Compression test results Questions

I've got a 1995 XJ that i've been battling an overheating issue for weeks now and this evening did a cylinder compression test to check for internal issues. Here's what I got starting with cylinder 1:
110, 110, 110, 90, 105, 90

Then I added 3 squirts of 10w-30 engine oil
130, 130, 120, 120, 130, 105

So what I gather from reading is that these results are relatively normal except for #6 being on the low side. Is it low enough to suggest a leak in the head gasket or in the water jacket? Do these results suggest anything else? I'll admit that engine work is one of my weaker points when it comes to jeep tech knowledge so any input would be greatly appreciated!

In regard to my overheating troubles I've already replaced the radiator, t-stat, water pump, serpentine, fan clutch, upper and lower radiator hoses (dealer purchased), flushed the cooling system (no oil in coolant), and changed the oil (no coolant in oil). I am STILL overheating at highway speeds primarily when going uphill.

when you say "overheating" what temp are we talking? and since you have replaced everything mechanical, have you checked the wiring? if your coolant stays bright green, and you cant smell coolant in the exhaust, i would assume there is no head gasket leak.

-Tim

Squirting oil into the cylinders will just let you know the condition of the rings and while getting weak they aint too terrible.... If it hadnt changed then you would more likely have a head problem , gasket etc.

xjmatt said:

I've got a 1995 XJ that i've been battling an overheating issue for weeks now and this evening did a cylinder compression test to check for internal issues. Here's what I got starting with cylinder 1:
110, 110, 110, 90, 105, 90

Then I added 3 squirts of 10w-30 engine oil
130, 130, 120, 120, 130, 105

So what I gather from reading is that these results are relatively normal except for #6 being on the low side. Is it low enough to suggest a leak in the head gasket or in the water jacket? Do these results suggest anything else? I'll admit that engine work is one of my weaker points when it comes to jeep tech knowledge so any input would be greatly appreciated!

In regard to my overheating troubles I've already replaced the radiator, t-stat, water pump, serpentine, fan clutch, upper and lower radiator hoses (dealer purchased), flushed the cooling system (no oil in coolant), and changed the oil (no coolant in oil). I am STILL overheating at highway speeds primarily when going uphill.

Click to expand...

Your dry compression numbers are low in all cylinders and since there's an average 20psi improvement when you squirt some oil into them, that's a sure sign you've got worn piston rings and they're allowing combustion gases to "blow by" into the crankcase.
The overheating could be related to the cooling passages in the head/block being blocked up by caked on crud that won't come out with a flush.
It looks to me like only an engine rebuild will improve things.

By overheating I mean it will get in the red going up hills. Around town it'll bounce between 210 and 230 but once i'm on the highway and start an ascent it'll just keep climbing until I stop. What wiring are you refering to specifically? The temp sensor in the t-stat housing or more?

Crap. This engine IS a replacement rebuild with only about 65K on it. Throttle response, acceleration, and gas mileage are all still great. Is there any other test I can do to help add some weight one way or another? Would taking it to a shop and having them pressure flush the block be worth a shot?

If it comes down to it how involved would this level of an engine rebuild be? As I mentioned I've never done any serious engine work but I'm willing to dive into anything. So long as it doesn't require a cherry picker to pull the motor out to work on it I've got the tools and patience to do just about anything.

While it's good to hear your enthusiasm, whith numbers like those it doesn't look good. About the only test you have left is a compression leak down test. The comp test is a good "guide" but it's not the most accurate test you can do. Alot can affect the results like how good you batt is, did you have all the plugs out or just one at a time, even if your using a good tester. As far as the overheating goes, I'd say it has to do with your weak engine numbers.

Sorry

That is terrible reading for an engine with 65k on her. Sounds like someone feed you a line of poop on the mileage or never changed the oil for the 65k. My 01 with 121k on her gets better readings then that. Because I believe the mileage is higher then 65k. I ditto the "The overheating could be related to the cooling passages in the head/block being blocked up by caked on crud that won't come out with a flush.".

A couple of things, Did you run the compression test with the throttle body at "idle" or wide open throttle, all plugs removed and a good battery? And have you used any other means to quantify the overheating condition? Your post did not mention replacement of the temperature sensor or if the auxilliary fan was working when the temp rises.

I ran the compression test with all of the plugs out, the throttle wide open, battery is a 7 month old optima red top with a full charge, and the starter is brand new. The motor turns over on the first crank every time. I did wonder just how good of a seal the tester could make since it's really hard to tighten down the plugs by hand on the inline 6 since the plugs are so recessed.

The rebuild was done about 2 years ago and the cause of the rebuild was my fault, sucked down a bit of water with the RPM's to high and threw a rod. Stupid mistake but crap happens and I learned. Ever since the rebuild I've run Mobile 1 10W 30 fully synthetic with a mobile 301 filter and changed it every 5K religiously. I am that way with all the fluids in my jeep, everything else gets changed every year or every 30K.

I have not changed the temp sensor. The aux fan comes on right when it should at 220 and runs until the motor gets back down to 210, which at idle it will do.

The compression readings definately indicate some loss of ring seal...BUT..remember that the temperature gauge reads COOLANT temperature, not internal engine temperature...the fact that the overheating is directly proportional to engine load points to > clogged radiator or some other problem that limits the ability of the coolant to REJECT heat.

My MJ was acting the same way....it turned out that the bottom 1/3 of the radiator was plugged up solid...the tubes run horizontally...when i replaced it, i cut the old one in half for post mortem inspection...now she runs at a normal 210*

The radiator is a new modine 3 core. I know 'technically' it could be bad even though it's new but I'm not quite at the point where I am second guessing the new parts :wierd: .

Another symptom worth noting is that even once the system has had time to cool, as in left overnight, if I open the radiator cap the next morning it's got pressure. This is what prompted me to do the cylinder compression testing.

That is a good radiator, I have them in my XJ and my MJ...hmmmm...pressure in system when cool...go to a good radiator shop and have the cooling system hydrocarbon tested...if positive....pull the head...may be a leaking head gasket, or (hopefully not) a cracked head...

Well, while your compression numbers are a little low, you said it yourself the engine runs perfectly fine aside from running hot under load. So with that respect you're not too different from the rest of the worlds 4.0 XJs

Focus on the cooling system, I wouldn't worry about the compression numbers too much myself until the engine starts running poorly. If you had cylinder gases pressurizing the coolant system you'd probably know it.

I have no problems with running in the 210-230 range but I do have a problem when it goes 240 and above.

Well sounds like I may need to break down and take it to a shop. I'll see what a hydrocarbon test turns up and maybe see if I can get it professionally flushed. If I'm lucky that'll do it, if not well... I'll be back. Would having a shop do a full leak down test prove anything for sure? Thanks for all the replies.

leakdown test is simply to test the combustions chambers integrity. I wouldn't go that route unless you suspect a valve that's not seating or you've got gases in the coolant.

my 4.0 MJ does the EXACT thing you're describing, 210-230 around town up to 255 on the interstate (never passed the far white line of 260 so far... knock on wood)
and ive done waterpump, thermostat, pressure bottle, and fan clutch.

however, when i flushed out my coolant it was absoluetly DISGUSTING brown rust colored water. couldnt see through 3inches of it.

id like to see what your problem is, mine could be the radiator i suppose, i was hoping to not have to do that.

-nick

EDIT: (accidently logged in on my brothers name, sorry forgot to check this is Oizarod115)

If it were me. I would pull the radiator and have it flow tested. This would not be the first time a NEW radiator was cloged! And it would be a great piece of mind to KNOW that it was good.

The other thing I would do is check the voltage of the gauges. You might find that a bad ground in the guages is producing false high temp reading. Even though I know that you said the electric fan turned on appropriately.

Both of these are far less work than pulling the head or doing a full re-build.

Best of luck,
Michael

I dont like it when mine goes over 210... on a hot day getting on it and then sitting in traffic it will go to 220 but that makes me nervous and if it goes over that I started questioning my cooling system...

and when I did my compression test I did one cylinder at a time and my compression levels were all around 140'ish...

if anything I just said helps at all good, and Im sorry if it doesnt... just figured I'd chime in...

FINALLY NO OVERHEATING!

So going on a number of suggestions I kept my focus on the cooling system. Since I had already replaced nearly everything I double checked everything. I removed, cleaned, and reinstalled the t-stat, water pump, and rechecked every belt, hose, and fitting. Nothing. Finally I decided to do a 2nd flush of the radiator and the block.

Mixed 1 gallon white vinegar, 1 gallon distilled water and ran it for 2 weeks. WOW did that stink when I drained it and what came out looked nasty. Then mixed a box of baking soda with distilled water and dumped that in and topped it off. Ran that for 2 weeks. Drained it, didn't stink but WOW the crud that came out. Refilled with 50-50 antifreeze distilled water mix. I took it for a couple test runs into the mountains on the highways and the temp gauge just reached 210. Then this past weekend I joined a group of jeeps coming up from CO Springs for a wheelin trip and between the highway getting there, 4-high on the approach, 4-low on the trail, the temp gauge never crossed 210. If anything I'd say its running on the cool side now!

So a handful of new parts and 2 flushes later I'm back on the trail. I'm thinking what caused this all was the deteriorating radiator. I'm thinking it began to fail and what broke loose chewed up the bearings of the water pump (It failed only after 30K miles), and blocked up the passages in the engine block. I don't know why the first flush didn't get it out but the second one did the trick. Lesson learned, keep on top of your cooling system scheduled maintenance!

My 94 is having the same problems, except mine does fine unless I'm driving in the city traffic or going under about 30 mph. Replaced the fan clutch, the t-stat, t-stat housing, water pump, radiator is a 3-core about a year old, upper and lower hoses. She runs fine if I drive with the heat on, but that sucks. I don't know the temp, since I only have a dummy light. I may try the vinegar/water baking soda/water technique and see how that goes.