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Looks like my engine is toast. Opinions on these compression #'s?

@dam

NAXJA Forum User
1995 4.0 with 233k highway miles. Oil changed every 3k for most of its life. Power up a particular hill near me has been the same for the last 10 years. It gets about 21 mpg and has for a long time. It has had a slight miss at idle, and has for years, but it baaaarrely squeaked by emissions this time, so I'm trying to track it down. I did a compression test a few weeks ago about 3 hours after driving, and just did one right now, wet and dry, an hour after driving. I poured 1 tablespoon of 10W-40 down each cylinder for the wet test.

I live at 6000' altitude, so divide these by 0.8369 to see what these numbers would look like at sea level

Here are the results

Cyl Dry1 Dry2 Wet
1) 123 110 143
2) 115 115 150
3) 123 118 163
4) 120 119 158
5) 127 122 165
6) 123 108 130

Weird. I was hoping to see more consistency between the dry tests. All the cylinders went down except 2 and 5 when the engine was warmer. Looks like I have a valve issue on 6.

So...time to put this engine out to pasture? I've never had a car long enough to warrant a rebuild. What would one cost for a 4.0, preferably with a stroker? Most likely though, I'll probably just sell the whole rig to someone out of town who doesn't have to go through emissions, and is perhaps mechanically inclined enough to rebuild it themselves.

Also, after I was all done, I let it sit for an hour and retested cylinder one. I got 120 wet, and 175 dry!
 
Those numbers aren't all that bad. When I run compression, I look at the first pump as an indicator of ring condition....it should be at least 50% of your final reading.

Personally, I'd keep driving it until something craps out or it actually fails emissions testing. Just have the money or the new motor ready by that time.
 
Those numbers aren't all that bad. When I run compression, I look at the first pump as an indicator of ring condition....it should be at least 50% of your final reading.

Personally, I'd keep driving it until something craps out or it actually fails emissions testing. Just have the money or the new motor ready by that time.

Hmmm...it drives great except for the miss at idle. What's the ballpark for a rebuild?

I'm doing this solo so didn't see the first crank pressure, but I'd go back and forth and it'd take quite a few cranks to level out.
 
We could probably get a huge debate going on this...but I think a fair amount of people here (including me) would say a stroker build would be a good idea for a rig at your elevation.

Check out this thread...lots of debate over the actual HP numbers, but most of us agree it's a good build. You can spend 2-3k on parts and machining if you don't get too crazy....you could spend double that too. ;)

http://www.naxja.org/forum/showthread.php?t=1140839
 
First off, I'd keep running it.
As for the idle:
Ever cleaned the throttle body and IAC?How long since new plugs, wires, cap and rotor?
I prefer NGK ZFR5N plugs.
Tested or replace the oxygen sensor? NTK only.
Ever replaced the injectors with upgraded 4 hole ones?
 
First off, I'd keep running it.
As for the idle:
Ever replaced the injectors with upgraded 4 hole ones?

First off, run a can of BG44 through the gas. Old clogged injectors cause this quite frequently. The same goes for the plugs and wires.
 
I'd say that engine is not toast. I've had very similar numbers on my MJ for years. I only drive it occasionally so in 5 years I've put about 10K miles on it. 2 yrs ago I did the head, mag'd, decked, new valve guide seals, TB gasket. Doesn't use oil, doesn't smoke, plenty of power, gets decent mileage, no leaks. If it were a newer engine the tolerances might be tighter, but a 4.0L with those numbers should be fine. JMO. Run it.

If you have any other issues down the road, swap it out.
 
"Drive it 'till it drops!"

Many good suggestions in this thread. If all the measured pollutant levels were high, use an IR Thermometer to see if the CAT is working. How many miles on the CAT?
 
I just got back from a 2 tank road trip...rural highways in New Mexico. I got 23.2 and 25 mpg- my best ever. I don't get why my mpg is so good if my wet vs dry compression numbers are so different.

I'd love to drive the wheels off it, but if I can't get it by emissions without a rebuild, I might buy something more comfortable and with modern safety features like ABS. A stroker with a higher compression ratio to make up for the altitude would be fun though.

Oh yeah- the following are brand new
4-hole injectors
Cap, rotor, wires, plugs
The actual complete distributor. Old one had a little play in it so I thought it might be the cause of my miss, but it didn't make a difference

It has quicker starts and better throttle response now (and apparently REALLY good mpg), but I took it to an emissions bay and there was no improvement in the emissions at idle- like 500 ppm at idle and 180 is the limit (which I squeaked pass by revving it way up to heat up the cat just before the test)

IAC motor is a year old. Old one was fine. I also ran a can of mopar combustion chamber cleaner through before I did the emissions test and compression test.

Got about 100k on the cat, but I doubt the cat is causing the miss

Oh yeah- fuel pressure is 33 psi and oil pressure is about 45
 
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I don't get it either, driving rather conservatively in s low mileage stock xj, best I've ever got was 22.
 
Got about 100k on the cat, but I doubt the cat is causing the miss
I'll agree with that, but disagree that the cranking compression has more to do with exhaust emissions than the condition of the Catalytic Converter.
 
It's the miss at idle that makes the exhaust emissions high, and since I've replaced everything but the coil and CPS on the ignition side, and have new injectors on the fuel side, that's what I'm left with: badness with either my piston rings or valves, and it's looking like rings except cylinder 6 (which could be the only cylinder that's missing). The cat can only do so much if it's not running properly in the first place.

Regarding mpg, it is a 5 speed and the high altitude helps (less drag), but I did spend a good portion of both those tanks going 75 with the AC on.
 
Coil if bad can cause general poor running. I don't see anything wrong with the dry compression. These engines can benefit from a head job every 160,000 miles or so.
 
I read that if wet compression was higher than dry compression by over 10%, your rings are considered bad
 
Regarding mpg, it is a 5 speed and the high altitude helps (less drag), but I did spend a good portion of both those tanks going 75 with the AC on.

That makes sense, also thinner air means less fuel for the proper ratio. Forgot about your altitude.
 
I read that if wet compression was higher than dry compression by over 10%, your rings are considered bad

what I was thinking since wet #s were quite a bit higher. I wouldn't say bad, just got a few miles on them. get a lot of blow by?

still wouldn't be afraid to drive it like I stole it
 
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