Ok here is my issue:
1991 V6 5 speed, motor just recently rebuilt, new timing chain, new head gaskets and heads rebuilt. New injectors, new tune up. Just installed headers, EGR blocked off. Gas Mileage is approx 10-11 mpg and my V8 gets 18-20. What can I do???????????

Any help is appreciated.

 

Verify the ecm grounds, usually on the back of the head(s), are clean & tight.
Make sure the ignition timing is set & advances when you rev it. Timing really effects intake vacuum. Low vacuum will cause it to run rich (when you have a map sensor).
The coolant temp sensor has a big impact on running rich, do a ohm test on the sensor at different temp. Google GM CTS ohm values for the temp/ohm rates.
Check fuel pressure & bleed down rate, may have a bad regulator or leaky injectors.

 

I also have added a 3 wire O2 sensor, because of where the pacesetter headers has it relocated. The distributor is new including the pickup coil and module.

 

Ive replaced the Fuel pump, filter, injectors and plugs and still only get 11 MPG.. HELP!!!!!

 

Did you check the stuff in my post?

 

do u know if the 3.1's have 2 cts? 1 temp sensor and 1 sending unit? i assume 1 right? i have similar issues, iv replaced the ctp but still got a a code for cts. i hope its not a defective cts, but that code still comes out. i got two codes. map sensor and cts. map iv replaced as well.

 

Did you ever figure this out JoeCool108?