I have a 93 vette. Ever since I started doing mods (hotcam, LT headers, worked over heads, Ed Wright custom fast chip) 3 years ago, Ive had some choking smell issues at idle and poor fuel economy. Split BLM's with left and right BLM values of 121 and 137. Integrators were both at 128.

I say smell issues because I dont want to say its rich (although I would expect that). It could be so lean that it produces NOx emissions that are choking

A quick check of fuel economy records shows that mixed city/highway driving usually gives me 13-15.6 MPG. Highway driving is 23 to 26 MPG

In an attempt to cure this problem, I installed heated O2 sensors 2 years ago. They had no effect (I did verify the heaters on them were hooked up properly). I installed a new IAC, and TPS sensor (wasted money, I think the old ones were ok) and reset them properly.

I also checked injector resistances (ok), compressin (ok), injector leakdown (by looking at how fast fuel pressure dropped after shutting the engine down. it was ok), temp sensor resistance vs. temperature (ok)...

I finally checked the MAP sensor today and found some odd results. I used an auto x-ray to look at MAP voltages. With the engine off/key on MAP voltage was 4.68 volts. It should ahve been about 4.9 volts (based on Chiltons fig. 74 on page 4-25) based on todays barometric pressure reading in my area (29.8 in HG)

I then started the engine up and hooked a vacuum gauge up to a vacuum port. The vacuum gauge indicated 17.5 in Hg. The MAP sensor voltage (based on my scan tool readout was fluctuating between 1.80 and 1.88 volts.

Based on my Chiltons manual, a vacuum of 17.5 in Hg should be giving me a MAP sensor voltage of 2.67 volts this is much different than the 1.80 volts I am seeing.

The MAP does fluctuate (as does manifold vacuum) when throttle is applied.

My question is this:

Should a SES light and code be appearing if their really is a problem with my MAP?

I see Codes 33, 34, and 63 all can relate to the MAP sensor circuit being out of range (According to Chiltons).

I hate to just throw a new MAP sensor at this thinking this is the problem (they are expensive, some $200+).

I

 

I am wondering if your custom chip could be the culprit. Maybe your parameters are not set correctly.

Just my .02.

 

It's rare that a MAP sensor goes bad. But, anything could happen.

You may try adding some timing at idle (closed TPS spark advance). This will help idle quality, and aide in a cleaner combustion.