Truck floors itself
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Truck floors itself
Ever since I installed the new 400m in my 77 f-150, the truck has had this hazardous problem where it throttles itself when there is a load on the engine. It only does it when the transmission is in 1st gear and just a little bit in 2nd. But usually when it shifts the pedal will suddenly come back off the floor and go to a normal idle, then I can step back into it and continue driving. When sitting at a stop light, just a little tap of the gas pedal and all hell breaks loose. I've tried putting my shoe under the gas pedal and it still manages to throttle itself heavily. I've tried switching carbs and linkage, no difference. Right now it has a motorcraft 2150 carb. The motor is slightly modified with flat top pistons, but I don't see how that could effect anything here. What I've noticed is it won't floor itself in reverse or neutral. I put the old 351m back in it and it doesn't have the problem yet it has all the same equipment on it ( carb, autotrans, EGR spacer). If anyone knows anything about this and how I could fix it, I would really appreciate some input.
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MatMan,
That almost sounds like a broken motor mount. Is there any kind of a clunk when it unfloors itself? I had a 351 Clevland in a 73 F-100 that would do that. Was a standard, so push the clutch pedal in and it would return to normal. Something to look at.
That almost sounds like a broken motor mount. Is there any kind of a clunk when it unfloors itself? I had a 351 Clevland in a 73 F-100 that would do that. Was a standard, so push the clutch pedal in and it would return to normal. Something to look at.
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I thought that too. But the motor mounts are brand-new when I put the motor in. I had someone stand on the brake with it in gear and I reached under the hood with the linkage unhooked and touched the throttle lever and it still did it. so its not the motor moving in comparison to the firewall. The motor doesn't really move much and there is no clunking or shifting of anything. It acts like there is some super manifold vacuum forcing the plates open or something. I've never heard of that though. That 400 was such an improvement I'd hate to think that I just can't use it.
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All of my vehicles are hooked up the same so I assume its correct. I had a huge 10lb spring on there and the normal size one, which make the pedal really tough, just to make it drive-able. Without the extra spring it would floor itself as soon as it started into gear.
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From: Spokane, WA
do you have some sort of low profile air cleaner on it or something? One of my buddies put an aftermarket air cleaner setup on his Chevy truck and the throttle linkage would get caught under it and it would do this sort of thing, he would have to slam the pedal with his foot to get it to come back under the cleaner.
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no, it's all stock. There's definitely nothing in the way. It will do it with the air cleaner off and the pedal unclipped. The gas pedal isn't sticking when I put it down, more like it pulls itself down when I barely touch it. The pedal will actually leave contact from my shoe and put itself to the floor until I pick up enough speed that it lets the hard torque of acceleration off. Then it comes back and everything is normal.
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From: Crestline, California
Is your truck's name Christine? ;-)
Seriously though, this sounds like perhaps some sort of vacuum problem? I can't imagine anything that would mechanically pull the linkage down to floor the truck like that. Sorry I don't have "The" answer... but good luck! I'm sure other more knowledgeable folks will chime in.
Dak
Seriously though, this sounds like perhaps some sort of vacuum problem? I can't imagine anything that would mechanically pull the linkage down to floor the truck like that. Sorry I don't have "The" answer... but good luck! I'm sure other more knowledgeable folks will chime in.
Dak
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I agree, it feels very vacuum oriented. I hope someone can help me find out how I can fix this. The motor is out and torn down right now (main bearing wore out), and there isn't any passages clogged or anything. I'm replacing the old manifold anyway so maybe that might help something. Any more thoughts on what could cause this would be very helpful though. It would really suck if I put this whole thing back together and it still does the exact same thing.
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ya, I'm stumped too. I guess when I rebuild the engine I'm gonna just hope for the best and that I changed something that will make a difference. plus the cam I'm putting in it will drop my manifold vacuum just a little so if it is a vacuum thing the problem should subside. Thanks to those who tried to help. If anyone can think of anything else that might cause this, feel free to post. Its going to be a while before I have it running again.
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Its an automatic choke. The crazy thing is, I've traded carburettors between all three of my trucks and all of them do the same thing when they are put on this motor. I found out when I traded egr plates with my 351m's that i think its trying to floor itself but just doesn't have the vacuum strength or something. the pedal just gets a little lighter when its under a load, but it doesn't go crazy and peal out like my 400 does. So bottom line: I've isolated most of the problem to the EGR plate/spacer. With a different spacer the 400 is almost safe to drive in town but it still has its moments where I have to stand on the brake and hope the car in front of me will move soon before I ram them.
I also noticed that when I put leaner main jets in it (48) it acts worse and i can't put it into gear at all without it taking off. with the rich jets (57) it only does it when i first touch the pedal, and then it lets up after i get moving around 20mph or so.
I think the 400 has some super power vacuum that rips the throttle wide open, but i know that's not physically possible. or is it?...
I also noticed that when I put leaner main jets in it (48) it acts worse and i can't put it into gear at all without it taking off. with the rich jets (57) it only does it when i first touch the pedal, and then it lets up after i get moving around 20mph or so.
I think the 400 has some super power vacuum that rips the throttle wide open, but i know that's not physically possible. or is it?...