Bought a 89 RS someone gave up on trying to get to idle right. Idle was all over the place and when you dropped it into gear it wanted to die. Tried a few things myself that I wont go into. Long story short when I unplug this thing my idle issue disappears (check engine light comes on but idle issue leaves)---plug it back in and problem returns. Has the electrical connection and then a vacuum line from it to the lower back of the TBI unit. Whats it called and why is it causing the issue. Thank in advance

 

I think it's the map sensor... I'll double check..

 

looks like it's the MAP sensor...

AC Delco P/N: 2131545

Double check that it indeed is the correct P/N... take yours off and cross reference your P/N with the current P/N and check to see with your P/N falls under the New Part Number Cross Reference List.... all can be found at RockAuto.com....

 

It is indeed a MAP sensor--After I posted here last night I surfed rockauto and found it.

Went down and bought a new one today. However the new one didnt fix the issue. If you leave it unplugged you get a check engine light and car runs fine-Idles around 1200 cold-You drop it into gear and RPMs drop to 750 just fine.

If you plug it in the idle is all over the place and when you drop it into gear RPM drops to 200 or so and wants to die. Sometimes it catches itself and sometimes it doesnt.

IAC was replaced by PO. It has new plugs wires and distributor. Has new alternator. Shows no codes. New TBI mounting gasket. New PCV with hose. TPS volts test good when moved by hand--havent gotten my ALDL cable to hook it up and watch everything yet.

 

the barometric pressure sensor looks the same

 

Possibly a vacuum leak as well. Grab a can of carb cleaner, and spray it around the vacuum lines, and gasket areas, see if engine speed changes.

 

That was were I was leaning until I found the MAP changed it. I replaced the PCV hose--replaced the TBI mounting gasket and a few other vacuum lines. Will keep looking at them.

When I get home this morning I'm going to measure the vacuum at the MAP. You can feel it has vacuum but I've not taken a reading. Anyone know what it should read???

 

Should be the same as full manifold vacuum, since that's what it measures. On a stock TBI I would think over 14-15" of vacuum at idle.

 

With the key on/engine not running I get 4.85 volts at the MAP

At the MAP end of the vacuum hose I get a steady 20inHg at startup thru about 10 minutes of it running. Is that too high then????