Head Gasket job
#101
04-19-2018, 09:24 AM
Veteran Member
#102
04-19-2018, 10:19 AM
If I recall the start of the thread correctly it was a normally aspirated V8, so compression ratio is 10.75.
With that CR a healthy engine should definitely get over 11bar (160psi). The 120psi (8.3bar) you reached in the best cylinder definitely means the piston rings or valve seats are done.
I remember when I fully restored my old BMW 320i inline six many years ago it started out with 6 to 7 bar compression pressure (87 to 101psi), the M20 engine had a similar CR of 10.5.
It was close to impossible to start this thing around freezing temperatures or below.
After bedding the valves and changing the piston rings it was back to >11bar on all 6 cylinders again, although I didn't touch the valve guides and they certainly had too much play to seat the valves perfectly.
I think you should have taken the chance to pull the pistons and change the rings when the machine was still open. Would still be the cheaper option compared to a new engine.
Pistons get perfectly clean with this spray oven cleaner stuff. And when all the coked crap is out of the ring grooves they also move freely again and seal to the wall.
Plus, when the engine is open on the bottom it is just a smile to change main and conrod bearing shells along, giving you safety for the next one and a half decades again.
Not sure what the parts cost in the US, but I paid like 300€ for all the parts at the time and the engine was really phantastic after that. OK, you have to add 33% for the additional 2 cylinders
And a lot of work ...
With that CR a healthy engine should definitely get over 11bar (160psi). The 120psi (8.3bar) you reached in the best cylinder definitely means the piston rings or valve seats are done.
I remember when I fully restored my old BMW 320i inline six many years ago it started out with 6 to 7 bar compression pressure (87 to 101psi), the M20 engine had a similar CR of 10.5.
It was close to impossible to start this thing around freezing temperatures or below.
After bedding the valves and changing the piston rings it was back to >11bar on all 6 cylinders again, although I didn't touch the valve guides and they certainly had too much play to seat the valves perfectly.
I think you should have taken the chance to pull the pistons and change the rings when the machine was still open. Would still be the cheaper option compared to a new engine.
Pistons get perfectly clean with this spray oven cleaner stuff. And when all the coked crap is out of the ring grooves they also move freely again and seal to the wall.
Plus, when the engine is open on the bottom it is just a smile to change main and conrod bearing shells along, giving you safety for the next one and a half decades again.
Not sure what the parts cost in the US, but I paid like 300€ for all the parts at the time and the engine was really phantastic after that. OK, you have to add 33% for the additional 2 cylinders
And a lot of work ...
Last edited by Aix; 04-19-2018 at 10:25 AM.
#103
04-19-2018, 10:35 AM
Senior Member
Wow. It is sort of hard to believe that the compression on those cylinders could have degraded that much, and with only 105,000 on the odometer no less. With the problem that you're now experiencing, it sounds like the engine is simply out of time.
So I'm pondering what I would do in this situation as we both have the same model and year Jaguars with roughly the same mileage. Perhaps if it were me, I might consider putting the car up for sale, and selling it 'as is' so it wouldn't be a total loss.
Purchasing a new engine for it would be more than the car is worth. On the other hand, at least you would know what you have so the bleeding would presumably cease at that point. You could easily spend thousands on repairs with any used vehicle that's up for sale nowadays so that in itself may be all the more reason to bite the bullet, and install another engine.
So I'm pondering what I would do in this situation as we both have the same model and year Jaguars with roughly the same mileage. Perhaps if it were me, I might consider putting the car up for sale, and selling it 'as is' so it wouldn't be a total loss.
Purchasing a new engine for it would be more than the car is worth. On the other hand, at least you would know what you have so the bleeding would presumably cease at that point. You could easily spend thousands on repairs with any used vehicle that's up for sale nowadays so that in itself may be all the more reason to bite the bullet, and install another engine.
#104
04-19-2018, 02:21 PM
I wouldn't give up on the engine just yet. But I suspect that the piston ring grooves are coked and that lets the rings stick when the piston is changing the cylinder wall side. My 190.000km 320i had thick coked residues in all ring grooves and the rings were effectively stuck in these deposits. So problem in this case is that all has to come off again.
Once the old rings were removed just spray the pistons thickly with oven cleaner, let it soak for a few hours and then wipe the stuff off. Solid deposits on the bottom of the grooves can be best scratched out with a broken part of the old rings, they have the correct width for the groove already and are quite sharp at the edges.
After that spray once more with oven cleaner, wipe off the rest and fit a new ring set.
That is in my experience the prime reason for lack of compression and can happen after comparatively low mileages already, depending on the duty cycle the car is operated in.
And the tip with the main and conrod bearings doesn't mean I think them seriously worn after that mileage, but if you once have the thing open on top and bottom it would be mad to not change these quite cheap and essential parts. Main bearings can be pushed out and the new ones slipped in with assembled crankshaft, bearing cap by bearing cap, The conrods have to come off anyway for the piston overhaul.
Once the old rings were removed just spray the pistons thickly with oven cleaner, let it soak for a few hours and then wipe the stuff off. Solid deposits on the bottom of the grooves can be best scratched out with a broken part of the old rings, they have the correct width for the groove already and are quite sharp at the edges.
After that spray once more with oven cleaner, wipe off the rest and fit a new ring set.
That is in my experience the prime reason for lack of compression and can happen after comparatively low mileages already, depending on the duty cycle the car is operated in.
And the tip with the main and conrod bearings doesn't mean I think them seriously worn after that mileage, but if you once have the thing open on top and bottom it would be mad to not change these quite cheap and essential parts. Main bearings can be pushed out and the new ones slipped in with assembled crankshaft, bearing cap by bearing cap, The conrods have to come off anyway for the piston overhaul.
#105
04-20-2018, 08:25 AM
Thanks for the input guys, I can get a low mileage motor delivered for about $1300. I really don't have the desire to pull this apart again, I'd much rather be fishing! I will try to figure it out once it's been swapped out. I will probably start a new thread, with pictures, of my attempt to swap out the motor, stay tuned.