TH350 Bogging when let off the gas
Thread Starter
Instructor
Joined: Jul 2010
Posts: 156
Likes: 43
From: Baltimore MD
TH350 Bogging when let off the gas
Hey all, I'm likely jumping the gun with this post since I haven't really dug into the cause myself but it's going to rain for the next 10 days and I won't be able to take the 80 out and investigate for a bit. SO, I figured I'd ask on here. I just got my car back 'on the road' after yeeeears and I'm having some trouble with the Trans I think, at least I never remembered it acting like this. I have stock 3.08 gears in it and it's cruising 65-70 at pretty high RPMs, maybe 3500-4000 as far as the trusty tach says. But aside from that, and my real concern at the moment, is that when I let my foot off the gas, it feels like I'm engine braking with a manual trans. The car immediately slows down due to the engine and you can hear it in the exhaust so I know its not brakes or something else holding it up. It's pretty significant, like I almost don't need to use the brake pedal at higher speeds.
I did some quick searching around and saw that vacuum modulators can cause issues, though I have yet to run across this specific one. Has anyone had this happen before?
I did some quick searching around and saw that vacuum modulators can cause issues, though I have yet to run across this specific one. Has anyone had this happen before?
Melting Slicks
Joined: Jun 2003
Posts: 2,802
Likes: 241
From: Niles, MI
3500-4000 rpms at 65-70 mph with 3.08 gears?! Sounds like its not shifting into 3rd gear. Check the vacuum line to the modulator. Turn modulator upside down to see if trans fluid runs out (leaky modulator). Then check modulator valve and governor valve to make sure they move freely.
Thread Starter
Instructor
Joined: Jul 2010
Posts: 156
Likes: 43
From: Baltimore MD
Well hopefully its not the converter clutch and its either the modulator or governor since those two seem like something I can check and replace in the parking lot.
Thanks for the input.
Thanks for the input.
Disabled by request 8 JUN 2025
Joined: May 2019
Posts: 329
Likes: 94
From: St. Augustine, FL
It's definitely not shifting into 3rd gear. Modulator is a good place to start. Make sure there is a vacuum line connected to the modulator. All other suggestions are on target.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
Back in the day, 1 trick to help a trans shift would be to let off the gas then shift to neutral, then back to drive.
I think it would be rare that this actually fixed anything, but it might allow you to drive it a while before you can get it fixed.
I think it would be rare that this actually fixed anything, but it might allow you to drive it a while before you can get it fixed.
Thread Starter
Instructor
Joined: Jul 2010
Posts: 156
Likes: 43
From: Baltimore MD
Last edited by big mort; Jun 18, 2020 at 08:08 AM.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
No, don't down shift. Accelerate (not wot) up to mid rpm (3000rpm), let off the gas, shift to neutral, give it some gas/rev match, so that it won't be violent if it doesn't shift, them back to drive.
Also, I have seen shifting problems solved with a simple filter change. Try the easy stuff first.
Also, I have seen shifting problems solved with a simple filter change. Try the easy stuff first.
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Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
Easy to check shifts to see what gear it gets to. Pull the shifter to 1st and start driving with light throttle. Once the car quits accelerating bump the shifter to 2nd and see if it shifts. Repeat for 3rd gear.
I wouldn't necessarily trust the stock tach. The tach in Dad's car seems to read high. 3.08 gears at approximately 60mph is just under 3k which is too low to be stuck in 2nd gear. With the 200-4R it runs at ~2300rpm in OD at 60mph without the converter locked. 700-800rpm of converter slip for both transmission with stock converters is too much when highway driving. We haven't connected the lock-up wiring to try it yet.
I wouldn't necessarily trust the stock tach. The tach in Dad's car seems to read high. 3.08 gears at approximately 60mph is just under 3k which is too low to be stuck in 2nd gear. With the 200-4R it runs at ~2300rpm in OD at 60mph without the converter locked. 700-800rpm of converter slip for both transmission with stock converters is too much when highway driving. We haven't connected the lock-up wiring to try it yet.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Your car sounds to me like it is working as it was designed. Unless the transmission has been swapped out, it does not have a lockup clutch (TCC) in it and all vehicles were "engine braking" type. It wasn't until later years when vehicles 'free wheeled' when throttle was closed at speed. If your carb has a dashpot on the throttle linkage which is intended to soften the effect of fully closing throttle at speed, it could be toast and need replacing. Otherwise, "situation normal" for that vintage of vehicle.
Race Director
Joined: Mar 2017
Posts: 11,953
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From: Virginia
If you have a digital timing light, you can confirm that your tach is functioning properly, even in the garage.
The proper way to fix amalfunctioning automatic transmission is to add a third pedal.
The proper way to fix a
Melting Slicks
Joined: Jun 2003
Posts: 2,802
Likes: 241
From: Niles, MI
Your car sounds to me like it is working as it was designed. Unless the transmission has been swapped out, it does not have a lockup clutch (TCC) in it and all vehicles were "engine braking" type. It wasn't until later years when vehicles 'free wheeled' when throttle was closed at speed. If your carb has a dashpot on the throttle linkage which is intended to soften the effect of fully closing throttle at speed, it could be toast and need replacing. Otherwise, "situation normal" for that vintage of vehicle.
I've never seen a dashpot utilized on a C3 Corvette.
The poster's transmission is not shifting into 3rd gear. This from the "gears.xls" spreadsheet.
2nd gear with 3.08 gears ar 3,750 rpm will have you at about 65 mph.
Last edited by Kid Vette; Jun 19, 2020 at 08:53 AM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If he isn't in proper range, that is definitely a problem...and there will be a lot of engine drag. I didn't catch that he was in 2nd range at highway speed....
Last edited by 7T1vette; Jun 19, 2020 at 11:50 PM.
Thread Starter
Instructor
Joined: Jul 2010
Posts: 156
Likes: 43
From: Baltimore MD
Thanks for the input all. I know I had 3rd selected on the highway. I was even popping it up one position into N and back into 3 at different revs to see if I could trick it into going into 3rd. But this week I'm going to start looking into the modulator and governor. I haven't driven this car on the highway in years but I don't remember this happening up until I had it put away last year for some engine/cooling work. I absolutely removed the vacuum line to the modulator but reinstalled it. Maybe the hoses on either side are toast and need to be replaced.