Ok, looking for some help with my fuel pump relay.
Background - 2007 Gen3 5.3L mostly stock. I used a PSI Conversions harness on it, which has the fuel pump relay already set up. I completed the swap in 2021, and have about 20K miles on it. It's been great.
Last fall I had a no-start problem in the parking lot of a shop. I pulled the return line off the fuel filter and saw that I wasn't getting any fuel. Checked the fuse, swapped in a new relay and it started. Figured it was a bad relay, so I tossed it and moved on.
Then about two weeks ago had another no-start, and again no fuel. Checked fuses, swapped another new relay - no joy. Waited 3 hours for a ride on the back of a AAA truck.
I figured maybe it was the fuel pump - probably original with 285K on it - so ordered a new one. But when I finally got to check it out today I found that the pump ohmed out ok, and there was no power at the harness when I turned the key on and cranked.
So back to double check the fuse & relay. I bench tested the relay - audible click & continuity across 30/87 when I connect the battery across 85/86. So the relay seems to be good.
Check the socket. There's power to terminal 30, and terminal 87 has continuity to the power pin on the harness back at the pump. One of 85 or 86 has continuity to ground and the other shows 12V when you turn the key on.
But the relay didn't turn on when it's in the socket and you turn the key on. No click, no voltage on terminal 87.
There was voltage on one of the 85/86 pins, but this was after the key had been on for awhile. I didn't realize at that point that the pump is only supposed to run for a few seconds to build up pressure before the engine starts. So I checked the amps across pins 85, 86 - 0.09 mA, definitely not enough to work the relay.
Did some more reading and figured out the bit about the pump only running for a few seconds when you turn the key on. So figured I'd check the amps when the key is first turned on - and promptly blew the milliamp fuse in my meter. Switched to the 10A limit and tried again and briefly saw about 2A. And yes, I've since learned that's not a good thing to do on an ECM circuit.
But, I saw plenty of current to trigger a relay. So I put the relay back in and cranked it again - got the click. Plugged the fuel pump back in and it started. Drove around the block, stopped and started it a few times. No problem.
But WTF is going on? The relay is clearly good. I figure the relay last fall was probably good also. Wish I had tested it rather then just tossing it. But occasionally the ECM doesn't trigger the fuel pump relay. And then it randomly starts working again.
Any ideas?