My 1991 Blazer S-10 w/4.3 TBI idles pretty rough especially when you first start it up. I just ran a diagnostic check using a jumper wire as shown at http://www.troublecodes.net/GM/

The service engine light flashed three times...paused then flashed three more times indicating a code 33 --- (MAP sensor signal voltage was too high (low vacuum) for 5 seconds when throttle opening was under 4%). Does this mean I should replace the MAP sensor?

 

Not necessarily. Be sure to check all the vacuum lines under the hood. My Blazer is a '95, and I had to replace all the vacuum lines 1.5 years ago - as they were all cracked, leaking, and falling apart.

G'Luck!
Rex S.

 

I already had to do that a few weeks ago. Some of the lines were actually falling apart. I disconnected the battery to clear the codes, and 33 is now the only one left.