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  #1  
Old September 1st, 2017, 12:50
@dam @dam is offline
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Looks like my engine is toast. Opinions on these compression #'s?

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!
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  #2  
Old September 1st, 2017, 18:00
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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.
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Old September 1st, 2017, 18:33
@dam @dam is offline
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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Originally Posted by JohnX View Post
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.
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Old September 1st, 2017, 20:34
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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
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88 MJ on 40's. 98 XJ on 35's. 98 Dodge, 52psi of boost. 94 Miata LS4. 70 Buick GS455. 01 Harley, been to 46 states. 05 KTM, 576cc. 06 Husky, girls need bikes too. 2011 R6, girls need sport bikes too. 99 Toyota Avalon, daily. 97 Miata, girls apparently need miatas too.
And everything fits in the garage!
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Old September 2nd, 2017, 04:33
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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?
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Old September 2nd, 2017, 10:05
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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Originally Posted by cruiser54 View Post
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.
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Old September 2nd, 2017, 11:05
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ParadiseXJ ParadiseXJ is offline
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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.
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Old September 2nd, 2017, 11:24
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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Originally Posted by old_man View Post
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.
BG44K was gonna be my next suggestion...LOL.
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Old September 3rd, 2017, 07:11
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Re: Looks like my engine is toast. Opinions on these compression #'s?

"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?
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Old September 3rd, 2017, 07:32
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ehall ehall is offline
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Re: Looks like my engine is toast. Opinions on these compression #'s?

rough idle can be all kinds of small things related to fuel, air, ignition

those numbers eliminate compression
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Old September 3rd, 2017, 17:28
@dam @dam is offline
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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

Last edited by @dam; September 3rd, 2017 at 17:38.
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Old September 3rd, 2017, 20:37
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Re: Looks like my engine is toast. Opinions on these compression #'s?

I don't get it either, driving rather conservatively in s low mileage stock xj, best I've ever got was 22.
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88 MJ on 40's. 98 XJ on 35's. 98 Dodge, 52psi of boost. 94 Miata LS4. 70 Buick GS455. 01 Harley, been to 46 states. 05 KTM, 576cc. 06 Husky, girls need bikes too. 2011 R6, girls need sport bikes too. 99 Toyota Avalon, daily. 97 Miata, girls apparently need miatas too.
And everything fits in the garage!
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Old September 3rd, 2017, 21:09
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Re: Looks like my engine is toast. Opinions on these compression #'s?

Quote:
Originally Posted by @dam View Post

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.
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  #14  
Old September 3rd, 2017, 21:57
@dam @dam is offline
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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.
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  #15  
Old September 4th, 2017, 03:50
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Re: Looks like my engine is toast. Opinions on these compression #'s?

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.
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