1998 V70 R - Low Intake Manifold Vacuum

Post by **scoobycant** » 26 Feb 2011, 08:27

194,000 mi on odometer. Starts, idles, and drives nicely. No check engine light. But I'm measuring a steady 14 inHg vacuum at idle with warm engine. Cylinder compression #1-5 are 178-180-175-178-180. New vacuum elbows at passenger side intake manifold, SAS valve, SAS solenoid. Tried propane to check for leak - found none. Any way to check for exhaust restriction? Thanks in advance.

Post by **JRL** » 26 Feb 2011, 09:07

Has the COMPLETE breather system been replaced?
How many miles on the Cat Converter?

Post by **scoobycant** » 26 Feb 2011, 10:12

COMPLETE breather system replaced with new factory parts less than 3,000 miles ago. I measure 2.5 inWC vacuum at dipstick.
Catalytic converter, pipes, mufflers are original.

Post by **JDS60R** » 26 Feb 2011, 10:19

You can drill the exhaust pipe before and after the cat and test for pressure. I got tired of drilling so I mocked up a metal plug that threads into the oxygen sensor bungs.

An easy check is to create and exhaust leak after the turbo. You can loosen the pipe or just remove the O2 sensor while checking intake vacuum. This should be done with extreme precaution towards fire being ejected through the O2 hole. Be prepared.

You might want to go under the car and smack the cat converter and muffler - only when cold to check for rattles and broken baffles. Many folks do not change the 02 sensor often enough and a damaged cat or bad muffler baffle may exist.

Post by **scoobycant** » 26 Feb 2011, 10:36

Thanks for the info. I'll do the smack test first. Then I'll drill the holes. What is the acceptable/bad differential pressure across the catalytic converter?

Post by **JDS60R** » 26 Feb 2011, 11:01

Measurement in front of the cat shows cat building pressure. measurement after shows muffler building pressure. A good performance exhaust should be far under 1 psi.

Post by **jimmy57** » 26 Feb 2011, 12:29

14 inches vaccum is not really low for that car. The turbo is loading the engine a bit at idle and if you measure pressure on the turbo control valve hose from outlet of turbo you'll find .25 psi +/-
High compression engines with lower idles speed and no turbo will go 20-22.
The S40/V40 4 cylinders are hard pressed to get 10 inches vac.

Retired MVS Contributor · Post by **Retired MVS Contributor** » 26 Feb 2011, 13:29

Low vac in turbos is the reason why heater controls are not vac, but electric...

Post by **Glencoeteen** » 26 Feb 2011, 14:46

hmmm still turbo or not only 10lbs at idle on a turbo doesn't sound right, I am thinking you have a pinched/leaking Vacuum line to your Gauge, my 98 T5 has 210k and @ Idle is at 20lbs then shoots up as soon as i accelerate. and i just installed it about a month ago,

Post by **jimmy57** » 26 Feb 2011, 19:27

I would not worry if there are no symptoms. Until I knew how accurate my vac gauge is.