low vaccum while crusing

[blue83ranger]

I recently put plates back on my '86 and took the vacuum gauge out of my '88 and while I'm driving steady on flat ground at about 55 my vacuum reads about 7, with my '88 i got about 16. I think my problem is just lift, and tires but I'd thought I'd see what everyone else thinks. The truck originally was a 1986, supercab, 4x4, 2.9, a4ld, 3.73 gears and i think the tires were 215 75 15. The stuff that changed is 4 inch lift, 31 mud terrains, engine bored 30 over and it has a rebuilt C4 with a shift kit and a new modulator. The only code i have is for the egr, the engine starts, runs, and drives smooth. and I couldn't find any vacuum leaks. The engine idles at about 17-20 depending how warm it is. any thoughts?

 

[Big Jim M]

Oh my! What a great post!

I recently put plates back on my '86 and took the vacuum gauge out of my '88 and while I'm driving steady on flat ground at about 55 my vacuum reads about 7, with my '88 i got about 16. I think my problem is just lift, and tires but I'd thought I'd see what everyone else thinks. The truck originally was a 1986, supercab, 4x4, 2.9, a4ld, 3.73 gears and i think the tires were 215 75 15. The stuff that changed is 4 inch lift, 31 mud terrains, engine bored 30 over and it has a rebuilt C4 with a shift kit and a new modulator. The only code i have is for the egr, the engine starts, runs, and drives smooth. and I couldn't find any vacuum leaks. The engine idles at about 17-20 depending how warm it is. any thoughts?

Just what one might expect going from a 27" tire to a 31" tire. With only 7 inches of vac the whole system isn't workinng as designed.
One question however.. When you let off of the gas does the vac go up to 20 as it should? If not then you have a gauge problem. Might be the place you are getting your vac from.
Big JIm

 

[blue83ranger]

I took the gauge off my '88 and that vac on that is great. On the '88 if i was going 55 and let off the gas it would get very close to 25. The '86 gets to around 20 and i taped into both in the same spot.
I also just remembered, a few years ago the exhaust rusted out so i just welded some spare exhaust parts i had in, and one peice is off a escort with a 2.3, so, could to small of an exhaust do this?

 

[Big Jim M]

Too small of exhaust will do that BIG TIME!
Big JIm

 

[blue83ranger]

So there's at least some hope for getting the vac better. thanks for the help, when i get around to fixing it i'll let you know how the vac is.

 

[blue83ranger]

I got my exhaust fixed with all 2 inch pipe and 2 glass packs, 1 was just to loud, and not the good kind of loud. I really tried to make dual 2 inch work but i went with single 2 inch and the vac is barely better but still not the greatest. But i guess without overdrive i can't really regear to make it easier on the engine. I'm already around the 3000rpm mark at 55 now.