To start the vehicle, fuel pressure needs to be at least 60 PSI. 55 won't do it. So if you can only get it to 55 with the key on, engine off, then that's your problem. Frankly, this is common. I had a 2002 Blazer (Same pump) that wouldn't start unless I turned the key on for a couple of seconds to let it prime the system, then off and on again, and then cranked it. After a few days of that it gave out altogether.
In short, my money is on the pump. Again it's common. Over the last 7 years I've owned 3 Chevrolet Blazers and 1 GMC Jimmy. I've replaced at least 5 fuel pumps in them. The third time I did one on my 2002 I got sick of it and cut and access hole in the floorboard. After that the entire job only took 15 minutes.
As a test measure, try spraying some starting fluid into the intake and then crank it and give it a little gas. I bet it starts. It might even hold idle and drive.
Another test (more of a backyard trick) is to talk the fuel tank while someone else cranks it. Sometimes that jarring effect on the pump would running will give it a little boost and get it over 60 PSI. But (1) you need a helper or a long striking object and (2) even if it starts it's not fixed, just well enough to get home or a shop.
Wednesday, April 14th, 2021 AT 6:13 PM