Would someone check my MAP math?

[6spdYJ]

You just reminded me of something. Remember that the 2.5 is the parent motor of the 4.0. I know a guy who had some random power drops, followed by all out failure to run and backfires through the throttle body. It turned out to be that his timing gear bolt backed off and the gear sheared off the stud. Once the cam gear spun, he would get different compression readings from the same cylinder - #1 first test 95psi, #1 second test 110psi, #1 third test 80psi.

[4wheelinmike]

I use to back fire through the throttle body lol

[6spdYJ]

Just did a bit of research. The camshaft position sensor is on 2000+ 6cyls. It replaces the function of the distributor body in the earlier models. Won't be the last time I'm wrong. Sorry.

[4wheelinmike]

BUT we will throw a CMP code if the conditions are right!

[6spdYJ]

I was reading back through this and remembered something. I once drove a 1988 Le Sabre with the 3800. It was MPI with all the bells and whistles. It was only a year old and ran flawlessly. On one trip, for whatever reason, it backfired and couldn't get over 35MPH all the way home. During the trip home, sometimes it would try to run smooth and then sputter again. It frequently died at stoplights. What the culprit wound up being was a small leak on a vacuum line caused it to malfunction and backfire. The pressure blew the hose almost all the way off. It had just enough vacuum to run, and with the right vibration it wold even try to re-seat itself. After the hose was replaced and the rest were checked and proven to be good, it ran like brand new until the day it was traded in. Just a thought.

[Jason]

I am currently replacing a few hoses and seriously looking at the exhaust manifold for a leak I think I have.

[6spdYJ]

The 4.0 is notorious for loosening up at the head. Grab a 9/16 combination wrench and a bunch of patience. If you find that it has cracked, check this out - http://www.ineedparts.com/auto-parts...-manifold.html
Mine worked out great. You won't find a better price. What it actually is, is a mid-length header. Look it up under challenge-Y - that's the manufacturer's name for it. It just doesn't have the prettyest finish.

[Jason]

After taking time to check this and that, I decided, against better judgement, to take the Jeep out on a 26 mile errand. Got about 3/4ths of the way and the Jeep backfires and dies on the freeway. Doh!
Got to my destination with only a few more instances of dead jeep and decided to stay off the freeway and take Azuza to Beach Blvd home. I must have been on the wrong Azusa as the one I was on hit a dead end.
Anyway, found my correct route and about every 1/2 mile, the engine would just die. Sit for a minute and start up again.
Got home and let everything cool so I could start working under the hood. When I started to remove the vaccum lines, the fresh inlet hose from the air cleaner just broke. Hard brittle plastic I guess.
I am going to start with replacing the vaccum lines and remove the CCV fitting and look at that. There is a sensor on the front of the fuel rail that looks suspicious.
One thing I did notice is a lot of carbon build-up in the throttle body. Probably from the engine running so rich. Each time the engine died, there was a strong gas odor.

[6spdYJ]

http://bc4x4.com/faqs/yj.cfm?cat=5
Check out the stuff here. There's a lot of collective info on the YJ. Cleaning the inside of the TB is recommended. I do know a flaky MAP sensor or TPS will cause it to run rich and/or just die.

[Jason]

Alrighty. This weekend I changed the TPS, changed the Idle sensor, tested the fuel pressure, test the coil, and nothing. The throttle body was covered in carbon residue and that is now all cleaned up. The engine now will start and after about 30 seconds it dies. It will start right back up, idle for a bit and die.
Next, I think I will drop the fuel tank and take a look at the fuel pump. I've never looked at the fuel pump. Does it have a pick up that might get clogged? With the tank empty, it will get some fresh fuel and maybe, problem solved. I'm running out of things to replace. I hope it is not the main computer.

edit, also took off the CPS and cleaned it off and shot some cleaner on the flywheel just for fun.

[6spdYJ]

Dumb question, have you done the fuel filter? The pump does have a sock over the pickup that can get clogged. The bottom of the tank has a bowl in it that the pickup sits in. If this is full of crud, it will suck up and clog the sock. It's definitely worth looking at.

[Jason]

Fuel filter was done a couple of month's ago. I was trying to do everything else before i drop the tank. I was thinking the same thing about the fuel pump. That might explain why I can get it started, runs for a bit, and then the pickup sucks up crud and starves itself. I don't know. I'm running out of ideas.
I told the guys at the parts store that since I had been in numerous times over the weekend and spent some coin that I wanted a personalized parking spot in front.

I'm kind of under the gun here as the Jeep needs to be smogged here in August. I may have to ask for an extension while I get it figured out.

[6spdYJ]

Your fuel pressure should be roughly 33psi with the vacuum connected to the regulator at idle and go to 39psi with the vacuum disconnected (to simulate full throttle). If you have it idle and disconnect the vacuum line, does it die any faster? Also, does the idle increase a few hundred RPM before it dies? A lean condition from fuel starvation will cause this. It's much easier to detect in a carbureted engine though. I'll go through my manual and see if there's anything we may be missing. I forget. 4cyl?

[Jason]

Yes, 4cyl. My fuel pressure readings were 39 with no vacuum and 32ish when connected. I did find it odd that without vacuum, the needle was 39 exactly. Now when I applied vacuum, the needle jumped around 31-33. Never once did it remain stable. I was thinking about that this morning. That just didn't seem right.
Each time the engine died, it was with the vacuum line attached. I was more interested in the pressure readings. It didn't seem to want to die any quicker without vacuum.
I did learn something new this weeked. I do not have a PCV. I have a CCV. There is no "valve". Just a really small hole for the gases to pass through. Air restrictive valve I guess. I wonder what keeps the gases from going the other way?
Last night I was looking at the fuel return lines and the charcol canister caught my eye.

[6spdYJ]

Go back to www.bc4x4.com there is a subject on YJ tech - engines that addresses this. There's a lot of good information there.