Have tried searching archives without any firm suggestions. Truck ('78 300cid w/C6) runs great when cool and great when hot. BUT if I sit at a light for more than a couple of minutes, when I let my foot off the brake the engine bogs, the revs dump, and if I time it wrong on the gas, I can kill the engine. If I just let off the brake and don't touch the throttle, it recovers and drives off smoothly. It only does this a) in gear b) when I am holding the brake - if I drop it into neutral during the light and then shift into drive when the light turns green, no problem. Accelerator pump seems good, I replaced the plugs and adjusted the carb and timing using a vacuum gauge, and have replaced every vacuum line. I am guessing it is somehow vacuum related since it seems to occur only when I am letting off the brakes? The booster shows no signs otherwise of being bad, brakes are good and consistent, and I have not found any vacuum leaks. Any ideas what else to try? Sorry to be a dumba*** but this has me puzzled.

 

I am not by far any kind of expert, but could it be something with the torque converter? I am not 100% sure on how they exactly work, or how you can test it, just was the second thing that popped into my mind, since I can't think of anything with vacuum that would cause it. Only other thing is accelerator pump problems.

 

I doubt it's vacuum related, but if you spray WD-40 on a possible leak, your engine will speed up. I don't think that's what it is, unfortunately

 

Double check your check valve

AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - Vacuum Brake Boosters

 

Will do...AutoZone says they are a whole $6, so I'll probably just replace it and report back.

 

I am having this issue when the ambient temp reaches over 95F Mine will bog down after any idling for a longer period such as a traffic light.. it is a culprit of vapor lock in my case. Possibility you are having same issue?

 

Have had vapor lock issues on another car and this seems to be different - it's a one-time bog per long light , and there is no stuttering after the engine recovers (which it does best by itself with no help from me.) The fuel line is routed pretty well as far as I can tel - I still have a mechanical fuel pump and
the line goes more or less over the top of the engine well away from the exhaust manifold. I'll check it to be sure, though.

 

If you're not losing brake fluid to the booster - it would show in the exhaust as smoke and indicate a bad M/C and booster diaphragm - have you checked the booster swivel/connections for vacuum leaks?

 

I think what you refer to as the swivel is the check valve mentioned above. It can be bad without the booster diaphragm being bad. I once had a bad booster which was sucking brake fluid into the engine on a '68 BMW2002 I bought in the '70s. When I let up on the brakes the engine would suck brake fluid (which doesn't burn well!), stutter, blow clouds of white smoke like a mosquito fogging truck, and took about a block to clear out and resume running right. Cost me a master cylinder, booster, head gasket, and head skim to get it totally fixed.

 

Yes, swivel = booster vacuum check valve. And yessir, it can be bad without any other booster problems. I didn't read where you might have checked it. If you did, I didn't see it - mea culpa.

Ha ha - clouds of white smoke....like SeaFoam or Marvel Mystery Oil burning off. Passers by, neighbors love it! Yeow on the cost and time it took for you to fix the Beemer....

 

well, I checked the brake booster check valve this morning....seems to work just fine but having already bought the replacement I just installed the new one for kicks 'n giggles.
Just starting to put the back of the house together (post-flooding contractors finally out!!!) so I guess I will jump into this again after I move the furniture etc back.

One nice thing about street flooding - suddenly the wife's vocal anti-truck comments sure went away when only one vehicle - said truck - could navigate.

 

Funny you say that, because my dad said his BMW 2002 did the same exact thing!

 

My sympathies to your Dad. My Beemer was a grey-market car and parts were sometimes problematic, but when it ran it was a blast. My experience with German cars is that they are reliable and great till they hit a certain point, at which time EVERYTHING seems to need to be fixed or replaced. When you finish that, it is a nice car again. I got my 2002 right at the wrong time....

Update - I pulled the old check valve, checked it, and it seemed fine. As long as I had it apart, though, I replaced the part - and today had the exact same bog (actually it died at the light.) So with no change, the brake vacuum booster check valve is eliminated as the problem cause. Any ideas?

 

I know you said the booster is good with no noticeable leaks....but how about a few more easy checks for "airtight-ness?" Does seem like a vacuum related issue to some extent.

If the pedal feels "hard" while the engine is running, the booster isn't operating correctly.

Test 1

1. With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
2. Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.

Test 2

1. Run the engine a couple of minutes.
2. Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3

1. Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

If you previously have done these checks and everything is in good working order I suggest looking at the distributor. Check the cap for cracks and that it's properly seated. Look at the breaker plate and check for any wobble...as well as the shaft.


This is a poser.......

 

Certainly a more exhaustive set of tests than I normally do for a booster...and it passed with flying colors. Nice to know the booster is good! Unfortunately, the bog problem is still there even with the new bypass valve (not a shock as the old valve also tested OK.) Next step I guess is to individually plug vacuum lines to see if another vacuum component could be contributing, followed by a retune.