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Did a compression test...

G

Glacialbass

NAXJA Forum User
Location
Morrison, CO
I'm not exactly thrilled with the results. 92 4.0 AW4 NP231 160k miles.

#1 100
#2 122
#3 130
#4 120
#5 121
#5 125

I added about a tablespoon or so of oil to each cylinder and repeated the test.

#1 123
#2 140
#3 156
#4 152
#5 140
#6 151

I'm hoping that someone can help me diagnose my results. Obviously, cylinder #1 is low, but I'm not sure why. Any help would be great.

Thanks in advance.
 
the increased compression when you added the oil points to a pressure leak-down somewhere, probably the piston rings, as suggested by j99xj.
if it still runs fine, jest let her be. otherwise, replace the rings. while you have the head off, take a look at the cylidar walls. if you can see any gouging, you'll want to hone it a bit.
 
It seems to still be running ok, it's just not getting the mileage that I figured it would, and I had the tester and had some free time. I suppose I may just drive it and see if it gets any worse?

Thanks for the advice guys.
 
I had a motor that ran lean due to vacuum leaks and a marginal O2 sensor. and affected my compression, my original numbers were a bit higher than yours.
After replacing the parts and plugging up the vacuum leak, my compression actually improved over the next year.
100 on the front cylinder sounds like it may come from a vacuum leak and the lean burn my be causing some of your ring problems. The normal glaze on the cylinder walls get damaged.
Switching to 10-40 or even 20-50 motor oil never hurt any 4.0 or 4.2 that I have ever noticed. Oil additives to help compression seem to have accelerated main bearing wear on some motors I've seen.
Did you do the compression test with the TB throttle plate tied open?
 
I might run a can of BG44K through it. It removes carbon deposits and will sometimes loosen a sticky ring, helping compression. If nothing else it will clean your injectors and for most vehicles, make a big improvement in MPG.
 
I've already run BG44k through it in an attempt to get better mileage (was at 12mpg at the time city, now about 14).

Yes, I did perform the test with the throttle wide open, and all plugs removed.

I changed the oil last night and put 15w-50 in it (Rotella) in an attempt to find an oil with more zinc content. I also added a quart of Marvel Mystery Oil in the crankcase at the same time. In regards to vaccuum issues, I'm fairly sure that I don't have any. I suppose I could hook a gauge up to it, but I'm not seeing any other symptoms of a leak.
 
My experience with low compression on a high mileage XJ 4.0, is busted rings! A cylinder with 100 psi is pretty bad. You may get lucky with a stuck compression ring, but I'd guess busted rings. Problem is, if you continue to run with busted rings, you stand a good chance of gouging the cylinder walls.
 
Try a little snake oil in a can. A engine rebuild is an easy thing to suggest when it's somebody else's $1800.

Restore is getting a reputation as an affordable additive that does what it says. I have a 90 Caravan 4 cylinder I just dumped some in. It had 25-60-25-25 on the tester. Chuffed like a one cylinder steam locomotive. Can't wait to start it and give it a weeks drive to see what the results are.

Ecomike has some threads on additives that are showing good reports. Obviously an severe test like I am trying may obviously not have a good ending, but others with readings like yours have shown positive results for the expense.

If I get any good out of it, I'm doing it to my '90 with 185K. It won't hurt.
 
TiRod said:
"--- I have a 90 Caravan 4 cylinder I just dumped some in. It had 25-60-25-25 on the tester. Chuffed like a one cylinder steam locomotive. Can't wait to start it and give it a weeks drive to see what the results are.

---.
Click to expand...

Can't believe an engine with that low of compression would even run.:looney:
And, unless you have to pay someone to do your work for you, it doesn't cost anywhere near $1800 to do a ring job, unless, ofcourse, you have to do a rebore.
 
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