I have a new-to-me 98 XJ with 210k miles on the clock. Mechanical history is basically unknown.

There was a P0301 (Cyl 1 Misfire) as well as a code related to the downstream O2 sensor (forget the code, but the wires got ripped out of the sensor, so it's not sending anything, lol). I thought this might be contributing to the misfire, but effects of sensor replacement have yet to be evaluated.

The Jeep is now basically parked for the winter (Wisconson) while I go through it and replace all the steering components, but I figured I'd start doing some basic checks of the motor in the meantime.

I got the $50 Maddox compression tester set from Harbor Freight (has Schrader valve at the plug end of the hose), and proceeded to do dry and wet compression tests, and got some weird results:

(Tested COLD, throttle at WOT, all plugs removed, cranked for 6-7 revolutions)

C1: 145 PSI DRY/ 135 PSI WET,
C2: 175 / 185
C3: 170 / 180
C4: 160 / 165
C5: 175 / 185
C6: 185 / 195

The weirdest thing was that the wet pressure actually went down 10PSI on cylinder 1. Unfortunately, I only checked cyl 1 dry once, and it was the first reading taken with the new gauge. It also repeated the 135 reading 3 times on the wet test.

The other strange thing is that the FSM says spec is 120-150 PSI with a 30 PSI max variance. Most of my readings are way above spec, so I'm also questioning the accuracy of my gauge. I'll probably wait a week and recheck cyl 1, just out of curiosity...

Either way, relatively speaking, it seems like cyl 1 is lower compression compared to the others, so this may explain the misfire.

TL/DR: Has anyone seen a wet compression test come back lower than the dry??

Seems like the next step is a leakdown test.

ETA: I also have a P0128 code, and suspect either the t-stat is stuck open or was removed completely. The system is full of rust and needs to be flushed (no heat either), so is definitely in need of some TLC. It takes forever to warm up and won't go above ~160 while driving, and idling won't go over 195. I verified the gauge readout was correct with a laser IR thermometer.

 

I'd say it's carboned up across the board with a leaking valve in #1. Do a cylinder wash by pouring water down the throttle body and bust that carbon out of it. Engine hot running at 2000 rpm, slowly pour a large glass of water down the throat and the water will boil off the carbon and allow it to blow out.

 

Thanks for the reply @dave1123 . I'm hoping it's that simple. I'm going to do a leakdown later today to see if I can confirm a valve.

The steam clean sounds like a good idea. That will be high on the list next spring after I get the cooling system refreshed and can actually get up to operating temp, lol.

 

Harbor Freight..........

 

Yeah, but this was the "expensive" version of the kit. 🤣

 

I had an ugly little miss only at idle in my newly rebuilt engine in my WJ and I was pissed! It cured itself after a couple of hundred miles because it was just the rings in one cylinder not seating perfectly. Maybe I had some deeper hone scratches in that one. Who knows? Anyhow, it's got 11K miles on the rebuild now and purrs. 263K on the beast.

 

Re-do the test at a later date. I trust 4.0s alot more than I do cheap chinese tools.

 

Quick update. I just did a leakdown test on #1. Fed the thing 100PSI input and got a reading of 91PSI. 9% leakdown... Seems pretty decent!

I heard a little bit of air through the oil fill cap. Could hear a little air out of the exhaust, and I heard a bit of gurgling/dripping faucet sound from the TB with the plate open. No bubbles at all in the rad.

There was a "dog mom" air freshener hanging from the mirror when I bought it from a little used car lot last week, and a couple of tampons under the seat (luckily still new), so I'm hoping it is just a bit carboned up from some easy miles. I'm going to shelve further investigation into this issue until the spring when I have time for a good steam bath, at which point I'll pull it apart and dig deeper if I have to. I'll be sure to post back with any updates or resolutions if I learn anything new.

Thanks for the input!

 

Nice work Colombo!! Funny stuff!!

 

LMAO. Reminded me of the bottom of my wife's purse, and she drives like she's 90. 😄

 

You're killing me!! LOL

 

Dave, we've heard about your girlfriend over and over and over! Give it a rest, willya?

 

I thought there was a Chrysler Master Tech video on dribbling water into the carb intake to clean carbon deposits from cylinders. Searched and browsed here, didn't find it (largest collection I know of):
https://www.youtube.com/user/mymopartv/videos

Are there any extra precautions one should take with the Mass Airflow Sensor when squirting water into an EFI system?

 

Nope. Just Do It.

 

I'm replacing my motor mounts, brackets, and noticed a crack near the collector on the ex. manifold, so I just pulled the intake and exhaust.

I peeked into the cylinder 1 intake port. The carbon buildup was fairly minimal, but there does appear to be some crustaceans around the intake valve seat that doesn't appear as obviously on the other intake valves. I tried to snap a pic, but it's tough to get a good shot in focus.

It's at TDC, so I shot some MMO in there in the hopes that maybe it will soften things up while it sits for the next few months. I'll definitely try a steam treatment in the spring, but based on the minimal buildup, I'm hopeful I can resolve this without digging deeper.