1970s THM425/400 erratic engine braking?

[TJ Hopland]

My 73 seems to have developed a new quirk.  On the highway so between 55-70 if I ease off the gas I'm feeling this erratic sort of stumble.  I'm not sure its the trans, could be the engine?

Reason I'm not thinking its the engine is I don't think the engine really knows that slight highway decell and the engine hasn't shown any signs of issues at cruise or acceleration.

Trans fluid level looks good.  Fluid is still so light color its hard to see on the stick so it doesn't seem like the trans is in self destruct mode.  Trans seems to be shifting in the normal spots and smooth so I'm not thinking a kickdown switch is stuck or a vac modulator problem.

It did pop into my head that maybe it was brakes dragging even though it seems to do it after 30 miles on the highway not touching the brakes at all.  When I got home I pulled out the heat gun and brake temps were matching side to side and seemed reasonable.

I got about 60 miles in yesterday and the only signs of an issue were that slight decel at highway speeds like when you start going down a slight hill.  It did seem to a little thing one time rolling to a stop.  It sounded like the engine was about to stall and there may have been an odd feel (or it could have been the road surface) but I feel like as the engine was really struggling and just about coming to a complete stop something just released and the engine went back to normal.

I tried to get it to do it closer to home and could not.  Not sure if its just the wrong sort of terrain or what.  I know over the last couple months I have had the sense that its not coming down to idle. Historically I have had issues with the floor mat riding up on the pedal and with the previous fuel system it was possible to get the air cleaner on in such a way that the throttle linkage would slightly rub and possibly stick.  Like a week ago I took some time to explore that and there is plenty of clearance with the current setup and no signs or hints of any sort of binding going on. 

On my test run at home I pulled out the little display unit for the EFI and monitored the TPS and a vast majority of the time its dropping all the way to zero.  Every once in a while it will be at 1.5 and one time I saw it at 3 out of 100.  I'm not sure why it sometimes does that but doing 55+ in 3rd gear I don't think 3% is going to amount to anything.  But since I was not able to get it to do it and then look at the TPS I guess I can't rule out something going on there.

Any other thoughts or things I can check or test?  Trans has been rebuilt at least one time so its not original at least.  Its been a while tho so its possible some seal is petrified and something is sticking?   Or are others thinking its engine?   Its a stutter like you would normally experience on acceleration if you were running out of gas, a fairly abrupt bup bup bup sort of thing. But the car is fine at lower speeds and accelerates fine both moderate and hard is why I'm thinking its maybe something in the trans not quite 'pressing the clutch all the way in' when it should.       

[Ralph Messina CLC 4937]

Have you checked for unwanted interference by your Cruise Control?

[TJ Hopland]

The cruise linkage was why I was checking the TPS reading.  If it was a linkage issue TPS would show the throttle being open and at highway speeds I would expect it to be significantly open. 

The linkage all seems alright and doesn't seem like it ever sticks at least just sitting in the driveway but the way either the pedal or cruise has to slack I suppose its possible it can find itself in a position it sticks things. If it was a mechanical issue like that I would not expect it to only happen on that slight downhill thing at highway speeds.  I would think it could happen pretty much any time the throttle is being moved?

I suppose I need to get my little EFI thing set up to display in large mode things like the TPS and maybe fuel command so I can glance at it and then go out and see if I can get it to do it again.  The closest place it did it to my house isn't that close and its interstate near a bunch of interchange stuff so not a great place to get stranded.  Was hoping it would do it last night on the 2 lane with the wide shoulder but it didn't seem to want to do it there. 

[tcom2027]

Good morning TJ.

TCC solenoid locking and unlocking uncommanded or erratically? Just a thought based on stalling comment. You didn't say if there was any lag or hesitation on acceleration. If so it would confirm a possible TCC issue.

tony

[TJ Hopland]

TCC clutch sticking would fit I think but problem is this being a 73 has not got one.  Acceleration is smooth and shifting is normal so that's whats throwing me.  I would think an engine issue or trans on its way out would show itself at other times especially when you are on it hard and I tried that, broke the tires loose at one point and it was strong and smooth.   

["Cadillac Kid" Greg Surfas 15364]

Stretched drive chain?
Greg Surfas

[tcom2027]

TCC clutch sticking would fit I think but problem is this being a 73 has not got one.

OK, I wasn't sure if the 73s had locking torque converters or not. Maybe take a look at carburation? Dash pot binding? Strange symptoms.

[TJ Hopland]

I did drive the route where I first noticed it today in another vehicle and it does appear to be slightly down hill so I may just have to pick a time with minimal traffic and drive there again to see if it does it again and try monitoring the EFI to see if that is doing anything odd and maybe popping it in neutral.  I wonder if its just a 3rd gear thing?  I could maybe try it in 2nd at a lower speed.

[klinebau]

The only things I can think of are a sticky PCV valve or maybe a vacuum hose starting to leak.

[TJ Hopland]

I will have to make sure I get the tach signal up on my display or wire up a tach to see if/how much the engine speed is changing when its doing it.  I'm assuming it will be because even if the trans is the problem its gonna be dragging the engine with it.

How does the engine braking over run part work in a 400?   

[The Tassie Devil(le)]

......How does the engine braking over run part work in a 400? 

Probably by just going back to idle, and the friction within the transmission parts themselves create the retardation.

I have a Predator 5000 on my '72, and when I back off the gas, the car slows a lot faster than when I had the Rochester on the intake manifold.

With the Predator, when the throttle is off, there is very little air flowing through the blades.   Low flow means that there is lo furl being delivered through the venturies, as the Predator does not have an inbuilt idle circuit.

Bruce.

[TJ Hopland]

If I come off the gas I think its OK but I will have to try and confirm that.  Hopefully I can find a spot that isn't interstate that I can do some testing.