Had no compression in #4 did leak down test. took head off long story that would be…but you all know how that feels. Come to find out the intake valve is a little bent. The history of this engine is unknown.how can th valve get bent besides someone for running of or changes head gasket and never took precautions. Can it get bent any other way?
HAD ir been running, ir was this after use?
I picked up a non-running 1985 XJ6 as a parts car several years ago. When I removed and partially disassembled the 4.2L XK engine I noticed that there were 2 or 3 slightly bent valves, probably the reason that the car was junked. Upon closer inspection I noticed that the intake and exhaust cam shafts were swapped. Interestingly the intake cam shaft had “Exhaust” written on it with nail polish, and the exhaust cam shaft had “Intake” written on it with nail polish. It was obvious that the engine had never been run since the cam shafts were swapped because there wasn’t a molecule of combustion products on the head and everything was squeaky clean. So someone removed the cylinder head, inadvertently swapped the cam shafts, and then when they put it back on they bent some valves. I removed and replaced the bent valves myself and installed that head onto the 4.2L XK engine in one of my other Jaguar with a 4.2L XK engine that had some compression issues where it had great compression and has been running nicely ever since.
So to answer your question, there are three possible ways (there may be more) that I can think of that would cause a bent valve in a Jaguar 4.2L XK engine. The first is bending the valve upon removal and/or replacement of the cylinder head because some of the valves sit below the surface of the head if it is removed with the cam shafts in place and contact with anything can bend an open valve. The second is rotating the engine while adjusting valves and causing contact between a piston and a valve. The third is a failure of some kind in the timing gear that allows the timing chain to jump teeth. In the case of the timing gear problem I would expect to see other issues. Based on my experience with these engines over the past 17 years, I suspect that the first two (maintenance errors) are the most likely causes by a wide margin and the third one (timing gear problems) is very rare.
I know that you have no records but my guess is that someone messed up a cylinder head swap or valve clearance adjustment. That is easy enough to do and is probably the cause of the bent valve in my opinion.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
1 Like
In a rare case a valve with a lot of crud on the stem might stick open and get bent by the piston crown.
X[quote=“Rob_Reilly, post:4, topic:358957, full:true”]
In a rare case a valve with a lot of crud on the stem might stick open and get bent by the piston crown.
[/quote]
In a properly-timed engine, not an intake.
If it stays fully open yes but highly unlikely to stay in that position IMHO
If it got stuck open…sure. Anything is possible!
I bought a non running 4.2 series one years ago. I stripped the engine and found it had one bent valve. On strip down of the engine, on removal of the core plugs, the waterways were blocked solid with crud, as were the waterways in the head.
In that particular case, the bent valve was caused by overheating.
Sawyer didn’t say if he had ever seen this engine run, nor how much crud was in it.
I rescued an abandoned XK140 engine that had so much crud and rust in the head that several valves were stuck open.
Wiggles I purchased the engine on CL and bloke said it was running came with transmission.
If you sit a head with the cams set on a flat surface without supporting the head you will absolutely bend a valve, not much, but it’s still bent and wont completely seat. Just looking it would appear that they are all at or below the head surface but all are not.
Bob
889076
Plymouth, Mi
Mr Novak I would agree someone had done a valve adjustment some time ago using non jaguar locking washers. So someone probably did some other work also possibly bending the valve then so I don’t really know the history of the engine has I said no compression in 4 hole but all others at 150-154.
cheers
the vavle was pretty clean in this case rob…
good point Davidm I noticed someone had removed the temp sensor under the manifold probably a clue cheers
Rob never saw the engine running…I removed the block plugs to chase the stud threads but was pretty clean inside.
quite true that’s why I always remove the cams when removing the head.
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To expand on Paul’s remark; “(there may be more)”, Sawyer - with only one valve bent, one may add exotics…
An already bent valve was reinstalled unchecked - bending reason unknown, but may conceivably manufacturing/transport occurrence. Too tight valve lash (‘negative lash’). Or engine rotated slightly with chains removed, accidentally or before installer saw the errors of his ways. Broken/weak valve spring causing piston contact at high revs - or incredible play in bearings, big or small…
In-car; only piston contacting a valve not fully closed is the fundamental reason, of course - the large valve, at an angle to the piston crown bends instantly upon contact. And very little bend is required for the valve to leak…
Uncertainties like this enhances the advisability to carefully check possibilities mentioned before a strip-down. Just finding a bent valve after the strip-down tells little - and may lead to a repeat…
Frank
xj6 85 Sov Europe (UK/NZ)
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Frank I checked the top of the piston. there was no indication of the valve hitting marks etc no movement in the piston. when I checked the compression in #4 with 0 compression first thing I checked was the valve lash. Intake and exhaust were set to .012-.014. one thing I have not compared is the valve springs so good point I will check. So might not know how valve got bent don’t know history of engine all I know is the guy had a shop in Van Nuys CA he said it was running the car it came out of 1971 xj6 now had a Ls1 engine. so basically I got it for parts the engine had all accessories attached even A/C compressor and BW transmission for 500 us dollars.i have a rebuilt head already to take place so I keep posting the outcome.
David could you educate me on how overheating can cause valve bending?
If I remember correctly, continued use of an overheating engine can cause closed up engine clearances. Also lack of lubrication, which may cause valves to stick in their guides.
There are probably more reasons I can’t think of right now.
I have also been told, in certain circumstances, a valve can contact another valve, which may explain why there is no mark on the piston.