So about 2 months ago I had some severe bucking after about 45 mins of highway driving. Towed it to a local mechanic and he couldn't find anything wrong or get it to recreate the problem. Since, I added dry gas, replaced the cap and plugs, replaced the muffler. Trucks been running great until tonight. It was the first time I've been on the highway for more than 15 mins and about 45 minutes in, it started to do it again. Got worse the longer I drove.

Basically, it starts misfiring/bucking and when I let off the gas, it goes back to normal. No symtpons at idle and less frequent at slower speeds. The warm engine does seem to be a factor because when I got it home and let it cool, I went for a drive and it didn't do it again. Also, it seemed to happen more drastically when I hit inclines.

I thought it could be the carb or a vacuum line but it's not consistent. Also thought maybe a fuse or a belt. The heat/45 min highway driving thing though seems peculiar though...

Any ideas???

 

Clogged filters?

Gives intermittent issues with fuel supply, lower fuel supply gives more chance to vaporize, add in the heat buildup of long high speed cruise?

 

Check the EGR valve and TVV (coolant sensor that controls the EGR).

 

Don't know if this relates but I also have a small backfire regularly when I let off the gas... Could the two problems be related?

 

I had a clogged fuel filter from fuel tank sludge. The pickup sock in the tank was clogged too. I would try something like Berrymans B-12 (worked for me) or other solvent in the fuel tank and see if it keeps happening. If that helps, try a new fuel filter and possibly check out the inside of your tank.

 

Catalytic converter?

 

The Aisin Carb does not like any vacuum leaks at all, I would check for leaks from the hoses to the base plate.

 

I swapped out the carb this weekend and replaced all the vacuum lines. Had some problems with tuning the carb but after I got that worked out- she is running great. Idle is much smoother and there is absolutely no backfires.

I took her on the highway again to see if it would start bucking like before. After about 30 mins or highway driving- BAM- started bucking just like before. So to recap- I've done the plugs, wires, cap, carb, vac lines, dry gas... what else could it be? fuel filter? fuel pump?

 

Alright guys, I replaced the fuel pump, pcv, and fuel filter. Still bucking when I'm full out on the highway- errrr... going to try the EGR valve and TVV as suggested. Any other thoughts?

 

I first, like mentioned above would have thought fuel. Any codes?

 

No codes. I'm also going to try the purge can before removing the egr... wondering if it is clogged. My tank seems pretty pressurized so I'm wondering if its an issue with ventilation.

It only happens under 2 conditions 1) I'm riding high rpms getting onto a highway in like 4th or 5th and I keep my foot on the gas. It will start to buck/miss/surge slightly but when I let off on the gas it'lll stop and as long as I dont ride it for a while on high RPM, it is fine. 2) After about 45mins of highway driving, it starts happening slowly and then more and more over time. If I shut off the engine and let is rest for a bit- it'll buy me another 45 mins or so. Quite strange.

 

I'm updating this b/c I fixed the problem and have seen many of these threads without a definitive answer. The problem was a combo of 1) clogged charcoal can 2) aftermarket gas cap without vent. Basically, I was getting vapor lock after a while of driving and so no fuel was getting out of the tank.