Running issues
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From: MD
Running issues
I'm trying so hard to track down the issue I am having, but I haven't lucked out yet.
I have a 351M/400 in a '78 F150 2WD. The engine itself runs good with plenty of compression, but between stone cold and warming up, it's impossible to stay running, let alone driving it when warming up.
If I start it up with the fast idle on, I can put it in drive and the vacuum will drop to 15 Hg from 21 Hg and drive it. But let it warm up a little before putting it in gear, it will die no matter what.
I'm not sure if I still have a vacuum leak - I've replaced/diagnosed/disconnected almost everything that requires vacuum to no avail.
If I fight with till it's completely warmed up - at operating temperature - it will drive fine, but it'd idle slightly rough with low vacuum readings - between 10 Hg to 15 Hg.
What gives?
I have a 351M/400 in a '78 F150 2WD. The engine itself runs good with plenty of compression, but between stone cold and warming up, it's impossible to stay running, let alone driving it when warming up.
If I start it up with the fast idle on, I can put it in drive and the vacuum will drop to 15 Hg from 21 Hg and drive it. But let it warm up a little before putting it in gear, it will die no matter what.
I'm not sure if I still have a vacuum leak - I've replaced/diagnosed/disconnected almost everything that requires vacuum to no avail.
If I fight with till it's completely warmed up - at operating temperature - it will drive fine, but it'd idle slightly rough with low vacuum readings - between 10 Hg to 15 Hg.
What gives?
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From: MD
New spark plugs, not even 10 miles on them yet. The old ones I took out were tan/black (rebuilt carb then replaced plugs). I think I did pull one recently, but i'll do it again.
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From: MD
Around 200. That's the operating temperature.
It has good oil pressure for a 351M/400. I haven't hooked a mechanical oil gauge to it yet (need to buy some copper tube), but the factory gauge nearly pegs at cold, and drops a little when warm. It's never went below middle of the gauge.
It has good oil pressure for a 351M/400. I haven't hooked a mechanical oil gauge to it yet (need to buy some copper tube), but the factory gauge nearly pegs at cold, and drops a little when warm. It's never went below middle of the gauge.
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From: MD
Heat expands, so I think it'd seal instead when it's warming up...
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If theres low vacuum when warm, you are either loosing compression somehow or leaking vacuum, since temps are fine and oils fine, ... it could also be electrical, check your ignition module, dizzy cap, hot wires, have you checked you plugs?
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From: MD
Yep. Same as before. Tan.
I did a compression test when the engine was warm, and I got 140-155 on all cylinders. It runs smooth at 1500 RPM. But at 700 RPM, it's rough. At warm idle, vacuum is at 19-20 Hg. It drives fine with plenty of power. Only when it's having an issue is when it's not fully warmed up where it's impossible to stay running when in gear.
I've fixed/disconnected every vacuum leak/source I found. It has greatly improved since then, I still suspect it's not completely cured of it. I took an unlit propane torch to the manifold and the carburetor mount. No change. I don't know where else to look.
I did a compression test when the engine was warm, and I got 140-155 on all cylinders. It runs smooth at 1500 RPM. But at 700 RPM, it's rough. At warm idle, vacuum is at 19-20 Hg. It drives fine with plenty of power. Only when it's having an issue is when it's not fully warmed up where it's impossible to stay running when in gear.
I've fixed/disconnected every vacuum leak/source I found. It has greatly improved since then, I still suspect it's not completely cured of it. I took an unlit propane torch to the manifold and the carburetor mount. No change. I don't know where else to look.
