Does anyone know what the vacuum should be at idle at the opti vent in the inlet elbow?

I have picked up a very intermittent DTC36 (which I think is a bad electrical connection) but want to rule out vent system and have been unable to locate a spec in the factory manual (or by search here.) I am seeing 2" vaccum at idle (which might be OK given the check valves) and would like to verify before I chase an intermittent open/short.

 

In the 17 years I've owned and worked on my LT1, and the 12+ years I've been posting here, I've ever seen a spec for the vacuum at the Opti.

 

I would guess the vacuum on the hose to the intake manifold side of the opti s/b equal to the MAP. The check valve is a one valve to prevent backfires from entering the OPTI, a small orifice fitting limits the actual air flow to a minimum. You should have a direct open (but limited flow) to the intake. 2" of Hg sounds low intake at idle s/b around low 30's MAP (9-10" Hg) for a stock setup.. I assume since you have a DTC 36 (missing high res or extra low res pulses) you are verifying the vented system air flow. Looks like opti or harness at this point. Old fashioned opti vent test is to pull vacuum line into the opti at the opti it should hold a piece of paper.

 

There's a flow restrictor in the vacuum line. You aren't going to see full intake manifold vacuum, which is typically 20"Hg at idle with a stock cam. The MAP would be 10"Hg. Vacuum = BAR - MAP

 

Thanks for your help guys. I am going to take a couple more measurements (MAP, manifold and opti vacuum) and will do all of them at normal operating temp to see if everything is in spec. Since I cleared the code, it has not come back though I have not driven it (just idling in the driveway for 30 minutes or so). I also tested the PCM voltage at the ignition connector and it was OK; maybe that connection was loose/corroded. Could also just be mad I don't drive it enough.

 

At idle and at operating temp, MAP was 30-31 kPa, manifold vacuum was 19.5 though I did find a factory test procedure for the vent system (at page 6E3-C4-10) which I failed due to a bad vacuum valve.

 

pgerst, I don't have a '95 SM, for future reference can you briefly describe how they recommend testing the line. Thanks.. Bob D.

 

Bobdec --

1) Disconnect vent system at manifold, apply hand vacuum -- vacuum should bleed off and not build. Fail: check for blockage/restriction.
2) Disconnect dist air supply at elbow and plug, apply 10" vacuum at manifold, vacuum should hold. Fail: check for vacuum leaks.
3) Remove plug from dist vent, apply 10" vacuum at dist vent with manifold disconnected, vacuum should hold. Fail: check for bad check valve
Also there is a note to test valves in both directions with vacuum, if either hold or don't hold in both directions, valve is faulty.

 

That makes sense, I keep forgetting how handy the hand vac can be..

 

Yes, makes me wonder how many optis have been diagnosed as bad sensor (DTC 36) and replaced when all that was needed was a $3 vacuum valve.