1967 400engine question - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 06-15-2015, 09:09 PM Thread Starter
 
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1967 400engine question

I came home today to decided to pull all the tune up parts. I decided to put new plugs and compression check I have the old plugs out for review. Here is the bad news The compression runs 140 to 150 psi on all cylinders, except for #1 cylinder. That one is 60 psi. I have re-ran the check a couple of times to see if it was my tool. I think the reading is accurate. The spark plug does not show any indications of oil fouling.
No external air noise. No exhaust gas in the radiator. No over heating. No water or oil leakage. #3 cylinder pressure is 140+ like the rest of the engine. This would lead me that the head gasket is holding. If the rings were bad, then there would be oil fouling on the plug. The plugs are fairly even with color. Usually plugs tell the health of the engine, but my plugs are more fuel wet as I don't drive it long enough to heat them up. So, the plugs looked like I expected them. The rings let some blow by but is even across all eight cylinders. The next guess would be lose rocker and I don't think so, as valve train noise seems normal.


My guess is the exhaust valve. This seems to fit the symptoms. The engine was not built to run on unleaded fuel and it has been running on unleaded since the 70's.
please any nadvice would be helpful
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post #2 of 4 (permalink) Old 06-16-2015, 08:54 AM
 
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Sounds like it could be that the intake valve is not opening properly, thus not filling the cylinder properly. This could be a loose or broke rocker, bad lifter, or cam lobe going flat.

When I broke an exhaust rocker, it would cough and pop back thru the carb, cause the exhaust had no place else to go. But with an intake not opening correctly, it would not have this symptom.

Hey, it's just an idea. I'm not a diagnostic expert.
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post #3 of 4 (permalink) Old 06-16-2015, 09:05 AM Thread Starter
 
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Thanks

Thank you for the thoughts/idea, just one more piece of fun
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post #4 of 4 (permalink) Old 06-16-2015, 06:41 PM
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My guess is a burnt exhaust valve, too. My '67 GTO burnt an exhaust valve on #1 in 1988. You can do a leak down test to verify, or if you have true dual exhaust without a crossover, you can go back to the tailpipe and listen to it pop and miss out of that bank. A leakdown test will tell the story, though. Good luck, and tell us what you find!
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO General Discussion

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