MAP sensor for crankcase vacuum?

[jaybnve]

I'm trying to get crankcase vacuum logged on my FAST EFI setup, and figured I would just use a MAP sensor connected to the crankcase. I have it mounted on the firewall above the engine, with the hose running down to a port that opens to the valley under the manifold, to avoid as much oil contamination as possible. After installation and calibration, I fired the engine and it seemed to be working fine. However, after five minutes or so the readings start going out the window, up to 26" of vacuum. This occurs even with the bypass valve full open, and it stays at this reading for a while once the engine is shut off, before slowly dropping back down to zero over a period of 10 minutes or so.

When I pull the line from the MAP sensor it seems a little oily. Is it possible that this oil contamination is screwing up the MAP sensor? Should I be using some other kind of sensor instead for this job?

Thanks in advance for any responses - Jay

[dieselgeek]

Re: MAP sensor for crankcase vacuum?

Seems weird, I've gotten motorola map sensors all kinds of nasty and never killed one...


Have you checked the voltage going into the FAST when it's reading high? I.e., I'd be interested to know if it's really putting out the equivalent voltage and thus the sensor itself is screwed up, or if maybe the FAST is configured wrong or broken, and misinterpreting what it's being fed by the sensor... also, is the sensor a 5v sensor, all the little stuff applies - make sure the ground didn't fade away or that the Vref isn't doing something goofy...

[CDMBill]

Re: MAP sensor for crankcase vacuum?

In don't know the FAST system, but our EMS-Pro units use either MAP or TPS to drive the fuel/spark for acceleration enrichment which means we can use the MAP sensor input on the circuit board to measure something else like crankcase pressure/vacuum.

I didn't on Friday but Brian, who still has a carb on top, did and saw 66 KPA max on his under load which translates to about 9" of vacuum. It naturally jumped much higher after going through the traps and letting off the gas.

Given my recent issues with power coupled noise I'd wonder about that before any concerns about a Little oil in the MAP sensor vacuum line. I'm using a conventional mechanical boost/vacuum gauge on the cowl to monitor the crankcase vacuum and I calibrated the vacuum break by watching the gauge while going through the traps. I have mine set to a max of 12". Its kind of jury rigged but its another way of getting the setting right if the datalog MAP isn't doing what you want it to. I'm going to add a new LM-2 with additional datalogging channels to the car which I can view in the Megalog viewer all at once (so I'm told)

I wish I was going to DW to see your Galaxie, with the new cam and changes I might have given you a run for the jacket.