bucking at hwy speed

[MountainManJoe]

So why isn't his tranny shifting to 3rd if the RPM drops too low?

The website didn't have the speed, but it shouldn't be too hard to figure out.
2410 RPM / 3.42 = 705 RPM at the wheels
circumference = pi x d = 3.14x31 in = 97.3 in = 2.47m per revolution
705 RPM x 2.47m = 1.74 km per minute = 105 km/h

[doyoulikeithere]

So why isn't his tranny shifting to 3rd if the RPM drops too low?

It does;
it just does it a wee bit too late (about 10% is my guess), because it thinks it has enough power to turn those smaller tires at that rpm.
That difference is when the engine labors, and it bucks.

[photo_van]

that chart is 1:1 right. so,
With 3:42:1, 31" tires, Thats 2410, which is barely in the yello.
But the 700R4 in OD turns at .7:1, not 1:1
2410 * .7 = 1687 which is way to slow.
under 1700 RPMs in OD with load would make my van buck if it didnt kick down a gear.

Your van's computer is programmed to fuel and shift for about a 26-27" tire on 3.42:1, which is in the blue squares.
Tanks for the cool chart TB, can you tell me the speed that it must represent but doesnt tell.
I'm suspecting that your computer, re-calibrated for the correct gear/tire ratios, will shift when it should, instead of a little bit after it should.
Just my 2 cents.....

Makes sense. I don't pretend to know anything about electronic automatic transmissions, but always figured RPM and vacuum would have something to do with when they shifted down.

[astroturf]

but always figured RPM and vacuum would have something to do with when they shifted down.

How long have you had your vent vacuum problem?

Jim

[photo_van]

but always figured RPM and vacuum would have something to do with when they shifted down.

How long have you had your vent vacuum problem?

Jim

Ha, don't believe in coincidences eh?
Quite a long time, but not as long as the "bucking" issue.... I don't think

[LiftedAWDAstro]

Do the tune up...plugs, wires, cap and rotor...especially the cap and rotor and the problem will go away. There is no need to re-tune the computer for the tire size as it will not have any effect on the bucking. If you want better mileage you must swap to 4.10's due to living in the mountains. Once you do that, get a programmer and tune the tranny for gear ratio, tire size and shift points and firmness. NONE of this will matter until the tune up is done.

[photo_van]

Thanks Dean, the list is growing for my first under the doghouse experience. Plugs, cap, rotor and wires are on my 100,000 mile list.

I haven't been back here in 5 years I think, great to see you still here. We've spent many, many nights in the van since then, great rig!

Thanks again for your help and input on this fantastic site.

John

[mdmead]

I agree with Dean about doing the tune-up. I'm just not as optimisitc it will solve the surging/bucking. (Hope I'm wrong though!)

[photo_van]

I remember reading about similar issues here quite a few years ago (can't find the threads anymore), but it sounded like many people went all through their engine without remedying the situation. Then somewhere, someone mentioned an electrical issue at a under body conector (on older vans apparently) that was subject to corrosion, that caused similar problems. That led me to the trans as a possible cause.

In any case, it seams like a good idea to do the plugs cap and rotor for 100,000 mile maintenance anyway. If that fixes it great, if not I've got lots of other leads.

While I'm on the 100,000 mile maintenance; I've looked over the forum. Is there anything preventive you long time owners would highly recommend doing to a '98 that regularly finds itself alone in remote areas, many miles from help? I read the "two guys and a van" thread, good info there.

Thanks

John

[photo_van]

A minor update. I haven't done the tune up yet, but have done a few road trip experiments leaving the trans in 3rd: no bucking and maybe a slight increase in MPG.

[photo_van]

Pretty old thread but here's a follow up. I did have hidden codes; misfire #3. I did a full tune up and found when replacing the plug wires that #3 wire boot was cracked and had electrical tape wrapped around it! This was probably done by the same guy that cross threaded the #6 sparkplug Anyway, been running now for a month or two, no bucking!

[astroturf]

Thats Great News

Thanks for the follow up

Jim