krisedwards

Has anyone ever put a vacuum gauge on the intake and measured what the vacuum is at idle?

I'm trying to determine if I have a vacuum leak. I have a vacuum gauge and I hooked it up to one of the vacuum ports in the intake. The gauge measures about 15 inches of mercury while the engine is idling.

Is there a specification out there somewhere that tells what the vacuum should be at idle (or some other RPM)? I've looked through Adrian’s book as well as the tech spec book and found nothing. I've seen this called out in manuals for other types of cars, but not yet for the 964. Does anyone know?

Thanks
Kris

Geoffrey

It will be about 55kPa absolute.

krisedwards

Thanks Geoffrey.

So my measurement of 15 in-Hg converts to 50.79 Kilopascal (relative)

Atmospheric pressure is 101.325 Kpa, so 101.325 - 50.79 = 50.55 Kpa (absolute).

I'm low by about 5 Kpa. Any thoughts on whether that's significant? Should I start searching for a hole in the intake somewhere?

Kris

springer3

No direct experience, but I think just a small difference in RPM would make that much difference. I also think a leak significant enough to change the manifold pressure would be easy to find. What are the other symptoms? What diagnostic codes are logged?

garrett376

An easy way to see how much difference a vacuum leak causes is to have you vacuum gauge hooked up, have the engine idling (giving you your 50Kpa reading) and pull off one of the flexible vacuum lines on the top of the manifold (vacuum canister for the resonance valve is the easiest) and squeeze it shut and vary its opening to vary the amount of "leakage" - see how much it changes and that will give you an idea of what sized leak to look for, or if 5Kpa is significant.

krisedwards

garrett376,

That's a good idea, I'll try it.

springer3,

The symptom is hunting at idle. I don't have a hammer so I don't know what the codes are.

I have done the following:
1. Completely disassembled the idle air control valve.
Nothing looked broken or worn
Cleaned the whole thing out
Inspected the coil, and checked resistance (8 ohms)
2. Reinstalled idle control valve, no change in hunting.

3. Adjusted air bypass valve on AFM
Backed it out until idle got better, 4 turns.

The idle is much better after adjusting the air bypass valve, but still hunts from time to time.

My O2 sensor is currently disconnected as it was causing all sorts of hesitation problems. I intend to replace it soon. Perhaps the idle will improve then, though the O2 sensor was disconnected months before the idle started to hunt.


Kris

cjoenck

Sorry for resurrecting an old post but I got curious and measured vacuum at idle (at operating temp) and got 14 in HG at the gauge. I am at 600ft and an ambient temp of about 80F so doing the math

Atmospheric pressure is 99.231 corrected for temp and elevation - 47.41 Kpa is 51.82 Kpa so I am low.

Idle is hunting just slightly. Assuming I have no vacuum leaks, what does a low vacuum reading signify?

Chris

cjoenck

Can someone do me a favor and confirm measured vacuum at the gauge for a healthy engine? Please let me know approximate temp and elevation when you take the reading.

Thanks very much.

Gus

I have a Varioram and mounted a vaccum gauge in the lines to verify that I have vaccum pressure to operate the Varioram. At idle (1000 RPM) I am pulling 18 -19 lbs of vaccum pressure- just as general info -

cjoenck

Perfect, thank you. I think I am definitely low then at 15 inHg at operating temperature and idle (880 rpm). I am looking for small leaks now.

I also noticed that the engine runs better when outside temp. is lower, ie around 20c - which is the default for the MAF when the actual signal is not plausible I believe. So I need to look at that too. Unfortunately the scan tool does not show actual intake air temp.