The best mileage I ever got on the highway at a constant 65 mph was 14 mpg, which I thought wasn't the best but it was great for my truck! Has a Jasper motor in it and it had just gotten tuned up by me using 87 octane gas. I never checked the fuel pressure but was told by a mechanic this week that it should read 55 psi(didn't tell me if it should've been running or cranking)and mine was reading 53 cranking so that meant my fuel pump was going bad! said the injectors wouldn't cycle on unless it was at 55 psi--even if it was 54 the injectors wouldn't cycle on. I paid over $700 4 years ago to get it replaced and it's costing me to fix it again! I'm not sure if he's being truthful but he's got me by the ball$$ so I have no choice but to replace it.
Just curious--how's your wheel alignment? That also affects your mileage. Your tires don't really have to show abnormal wear for it to be off just enough to where it'll affect mileage. Just a thought.

 

Without a doubt, low fuel pressure is your problem. If fuel pressure is less than 60psi you will have performance problems. If pressure drops below ~43psi the poppets will not open... no fuel, no start, no run Key ON, engine OFF, fuel pump running, fuel pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off. If it fails either one of those tests, following the flow chart in this thread will find the problem: https://blazerforum.com/forum/tech-a...agnosis-23038/

GM does not publish any "engine running" fuel pressure specs for the 4.3L engine due to far too many variables involved with engine load, elevation above sea level, general engine condition, etc etc.

 

replaced fuel filter.. was pretty plugged ... but fuel pressure is still at 50 psi running.... looks like i need to change the fuel pump... this sucks ... now i also need to figure out why it keeps tearing out motor mounts... new ones.. i almost want to put a solid mount kit in

 

GM does not publish any "engine running" fuel pressure specs for this engine. There are far too many variables involved with running pressure. Key ON, engine OFF, fuel pump running, fuel pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off. If it fails either one of those tests, following the flow chart in this thread will find the problem: https://blazerforum.com/forum/tech-ar...agnosis-23038/