Interpreting compression test results -- please help!

Dear all,

Some days ago I did a compression test on my XJ12 H.E. 1983, engine at operating temperature, all spark plugs out. Due to me and my helper being daydreamers, we forgot to step on the gas pedal. Anyway, the results are interesting despite this error, and they are somewhat worrying:

Dry test gave about 120 PSI on all twelve cylinders. Pouring a small amount of engine oil into the cylinders, had the readings jump up to around 190/200 PSI. I guess that the values would have been higher with the throttle wide open, but surely the huge difference between dry and wet test would still remain.

The variance between the cylinders is well within the range (< 15% of variance).

As far as I know, the values of the wet test are OK for the H.E. engine, especially if I take into account that the values would have been a little higher with open throttle. But the values of the dry test are really low, even with the throttle closed. All in all, this points to worn piston rings. A closer look at the cylinders with an endoscope reveilled no visible damage, the cylinder walls are in good shape, no scrapes, and the same applies to the pistons and the valves.

So far, so good – or rather, bad. But what are the consequences? Does the engine need an urgent overhaul yet, or would you advise me to just keep on driving? What do you think? The engine runs smoothly, no strange noises, no lack of power – no sign of a problem at all, if it were not for those low readings at the dry compression test. Better shouldn’t have done it. :slight_smile:


As long as they’re all the same, I wouldn’t worry too much. Throttle open woulda been better, obviously. If you have a legit issue at all – low compression corrected by adding oil – it would mean that all of the piston rings are leaky. And the most likely reason for that – presuming the car doesn’t have 400K miles on it – is that all of the piston rings are jammed in their grooves. And the fix for that is an Italian Tune-up.

Thank you for the quick reply! That sounds really encouraging! The engine has done its work for around 130K km. First owner had all work done by a Jaguar garage, all documented (no engine issues back then). Second owner had the car for 6 years and drove it for barely 2k km, so the car has mostly been sitting for more than half a decade. Third owner is me, since September last year. Italian tune-up has not been done yet – if you say it could prove to be useful, I’ll go for it this Spring.

I think you have a sound engine, and if it smokes a little, tell observers that all Jags smoke. Leaks are more of a problem than cheap oil.

I know its a pain, but I’d really try again with throttles wide open. It would be good to know full compression numbers for future comparison. My first cylinder was also low. Then i remembered to open throttles. Shot up to 200. Dry.

I have a question, some say only four cranks, some say six, i just keep cranking until psi wont go up anymore. Usually takes 7 or 8 cranks. Is this ok?

Yep, that’s the way to do it IMHO.

There’s also the question of cold vs. warm engine. On the Jaguar V12, I simply don’t see how you’re gonna perform a compression test on a hot engine. If it’s hot when you test the first cylinder, it’ll be cold by the time you test the last cylinder. My preference is to do them all cold for consistency.

I crank until the guage stalls, I would throw some ATF and acetone down the plug holes and let that sit for a day or two. Then fire the engine up to clear out what remains and take it for a short run and do the test again, and then do an oil change to clear out anything that went past the rings.

I have no AC or cruise control. I can get throttle tower and coil out in 15 minutes, and plugs out in 15. So its usually only 30 minutes when im ready to test. My engine temp went from 180F to 160F when i was ready. It was still pretty close to 160F at end of test.

Being cold, i would think the results would be more skewed.
The first couple cylinders are cold, but the other pistons are warming up the seals a bit as they’re being cranked?

Thank you for your opinions! I did the test with the engine as warm as possible, took me about an hour and a half to get all the plugs out (hate it). And we cranked until the gauge stalled, maybe five or six strokes for each cylinder. Battery is new and was connected to a charging device.

What do you think of this product called Anticarbon? It is especially designed to decarbonise piston rings and grooves.

I’m afraid the English version of the site would not present it, maybe that’s a legal issue, I don’t know. The same applies to the safety data sheet:

The product mainly consists of Dimethylformamide, which is said to be an excellent (and quite toxic, I have to admit) solvent, and hence a very good engine cleaner. Anyone tried this stuff on a V12? I read so many positive comments on it that I feel tempted into giving it a try. Plugs will have to come out anyway for the next compression test. :slight_smile:

Instructions say to just spray some of the stuff down the plug holes, let it sit for a while, crank the engine, let it sit again, and finally fire the engine up to burn the residues with high RPMs. Oil change is not needed, at least that’s what they claim.

The product is purportedly extremely harmful to any sort of paint. Do you know if the oil sump of the V12 has any coating on the inside? That might be a severe contraindication (or one would have to go for an oil change instantly afterwards).

I’d try the Italian Tune-Up first.

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Yup, it’s also a lot more fun that spraying stuff down spark plug holes.

OK, then an Italian tune-up it will be.

A few oil changes should help over time. I’ve used marvel mystery oil with oil change, which supposedly helps. 20%, so 2 qts for our 5.3L. Ran it 500 miles and then changed oil. (Dont do it middle of summer, because it will thin the oil)

Im also now running synthetic oil over the last year. Oil leaks got worse the first few months, but now are almost nothing. The odd drip now.

Do you add Marvel Mystery Oil just a few hundred miles before the oil change, or do you continually use it as a regular additive as well? In Europe it’s hard to obtain – thought about giving Seafoam a chance, which seems to be a very similar product (and well available at online shops here).

Try running it hard first.
Adding anything that dissolves crud to the oil directly can have unwanted consequences. If it runs fine after a long hard trip you can skip adding stuff. The top end product looks interesting if it works, and since it’s blown out the top it doesn’t do anything bad below.
Running it with atf or diesel just before the change, why not, especially if dismantling later…

I drained 2 qts and then added it 500 miles before oil change. Similar to using ATF, but it has a mysterious ingredient! :wink:

Thats why i run it only 500 miles and change, filter will catch all the junk.

I would NOT put seafoam in crankcase.

I’ve tried seafoam through intake, IMO all it does is give you colorful exhaust. To clean top end, i use Techron and BG44K. I believe they do help, slowly.

A small update: I just felt tempted (forgive me) into trying this Anticarbon product, and guess what? It worked sensationally well. (No, I’ve got nothing to do with this company.) Spraying a little bit of it down the spark plug holes and cranking the engine a couple of times produced a significant amount of black carbon dirt on the cylinder walls, in worm-like structures, clinging to the cylinder walls until starting the engine blew them out through the exhaust. The dirt must have come from below, as the upper parts of the cylinders were clean before (I checked that with an endoscope). As the ingredients of the spray seem to be rather aggressive – and as I decided to let the solution sit there for a couple of hours (instructions say: some 20 or 30 mins.) --, I went for just a short 20 km ride afterwards, to get all the residues out of the engine, and did an oil change (incl. filter change) immediately.

I didn’t find the time to do a compression test afterwards (will do that sometime in the future), but what ist interesting is that my fuel consumption has come down to no more than 14.5 litres on 100 km, which is rather low when compared to the 17-18 l/100 km I had before the experiment – and, I think, rather good, even for the H.E engine. Ambient conditions were the same, as were the routes I took, mainly highways, and (I guess) my driving style. I also got the impression that the car gained some horse power, but that might well just be a subjective impression and, other than the fuel consumption, nothing I could prove by means of figures.

Also did the Italian tune-up, before and after the experiment. That seemed to have helped as well, but the Anticarbon thing still got out a lot of carbon deposits even after the first tune-up, and the fuel consumption went down after the spray treatment. Might easily be that the combination of both did the trick.

When I ran my workshop I used to offer an engine oil flush and as an experiment we did a comp test before and after. There was a rise of a few psi after the oil change which I can only put down to the oil flush easing the rings a bit?

Did the engine turn over faster on the starter? Was it colder?

For what its worth, i ran seafoam in my gas tank on another car. Bit more pep, gas mileage went way up. After that tank, things came right back to normal.