I put this at the end of my other (very long) post about having issues getting my 383 running, but since this may be a separate issue I wanted to start a new thread. Just a quick recap, my car is a 1985 that I swapped out the harness and ECM to the '730, I also purchased a MAP sensor that was recommended by the people I bought the harness from.

Anyway, I've had a bunch of issues getting my car running and started looking at the MAP sensor. With the key on, I used my brother's scanner to see that the MAP voltage was 4.68 and the 29 inches. I used a hand vacuum pump hooked up to the MAP sensor, as I pumped it the inches dropped to about 6 and the voltage went down to almost 0. As I slowly bled the gauge, the volts and inches went back up.

This seemed odd to me. First of all, not sure what sensor is reading but if it's engine vacuum, shouldn't it be a negative number and not positive? Anyway, not sure if any of this makes sense or if I should be checking the vacuum/voltage directly at the sensor (instead of through the ECM)?

 

I know people are going to freak out but I would have kept it a MAF system.
I'd have to look at my 92 service manual to check the specks of the map sensor

 

http://www.aa1car.com/library/map_sensors.htm

 

Another question...I know I've asked this before and was answered, either in a thread or through a PM but I just spent 30 minutes looking and can't find it. Is there a good or preferred place to get vacuum from to go to the MAP sensor? Right now, I have three ports on the very back of the upper intake: one is going to the PCV, one is going to the brake booster and the other I have teed off to go to the fuel regulator & MAP sensor. Is this okay?

 

There are 3 types of MAP sensor for GM's I've seen. 1 bar = 30" vacuum to 15 psi, 2 bar = 30" vacuum to 30 psi, 3 bar = 30 " vacuum to 45 psi. They are 3 wire sensors, the PCM/ECM sends it 5 volts (reference voltage), ground and signal return to the computer. so the voltage will be between 0 to 5 volts. The scan tool can show voltage or kPa . Vacuum readings in an engine ranges between 0" to about 26" of vacuum. The absolute pressure kPa readings in an engine ranges between 103kPa to about 9kPa . When the throttle is opened all the way the outside pressure of 14.7 psi at sea level is pushing, the inside of the engine is about 0" to 2", everything rushes in, at that point the voltage is above 4 volts. when the throttle is closed the voltage is close to 2 volts. Most Smog cars idle between 1.5 and 2.5 volts with OEM parts. The injectors are mounted so that the nozzle is inside, when the throttle is wide open the fuel pressure is up to the maximum that the fuel pressure regulator will allow, when the throttle is closed the nozzles have a suction on them and the fuel pressure should be down 5 to 8 psi lower, due to the vacuum hose attached to the fuel press regulator from the intake manifold. If your putting a vacuum pump on the map sensor and if it is a 1 bar it will be less than 1 volt near 28" of vacuum. Cars with very large cams that have high valve overlap have less vacuum, they will drive you nuts tuning because the MAP is quite different than a car with a non stock engine.

Dave

 

yes, until the regulator goes bad. if the three are too close together, the vacuum will be off a little, like when using your brakes. pcv has suction all the time, make sure your on a good source, point the map vacuum downward if possible.

 

sorry the 4 map sensor is stock 0" to 30" vacuum, too long of a day working on cars!

 

Preferred to have the MAP as closes as poss to vac source with it's own vac port for best response.
Your HSR should have 2 x 3/8" and 2 x 1/8" threaded holes at rear ; use the spare 1/8" hole for the MAP

 

I just looked at the link Vettoez posted. The chart in the middle...is opposite of the readings I get. I think. With just the key on, I get 4.7 volts or whatever. When I start the car and press on the gas, the voltage goes down. I can't remember now, but that sure seems like what happened...is this even possible? I'll go back tonight and check again & post later. I could totally be wrong on this.

Also looking at the chart, my Hg reading off of the scanner pretty much matches the Manifold Vacuum chart so I assume that's what it's reading.

As for hooking up the MAP sensor...I have no spare holes in the HSR. I have one 3/8" hole for a sensor - air temp, I believe - and one 3/8" hole for the brake booster. Then, I have one 1/8" hole for the PCV and the other 1/8" hole I have teed & going to the MAP & fuel regulator. Is there a better way to organize these?

 

Yes you do , the 3/8" hole in the underside of the plenum
Either put the IAT sensor in there ( where it is on stock TPI ) or use that hole for the PCV line ( preferred )
freeing up a port at the rear
3/8" for brake and IAT
1/8" for reg AND map

 

Right. Totally forgot about that port in the bottom. I will change these around later today.

Thanks.