I have a 1990 F-150 with about 100K miles. With the truck in neutral and pushing ever so slightly on the accelerator I can nearly kill the engine. If I rev it hard I don't hear any misses or backfires. Going down the road in gear the truck bucks every time I either accerate or decelerate unless I gun it. What is the best guess to what it is and where do I start to fix it. I changed the fuel filter last winter (about 4,000 miles ago) as well as the plugs and cords. also I noticed that there is black substance buildup just above the muffler that has just recently started. Is this a Vacuum problem.

Not a mechanic in New Mexico-

 

Sounds like it to me. I would suggest it may well be the EGR system if it only does it after warming up and above 50 mph.


I would check every vacuum hose on it.

With the engine running you can spray WD-40 on things if the engine speed increases with that there be the air leak..

 

thanks for replying, I did change the ERG valve first since that is what the code came up with, but it did not help as well as look for vacuum leaks initially. Anyways maybe I will continue plugging away at it until I get frustrated enough to take it to a shop.

-not a mechanic in New Mexico

 

its a 1990? is it a 302 or 351? it could be the distributor.

 

Its the 6 cylinder I don't have the book on it, but I think it is a 300 4.9L engine.

Anyway I thank everybody that responded for their input, I have suspected it could be the distributer, but wanted to get some feedback from everyone before I changed anymore parts, because someone said it could be the coil, someone else told me a vacuum leak and I thought maybe a cracked distributer. I will start with the distributer since it probably is the cheapest to replace.

 

Seems that we might have the same problem...
My 97 f150 4.6L bucks if I'm standing still with the shifter lever in Drive, as well as when I'm coasting or accellerating moderately. The only time it doesn't buck is if I step onit and let it down-shift.
I also, have replaced plugs wires fuel filter and dumped injector cleaner in the gas. Those changes haven't helped. I just took it to a parts store that check for trouble codes free. I have a P301 - cylinder 1 misfire. I suspect it's something electrical and will find out if I can test the coil pack myself. There's also a light fuel smell at start and by tail pipe...