400 wet plug, erratic vacuum
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400 wet plug, erratic vacuum
My '79 400 has always had a bit of a valve tick since I pulled it out and started working on it last spring. I chalked it up to a varnished lifter, put some detergent in the oil and kept running it. Earlier this summer, I was cruising down the freeway and all of a sudden, the ticking stopped! It was a miracle! Except when I pulled up to a stop, I felt a serious vibration through the drivetrain. After some diagnosis, I discovered that the #5 cylinder is misfiring; I pulled the plug out, and it was black and wet. The vacuum needle vibrates rapidly between 14" and 18" at idle. All of the other cylinders have checked out OK as far as spark plug reading, and I have checked to make sure that it's not the plug wire or the plug by switching both to a different cylinder. At this point, I'm assuming it's something in the valve train. It seems to run smoother at RPM, and misfires at slower speeds. Any ideas before I pop open the valve covers?
Here's a link to the vacuum gauge test:
Here's a link to the vacuum gauge test:
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Joined: Mar 2011
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From: Taxachusetts
From the looks of the needle it seems you have loose or worn valve guides, especially on #5.
Have you found what vacuum readings and their corresponding meanings are on the 'net? There's a boatload of 'em out there. Should have come with your gauge, too.
Have you found what vacuum readings and their corresponding meanings are on the 'net? There's a boatload of 'em out there. Should have come with your gauge, too.
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I did, but I didn't find one that was specific about this type of movement... Until I opened my service manual. It said: 4 in. Hg rapid needle movement is probably worn valve guides. Now I feel silly. I'll open her up and see what the heads look like later. Sorry and thanks!
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Looks like I had a bent pushrod that finally broke. I'm hoping I didn't beat up the heads or other rods too bad by having the broken pieces laying in the bottom of the head for what amounted to a couple of hours of running time. I think, based on the behavior, I may have bent the rod when I first started working on the engine, and it finally just snapped. I recovered the whole rod, so no missing shards dropped down into the engine. I'm going to replace it and roll over the engine a few times by hand to ensure that the valve isn't binding or sticking.
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