bucking at hwy speed

[photo_van]

I've been having this issue for years, intermittent though it seems to be worsening. I used to think it was an engine mis-fire, but check engine light has never come on (though I haven't checked codes).

What I have noticed is that it only occurs at hwy speeds under load just prior to downshift to 3rd it will start bucking intermittently. If I step on the gas it downshifts and all is well. Likewise if I manually shift down everything is fine. Sometimes I'll drive in third just to get rid of it. Doesn't always happen, but enough to be quite annoying.

Could this be a trans issue? If so any repair ideas?

Thanks

John

98 AWD, 31x10.50s

[doyoulikeithere]

Is your speedo out by 10% or so?
My dumb guess would be to re-tune your computer to know what size tires and gears its turning.
If it thinks its turning 205's at 100KMH at 1800 RPM, but its 'actually' turning 31's at 1800 rpm's its gonna have troubles.
If your computer still thinks its turning those weeny stock 205-75's, it probably doesnt realize that its having troubles at lower RPMs with your computer controlled auto tranny.
Being an OBDII computer, it is easier to re-tune than the older OBDI.
Maybe Look into doing it? It might just fix it.

[photo_van]

Speedo is out by exactly 10%.
I don't know a thing about re-tuning the OBDII, but I bet it's on this forum!

Thanks for the insight. Maybe it's time to finally get those 4:10 gears ($$$$$).

John

[doyoulikeithere]

Speedo is out by exactly 10%.
I don't know a thing about re-tuning the OBDII, but I bet it's on this forum!

Thanks for the insight. Maybe it's time to finally get those 4:10 gears ($$$$$).

John

The gears sound great! What gears are you running now?

My money says it will be a lot cheaper to get someone to tune your computer to correct your speedo.

If you do put in other gears, you probably should still tune the computer to know what its dealing with, and correct the speedo by doing so.
If your paying someone to do it, he can probably also tune it for towing or performance etc. Research it a bit. Can be done with a just laptop and a cord I think...

If I was experiencing that symptom in my old OBDI - 5 speed standard van, I would hear the engine lugging, feel it start to buck a bit, and I would have to shift down.
Those big tires can make that happen at a completely 'new' rpm as far as the computer is concerned, and when the computer compensates, it does so believing it has to turn a weeny little tire.
Its probably happening right at the speed limit , annoying you every day?

I just bought a chip from harrisstbi.com, but those are for OBDI. Anyways, good luck, let us know what you find out, k?

[MountainManJoe]

Can you explain that a bit further, doyoulikeithere?

I understand that bigger tires will shift the engine to wheel ratio.
For example, going 100km/h, the engine will be turning 2,800 rpm instead of 3,000 (not accurate but you get the idea). But why would the drivetrain care what speed the vehicle is travelling? 50 or 100 km/h is all the same to it.

[photo_van]

I don't recall what gears off hand (I seem to remember of the 3 stock ratios it was the middle; 3:50 something?)

In any case, it happens on long gradual uphills (or stiff headwind), not daily, more on longer road trips when the vehicle is good and hot.

[doyoulikeithere]

Probably 3.42:1 ...

Well TB.
Personally, I like the older vans that arent so computer dependent, have window cranks, manual door locks, and a clutch!
But.....
Speed isnt actually the issue, its the amount power required to turn the wheels at those lower rpms in top gear.

Turning the wheels at 1500-1800 rpms in OD is a heck of a lot tougher if the wheels are actually 31s, not the 205s that the computer is programmed to account for.
The computer actually doesnt know the diff in tire size, so has trouble turning the bigger tires in top gear at lowest rpms.
It bucks because it should be able to turn those little tires at that rpm,l but its just too much with the bigger tires.

Re-tuning can not only correct the speedo, but can adjust the shift points to a more useable rpm range.

He said if its bucking, he just needs to give gas, it shifts down and goes fine in lower gear. or, If he is going a bit faster (higher RPMs), top gear no longer bucks.
To me, Sounds like the shift points need to be adjusted, and the speedo corrected. Then the computer will give a smoother ride, especially at lowest rpms in top gear.

Just seems like common sense to me...

[MountainManJoe]

Personally, I like the older vans that arent so computer dependent, have window cranks, manual door locks, and a clutch!

I saw the perfect car for you ...

It bucks because it should be able to turn those little tires at that rpm,l but its just too much with the bigger tires.

So wouldn't a van with 205's pulling a boat buck too then?

photo_van, what is your axle ratio? I run 30's with a 3.73 axle and no bucking.

[photo_van]

[quote- photo_van, what is your axle ratio? I run 30's with a 3.73 axle and no bucking.[/quote]

I was trying to remember that, not the 3:73 (that was the lowest stock gearing right?) Mine was the next one up; 3:50 something?? Mine did it when I had 30's as well.

[doyoulikeithere]

So wouldn't a van with 205's pulling a boat buck too then?

No, because it doesnt require as much torque to turn the smaller tires no matter how much the load.
It would do it, if say, you were driving a 5 speed van, going too slow (low rpms) in 5th gear no matter what tires, but mattering how much power needed to turn the wheel in top gear at low rpms.
Hey no sense in being nasty there ! I did start my reply with "my dumb guess is...." But I'm not a caveman. My van has rubber tires .

I simply think that it sounds like it happens at really low rpms in top gear only, because the engine is lugging a bit and should downshift itself just a tad sooner.
Sounds like that happens right at cruising speed, annoying...
I'd say, Get a proper tune up and look into getting the chip tuned.
A proper tune up on those newfangeled techno droid OBDII units includes scanning for codes etc anyways.

[silver mist AWD]

I wouldn't rule out a fuel delivery problem, especially if it has become progressively worse over time.

Could it be a fuel filter or pump?

[6spd_monte]

I simply think that it sounds like it happens at really low rpms in top gear only, because the engine is lugging a bit and should downshift itself just a tad sooner.

I wonder if it has something to do the the torque converter clutch? Maybe not unlocking when it should?

[mdmead]

My van suffered from this for a couple/few years before I took it off the road. My van was running stock tires/gears so you can rule that out.

I brought the issue up on here way back when and a few others also experienced the same condition. But at the time, nobody had come up with a cause nor found a cure.

The fact that my fuel spider was old, I thought that could be a potential cause. Also maybe a vacuum issue do to old/worn parts. I think I ruled out spark, at least as far as plugs/wires/cap, because I believe I had the same condition both before and after changing them.

[photo_van]

For a long time I was thinking it was engine related, but because of how and when it manifests itself, I'm starting to think trans. Also, if any one or more cylinders is mis-firing, wouldn't it throw a code?

[photo_van]

[quote=
I wonder if it has something to do the the torque converter clutch? Maybe not unlocking when it should?[/quote]

Anyway of checking that??