."He" replaced the engine....and uhh ooh man look at that a shiny brand new exhaust system too!!???!?! wonder how you would explain that one.