Dear collective Jagsperts,

Decades off road Series 2 XJ V12. Engine locked when purchased. After many dead ends (piston rings rusted in cyl. bores? Crank mains frozen to bearings?) found starter pinion jammed hard into flexplate teeth.
Starter rebuilt. Slowly testing & bringing other systems into state of good repair before attempting running.
Engine had rotated OK on starter. During cylinder compression testing starter jammed. Got pinion retracted, discovered engine locked. Cannot be barred in correct direction. Tried reverse few degree's, moves. (I know it's a bad idea reversing)
Not starter, cannot bar engine over. I guesstimated engine had rotated maybe 150 - 175 revolutions at starter speed over many separate short attempts. (8-10 revs per). Over different days, not done in continuous stretch.
Plugs out, injectors out, throttle plates latched open.
What would - could - might - lock rotating mass at say, rev 180 that wouldn't lock it at rev 1, 2, 10, 20 ?

Crankshaft Conundrum?

Appreciate anything from actual experience to lucky guesses.

 

TF,
The engine locking suddenly, but only in one direction, would need to be a blockage in the rotating assembly, most likely in a cylinder. You managed to free-up the crank initially and the engine would turn over with the rebuilt starter. Then you started compression testing: did you remove all the plugs at once before starting the tests; what was the compression on the cylinders you were able to test (if any); if it locked up on the first cylinder tested, something dropped into a spark plug hole while removing the plugs, could be in any cylinder; could a piece come off the compression tester itself; I have read here in the Forum that the valve guides (I think that was the part) can work loose and come up and hit the cam lobe, I don't have a Jaguar engine so someone else will chime in hopefully. Something more unreasonable would be a dropped valve, broken valve spring.

Beyond something that got into the cylinder between rotation #1 and #179 could be an unusual instance of a loose bolt backing out and jamming against a moving part. The starter that jams repeatedly is a red flag, why is it doing that? Does the flex plate still look straight and true, starter ring not broken or bent?

Yes, it is generally preferred that an engine be rotated in normal direction, but that is almost entirely done while performing critical engine setup such as valve adjustments, cam timing, ignition timing, piston deck clearance and a couple of others. I see no reason why you couldn't carefully turn the engine in reverse while watching the valve/cam lobes; listening for a "clunk, tink, ping" or feeling a tight spot as you are turning the crank by hand. If possible, shine a bright light into the open plug holes as each piston nears top-dead-center. You might see something.

Dave

 

If you rotate the engine backwards you risk the very brittle cam chain tensioner to fail and may have jumped a few teeth cranking it and have a valve hitting a piston or a valve head has dropped off.

 

Sounds like strip down required anyway so maybe just start that to find a rectify the problem?

 

Did you drop a valve?
look for some loose hardware/tool that might have slipped into gears or other parts that are supposed to move together.
Thats a tough one but I'm sure there is a simple solution once you find it.
You need to be the JAG whisper, and she will tell you her ills.
Bobmo

 

Thanks for the post Jim, that's exactly the kind of specific knowledge I don't have.

Dave

 

LT1, Bobmo, et al;

- Compression tests were between 125psi & 40. Most between 120 & 90. All cyls. tested. Plugs, injectors out of engine before compression tests begun. Was beginning to repeat compression test on low cyls. when starter pinion jammed in flywheel. Thought "Battery low?" Left on charge longer this time. Cranked few times, stopped. Thought "Oh, no, darn starter wasn't rebuilt properly!" Then found engine was jamming. Once flywheel moved one tooth back & forth, pinion pops out on own.
- Engine hasn't run yet under my or PO's ownership. If a valve dropped, wouldn't that have occurred during or immediately after hot running? Would they really wait 20 years & fall out while being cold cranked?
- Had bottom of engine off to examine crank, bearings & connecting rods. Never had cam covers off. While bottom off, looked up both sides of cam chain as far as possible with digital inspection camera. At that point chain and tensioner looked surprisingly good for mileage. No extra slack or 'extra bits' visible.
- Flex plate looks OK. Stretch of teeth where pinion was jammed for years (while clueless owners like me attempted to bar engine over) has some visible 'wrong pattern prints', nothing scary. After replacing rebuilt starter I barred engine to dozens of different positions so starter could engage every stretch of flexplate teeth, in case a specific quadrant of teeth were 'off'. Starter engaged flawlessly every time.
- Starter fully rebuilt last year by very experienced tech, at small motor shop which was a North American BL warranty repair shop in '70's. Still had BL starter parts on shelf. I believe starter is OK, but stalls when engine jams. For all the hate'n I've seen for years towards BL, this starter really spins this crank. While it sounds as if its rotating rather leisurely, trying to bump it part of full rotation is almost impossible. It spins that rotating mass at a great clip.
- I know a full stripdown would reveal all, am really really trying to avoid that, at this time. Wasted huge amount of time (& took LOTSA stuff apart) chasing non-existent phantoms when the original problem was a jammed pinion starter. Really hate to tear whole thing apart when, out there somewhere, someone knows "... oh, yeah, just open X & do Y.." Which I won't hear about until the whole V12 is in 10,000 pieces across garage.

Thanks, all, for time & intellectual effort!

 

I think your next job should be to take off the cam covers and look to see if a valve is stuck down. Having said that, the valves are vertical, unlike the twin-cam six cylinder XK, and unlikely to stop a rising piston, but y'never know !

 

If we are talking about a V12, then it is my understanding that this design is known as a Non Iterference one. Even with a valve stuck fully open, the piston will not strike it. I know this because I had a 72 XKE V12 and drove it for 3 years with an exhaust valve stuck fully open. So, a stuck valve should not prevent the engine turning over. Ian

 

Nice to know !

 

I think this might not be true of the flathead pre-HE engine, only of the May Fireball head design of the HE version?

 

40psi and 90psi are not good pressures. Are (is) the 90psi and 40psi in adjacent cylinders? If so, probable bad head gasket. Is the engine still locked up now, with all spark plugs removed? Is the starter properly adjusted to give tooth clearance with the the ring gear? I think Fraser's suggestion to remove the cam covers and study the moving parts is a very good idea. Open valve, hole in piston, bad head gasket, extremely worn piston/rings are about the only way to loose compression. Regardless of the cause of the locked engine, a tear-down will be necessary. Low compression never fixes itself.

Dave