Vacuum dilemma

[Knuckledragger]

I have recently had an odd feeling about my vacuum system and would like some help pin pointing the source of the problem.

On startup, the power brakes have one pump then lose vacuum for assist, then get back after a couple of pumps of the pedal. Cold start driving, the transmission waits to shift way too long. After the truck warms up a bit, all of this goes away and it performs as expected.

Perhaps connected, perhaps not: when cold, the C6 takes up to a minute to shift into reverse from park. Not so often when warmed up and after having been driven a bit.

The question is: is my vacuum pump going out? Is there a leak in the vacuum system that somehow seals itself when warm?

I have not spent any time verifying any leaks yet, maybe the power brake booster unit has a cracked grommet or valve. The odd thing is that after getting warm, everything seems to perform as expected.

 

[IDIBRONCO]

I'd guess that you have a leak. It's possible that something's getting old and shrinks slightly when cold, expanding slightly to seal better when warm. It's not quite the same thing, but this was my experience. I got my 1981 F150 with a bad 351M in it. Since I don't care for the M series engines, I put a used 302 and a FMX (odd but it was the only small block transmission that I had at the time) in the truck. It did ok until it got cold out a few months later. The mileage was horrible at about 9MPG (with a 2v carb and 2.47 gears!). In the mornings, the transmission would shift ok at 15 degrees, but at 14, it would stick in first. I could manually shift and get it into second, but it would not go into drive for anything until the transmission got pretty warm. It would take from 10 to 15 miles of course by thee 15 mile mark, I would be at work. I fought and fought this until it got warmer out. Since the warm air only came out the vents on the dash no matter where I had the controls set, I decided that it must have a vacuum leak behind the dash. I capped off the line that goes into the cab and it was instantly better. The transmission shifts fine even at negative temps and I gained 3MPG. Not related, but to finish the story, when that 302 went down, I put in another used one that I had and I gained another 6+MPG. I thought "I'll take it!" Now with ported heads and shorty headers, it makes about 19.75MPG. Back to the original point, all I can come up with as a reason for the transmission to shift like that is the oil viscosity. When it gets colder, the transmission fluid gets thicker. I guess there wasn't enough vacuum, to overcome the thicker oil at less than 15 degrees.

 

[snicklas]

My first suspect would the the vacuum pump. Thew 92 we had, had a "lazy" vacuum pump. When it was cold it would do the same thing with the brakes (it was an E4OD). After a minute or two of running it would start to come around. Once warm, it was fine. If the ambient temperature was cold, it took longer.....

I didn't have vacuum gauge on it, but if you put your finger on the hose, you could feel it take bit really suck on your thumb. Not and immediate pull like it should.

 

[subway]

i would throw a vacuum gage on the pump only and see what it is doing.

you could then plug it into the system to see if it is acting the same or you have a leak.

somewhere there is a spec that if the pump is not hitting this many inches of vacuum it is on its way out. unfortunately i dont have that info off hand.

 

[Garbage_Mechan]

I think it is possible the two are not related. I’ve had many cases of old hard trans seals that begin to work once warmed up a bit. Will it go right into drive from park cold? If yes try going from park to drive and hold 1,000 rpm for 5 minutes then try reverse. If it goes into reverse when the oil is warmed up a bit it is likely shift drum seals. Seal conditioner might fix it for a while.

 

[sjwelds]

This thread sucks.

 

[Knuckledragger]

I think it is possible the two are not related. I’ve had many cases of old hard trans seals that begin to work once warmed up a bit. Will it go right into drive from park cold? If yes try going from park to drive and hold 1,000 rpm for 5 minutes then try reverse. If it goes into reverse when the oil is warmed up a bit it is likely shift drum seals. Seal conditioner might fix it for a while.

I may try conditioner, but this particular problem has existed since the trans was rebuilt about 40,000 miles ago. It is just getting worse with age. When I relate "cold" that only means it has not been running for at least 24 hours. In SoCal, cold is under 60 degrees and that does not happen often.

 

[BrianX128]

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Here's a dead one. A good one with a drill will be around 25-30 which is the same as in the engine at any rpm really.