Has spark, has fuel, cranks but wont start...I'm desperate

[SilverXJ]

I don't think that narrows it down to the PCM. From what I gather you have Cali emissions and you said he has Fed. Yours has 4 O2 sensors, his has 2. I am too lazy to open up the FSM and see where the O2 sensors go on the PCM, so the pins they go to may be different and not reading the sensors 100%. I have a spare Cali PCM, but unfortunately it was attached to my XJ which has key sentry. Did you try to swap his PCM in yours?

Are you 100% sure your sere tdc on the compression stroke when you dropped the cam sensor in? Did the drive gear look good?

What color was the spark when you checked the coil rail? Blue, orange?

Have you checked the CTS?

Also, consider removing the plugs and let the plugs and cylinder dry out of fuel.

[nicpaige]

No both of ours have federal emmissions. Only difference is his is a 99 and mine a 2000. CTS checked, replaced also, had spark arcing from plug to valve cover about an inch in length. I wasnt comfortable putting his good pcm in mine for fear it may damage his pcm out of his daily driver.

[SilverXJ]

Oh.. you did say 99.. i kind of just glossed over that part and wasn't thinking. That test is even worse. lol. His is distributor and yours is coil rail. The coil rail has 4 wires. B+ and 3 ground for 6 cylinders. What was probably happening when you put it on his is that only two plugs were firing being that his wiring harness was only hooking into one of the ground wires.

[nicpaige]

You are correct, back to the drawing board. For some reason I thought the 99's were coil rail as well. Didnt even look. Live and learn again.

[nicpaige]

Update....matching year-part number computer from salvage tried in car and exact same results. Have fuel, spark, but will not fire and run. Back to searching for shorted wires.

[nicpaige]

Here comes the gut wrenching part. AFter going over all sensors, battery, timing, etc we tried some starting fluid down the TB. We got a few very large backfires but did not run. SO then it seemed logical to check compression.....not good on all cylinders. Added some oil into cylinders and retested....better but still only 100. Went straight to leak down test and had 85%. Looks like the rings are shot. Blow by coming from crank evac, dipstick. None out exhaust or radiator. I am baffled how this happened the damn thing was running fine, never once did it blow smoke or foul plugs or even run rough. Im going to run a scope in the cylinders just for giggles but it's a foregone conclusion that its coming back out of the Jeep.

[SIXPAK]

I dont get it, that shop does great work. Puzzled

[nicpaige]

Im not laying any blame toward the machie shop. I assembled the engine, they did the machine work. Im just really confused as to how it can be running fine (although it never left the garage and onto the road). We shut it off and finished buttoning up some interior items. It has never started up after that time. Checked and double checked all sensors, spark, fuel. Starting fluid made it act like it may barely fire and then multiple backfires through intake. Very low wet compression numbers (cold of course) in the 90-100 range. Terrible leak down numbers in the 80-85% range. All leakage coming though evap tube, dipstick tube. I'm heading down to look at the cylinders with borescope now. How can an engine run fine, idle fine, 14.7:1 a/f ratio, great oil psi, no over heating, proper cam break in go from that to no start and 85% leak down. I am assuming not enough cylinder compression to ignite fuel? And what with the backfiring with starting fluid? Suggestions would be awesome before the engine gets ripped back out for tear down.

[jsawduste]

Nic, only thing I can come up with that would be common to all 6 would be the rings. If by chance they grooved the hell out of the walls or are somehow stuck in the grooves.

The backfire confirms you have an ignition source. When and how TBD.

Curious on what you find with the scope.

[nicpaige]

The scope did not reveal anything catastrophic, other than what looked like some rusty deposits on the undercut of the valves and possibly deposited on top of the piston. The video was not super clear but I took some snapshots and will post them tomorrow hopefully. Right now the plan is to pull the engine and tear it down. Still a complete mystery how this came about but the leak down test was pretty damning. You would have thought oil would have been pouring out the exhaust. Really dejected at this point. Looks like the original 4.0 may be pressed back into duty.

[nicpaige]

skirt1.jpg

Nic, only thing I can come up with that would be common to all 6 would be the rings. If by chance they grooved the hell out of the walls or are somehow stuck in the grooves.

The backfire confirms you have an ignition source. When and how TBD.

Curious on what you find with the scope.

DING DING DING DING we have a winner. Now time for full disclosure. From what I see pulling the pistons it can only be installation and assembly error. Plain and simple, nobody to blame here but me. To me it looks like one or a combination of three possible things. 30 minutes run time, maybe thats a record.
1. Ring gaps way too tight
2. Ring thickness not correct
3. Engine not properly washed before assembly
All of these possible causes are operator error. Pisses me off because I know better and just got in too much of a hurry. I did not check all the rings, and the ones I did I must have read my feeler gages wrong. Doesnt matter. Flame away

[nicpaige]

Well to close this thread out I can report that the Jeep is back to running again. File fit the new rings set, cleaned up the piston skirts and ring lands from the frozen rings. New main and rod bearings installed as well. Started up fine, idles fine, good temp and good oil psi. Hopefully I can actually get hold of a tuner to work with and finally get back to enjoying this Jeep!

[jsawduste]