1988 MJ 4.0L was running - now nothing but fouled plugs

[terrawombat]

1988 Comanche w/4.0L and 5-speed transmission. Truck ran a couple years ago, but kept getting fuel mixed in with the oil. Parked it, bought another MJ and basically forgot about the truck. Just came back to it a couple months ago and had the injectors professionally cleaned - someone told me stuck injectors could be leaking fuel into the engine and past the rings, hence the gas/oil mix in the crankcase.

 

Fast forward to now - replaced the alternator, installed new plugs, truck started and ran, but with a very noticeable hesitation coming off idle and some ferocious backfires as the truck came down from the upper RPM range. Plugs were black as night indicating a rich mixture. Was running open headers so I put the front pipe and cat back on. Tried to start the truck to see if the hesitation ceased, but the truck will not start for me. Pulled plugs and they're soaked in fuel - used a torch and burned them clean again, reinstalled and the truck will maybe start up once, bog a little, and die. Subsequent restarts only end with the truck cranking over endlessly with the occasional "gunshot" backfire - definitely coming from the exhaust.

 

Truck has 38 psi of fuel pressure when cranking - but fuel delivery was never really an issue here, obviously. Grounded a spark plug to the valve cover, cranked, and I've got spark, although it was tough to see how strong it was. Replaced the crank angle sensor with a known good one, swapped the ignition coil from my other MJ, but nothing seems to be doing the trick here. Even went back to open header since the truck ran fine when I had it in that configuration, but nope - still getting fouled plugs and the pleasant aroma of gasoline in my shop.

 

Anything else I'm missing? I suspect the oxygen sensor, but I feel that the truck should at least start up for me, even if the current O2 is bad. I read somewhere that water temp sensor might have a play in all of this? When I go to crank the truck, I notice that the water temp gauge will peg all the way to the right - is this normal or a possible indicator that something is amiss?

 

Help me out here - I want to get this truck running good so that I will have motivation to repaint it and restore it...otherwise I may just scrap it. Thanks.

[Eagle]

Two year old gas?

 

Two year old gasoline = varnish.

[BLHTAZ]

Two year old gas?

 

Two year old gasoline = varnish.

:agree: You didn't mention putting fresh gas in it so we are left to ASSume that you are trying to run it on that old gas that is not really gas any more ;) .

 

I would try fresh gas and maybe some new plugs too depending on how old yours are.

[mvusse]

It sounds like a timing issue, but since timing is not adjustable I also put my money on the gas.

[terrawombat]

Did a lot of wrenching/testing/diagnosing on the truck this past weekend and I'm happy to report positive results! Disposed of ALL old fuel in the tank and fuel lines and put 5 gallons of fresh stuff in. Also pulled the plugs and used a propane torch to "clean" them up. Reinstalled plugs, cranked it over and it fired right up. Let the engine get to operating temp, revved the engine a bit - small backfire still prevalent, but NOTHING what it was before.

 

Noticed my oil pressure was WAY down and I was getting a consistent oil drip from the back of my pan. The stuff was thin and smelled like gasoline - looks like I'm right back where I started. Drained the "oil" and put some fresh 20w-50 in. Also, on the NAXJ forums, someone suggested that I may have a lower temp thermostat in since I never really got up to the proper operating temperature, so I bought a 195 degree t-stat for good measure. Upon swapping that in, I found out that person was right - someone stuck a 180 degree thermostat in this truck at one point and since the ECU uses the coolant temp sensor as a method of determining injector pulse width, it could have been one reason why the truck seems to always be running rich. Also, while I was wrenching on the engine, I bought a fuel injector reseal kit since I noticed a small drip from one of the injectors and found a tattered o-ring - decided to replace all 12 for peace of mind.

 

So, new oil, thermostat, and injector o-rings - truck fires up immediately and gets up to its PROPER operating temp of 195 degrees. I purposely only put in 4 quarts of oil so that I could more accurately monitor the crankcase fluid level. I'm looking to see if my gas in oil problem is still lingering. So far, the truck has had about an hour of run-time and I'm happy to report that the crankcase oil level has remained unchanged and still smells like OIL! Hooray! I'm still skeptical, so I'll be doing a lot more testing on the engine in the coming days. Also, while I was letting the engine run, I did a check on every sensor and they all check out fine (I used a webpage to verify my results, but I can't seem to find it. I'm at work and its saved on my home comp). Even my O2 sensor is cycling like it should! Only sensor that MAY need to be replaced is the coolant temp sensor - the ohm reading on a warm engine was about 360-375 ohms, which is a little high, but nothing crazy.

 

Also ran a compression check on the engine to put my biggest fears to rest. On a warm engine, no spark plugs in, throttle fully open, I got readings of 140-160 across the board. Seems a little high from the spec I found in the service manual, but I suspect my gauge reads a little high and there is also a good amount of carbon buildup from years of running rich. I still want to do a leakdown test to get an idea of what condition the rings are in.

 

Next up - dropping the pan, replacing the rear main seal and oil pan gasket. Going to clean up as much of the crud as I can that has accumulated on the bottom of the engine from years of neglect. Thinking about replacing the oil pump while I'm in there, but I currently am getting 25psi at warm idle and about 40-45 psi when I rev the engine up a bit so I'm thinking a new oil pump might not be necessary. Anything else I should look at while I'm down there?

 

I've also been prepping the frame rails for a nice, thick coat of POR-15. I'm also going to hit the entire bottom of the bed with an undercoating spray that I just ordered. Also been sanding the rest of the body in preparation for a full repaint. This would have all been done a year ago if the dang engine would have just cooperated. Not really sure if it's fixed yet and if it is, what I did to fix it. I think it heard me say, 'if this damn engine would function properly, I'd go ahead and put a Motion Offroad 6.5" lift kit on this truck.' Since then, it has behaved VERY well.

[BLHTAZ]

:clapping: Excellent job :thumbsup:

 

Your oil pressure is fine. If it were low, I would be looking at the rod & main bearings before the oil pump as the bearings are VERY often the problem where oil pressure is concerned and the pump is rarely an issue. If you are running the pressures that you just wrote...I woudn't touch anything. Mine was running at about 12 psi @ warm idle and would not go over 28 at 3000 RPM. My bearings were shot and I did the pump for the hell of it while we were in there and it now idles at about 20 - 25 and runs at about 40 on the hwy.

[terrawombat]

:clapping: Excellent job :thumbsup:

 

Your oil pressure is fine. If it were low, I would be looking at the rod & main bearings before the oil pump as the bearings are VERY often the problem where oil pressure is concerned and the pump is rarely an issue. If you are running the pressures that you just wrote...I woudn't touch anything. Mine was running at about 12 psi @ warm idle and would not go over 28 at 3000 RPM. My bearings were shot and I did the pump for the hell of it while we were in there and it now idles at about 20 - 25 and runs at about 40 on the hwy.

 

Thanks for the reassurance. I'm always a little bit leery of touching "high profile" engine items like the oil pump or bearings if I don't REALLY need to. The pan does need to be dropped though as there is a significant leak coming from the rear - not sure if it's just the gasket or the rear main seal, but I ordered and will replace both. I think once the leak is fixed and I can confirm 100% that I no longer have fuel leaking into the crankcase, I will be confident enough in this truck to start doing some "fun" upgrades ;)

 

I need to check the clearance of my shop entrance door to make sure I don't lift the truck so much I can't get it out!

 

Oh, and while I have your attention, would you happen to know the best place to get random body parts - such at cabin/windshield trim, those little "crows feet" bed spacers, fender flares (and the metal backing brackets), Comanche/Jeep emblems, etc. I realize I can probably pick all of this stuff up at a dealer for a premium, but if there is an online MJ vendor, I would love to support him/her over a dealer any day.