Hey gang, I need some help interpreting some compression numbers and the resulting "life dilemma"...

First the numbers. I tested for compression this weekend on my '93 XJ 4.0 HO. 208K miles. I rented a gauge from the 'Zone (Actron CP7827). The directions I found both here and online (averaged) were as follows:
1. Test when engine is warm/hot.
2. Remove ALL spark plugs (not one at a time), disconnect ignition, disconnect fuel injector clips.
3. Thread the tester into the spark plug hole until hand tight. As y'all know, the head has no room to get fingers onto the fitting because of the hose and other accessories, so the best I could do was turn the house until the fitting stopped turning. There was NO leakdown of pressure after cranking, so I assume that must have been tight enough...
4. Turn the engine over 3-7 times. Some say 3, others say up to 7. I ended up using 7, but the results will tell you what you need to know...
5. Record the results for each cylinder.
6. Add a couple tablespoons of oil into each cylinder, then repeat the test in order to determine whether low compression is due to bad rings.
(NOTE: Having forgotten, I redid the DRY test on Cyl #6 with wide-open throttle, but it only made 5 PSI difference, so I don't believe that matters much. Correct me if I'm wrong.)

Here are the results from front to back:
Dry Wet Diff
#1: 65 80 +15
#2: 85 110 +25
#3: 90 130 +40
#4: 30 80 +50
#5: 70 95 +25
#6: 75 100 +25

Interpretation: I read that the acceptable range should be 120-150 PSI with no more than 10-15% difference between any two cylinders. First off, none of my cylinders broke 100 PSI on the dry test, which would suggest that it's shot to begin with. Second, cylinders 1, 5, and 6 are much lower than 2 and 3, outside the 10-15% range. Cylinder 4 was unbelievable - I did that test 3 times after doing the other cylinders just to be sure it was working. Third, on average, adding oil to the cylinder caused a jump in compression of about 30 PSI. This would seem to indicate that the rings are shot, but I could use some input from folks who know a bit more than I.

Q1: Is my testing procedure correct, including threading the tester hose, engine temp (it wasn't hot, just warm, about an hour after shutoff) and so forth?
Q2: Should this engine even be running with compression as low as the readings I got? The Heep still has great low end power and acceleration, but it does suffer high-end acceleration. While towing a trailer on the highway, (65+ mph) it struggles up hills, which tells me compression is bad, but I do need some confirmation on that.
Q3: Does the "wet" test confirm that my rings are shot? Even if the valves are bad, is there any life in this engine without overhaul in the next few years? As I said, it pulls hard, but burns a quart of oil every 500 miles, or two tanks of fuel. Also gets bad fuel mileage (even for an XJ).
Q4: With cylinder #4 having such low compression, shouldn't it be misfiring all the time? It runs and idles quite smoothly except for when I start it with a hot engine after sitting a short while. That seems like more of a heat soak problem than compression, as it results in a rough idle for a couple minutes of driving, then smooth again.
Q5: Not to salt the mine, but my brother had this car 10 years ago and it ran out of oil twice due to a batch of faulty oil sending units. Shop at that time said it probably scored the cylinder walls. The first time he didn't know what had happened and drove it 5-10 miles home with no oil. Should I just be happy it's driven this long (plus 70,000 miles) after something awful like that?

Life dilemma: Just bought a house, recently married, kids in the near future, new dog, also have motorcycle = plenty of other projects besides wrenching every other weekend. I always said I'd rather fight my XJ than be comfortable in anything else. Truck's been in the family 20 years, but it seems like the numbers are telling me what to do....

Has anyone ever stopped being an XJ owner and lived to tell the tale?

 

when you did a wet test and they shot up that much it means your rings are pretty well done with. After completely reading th post, yes you need an engine or overhaul. Dont give up on it, save a little here and there if need be and fix it. I am trying to save to buy a house, and have kid number 2 coming, and mine needs a trans

 

That's what I was afraid of. I should mention that, living in the rust belt, the underbody is in pretty bad shape. Floor pan is rusted through in a few places, e-brake cables are shot, rear drums are crummy, rockers are completely non-existent, which leaves a sharp edge for anyone including the dog getting in and out. Rear quarters are rusted behind the wheels, also coming through over the fenders. Windows moving slow, door seals and window seals are aged and cracked. Tailgate has a pretty good crack in it. Exhaust system is limping.

Just wanted to be clear that we're not dealing with a "great-looking Jeep with a bad motor", she's just getting rough all the way around. That's why I'm not sure it's worth doing the motor work.

 

Give it a mercy killing. It has served your family well. Probably cheaper and easier to get one in better shape. I am sure the posts are about to start about how you should save it. But really they made them from 1984-2001, they are not rare and there are lots of good ones available for fair prices. Just my .02 worth.

 

It has been limped along and had a lot of money put in already to keep going for the sake of love and all that crap. It's at the point now where it's just a truck and needs to do some good for someone else. Also it is a daily driver, and so I can't afford to risk blowing the engine or paying for an overhaul job and not having anything to drive. I have a line on a great deal on a good car, and I can't pass that up. If I can get a chunk of change from it, I'll sell, but if not I might just keep it around to use/abuse.

I think in a few years I may be in a position to have an XJ I wouldn't mind spending money on. If that's the case, I would focus on finding a low-mile, little/no rust late model and build it up right. Then I think I'd have something worth taking care of for the long haul. Anyone have any suggestions on what model year/package is proven the most reliable? Would an '01 be the best you can get? If I remember correctly, you lose the SelecTrac xfer case after '96 or '97, and I would miss the Full Time 4 option...

 

At the risk of sounding dumb, try a decarb and test your compression again.

Decarb: pull all your plugs out and shoot some Deep Creep in the cylinders. Let it sit overnight. When you fire it up it's gonna smoke like a house on fire. The Deep Creep will loosen up any and all carbon on the pistons and rings and then blow it out the exhaust. I have done this a few engines and gotten the compression numbers up and more even.

It's up to you. I don't know if it's worth it to spend the $8.00 on the Deep Creep to see if you can improve it or not. I would at least give it a shot.

Does it burn oil?

 

I might double check the whole process with the throttle open, being it's sort of a "high stakes" deal.

I've heard maybe there is some "merit" to the product, "Restore". Wanna be "guinni pig?" I'm over 275K and need to gap my plugs...maybe I'll check mine. I'm adding a quart of oil about every 1,500 miles. How about you?

 

Yeah, I think for an old, tired engine, I'd try a can of Restore. Probably couldn't do much harm.

 

Just to be clear, I've never needed to put anything in mine except Castrol 30W. It's warm here and I'm the only driver, so I can get away with that.

 

Hole #4 is definitely suspect. My next step? Secure a borescope (head no larger than 1/2"/12mm) and use it to examine the cylinder with the piston at BDC. Anything that's significantly higher may warrant inspection of the "low holes," but anything that runs more than 15% or so below the calculated average definitely warrants further investigation!

Your method looks generally good, but a few things to note:
- Compression tests should always be performed with WOT (Wide Open Throttle.)
- The "nominal" compression ratio (SCR) of the 6-242 runs about 8-7-8.8:1. With atmospheric STP being 14.7psia, that means that compression should come out to 129-130psia, if everything's up to scratch. However, the actual compression ratio (DCR, factoring in the IVC event, when compression actually begins) should be used to calculate. Unless the weather is extreme, you can leave "base pressure" at STP figures (14.69psia @ 68*F at sea level.) Given that, #2 and #3 look a touch high, but are still acceptable (could be due to deposit buildup on the inner chamber surfaces, for instance.)
- When calculating your average, discard any obvious "flyers" (out-of-range data points.) This isn't a global statistical analysis - you're just trying to decide if something warrants further investigation. Once you've got the average (above, bolded,) look at your individual numbers in relation to that - take the difference between the two and divide that by the calculated average to get the % variation, then + or -, obviously, would be above or below. Bearing in mind engine wear and leakage due to ring fitment and piston-to-bore clearances for assembly, and fudge in a bit for mileage, the general guideline is that any cylinder that falls outside of 85%-125% of the average should be investigated, and any hole that falls outside of 75-140% of average damned well better be investigated! (Modify the range outward by 2% per 100,000 miles on the odometer.)
- Compression tests should be performed on the engine as warm as you can stand working on it - if you can wear heat gloves and get to it immediately after shutdown, that's best. Part of the reason that piston rings are fitted the way they are is to allow for expansion under operating heat - which improves the seal, as the ring end gap shrinks (this is important because, if the "cold ring end gap" is too small, the rings can actually bind the engine and cause damage! Too great, and you won't get a good seal. Properly-fitted conventional rings offer a 93-95% seal. Even "gapless" rings - two-piece compression rings - only offer a 98-99& seal...)

Recommendations:
1) Repeat compression test, with HOT engine. Compare numbers to "one-hour cold," as they could still be instructive.
2) If possible, BUY your own compression tester. I don't like to borrow or rent measuring tools - I don't know what they've gone through before I got to them. I've never used "shop tools" for measurements for that reason, I know the history of my tools!
3) If possible, secure a borescope and check holes #1 & #4 at a minimum. #1 is a bit low, #4 has something likely wrong. (Given the disparity between wet and dry readings, I'm inclined to go with ring/bore scoring.)

Good luck!

 

Well, depending on the weight and size of the trailer, that may or may not be normal. The oil consumption is not normal. (At 240K miles I don't use any between changes)

You most likely just have a worn motor. You can rebuild it, slap in a salvage motor, or just drive it until something finally gives up. Given the other issues you mentioned, I'd be inclined to just drive it as-is.
Worn cylinder walls and rings don't normally cause a sudden, catastrophic failure, just a continuous slow degradation of performance until it gets difficult to start, idles rough, etc. You might have quite a few miles left in there if you don't mind adding oil and dealing with the power loss, but the writing is on the wall.

 

Hello all, I did a compression test on my Jeep today and the numbers show roughly dry:150psi across the board (give or take 5psi) Wet:160psi. I have the same type of running conditions as well.
1. When at normal running temp after getting off the freeway, if you stop and start the vehicle shorty there after, it runs rough and has a sort of hicup like it is misfiring.
2. Loss of power going up a long steep incline. It used to be no problem in 3rd gear at 60mph, now it is 2nd@60mph around 4000 RPM.
3. Low oil pressure once it is at normal temp. The gauge drops to 0psi and the light comes on. I had put a manual gauge to test the accuracy and it would drop, but not to 0psi maybe 3-5psi. I had read on a forum that Jeep says 3psi is acceptable, but then why would the check oil come on.

I know this is an old post, but hopefully still visited. Could the transmission possibly be the cause of the power loss? Others have said: fuel filter/pump, injectors, air intake.

 

Look up zombie threads, and why they are not a good thing.

Then go start your own thread.

 

It's alive!!!