1991 Sonoma - bad (new) fuel pump or relay?

gasoline
pump

#1

This is my first post to the Car Talk Community. I am an irrigation tech in the Silicon Valley. I drive a 1991 GMC Sonoma pickup with a utility box bed for work… basically it’s a rolling tool box, with 262 K miles on it.

I had the fuel pump replaced last week after it died.

My mechanic told me that he tested the circuit, and that there was power out to the fuel pump, but the fuel pump did not work. He also tried spraying something (into the throttle body I guess) and it started right up. So he replaced it.

I piked up the truck yesterday, and it ran fine. Then did a repair job this AM, and it quit on my way home. Fortunately, traffic was light and I coasted to a stop on the shoulder, only a couple of blocks from home.

The symptoms are the same as before the repair work:
I can hear what i think is the fuel pump relay (on the firewall, driver side) click a second after I turn the key. But I don’t hear the whir or whine that the fuel pump normally makes at the same time.

Could it be that the relay contacts are worn? Or, with less than 20 miles on it, could the new fuel pump have gone bad?

The 10A “ECM B” fuse that supposedly leads to the relay on the firewall seems to be OK. The other 10A “ECM 1” fuse next to it is a little loose in its holder/ socket, as is the 3A fuse labeled “CRANK”… but the starter and engine turn over just fine, it just seems to be starved for gas, and the fuel pump is not working.


#2

*picked * up the truck yesterday, not piked up…

Thanks!


#3

@‌irrig8r

Do you happen to know if your fuel tank was full of crud?

Did the new fuel pump come with a sock?

Did the mechanic also replace the fuel filter?

If you want to check that theory of the relay, unplug it and bridge 30 and 87 on the fuse box. If the pump is running, you might just have a bad relay


#4

Apparently the fuel tank was not full of crud, and they checked the fuel filter and said it was OK, and it seemed to be as it was running yesterday and today, then all of a sudden it quit while I was driving.

Overall, during the last day and a half, it ran better than it had for the last year. Didn’t need to warm it up much, seemed like the idle felt more “steady”. Where before, sometimes after running an hour or more, when I came to a stop and idled at a stoplight it would sometimes “lug” and threaten to stall unless I put it in park and gave it gas. Basically, it feels like there’s more fuel flowing, so I thought maybe they changed the filter too, but the receipt says no.

As for “bridging 30 and 87”, do you mean using something like a jumper wire?

There are two relays on the firewall, side by side. The one wired to “ECM B” is supposed to be the fuel pump relay, I don’t know what the one wired to “ECM 1” does. I thought maybe I could switch the two. Switching relays like that in my '94 Ford Explorer got it started once when I had a similar problem.

BTW, forgot to mention, 4.3L V6 version, auto trans.


#5

BTW, the receipt looks like it says “Found & replaced bad fuel pump & striance in tank” but it could actually say “strianer” (strainer) I guess.

Also says “updated wire harness” because he told me the new pumps come with them.

Thanks!


#6

@‌irrig8r

Yeah, I meant using a jumper wire

Swapping another relay would do the trick . . . I would swap something “unimportant” such as the horn relay, if it’s the same type

As for the wire harness . . . yes, GM has long had problems with their fuel pump pigtails melting.
A new pump often comes with an updated pigtail.

Glad to hear the tank was clean . . . one less thing to worry about


#7

Do you suppose there’s some way for me to test the relay when it’s out of the vehicle with my multi-meter, for instance, is there a resistance reading I should be able to measure across the tabs? Any idea where I could find that what that value should be, or if there’s at least a correct range?


#8

Found this online. Looks helpful:


#9

@irrig8r‌

If you can read the relay part number, this might help

http://www.wellsve.com/test_spec_diag.html


#10

Thanks!. That was helpful! I will post the results after I run a test.


#11

image


#12

If you can access the point where the wiring harness connects to the fuel pump, that would be where I’d start. If the voltage there is 12 volts (with the everything connected as normal) and the fuel pump isn’t running, you know the pump (or the electrical connector) is bad.


#13

I never got to testing the relay(s).

There are 2 10 A fuses I mentioned in the last paragraph of my first post. Wiggling the one on the right, which was loose, with my key turned “on”, made that familiar fuel pump sound as it turned on and off. It’s now clear the the problem is the fuse block or the wiring that goes to and from it.

Some of the fuses show corrosion, and there was a water leak at the top of the windshield that took a me a while to get to last year, when I couldn’t pinpoint the source.

The strange thing is that it’s the fuse labeled ECM 1 that’s loose, and it supposedly doesn’t go the fuel pump relay, but does other stuff. My mechanic says he is trying to get a replacement fuse block for me.


#14

Here’s an idea . . .

Unplug the battery negative cable

Flip the fuse block upside down and check out the connections. Might be burnt or corroded

I’ll go a little off topic

A few months back, I had a Jimmy that was missing the blower high speed. To make a long story short, the fuse block itself was partially melted. I would never have guessed, until I visually inspected the bottom side. I wound up running an external inline blower high speed fuse. It’s been back a few times for regular scheduled service, and the blower is still working great, on all speeds

Come to think of it, GM seems to have its fair share of melted pigtails and fuse blocks. The fact that they make the pigtails so easy to get seems to say something . . .