351M low compression
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Fleet Mechanic
Joined: Jul 2002
Posts: 1,297
Likes: 34
From: S/W Virginia
351M low compression
Hi,
I did a compression test today on an engine that I bought to go in my truck, a 351M. I bought the engine as good, but you know how that goes. 5 cylinders were about 115-120. Cylinders 7 and 8 were about 30PSI. Cylinder 2 was 70. Oil didn't seem to help much, so that would make me thing valves. I hooked an air compressor to the cylinder at TDC compression stroke and there was a lot of air coming out of the intake. It seems odd that a valve leak would be that bad though, no so much on the 70 PSI cylinder, but on the 30PSI ones. By looking at them move it doesn't appear that they are sticking.
The plugs I took out all looked like they'd been running good. Good color, no oil.
Anything else I should try before pulling the heads? I'm trying to get a running engine as cheap as I can.
I did a compression test today on an engine that I bought to go in my truck, a 351M. I bought the engine as good, but you know how that goes. 5 cylinders were about 115-120. Cylinders 7 and 8 were about 30PSI. Cylinder 2 was 70. Oil didn't seem to help much, so that would make me thing valves. I hooked an air compressor to the cylinder at TDC compression stroke and there was a lot of air coming out of the intake. It seems odd that a valve leak would be that bad though, no so much on the 70 PSI cylinder, but on the 30PSI ones. By looking at them move it doesn't appear that they are sticking.
The plugs I took out all looked like they'd been running good. Good color, no oil.
Anything else I should try before pulling the heads? I'm trying to get a running engine as cheap as I can.
Hotshot
Joined: Feb 2004
Posts: 10,830
Likes: 2,611
From: The Shenandoah Valley
Good bet it's a blown gasket between 7 & 8. If you just bought the engine, how did you rig a compression test .... is it already in your truck.
Air would come out of the intake if the head gasket was blown between 7 & 8 if either intake valve was open.
Air would come out of the intake if the head gasket was blown between 7 & 8 if either intake valve was open.
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Fleet Mechanic
Joined: Jul 2002
Posts: 1,297
Likes: 34
From: S/W Virginia
I didn't think about head gasket being bad on those two. It's not in truck yet. I have it hanging from a chain hoist and put the bell housing and starter on, and used some jumper cables to starter.
Hotshot
Joined: Feb 2004
Posts: 10,830
Likes: 2,611
From: The Shenandoah Valley
I once did a 454 BBC top end (that I had done the bottom end work on with engine up side down, then built a stout wooden cradle on wheels) and pulled the heads with it right side up and then worked on the top end in my spare time, I just never actually pulled the crank, pistons, or rods out of the block.
Cross-Country
Joined: Jun 2012
Posts: 70
Likes: 4
From: LaGrangeville, N.Y.
I'm not trying to be an *** but the best thing you could do is junk the motor and get a 351W. A lot more parts available for them and a MUCH better engine. The 351M was a problem child from the start with head gaskets and cracked blocks. It's been a while since I've worked with this era but I did a swap in my '81 F350. I went from a 4 speed to a C6 auto at the same time. If I remember correctly the bell housing from the 300 and 351W are the same, but the 351M will require a different bell housing if this is going in your F100 listed on your profile.
The 351M was a smog motor they threw together when they couldn't keep up with demand for the new 351W. They only made 160hp and performance parts are few and far between.
The 351M was a smog motor they threw together when they couldn't keep up with demand for the new 351W. They only made 160hp and performance parts are few and far between.
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Fleet Mechanic
Joined: Jul 2002
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From: S/W Virginia
No, it's going in a 78 150 4x4. It seems that motor mounts are the issue. I was going to put a 300 in it but couldn't seem to find any mounts. Stuck with the 351m so I'd already have the right mounts, Bell housing, etc
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Fleet Mechanic
Joined: Feb 2004
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From: Arlington,Wa
The 351M gets such a bad rep...but it can be turned into a decent engine. Bigger cam... better timing chain, 4bbl intake and dual exhaust and you'll have people asking you if it's a 390. Ask me how I know.
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Fleet Mechanic
Joined: Jul 2002
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From: S/W Virginia
Yesterday I added some ATF to those 3 cylinders. Put the plug back it and spun it over a few times. Let it sit a while, pulled the plugs and spun it over to let it purge the oil. Compression test was then showing about 150 on those cylinders. Do you think maybe it's got crud out from under the valves?
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