Hello I decided to put a vacuum gauge on my 2004 blazer with the 4.3 today. It has been running fine and no problems after replacing intake manifold gaskets. Anyway when i put it on the gauge it is reading a steady 15 in of hg which according to my gauge means that their is incorrect timing. I know that timing is not a issue since it is a electronic ignition and timing cannot be adjusted. and i would be able to tell if the timing was off. After doing so research i found that some said 15 in was normal for this engine but could not find an exact spec for it. Does any know if 15 in is normal for this engine and if not what could be causing it to be slightly low like this. Thanks

 

Restricted exhaust can also cause low vacuum. What symptoms are you experiencing?

 

none as of right now, only slightly low long term fuel trim like -7 or -8. runs fine though and exhaust is flowmaster 40 series delta flow cat back. I'm thinking that 15 in of hg is normal for this engine. when I rev the engine the gauge responds normally.

 

Negative fuel trims mean the engine is running slightly rich, and the PCM is leaning the fuel mixture to achieve 14.7:1 Generally when fuel trims exceed 10, in either direction, there is a problem, (ideal is zero). These things idle around 550 to 600 rpm and 15" is about right. The big clue is if the SES light is off while driving, things are where they should be.


It might be worthwhile checking exhaust back pressure before and after the converter. Idle or steady cruise it should not exceed .5psi and should not exceed 2.5psi on brief wide open throttle.

 

great thank you very much for the info, how would i go about checking exhaust back pressure before and after the cat.

 

Remove an upstream oxygen sensor and check the pressure. Then remove the downstream sensor and check it there. If the converter is restricted, the upstream pressure will be higher than the downstream. If they are nearly the same, but high, the problem is behind the converter.