bad stumble in overdrive

[Sovereign]

My truck has started running weird. When it shifts into overdrive, it starts to stumble like there's a missing cylinder. It feels the way you'd expect a manual transmission vehicle to feel when you've selected too high of a gear. It's fine running around just in drive. I even floored it going up a hill in drive only without issue. Still slow as dirt but runs smoothly. I was afraid it was an engine load issue and maybe the carb needs to be rebuilt since I don't drive it much and use regular gasoline with has some ethanol in it. It seems tied into the transmission though. I have adjusted the TV cable. I also recently finished installing 3.73 gears. The truck did not have this issue with the 2.73 gears. Could be just coincidence. Are there are transmission changes I should consider or needed to do when I changed to 3.73s? I'm not interested in performance with the truck. I haven't had a chance to investigate anything yet. Does anyone have any suggestions on what to check beyond the basics like plugs, plug wires, timing, etc?

The distributor appears to have been changed to a standard HEI unit. I've removed all emissions valves and solenoids. All vacuum hoses are good and unused ports capped. All of that has been done for a bit. This issue is new.

Thanks

[Accelo]

A converter clutch going out can cause these symptoms.
It's possible to isolate a engine stumble from Torque Converter lock-up issues.
While it is trailer hitching, try stepping on the brake while maintaining the same throttle with the right foot.
Just enough to turn on the brake light. See it if quits.
Cheers

[chris989]

Is there any visable smoke under load when this happens?

If you you are using a carb I highly recomend installing a AFR gauge if possible.. It takes the guess work out of one varibles when tuning.


If it is a TV cable,, make sure it is adjusted right. I had an 700r many years ago and driving cross country it was acting up on the highway.. It has been 25 years ago but i remember having to mess with the cable.

Chris

[Rich84]

I had a similar issue. Truck would stumble every time the converter went into lockup. A slight mis-fire is amplified when the converter is locked. It was one bad spark plug wire.

[Sovereign]

I appreciate the replies, everyone. I'll try to give these suggestion a shot at some point.

No smoke that I can see while this is occurring.

I adjusted the TV cable already thinking it might be causing issues with no change.

If it is converter lockup causing the issue, does that require a new converter or is it a wiring issue? It's been a long time since I've dealt with an auto transmission.

Thanks

[cadillac_al]

What speed and rpm is this happening at? I had a vehicle that would go into overdrive at 35 mph and it would buck and kick until it got to 40 mph and the bucking went away. I always wanted to re-flash it to go into OD at 40 mph but never did.

[Accelo]

If it is converter lockup causing the issue, does that require a new converter or is it a wiring issue?
Identify the issue first. Like Rich84 mentioned, it could be many different causes which includes a worn valve in the transmission.
Track the issue by eliminating one thing at a time. Interesting it manifested it's self after a gear change.
Typically moving to a higher (numerically) gear would make this better not worse.
A scan tool really helps, even the cheesiest will have indicators for lock-up and not locked up. The year of the truck will help.

[Sovereign]

The truck is an 86. Where would I plug a scan tool in on this truck? There's no ALDL port.

The bucking was happening from 35-50mph. Haven't tried going faster.

Not sure the gear change has anything to do with the issue. Could be but I don't know for sure. I agree it should be better if it's a load issue, but the issue wasn't present with the 2.73 gears.

[Just call me Sean]

You can unplug the square electrical connector from the side of the transmission to keep the torque converter from locking up.

[Sovereign]

Checked the plugs and wires yesterday. No damage to any of the wires and they were all connected well. Firing order is correct. Gap corrected if necessary though none were "bad". Distributor is newer and a non-ESC HEI unit. Basically, I'm confident there is not a miss occurring. Hopefully, I can get out this weekend and drive the truck to test some more.

Thanks!

[Sovereign]

It appears to be a lock up issue. I was able to get the truck out today for a quick drive. I waited until it started stumbling and then pressed the brake pedal with my left foot. Sure enough, it went away instantly. I did this repeatedly and through different speeds and throttle inputs just to make sure I wasn't imagining anything.

So is my torque converter bad or is there another part I could inspect?

If my memory serves me well, the transmission has gears 1 through 4 and lock up after that, correct? I could unplug the connector on the transmission and just not utilize lock up then? I don't drive it far and certainly don't tow. I doubt the fluid temperature would become an issue.

Thanks for the help.

[Just call me Sean]

I ran mine for years without lock up until it quit.

[v30crewcab]

can you brake/stall it and make it miss? I bet you got a plug going bad. seen alot of them work fine till they were under load in od. changed alot of plugs at the dealer.