bucking at hwy speed

[MountainManJoe]

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?

Don't quote me on this, but I don't think it'll throw a code for every misfire.

[Cobra]

TimelessB: you're right AFAIK weak ones it won't detect ones stronger then that will set a pending code in the computer but won't light the CEL, if it continues missing or misses really badly it will set a code and light the CEL. I'm pretty sure thats how it goes

[photo_van]

I didn't realize you can get a code without the check engine light coming on. I'll put the reader on it....

[astroturf]

Miles on van? Last tune up? Last fuel filter change? Last air filter change?

Miles on transmission? Last transmission service? Last transmission filter change? After market trans cooler installed?

The failure occurs under-load, but goes away when more throttle is applied, Correct?

Thanks, Jim

[photo_van]

Miles on van? Last tune up? Last fuel filter change? Last air filter change?

Miles on transmission? Last transmission service? Last transmission filter change? After market trans cooler installed?

The failure occurs under-load, but goes away when more throttle is applied, Correct?

Thanks, Jim

I've had the van since 24,000, it now has 93,000 miles.
No service other than atf/filter changes last one 79,000. Air filter either changed or cleaned at every oil change. No other "tune up" other than fuel filter changes.
I have not installed the cooler yet (it's in my shed; I know, first thing I should have done!)
Yes, under load, in 4th (not constant but now and then; when bad it might be several times in a 15 second period). Goes away when I get annoyed and either let up, or step on the gas, or manually drop into third.

Its been doing this for years, and is either getting worse, or I'm losing patiences, or both! I don't recall it happening before going to larger tires, but that was back at 28,000 and patiences isn't the only thing I'm loosing .

Thank you
John

[doyoulikeithere]

I'm still thinking all can be well if you tune it.
the computer that is.....
http://www.tunercat.com/

[astroturf]

Fuel pressure check, and Distributor gear check.

That's where I would look first.

Keep us posted, Jim

PS You may want to pull the doghouse at night and mist it with a spray bottle of water, to make sure none of your electricals are leaking.

[photo_van]

Thanks you guys. I'll look into the programing, as I would like to correct my speedo anyway.

I've been staying out of the doghouse, but have plans for a good going over for the 100,000 mile anniversary. I'll look for fireworks and see if I can figure out how to check fuel pressure.

I know I've got a vacuum leak some where, as my heater control goes from "dash" to "floor" when I loose manifold vacuum (on the gas pedal); those controls are vacuum servos aren't they?. But thats another question for another thread.

John

[astroturf]

Thanks you guys. I'll look into the programing, as I would like to correct my speedo anyway.

I've been staying out of the doghouse, but have plans for a good going over for the 100,000 mile anniversary. I'll look for fireworks and see if I can figure out how to check fuel pressure.

I know I've got a vacuum leak some where, as my heater control goes from "dash" to "floor" when I loose manifold vacuum (on the gas pedal); those controls are vacuum servos aren't they?. But thats another question for another thread.

John

Vac Leak... Start here:

viewtopic.php?f=55&t=746&hilit=no+blower+from+vents

Jim

[Cobra]

be warned only good scanners will pick up pending DTC's i know mine will not

[photo_van]

[/quote]

Vac Leak... Start here:

viewtopic.php?f=55&t=746&hilit=no+blower+from+vents

Jim[/quote]

Thanks Astroturf, that's exactly the problem (with the heater)! One more thing to get done while I'm in the doghouse.
It's been years since I was on this forum, so long that I lost my user name and password! It was one forum back then. You guys are all great!!!

John

[photo_van]

be warned only good scanners will pick up pending DTC's i know mine will not

Ok. thanks Cobra.

John

[MountainManJoe]

These values are for a 1:1 transmission ratio (3rd gear).

I don't think he's too off-the-chart. (RPM's shown will be slightly lower in overdrive).
Too bad he doesn't have the tow/haul button.

[photo_van]

Cool chart

3.42 with 31" tires looks to be good for fuel economy. According to the chart, maybe better in 3rd gear. Perhaps I should do a test sometime. No bucking and better mileage would work for me!

[doyoulikeithere]

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.....