TH350 Seems Unhappy
 

Got a '65 Suburban that a previous owner stuffed a SBC and TH350 into; it's been running great the ~2 months I've had it. Took it to work today and it was fine like always. Went out at lunch to grab some grub (first time it's been close to ~80F out and I've driven it) and as it got close to operating temp it wasn't happy. General idle at stops seemed really rough, along with coasting/decelerating. On the gas (light and hard) and seems like normal. I discovered that shifting into neutral or park solve the problem. But now I'm worried about my trip home.

Fluid levels seem fine. Coolant gauge reads 195~200, which is normal. Don't have a tach but revs seem normal when I have some (light/mashed, doesn't matter) throttle input and I'm moving. Just decelerating is a rough idle, and once stopped the transmission seems like it wants to grab. Like I said, shift into P or N and everything is dandy.

Anything I can check before I head home?

Re: TH350 Seems Unhappy
 

Rough running doesn't sound like a transmission problem. It sounds like an ignition or fuel problem.

Re: TH350 Seems Unhappy
 

That was my original suspicion, but my "see if it's correlated to anything while I'm still in the driver seat" test keeps pointing me back to the transmission. If I shift into P or N while the engine starts running rough--which happens while stopped or coming to a stop--the problems all go away. The rough running felt very similar to when you lug an engine with a manual transmission; staying in too high of a gear than what's appropriate.

I made it home and midway home the problems went away completely and the rest of the trip was fine.

My new plugs, wires, and timing light just arrived, so I'll verify that's all good this weekend. I really wouldn't be surprised to find it's actually the engine, but my "testing" just happened to mess with it a bit.

Re: TH350 Seems Unhappy
 

Check the vacuum line rubber sleeves on both ends the modulator line that goes down to the transmission. They can split and leak vacuum especially at idle.

This sounds more like an engine issue rather than a transmission issue.

Let us know how your results from your plugs and wires etc.

:chevy::chevy::chevy:

Re: TH350 Seems Unhappy
 

It’s your initial timing or better said, your lack of initial timing that is the problem.
What is your initial timing in park and also in gear?
You’ll need a helper while checking timing in gear.

Basically my train of thought is your mechanical timing is kicking in when in park.