Please help! bucking at highway speeds

Dagobah

Dagobah

New member
I've brought the truck to my mechanic two different times now, and we still cant figure out whats causing this.
At highway speeds (50 or higher) the truck "bucks" and "skips" randomly under engine load. It doesn't happen every turn of the crank consistently, so my first thought was "oh, fouled plug", but my mechanic changed all the plugs and wires less than a year ago. After I brought it to him the first time (two weeks ago), he found a large crack in the intake pipe near the throttle body, and was convinced it was an air issue after that. He patched the hole, but I noticed immediately on the way home that the problem hadn't gone away.
Took it back to him, and he had it for almost a week, before calling me and telling me that he couldn't get the truck to display the symptoms I was describing, but that in looking around he noticed it was definitely time to replace the pcv valve which he went ahead and did. He said he would think a bad pcv valve would have more effect at idle than it would at highway speeds, but after taking it out on the highway, I couldn't get it to buck again so I figured that had to be it, and took it home.
Yesterday, I drove around all day on my normal rounds, and was glad by the end of the work day when I hadn't felt a single jerk or loss of power. Then I go to drive to my dads in the evening and it starts back up again!
This makes me think that it will only happen after being driven a while, which would explain why the mechanic couldn't get it to act up for him.
The only thing I can think to do at this point is to drive it around until it starts bucking again, and then drive up to the mechanic again, and have him ride with me while I point out the symptoms.

now to the question: has anyone else had this problem? Does anyone else know whats going on here?

'96 with 239xxx miles
Please help! Thanks in advance guys.
 
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Perhaps a coil pack on its way out? But not sure it would have this affect intermittently. I assume its not throwing any codes?
 
The symptoms you describe sound just like what I had been experiencing..."The bucking at highway speeds". It lasts only a few seconds, and resides when I let off the accelerator.

I did recently replace the fuel filter and have not had a re-occurrence of this problem.

However, I've only had my truck a few months, and it is not a daily driver. I've put about 300 miles on the truck since the filter change, so I'm not sure if this is enough to be convinced the issue has been resolved. I've put about 2000 miles total on the truck since purchase, and I would say it has occurred about 4-5 times in the time I have owned the truck.

How often does this occur for you?

Definitely interested to hear from others if they have had experience with this issue.

I have not changed plugs/wires, O2 sensor, or cleaned MAF. These were my next items to check if the problem returned. Other than this intermittent issue, the truck runs flawlessly.

Anybody else have any other suggestions?

BTW - my truck is going to flip 200K tomorrow when I drive it to Barrett Jackson. I was saving it for a special occassion :)
 
Ordinarily you'd get misfire codes, but since you're not getting any, I believe you can check for continuity, however someone more knowledgeable will have to chime in.
 
I had that same problem but I had a CEL pop on...

Turned out to be a compression issue.

Having a valve job done as we speak... will keep you posted :)
 
Your truck is missing on one or more cylinders. It often won't throw a code until the miss gets to the point where it happens more often than not. I know because I "chased my tail" for years on mine. The mechanic should have figured out that that it's a miss right away, much less after several weeks. Why is it missing? Here are the possible causes from cheapest to most expensive:

1) Bad plugs or wires (especially if you Have Bosch brand - ditch them and get OEM NGK or Denso) - I doubt that's what it is, but I would recommend running OEM plugs and wires regardless.
2) Bad coil pack. You can test the resistance with a volt meter to see if it's in spec, but you could also wait until you get a CEL (which you will at some point). Once you know what cylinder(s) is missing, you can swap a coil pack with another one. If the miss moves to a new cyl, it's the coil. If it stays with the same cyl, you have a problem with that particular cyl.
3) Clogged fuel filter.
4) Bad fuel injector. You can also test the resistance on the injectors and sometimes you can hear that it's not clicking (if you have a stethescope). Harbor Freight sells mechanics steths for really cheap. Sometimes an injector can get partially clogged, but it will test and sound normal.
5) Bad head gasket. Check for other symptoms like water in the oil, blowing white smoke out the tail pipe, excessive water out the tailpipe. Lots of bubbles coming to the top with the radiator cap off, loss of coolant with no leaks, and of course missing.
6) Burnt misadjusted/valve
7) Cracked head. Same symptoms as a head gasket, but you won't know until you remove the head.

After a couple years of dealing with this on mine and ruling everything cheap out, it just kept getting worse. Compression tested good in all cyls, leak down test didn't show anything either. Finally just had the engine rebuilt and found both my heads were cracked. The cracks were small enough to cause an intermittent miss at first, but in the end it was a constant miss.

Good luck, hope it's just a coil pack. Often it is.
 
Son of a *****!

Well, it's not the fuel filter on mine...:)

It happened over the weekend.

Brian, thanks for the other recommendations. I'll keep checking into this also.
 
lifted up said:
Son of a *****!

Well, it's not the fuel filter on mine...:)

It happened over the weekend.

Brian, thanks for the other recommendations. I'll keep checking into this also.
Click to expand...

Haha, yeah lifted up, when I mentioned your fuel filter fix to my mechanic, he looked at me like I just told him the earth really was flat after all...

But we'll get this thing figured out. How much you wanna bet were both gonna have to replace the exact same thing lol.
 
I had a similar issue that got worse as I let it go, but it ended up just needing plugs, so I did plugs and wires all Denso. Running great again. I have been told that there are several possibilities, such as the Plugs, Wires, O2 Sensor, and Crank Positions Sensor. I started with plugs because there was no code being thrown.
 
Dagobah said:
Haha, yeah lifted up, when I mentioned your fuel filter fix to my mechanic, he looked at me like I just told him the earth really was flat after all...

But we'll get this thing figured out. How much you wanna bet were both gonna have to replace the exact same thing lol.
Click to expand...

I bet you are right!...:)

How often does this occur on your truck?

It just seems crazy that there is no CEL, and it happens whether the engine is warm or cold. And, it occurs very irregularly.
 
Not so irregular for me, it happens every time I drive it. And I also figured out that it happens more often the more the truck is driven. It happens not at all upon cold start, and then by the end of the day it happens at lower speeds as well as highway speeds. I also think it is only happening at lower RPMs, it seems to run smoother at higher RPMs.
 
Dagobah said:
Not so irregular for me, it happens every time I drive it. And I also figured out that it happens more often the more the truck is driven. It happens not at all upon cold start, and then by the end of the day it happens at lower speeds as well as highway speeds. I also think it is only happening at lower RPMs, it seems to run smoother at higher RPMs.
Click to expand...

Did you ever find a resolution to this? mine is doing the exact same thing at the moment any help would be appreciated.
 
Oneluv13T4R said:
Did you ever find a resolution to this? mine is doing the exact same thing at the moment any help would be appreciated.
Click to expand...

Yes, it was a non-factory coil pack that went bad. Since these engines' coil packs each provide spark for TWO cylinders, when one goes out, you have TWO cylinders missfiring, which, as you can imagine provides for one hell of a buck when it happens.

EDIT: I swapped in a Toyota pack from a junkyard 4Runner, and she ran like cake for the rest of the time I had her.
 
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Dagobah said:
Yes, it was a non-factory coil pack that went bad. Since these engines' coil packs each provide spark for TWO cylinders, when one goes out, you have TWO cylinders missfiring, which, as you can imagine provides for one hell of a buck when it happens.

EDIT: I swapped in a Toyota pack from a junkyard 4Runner, and she ran like cake for the rest of the time I had her.
Click to expand...

Thanks brotha,
this has been so annoying and everyone has an opinion and soon as they say "I'm positive" needless to say it becomes costly and I seem to develop holes in my pockets.
Quick question how did you come to find out it was aftermarket coil packs went bad? And of course with that being said how would I go about checking mine?
Sure does seem to make sense and jive with the hesitation and jerking I get. Especially when I hit the gas as if I were to pass someone.
 
mine recently started doing the same. I will be installing new fuel filter, spark plugs and wires as general maintenance. Hopefully this resolves my issue. No CEL for me either, if i'm easy on the throttle it does fine, if i stab it it will misfire.
 
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