Thread: Why is my MAP sensor causing my car not to start?

I had originally been running an L67 MAP along with my SS M90 kit and was running very rich. Zooomer and several others recommended running the L36 MAP along with the kit... so i decided to swap back to the L36 MAP. With the L36 MAP connected, the car will crank but not start. As soon as I disconnect the MAP the car will start just fine. I reconnected the MAP after the car had been started and pulled out my Autotap. The MAP registers just fine. It read 18 inhg at idle and the vac decreased upon acceleration.

In my experience Vac should be more than 18 in on a good running motor(you may want to check for leaks)

see if the map spikes at a low vacuum range, unplug the map from the intake, cap off the intake line to the map, and see what it says on your scanner, it may just be that it "freaks out" at a certain pressure and with engine running it doesn't get that low of a reading

You reflashing the pcm for the other map sensor? The voltage scales are completely different.

Okay thanks, I'll give that a try and see what happens.

The different voltage is the reason I switched back to the L36 MAP. I am essentially running an L36 PCM with the neccessary changes for being supercharged. I figured the L67 MAP was the cause of the car running so rich before. I was able to find out this information about the different MAP sensors, but I am not quite sure how this would affect anything.

The MAP sensor on the L36 reads -14.7psig to 0psig.
The MAP sensor on the L67 read -14.7psig to +14.7psig

Both sensors output 0 to 5 volts.

So, the L36 sensor will output 1 volt for every 2.94 psi above absolute vacuum.

The L67 sensor will output 1 volt for every 5.88 psi above absolute vacuum.

Currently the MAP is connected to the vac line off the TB. Could it be that the PCM adusted itself to the L67 MAP, and the L36 MAP is throwing things off ? I left the battery disconnected for several hours before making any changes.

The MAP needs to come off the port under the snout on th LIM.

Both Zooomer and the SS M90 instructions say to connect the L36 MAP to the vac line off the TB.

The MAP sensor needs to be removed from the GT upper intake. Remove the rubber grommet on it and use a few inches of the supplied vacuum line to connect this to the vacuum line that was originally going to the factory fuel pressure regulator as shown in pic “vac lines 1”.

The FPR on the L36 is connected to the TB.

The car did start if I capped off the MAP sensor and TB line. So IDK what that means. I also happened to find that there was a slight crack in the rubber vac line piece directly off the TB. Can a vac leak cause a car not to start?

Originally Posted by 1998gpgt

Could it be that the PCM adusted itself to the L67 MAP, and the L36 MAP is throwing things off ?

No, it didn't just "adjust itself".

Originally Posted by VinnieGTP

The MAP needs to come off the port under the snout on th LIM.

Not if his PCM is still basically for an L36. It would throw off its readings, especially since that MAP isn't designed to see boost.

Originally Posted by 1998gpgt

Can a vac leak cause a car not to start?

Ask anyone who ever accidentally left that piece off when working on their car. It doesn't start.

You can't trouble shoot your car properly with other known issues. Fix the leak and we can go from there.