351m burnt vavles?
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351m burnt vavles?
I just bought a 79 f150 4x4 with about 150K miles. It seems to have a pretty consistant miss. A friend of mine said the 351m's have a history of burnt valves. True? Not true? I will be doing a compression check in the next couple of days to confirm. The motor appears to be stock.
MSEE
Joined: Apr 2004
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From: Austin, TX
I've heard that the M blocks aren't any worse than other engines, but then again many 351M's are loaded with smog equipment, which means hotter engine temperatures and therefore the greater the chance of burnt valves. My 400 with 150K has a burnt exhaust valve on its second head rebuild. The first time the heads were rebuilt, it had six burnt valves.
I'd not only do a compression test, I'd do a leakdown test too. That will pinpoint where you're leaking compression.
I'd not only do a compression test, I'd do a leakdown test too. That will pinpoint where you're leaking compression.
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Burnt Valves.
Thanks,
If the leak down test reveals burnt valves do you think it unwise to just redo the heads without doing the bottom end? I dont plan on doing any mods to the motor and just use the truck sparingly. How long could I drve on a burnt valve?
If the leak down test reveals burnt valves do you think it unwise to just redo the heads without doing the bottom end? I dont plan on doing any mods to the motor and just use the truck sparingly. How long could I drve on a burnt valve?
Laughing Gas
Joined: Nov 2002
Posts: 1,198
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From: Fairmont
It seems the 335 series are bad for guide wear. Once you get guide wear then the valves don't seal and hence burnt valve. Yes I have seen a number of burnt valves in these heads. Also what goes along with a burnt valve, sometimes, is then the head may crack.
It is very common to do a VJ on a pair of heads and not do anything to the bottom end. I would safely say over 3/4 of the VJ we do are that way, no bottom end work.
It is very common to do a VJ on a pair of heads and not do anything to the bottom end. I would safely say over 3/4 of the VJ we do are that way, no bottom end work.
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
Personally, I'd check your oil pressure with a mechanical gauge before doing just headwork on a 351M with that many miles. High-mileage 335-series engines are bad about oil pressure. Also, freshly sealed valves will put more pressure on your rings, since they'll most likely be the weakest link afterwards. It just depends on what kind of shape your engine is in. A wet and dry compression test can help you determine what kind of shape your rings are in.
Driving with a dead cylinder will speed up wear on the bottom end, kill your gas mileage, and make the engine hard to tune. The 400 in my '73 Galaxie has had a burnt exhaust valve for years. I've never pulled the heads off because I can barely get 28 pounds of oil pressure out of it. But it just keeps going, and going, and going...
Driving with a dead cylinder will speed up wear on the bottom end, kill your gas mileage, and make the engine hard to tune. The 400 in my '73 Galaxie has had a burnt exhaust valve for years. I've never pulled the heads off because I can barely get 28 pounds of oil pressure out of it. But it just keeps going, and going, and going...
Posting Legend
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That sounds good except for the 130# cylinder which is not within 10% (15#) of the highest cylinder. It is just a shade lower tho. There may be just a score in the cylinder wall or something could be wrong with the valves. A mechanic may have a borescope to look into the cylinder and tell.
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I just squirted oil into the low cylinder and it came right up to the others. It seems like I should be able to get this motor to run okay.
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