Some of you know me, I've been pursuing a mild engine noise that comes in around 2000 RPM and leaves around 2,700 RPM, or so it seems. When driving the car the untrained ear may just think it's a mean sounding Jaguar, but to me it does not sound right.

I have a compression test gauge but, unbelievably, I have never used it on this car. Tonight I took out all the plugs, blocked open the throttle valve with a toothbrush, removed the fuel pump relay....and tested all 8 cylinders.

six (6) cylinders read between 150 and 170, two (2) however only read around 90. The two cylinders on the drivers side (left) closest to the firewall. I put a little oil in each cylinder and they both went up to over 150. I put the car back together and went for a drive. The noise was reduced while driving, but when I put my ear on the engine and revved it, it really does seem the sound is coming from the area of those two cylinders, and then echoing under the intake manifold.

Any prognosis? Any advice on what to investigate next? I'm a little out of my depth on this one. Could it be that the valves need to be shimmed/adjusted? I've adjusted the solid lifters on my BMW, I suppose I could work on these. Is it worse than a valve problem? I am pretty sure this is a nikasil engine, are these cylinders shot?

Ok, enough questions. Thanks in advance for your help.

First off, how many miles do you have on her? The noise may roll out seemingly just because of the amount of rpm's doesn't allow you to hear it, its' probably still there throughout all rpm ranges.
Do you have an engine stethoscope? Same as a regular one, it just has a 6" rod on it instead of a disc (probably just as cold to an engine though....lol). These are cheap and extremely useful when chasing an pin pointing noises such as this.
Without observing the sound myself its difficult to know and from your part to describe. If those 2 cylinders are next to each other, I suspect a head gasket in that case. Check your cooling fluid level (doubtful you'll notice a difference though) and monitor it and also check your exhaust and smell it for antifreeze. Those are simply precautionary procedures if those cylinders are together. Also, check the plugs, you should have a nice white ash color on the electrode, if its at all orangeish (new word) or discolored I'd be suspicious of the head gasket.
As for valves, its poooossible, but I've not read on valve issue for Jags?. The oil trick almost indicates that is almost for sure a ring thing or a very slight gasket failure.
Other than that, it could be a number of things a good mechanic would have to check or someone here who's dealt with it (I'm speaking from American engine experience). That is, unless you have some time on your hands and want to break the heads off of her?

Thank you Highhorse, I appreciate the thoughtful response.

The car itself has almost 109,000 miles. I was told at purchase, when the car had 94,000 miles, that the engine was replaced 1,000 miles before and the receipt they provided for the engine said that it had 41,000 miles at the time of installation. So, the engine is SUPPOSED to have about 57,000 miles on it at this point. That's assuming the receipt was an honest one.

I do have a stethoscope and its still amazingly difficult to find the sound. I will continue to better pinpoint it though. I also have an inspection camera (boroscope?) and the tops of all the piston look the same to me... for whatever that is worth. lol The spark plugs all looked the same, white ash color with no orange discoloration. I check the coolant level about three times a week and have done so for many months (it's an old car). I have not had to add any coolant in several months.

Since a worn cylinder or ring situation seems to be indicated by the improvement when oil is added, I have been reading dozens of testimonials regarding Restore engine treatment. Some have even put it down the spark plug holes (small amount) before also adding it to engine oil. Since the alternative is major engine overhaul, I am strongly considering trying some Restore to see if the compression improves and/or the noise disappears. The noise seemed to come about shortly after I started running Marvel Mystery Oil in the engine. Could the MMO have cleaned out deposits that were improving compression?

I've never heard anyone who has use MMO except in a lawn mower engine. I really have my doubts on that stuff. Running a synthetic oil is by far the best way to go and no special additives will fix a broken part....broke is broke.
Have you checked your oil pressure and level? I find it hard to believe, but initially I only come up with is not enough oil to splash the wall or either a clogged journal and/or low pressure preventing oil from getting up onto the cylinder wall. Don't take that as what it is, again....I'm working off pure speculation.
As for that so called 41,000 mile motor, I think they forgot a 1....was it a wreck it came from? Jag makes a very good vehicle and you pay for that and expect that, that's why I doubt a now (1?)57,000 mile motor going south.
The oil pan is real easy to access, I'd drain her and take a look from underneath. Check your oil and smell it, does it smell burnt or look excessively black? That MMO may make your oil too thin also and cause your issue as you suspected....I'd dump it. I'm not sure if I'd go with a regular 5/30 change or thicken it up with a 0/40 to see if that quells that noise?

+1. Dump the mystery oil. Mobil 1 foreign car 0w 40 should help.

The sound seems more ominous in the video, but the "quality" of the sound is pretty accurate. It's hardly noticeable while driving the car, but I have to admit, with my head right at the engine, it sounds unpleasant.

Sounds like a rod bearing to me. I would unplug one ignition coil at a time, (it takes the load off of that rod and bearing) rev it up like in the video and see if the noise went away. If it did go away, it's that rod bearing. If it didn't go away, it could be a main bearing.

Neither one sounds minor.

Wow, that sounds like it could be a wrist pin....but, it could be a bad cam. That sounds like its coming from right under the valve cover. If it had lifters, I'd say it was a collapsed one. But since it doesn't, a quick check would be to pull that valve cover and check those cams and cradles.

The MMO came highly recommended by a veteran member on here. It appears the engine went a little too long between oil changes before I bought it, so the MMO was recommended to clean it out.

I think the main problem here is a shady salesperson. They said the engine was an 03 with metal tensioners and 41,000 miles. I naively believed him. After purchasing the car I figured out where the engine number is located and determined it was built in March of 99. I replaced all the plastic timing gear, but now have less faith in the engine since most of what I was told was fiction.

What about Nikasil? That 99 engine will not have steel cylinders

I presumed it was nikasil as soon as I learned the real manufacture date. Since it idles perfectly smooth and starts up on one turn of the engine or less, I figured the liners survived the high sulfur days. Could I be wrong?

I'm not an expert on every possible engine malady. But, years ago when engines aged faster vs todays engines, low compression in a cylinder or two could make some harsh noises -- so harsh that one would swear that something else was wrong.

I had a knock last summer in our "office" car -- an old, but well preserved Lexus sedan. We all thought it sounded like the end of the engine ... maybe because it had 250k. Ended up being a bad coil -- we were all shocked. Never had a miss make that noise.

I'm trying to understand how a bearing or crank problem would drop the compression in two of the cylinders. Maybe it would be worth taking out the plugs and taking a look inside>

Highhorse, if I have the energy after work tonight I might pull the cam cover and check things out. If I do I will report back. I feel like I over-torqued the cam caps last Summer, but I don't see how that could have caused a problem.

If the rings are worn and oil lubrication not substantial enough, it won't coat the rings to create pressure. He did state he poured oil into the cylinder and it recovered compression.

Its the age old adage, once you fix that weak link, then you find the next weakest link. In my case a few months back, I replaced the noisy secondary tensioners and found my primaries and chains were bad also.

I had a late meeting come up last night, and tonight after a family dinner I'm just too tired. I will delve into this either tomorrow night or on Thursday (my ONE day off).

Question though, for those diagnosticians out there...

If my fuel trims are near perfect, which they are, does that rule anything out? For example, if the timing is off a little could the fuel trims still be good? Do the excellent fuel trims narrow my focus on the potential cause of the noise?

Thanks.

I would bet you have 20K miles of enjoyable motoring before any real issue shows itself. Sounds like any other car that old!

I hope you are right Jim. I have a STRONG desire to trade it for an XJR anyway but don't want to sell someone a car with a known problem.

I did put a can of Restore in the oil, I even put a few drops directly into the spark plug holes on the two suspect cylinders. I've driven about 175 since I added it and the sound does seem much less pronounced. I can still make it audible with my head next to the engine, but it's very difficult to hear when driving the car.

There s a slight wobble in the engine upon start-up about half the time, not sure if that's related to the noise.

Also, I noticed when the gas tank is at 1/4 or less the fuel pump makes a loud struggling noise when the car is started. When I top the car off the fuel pump is quiet again. Supposedly the fuel pump was changed just before I bought the car, it was on the engine swap receipt. Who knows....

Did you listen to that video Jhartz?....after the 10 second mark, something wants out of that cat. I don't know how enjoyable it will be driving around with that, if it made 20k?

I would like to narrow to one question. If someone could please answer I would greatly appreciate it.

Is it possible for the timing to be off, yet still have excellent fuel trims?