Vacuum Source for MAP Sensor

[texasprd]

I've got an '88 1500 Silverado w/ 350 and TBI (w/ MAP sensor port at back of TBI unit). It appears that the vacuum port for the MAP sensor is plugged - I have 0" vacuum on vacuum gage w/ engine idling or above idle, good 5V reference to sensor, 5V coming out which drops to 0V when I remove harness connector, voltage varies as expected when I apply vacuum w/ hand pump. I seem to have inflicted this on myself yesterday when I pulled the TBI off to do some maintenance, and decided to make sure the MAP sensor vacuum port was clear - if there was any crud in there, I must have compacted it. From the back of the throttle body, it apparently goes forward internally - my question is: where in the TBI does the MAP sensor port originate? I know the PCV/evap/EGR ports originate at the front of the unit beneath the butterflies. Where does the MAP sensor port originate and actually get exposed to vacuum?

Thanks for your help!!

[2000CAYukon]

The MAP vacuum comes from below the butterflys (i.e. manifold vacuum). The EGR and PCV is also manifold vacuum but the Evap is ported vacuum (not manifold vacuum).

I would pull the TB and use compressed air to see what is blocking the port.

//2000CAYukon

[texasprd]

2000CA - Can you tell me where, under the butterflies, it begins?

I have the three ports on the front of the TBI that are horizontal (PCV, EGR/AIR, evap) - the only other thing I see that looks like it could be exposed to manifold vacuum is between the butterflies, just behind the throttle shaft (a vertical passage when the TBI is on the intake). If this is it, I may not be able to do much with compressed air because there are side-slots that will allow the air to escape. I did try going up through it w/ stiff wire, and also spraying injector cleaner - it didn't appear to do any good. Going from the outside (vac hose end) of the MAP port, I sprayed cleaner in and it didn't come out anywhere else.

[MT-2500]

Originally Posted by texasprd

2000CA - Can you tell me where, under the butterflies, it begins?

I have the three ports on the front of the TBI that are horizontal (PCV, EGR/AIR, evap) - the only other thing I see that looks like it could be exposed to manifold vacuum is between the butterflies, just behind the throttle shaft (a vertical passage when the TBI is on the intake). If this is it, I may not be able to do much with compressed air because there are side-slots that will allow the air to escape. I did try going up through it w/ stiff wire, and also spraying injector cleaner - it didn't appear to do any good. Going from the outside (vac hose end) of the MAP port, I sprayed cleaner in and it didn't come out anywhere else.

Take a small drill bit and hand screw it threw the pipe/hole.
When screwed threw you should see the end of it pop threw there.
That old carbon gets tough.
MT

[texasprd]

The external passage is cureved, so the drill-bit idea only worked for the vertical passage down into the baseplate. The most effective approach was a can of Valvoline intake-cleaner spray - the carb/choke cleaner spray was just not cutting it (literally). The Valvoline did wonders. The big lesson for me on this one was it's best to just remove the vacuum line from the map sensor and spray the cleaner in through there, rather than to try any "mechanical" cleaning method. Thanks for the responses, CA2000Yukon & MT-2500!