When I'm drivin down the freeway in my 77 f150 with a 400 2bbl, it seems to run real rough at around 55-60mph and higher. Driving thru town on surface streets doin 40-45mph it seems to run fine. When it runs rough I can feel almost the entire truck shaking it seems like. I have no idea what could be causing this. I know the engine needs to be rebuilt but I don't have the money yet to have it done. Anyone have any idea as to what might be causing this and how I might fix it temporarily until I can get it rebuilt? Thanks.

 

If the engine needs rebuilt that badly it could be low oil pressure and the lifters are bleeding off at high RPM's.

 

My guess would be a bad vibration from either driveshaft or wheels. Check the fuel filter, it may be starving for fuel at that speed too.

 

i had the same prob with my 77 f150 u need to flush the radiator and check to see if ur distributer cap has any corrosion on it and clean it.

 

"Flush the radiator?" Come on...let's not waste Mike's time. I don't see how that can have anything to do with it. Think about it.

Can you feel where the "roughness" is coming from? Does it feel like it is isolated within the engine, or is it throughout the truck? If it's the latter, I'd go with what the rest of the guys are saying. If you feel like it's isolated within the engine, I'd look at your timing with a light and look for timing chain slop, which can cause misfiring at wide open throttle sometimes. The fuel filter was a good one too - I'll throw in fuel pump as well (assuming it's just the engine).

 

Can it be replicated sitting still? Slowly go up with the rpm's, or is it only with you moving. (Any moving part that revolves around itself..brake roter/drum-drive shaft-tire/rim-fan blade will make a vibration at a certain rpm unless it's balanced.) Hang in there and keep in touch here. A lot of knowledge can be found.

 

It only does it when it's moving. Could having non-fowlers on all the plugs have anything to do with it? It also has a thermal fan clutch and after driving a while the fan does start clattering.

 

I personally don't like thermal fan clutches, but, if they go up, they will throw off the balance of things. (When the engine is warmed up, and turned off, reach in and grab a blade opposite of each other, and try and move it from front to rear(twisting). Lots of play isn't good. They have bearings in the works that when heated up, along with the thermocouple, they tighten up when connected to the waterpump to run faster. When at hy-way speeds(ram-air)it releases the grip on the shaft, and could cause vibration. Anti-fouling plugs won't have an effect that I know of. Do another road test and try to narrow it down to the front/center/rear of the truck, tire tread/front end play(slop in the steering)...Have patience my friend. We're here for you.

 

Thanks alot. I'll give it a shot tommorrow if I get the chance. I've been wanting to replace the fan clutch to a non thermal since i got the truck almost 2 years ago but just haven't had the time(or the money). I'll see if I can narrow it down a little better within the next few days.

 

All of the above suggestions are good and the same things I was thinking. I don't think your non-foulers would be a problem. I would think you would notice that at lower speeds as well.

Not sure why it was suggested to flush your radiator to stop it from running rough, but when you are doing that you could change your tires over to winter air as well . Sorry but that was a bad suggestion.

I would start with balancing your tires if your steering wheel feels like it is vibrating.

 

happened to me (351m)....i couldnt frgue it out soi took off the valve cover and saw that one of my rocker arms wasnt conected. I wasnt bent or anything, just "fell" off.

 

If you've got the gearing to try it, keep the tranny in 2nd while trying to duplicate the problem - might isolate it to 3rd gear, or eliminate the transmission as the culprit. Double-check your carrier bearing on the driveshaft and be certain that u-joints are in proper phase. If those things are all good, try rotating tires front to rear to see if problem worsens or gets slightly better - might only be one tire out of balance. If none of the above, and the front suspension components check out good, it is more than likely engine-related, however, I've never experienced clutch fans shaking "the whole vehicle", unless you were exagerating a little. They can produce a pretty good vibration, though. Possible that up at that speed, you're experiencing some engine vacuum issues which are confusing the vacuum modulator and the trans is trying to decide which gear it wants to be in. Be patient, these things can take awhile to isolate sometimes, and keep checking back with us all - lots of ideas come from here.

 

I spent alot of time checking my vacuum lines a few weeks ago and came up empty handed with it. Still think there may be something I overlooked. I noticed yesterday when I got off the freeway after driving to Tempe and back that as I was slowwing down on the exit ramp the idle was real low and and a little shaky. I'll probably check my spark plugs cuz there's alot of oil blow-by that causes at least one of them to short out oftem. But I just replaced all of them just a few weeks ago so could one already have gone bad and be causing a misfire?

 

it could have but as i said try flushing ur radiator by taking the tubes off and rince the radiator out with a hose. also u can seal the vaccum hoses with screws or advance auto has vaccum seal caps to seal them. if that doesnt work indeed turn the idle screw up a turn and a half and see what that does but if u have a misfire and a very rough shaking check the distributer cap to be sure its not loose bc if it is the rotor is hitting the cap it will cause a misfire. ur plugs just may need to be cleaned, i recommend a wire brush and just clean the engine part of the plug and the core and whin ur done see if the core isnt damaged. also u may try checking the plug gap..untill i hear from u again have a nice day...ty

 

How will flushing the radiator and turning up the idle screw help a problem @ 55mph? Could be a low float level, too much timing, partially clogged fuel filter or even a dry rotted rubber fuel line between the tank and frame causing air to be sucked into the line causing the carb to starve for gas at heavy loads.