98 XJ No start - has spark, fuel, and air. Backfires

Hi all, new member here. I recently bought a XJ with a bad fuel pump that would turn over but not start. After changing the fuel pump, it would backfire from the throttle body but not start. Only code was for the cam sensor. I disconnected the exhaust downpipe and I can visibly see fire shoot out from the exhaust manifold when cranking (disconnected because I thought it could be a clogged cat)


Changed the following parts:
-New battery
-Fuel Pump
-ASD relay
-All 6 spark plugs
-Crankshaft position sensor. TWICE after hearing its a common issue
-Distributor (due to cam position sensor code), cap & rotor
-Starter (previous starter was weak to crank)
-PCM/ECU whatever its called. Junk yard part from different year Jeep 4x4 (mine is 2WD)
-New ignition cylinder

Tested:
-Fuel pump primes and pumps just file. Fuel at Schrader valve
-Battery steady 12.4 volts
-Tested spark on each cylinder and at the distributor. All test good
-No codes/CEL (after fixing the 1 I had)
-Checked for vacuum leaks at the intake manifold (using smoke through the throttle body) and could not find any
-Tested all the ground I can find. Cleaned and tested good


I havent tested for compression or engine timing yet. I did notice oil on the spark plugs when i changed them. My thoughts is either bad PCM, blown head gasket, or stuck valve, but im really not sure at this point.

Does anyone have suggestions?? Thanks in advance!!

Welcome to the forum possibly it jumped time?

What brand crank position sensor did you try? I have read that some brands mostly don't work.

Did you check fuel pressure?

One should always suspect the most recent repairs or modifications when a fresh problem pops up. Go back over all the recent work and confirm everything is installed correctly, all the wire plugs are firmly connected, and that no wires or wire plugs were damaged or allowed to dangle onto hot parts or sharp metal edges.

Since the distributor was replaced, and "jumping time" is not not a very common 4.0L occurrence, the base timing at TDC should be double checked and re-set correctly if necessary. Inspection for proper spark plug wire installation should also be performed to confirm no wires are mixed up and on the wrong cylinders.

The CKP sensor was replaced the first time with a cheap $8 SKP brand and the second time with a better one (forgot the brand), but shouldnt a failed CKP prevent spark and fuel? Can't justify a $90 Mopar CKP sensor when Ive heard others received defective ones.

I triple-checked the plug firing order and made sure each plug on the distributor end and spark plug end are tight. There's spark on each plug too. If timing was off TDC, wouldnt the engine still start but misfire and run sluggish?

I don't know that PSI at the fuel rail. Gotta check to make sure its above 50. I'll check this in the next day or two

Sensors are good if you're getting spark. I agree on checking the distributor timing and firing order. Some of the firing order pictures on the internet are actually wrong, btw.

Cruisers page on indexing the distributor:
http://cruiser54.com/?p=68

xchromata said:

The CKP sensor was replaced the first time with a cheap $8 SKP brand and the second time with a better one .............

Click to expand...

Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even defective, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors.

Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Cheap parts are cheap for a reason.

If it's backfiring, you have all the things you need to run, just not in the right order. Pull the fuel pump relay and discharge the fuel pressure. Pour a little oil in the cylinders through the spark plug holes to build compression again, since the injectors have been shooting enough fuel in there to wash it all away. Do a compression test. If the results look good, roll the engine over to TDC on the compression stroke on cylinder 1. You know it's compression stroke by placing your finger over the spark plug holes and feel for pressure as you turn the engine over with a ratchet. Once you line up 0° on the timing cover, pull the distributor cap and make sure the rotor is pointing to the #1 contact. I suspect it's more than likely not

Hi all, here's an update. I removed the distributor and rotated the crank pully to TDC. Timing was off a bit. I put the distributor back in making sure the rotor was pointing to cyl 1 and I pull a small amount of oil in cyl 1 to build compression.

I also put starting fluid since I dont have a fuel gauge to check PSI.

This results white smoke blowing out of the valve cover (air intake return?).

Here's a 10 second clip showing what happens.

https://drive.google.com/file/d/18A0vETX2iEJD9txq8QOqzuIOmdoDYK67/view?usp=sharing

I'm thinking its a blown head gasket. What do you all think?

You probably don't have #1 Compression TDC anymore( most don't realize there are 2 TDC's).

Update: I found the problem. When I drained the oil I noticed coolant mixed in it. I didnt notice earlier through the dip stick or oil cap. I'm going to check the block for damage and rebuilt the head + new head gasket.

Thanks everyone who chimed in to help

xchromata said:

Update: I found the problem.

Click to expand...

I think you found A problem. That video is troubling. With that amount of blow-by while cranking, I suspect you're going to find more than a bad head gasket or stuck valve. Done a compression or leakdown test?

Took off the head and found that cylinder 4 is blown and cylinder 2 is damaged.

I cant post a pic but here's a link showing the damage
https://imgur.com/a/JyyFKNl

Wow. My first thought was how did the hole in the piston get there, and then I saw the massively clogged coolant passages. I guess the cooling system was badly neglected.

I bet the previous owner did a compression test, then sold it real fast, claiming it won't start.