Hey everyone, I have been suspicious that I have some vacuum issues recently. There are a few symptoms of a leaky vacuum that are driving me crazy. The two biggies that I have been attributing to a poor vacuum level are that there is a good hesitation when I first begin to accelerate, and that my tranny is shifting really rough and late.

So, I bought a vacuum gauge and dropped in in the engine compartment. I hooked it up to a port on the vacuum reservoir or whatever it's called. Anyways, I was expecting to get a low reading and then try to find the leak. However, when I first started the engine, the vacuum level went down to about 17.5 in Hg. I revved the gas a little and the vacuum dropped down to 20 in Hg and stayed there. From what I have read, 20 inches is a pretty good vacuum for these guys, right?

I haven't hooked the gauge up so that I can drive around and watch it yet. But, I wanted to know a couple of things. First, what SHOULD the vacuum do when I drive. I know that it should go to zero theroretically when I go WOT. But, if I am grannying (i doubt that's a word) around town, where should the vacuum drift? Where should I expect shift points on my tranny?

Finally, anyone have any suggestions on what I should check next for late shifting tranny or hesitation at acceleration? I have adjsuted the vacuum modulator on the tranny (I have a C6). That seemed to change my shift points a little in my favor, but I feel like something bogs down when I let off the gas at like 55mph. It feels like I have a small parachute out back. I can feel something sort of kick down (the tranny doesn't shift) but I feel some kind of additional load when I am decelerating.

Anyways, any help would be greatly appreciated.

Thanks

 

a little bit of an update.

I got the gauge mounted in the cab. It's hard to see, but I can sort of look at it now. When I am idleing in Neutral, the vacuum level hangs at between 18 and 20 inches of Hg. When I put the thing in gear, the vacuum drops down to like 15. Is that normal? Does that sound like a possble vacuum leak between my manifold and my tranny? Are there places besides the vacuum hose itself that can leak on the tranny?

I also noticed that revving the engine now will cause the vacuum to go up to about 15 and then back to 20ish when I let off the gas (in netural). I can get the vacuum to 10 if I rev it pretty hard (I imagine this contines down to 0 at WOT).

Anyone know what typical vacuum levels should be in drive? Anyone know where the C6 should shift (what vacuum level)?

thanks

 

my truck does the same exact thing that you just described (1990 F-250 4X4 351W EFI)

 

If you are looking for a fool proof vacuum leak detector you need a smoke machine.

Vacuum is also proportional to load...the greater the load the less the vacuum and putting the trans in gear and sitting there with your foot on the brake puts a load on the engine and that explains the loss in vacuum....going from 20 to 15. That is why the MAP is used to tell the computer what the load is on the engine with a speed density system. While driving it should fluxuate proportional to throttle and load.

A small vacuum leak is not necessarily going to result in lesser vacuum as displayed on the guage...especially with speed density where fuel is added to match the air.

 

I was using my cruise control and going up a small hill the tranny dropped down to second creating high rpm. It blew off a rubber hose { found this out later after a whining noise as I accelerated] I replaced it and it blew off again, I put a hose clamp on it and now it runs really really rough.
Any ideas?

thanks

Michael