Trust everyone is doing OK. Here is a problem on my 1992 XJS V12 5.3 Litre with 44 K miles documented. Car sat for many years inside. Got it running. Serviced the fuel system, ignition system. Fresh fuel. No rust in tank. Distributor / coils / wires / new plugs, etc... But it runs rough on Bank B. I have good spark. Good fuel pressure. No issues with the fuel pressure modulator diaphragm. All grounds good.

B2 cylinder shows zero compression when checked with a compression gauge, plugs out, starter spinning the engine. The rest of the cylinders on the same bank are at 175 PSI with the same compression check.

But I can pressurize the B2 cylinder and hold 120 PSI from an air compressor. which tells me the valve seats are intact and the piston has no hole in it. Mind boggling.

Checked valve clearance and it is at 0.010 on exhaust and 0.009 on intake. similar to the rest. Can lobes not worn. Watched the cam lobes push on the valves, normal operation Checked the B2 runner on intake. It is not blocked.

I have scoured the site and the internet and never found a problem like this. Zero compression while spinning the engine to do a compression check. Can hold a 120 PSI of pressure in the same cylinder with valves closed.

Thoughts anyone? Has anyone seen such a problem. Zero compression but can hold 120 PSI in a cylinder pressure test.

 

When performing the compression test, have you tried adding some ATF to the cylinder?

What does the fuel injector for that particular cylinder look like? Does it have a good spray pattern?

 

What a puzzle!
For starters, get the piston to BDC about to come up on the compression stroke. Then start blowing compressed air down the intake runner (rig up some sort of seal round the nozzle) and carefully get a mate to turn the engine at the crank front nut slowly CLOCKWISE as you look at the front of the engine. See if (a) air comes out of the plug hole and (b) when it stops doing so.
I suspect the cylinder is not getting anything through the intake.
If it is then try pressurising the cylinder down the plug hole as at the same time you repeat the test from BDC on the compression stroke, which will initially force air out of the valves, and see when pressure starts to build, ie when the valves close..
Then compare this last test with a good cylinder.
Report back!

 

Read this 4 times, just to be sure.

44K miles is WAY low, so this poor engine has spent most its life sitting and being started sparingly, and then shut down, before getting to operating temperature, BAD. NOT your fault.

The oil in the cylinder is standard procedure down here.
Throttles jammed WIDE open, Fuel pump relay removed, Coil/s HT lead/s removed.
1) Dry comp test.
2) Squirt of oil down each plug hole, repeat test, known as a wet comp test.

Compare the numbers.

I am also assuming the engine is AT TDC Comp Stroke B2 when the compressed air is added??

My suspicions, and only that, is glazed bore, and gummed rings.
Lazy valve is out of the door, you observed that, and the Inlet tract is open and clear, you did that also.

The down stroke creates a SLIGHT vacuam, air from the outside rushes in, and even with NO vacuam, atmospheric air would rush in to fill the void. Compressing that air on the UP stroke is the issue, BUT, the holding of 120psi is odd at best, never had that in all my engine adventures.

Lazy Inlet valve not closing QUICK enough, NAH, you would hear the tappet I would think.

If it were mine, I would have run it as was, up to temp, and then another hour, let the engine descum itself. Maybe NOT drive it, but work the revs, it is a race bred style of engine after all, not meant to sit around.

Hind sight, ALL my Jags never get started just to move them, or see if they run, and once started, are always left running until operating temp is reached before shutting down, especially the V12 cars.

 

Hey Greg, thanks for the ideas. in the pics I shared above, it shows both lobes up = B2 piston should be close to TDC, turning the crankshaft pulley opened the intake down to BDC. Can see the intake valve move away from the seat while piston goes down. Air goes through the intake and out of the spark plug opening until valve closes.

The B2 intake runner is clear. I pushed a fuel hose through it.

It seems that the valves, camshaft, piston for B2 are functioning as they should when the engine is rotated by hand.

I plan to pull the ECU out and jump the starter relay to spin the engine faster and do a compression check while watching the valves cycle. .

Could it be that the valves for B2 are sticking open when engine speed increases and engine warms up?

Should I venture into removing the cam sprocket and camshaft to check the condition of the valves, the caps, retainer clips, etc. Clearance presently on intake is 0.009 and on exhaust is 0.010?

 

Unimog
I would follow Grant's advice first! If it is not valves (and it seems not if you can see them moving, it has to be rings. So all the plugs out to ease the load on the starter, oil the bores and ask a friend to operate the starter while you carefully watch the dial for any flicker of compression.
If there is, that indicates rings are the problem, so drop loads of neat injector cleaner down the plug hole and leave it overnight, then repeat the compression test the next day and see if any improvement. If there is, button it up and drive it at max load and max revs for an hour and that should do it!

 

Hey Grant. thanks for the confirmations on the valves. I have had this one for a couple years, one owner before me who barely used it. then it sat for 11 years in his garage until I bought it. Thus the 44 K Miles. He had a british auto repair shop get it running again. (All the work that i mentioned above) but it ran extremely rough.

So here I am now, with time on my hands thanks to covid, trying to fix it.

Yes to the B2 cylinder at TDC with both valves closed when I applied 120 PSI compressed air through a hollowed sparkplug contraption that I made. No leaks through the intake valve. No hissing through the exhaust or the crank case.

I did not do the wet compression check since all cylinders registered 175 except B2 which did zero. I mean zero. The needle would not even bounce a bit. I even ran the engine at 2000 RPMs with the compression gauge connected on B2 ( injector plug disconnected).

Like I mentioned to Greg in my other reply, could it be that the solid lifters are galled up and get stuck when the engine RPM is increased and cylinder head temperature is up?

I am now at a fork in the road, do I pull the camshaft off and check the lifters? Any purpose in that? Any other tests I can do without pulling the camshaft off?

By the way, you are absolutely right on not starting them and turning them off without driving them 10 miles or so. Here are the few that I have. Favorite is the 1993 6 cylinder coupe. The 2 V12's have been painful.


4 with 6 cyl and 2 with the V12

the best one in the bunch

 

NICE fleet you have there.

Some more thinking, and Batphone discussions with Greg.

I cannot see it being valve related at the moment.
1) Lazy etc Inlet, spit back via the Inlet, no.
2) Exhaust valve issue, unlikely, as popping via the exhaust would be heard.
3) OK, that cylinder is NOT firing, so exhaust popping may be hard to hear.
4) Tappets are a tad tight, but NOT causing this for the above reasons. To sort the clearances, oh boy, heaps of work, and playing with that Plastic Boomerang chain tensioner, could be more trouble than you want at the moment.
5) Running with the comp tester installed, as you have done, and still zero, is almost mechanically impossible, as even with a valve seat dropped, and a holed piston, at 2000rpm, I would expect a small reading at least. Even with gummed rings, I would expect a partial reading.
6) Glazed bore, mmmm, maybe, but again a partial reading would be expected.

At the tear down state you are at, pour an egg cup of oil, any oil, into that cylinder, and retest it.

Lets know, and I will keep thinking, and drinking of course.

 

If you had a piston or ring problem causing 0 compression I would expect massive blow by. If you don't have significant blow by, I would further suspect a valve issue. Either a very stuck valve or lifter, that does not close once the engine starts rotating, or a broken spring. If you try to open the valves with a prybar you may feel a difference between the two. Use caution not to scratch the cam lobe or follower.

 

OK, you can’t see the valves opening and closing. What you see is the lifter opening and closing. With the recessed exhaust valve it might be possible for the top of the valve stem opening and closing. And the exhaust valve shutting under pressure of the leak down test.
Sorry, I’m grasping at straws here.
the discount tool places ( harbor freight) ) are now selling a cheap ( about $50 ) bore scope. Maybe you can find something similar Or try EBay and take a look?

 

Fellows, trust you had a great weekend. decent news to report. Thanks for all your advice and feedback. I continued cranking the engine while applying 120 PSI pressure. After few cycles, I heard a loud click like a spring coming loose or unseated, deep in the engine or in the cylinder head. I could hear it over the noise of the engine spinning.

So I thought might as well do a compression check with the same gauge, low and behold it registered 172.

I have no technical explanation for gaining compression on that cylinder. But one fact is that it sat for 11 years. The oil was bad, which I changed today. Maybe one of the valves was sticking in the valve guide due to corrosion on the valve shank. Who knows.

Oil pressure on start up was maxed out (15 W 40 oil), after warm up and running for 10 minutes at 1500 RPMs, it now sits in the middle at idle.

Temp holds in the middle. Both thermostats opened after few minutes, both upper hoses got hot the B side more than the A side. I plan to do the "Grant" radiator and thermostat service, probably go with the Motorad 233-160 but will have to measure the height first. Also, clean the radiator and do the belts.

Air compressor procedure pressurizing the B-2 cylinder while it is cycling.

Running after re-assembly and startup :

 

Most of these cars are driven by old men in retirement. Then they are driven less and less until the owners passing.
With the Common final drive ratio Of 2:88-1 even at 45mph the engine isn’t working much above an idle.
short stroke ( 2.756 ) the stress on the engine at 100 mph isn’t as much as A Chevy engine at 65 mph.
This is a car that begs for an Italian tune up.

Change the oil based on time not miles. And it will last your lifetime without the need for rebuild.

 

WELL DONE.

I agree, and had thought a lazy valve to close off. Clearly doing so while you gathered the gear to add compressed air, etc.

I suggest a GOOD long, semi hard drive, get some heat into that engine, and after, say 4 hours, find a pub, and celebrate, only enough to still drive home of course.

Change that oil again after that initial run, just be on the safe side.

WELL DONE again.

Watch those 160 stats, too cold for my liking. 180 are the best for me. That engine too cold is as bad as running too hot.

 

Thanks. Drove it in the neighborhood for a bit. Surprising, trans shift well, brakes not grabbing, steering tight, it drove as nice as my 1993 coupe with the straight Aj6.

Drove good overall despite the fact it sat for 11 years. Ambient temp was in the 70+ / 21 deg C.

Coolant temp held just past the middle - that worries me - wou like to see it at 40% on the gauge.

Oil pressure moved between 6 and 8 on the gauge. Fuel level sending unit is probably stuck or corroded thus the fuel warning.

Check engine light is on because the O2 sensors were unplugged temporarily to see if it runs better with or without them.

Next: new tires, O2 sensors, thermostats and the cooling system service. Anything else that should be serviced?

Driving it:

 

Do not try to run the car too cold. It’s best at about 190-195 (f)
buy one of those cheap ( $25)digital Infrared temperature detector and aim it at the bottom right of the radiator where the hose goes up and into the water pump.
Youll find dozens of uses not only on your car but also the houses etc.