Was the motor still at TDC with the marks lined up when you tried the compressed air? If so, #4 would be on the intake stroke with the intake valve already open (partially), allowing all the air to come out the intake/jboot (and possibly still some out the exhaust -- I'd need to look up the cam specs). If that's what you did, rotate the motor exactly one revolution so that #4 is at TDC compression, and try again...libarra82 wrote: Wed Jan 21, 2026 6:48 pm Received the air compressor today. Shot air into the spark plug port and it all went out of the jboot, dang.
Odd that it was just that one valve, I would think cylinder one would be bent also, but that was good. Lifter on that valve is very chatty, maybe it too stiff and didn’t let the valve close all the way and hit the piston?
I’ll tear it all down this weekend to assess the damage.
Cylinder Head rebuilt recommendations
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Tom
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Tom
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Well that would indeed suggest a bad intake valve... Definitely let us know what you find!libarra82 wrote: Wed Jan 21, 2026 7:31 pm Air escapes through the jboot no matter where I am in the rotation. Exhaust valve does close.
Found something interesting today. I injected air into each cylinder with all the spark plugs out. Found that air from cylinder 4 is leaking into cylinder 3.
I think when inserting air into cylinder 4, it’s leaking into cylinder 3 then up the intake. I may not have a bent valve after all. Head gasket hopefully, fingers crossed.
I think when inserting air into cylinder 4, it’s leaking into cylinder 3 then up the intake. I may not have a bent valve after all. Head gasket hopefully, fingers crossed.
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PSU_Crash
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If that's the case then you should have measured bad compression on both 3 and 4. Waiting for pictures with the head offlibarra82 wrote: Fri Jan 23, 2026 11:27 pm Found something interesting today. I injected air into each cylinder with all the spark plugs out. Found that air from cylinder 4 is leaking into cylinder 3.
I think when inserting air into cylinder 4, it’s leaking into cylinder 3 then up the intake. I may not have a bent valve after all. Head gasket hopefully, fingers crossed.
'86 Zermatt Silver 944 N/A 
'86 Mitsubishi Starion - Purpose built SM class Autocross car
'87 Chrysler Conquest - Mid LS Swap
'86 Mitsubishi Starion - Purpose built SM class Autocross car
'87 Chrysler Conquest - Mid LS Swap
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Tom
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Good work for one day! Now that the cam tower is off, did you inspect that valve spring/stem from above for anything obvious?
Looks like a very tidy, original engine bay. I worry about any car with original fuel lines, however, as those appear to be. Consider installing the Rennbay fuel line kit while you're at it....
Looks like a very tidy, original engine bay. I worry about any car with original fuel lines, however, as those appear to be. Consider installing the Rennbay fuel line kit while you're at it....