Engine Vacuum Drops

[bfarroo]

Here's an odd one. I start up my 87 Shelby Z and the vacuum will go to 20inches or so. after about 15-20 seconds it will instantly drop to 15. This is on a aftermarket gauge and doesn't coincide with engine RPM. I've looked over the vacuum harness and haven't found anything out of the ordinary. Anyone have any ideas? I'm going to hook up the scanner and see if that shows the same at the map sensor. I may just rip out all of the vacuum lines and start over.

[cordes]

Could it be the AIS? Have you checked the timing? When I start up my cars they idle a little higher and then it drops down. I have had a car skip a tooth on the timing belt and pull normal vac if I would rev it up a little, but drop to 15" at 1K RPMs when warm.

[bfarroo]

Doesn't do it with RPM. The motor will rev like it normally does, drop down to Idle RPM and about 10 sec later or so the vacuum will drop. I've gone over the timing in the past but I may check it over again this weekend. the sudden drop is what confuses me. After it does drop reving the motor will increase the vacuum like it would in any motor but is drops right back to 15 where it was instead of at 20.

[cordes]

Doesn't do it with RPM. The motor will rev like it normally does, drop down to Idle RPM and about 10 sec later or so the vacuum will drop. I've gone over the timing in the past but I may check it over again this weekend. the sudden drop is what confuses me. After it does drop reving the motor will increase the vacuum like it would in any motor but is drops right back to 15 where it was instead of at 20.

That's pretty wild. Perhaps there is a loose flap of gasket on the TB or something? That is a weird one.

[puppet]

How's the oil pressure Benji? I ask because usually in a semi-worn engine you'll see lower vac readings on cold start up .. oil pressure builds and/or gets to the head and vac readings will recover and hold. Your seeing the opposite. Worn valve guides? Marginal piston rings?

[bfarroo]

It's a stock TII motor in my 87 Z. Fairly low miles, Oil pressure is good. I'm going to hook a gauge directly to the manifold to verify it. The engine has the typical TD sounds, no loud knocks but does have a "normal" tick. I'm wondering if a check valve or something could be giving me a false reading.

[turbovanman²]

Does it run ok, if so, I wouldn't worry about it too much.

[BadAssPerformance]

My T2 shadow idles at 18" until it loads up on excessive amounts of race gas then idles, er chugs at 15" or less :o