TBI fuel pump unresponsive

[Matt Buffington]

Hi gang,

Been chasing down a crank-no-fire situation on my 1990 burb (350 stock TBI). After running around 50 miles or so around town one day (running perfectly, as far as I can tell), she would not start when I came out of the supermarket...just cranks away, but no firing!

I can get the engine to fire with starter fluid, so spark appears to be fine; noid lights flash so I know the injectors are firing. I changed the ignition module under the cap, no change there (pre noid light test....). Also checked connections in general, and I just don't see anything outta whack; everything is plugged in solid and clean. I've done the 'big 3' upgrade, so grounds are clean and strong.

However, I cannot hear the fuel pump running at all. Fuel pump relay appears to be working normally, and I checked the fuses and they are all fine, too. So....I am hoping the fuel pump is not shot....but I'd like some advice on what to check BEFORE dropping the tank, etc. Aside from the relay under the hood, is there another relay to the fuel pump? Or a fuse, perhaps?

thanks in advance for any advice!

Matt

 

[Vbb199]

On the relay bulkhead, jump orange over to tan/grey and see if it turns on, if so, then relay is ****.
You should be able to turn the key to "on" and hear the relay, anyways.


If relay is ok, check fuel sender ground. Its on top of drivers frame tail, behind drivers back tire.


Verify youre getting 12v to the plug for the fuel pump with a multimeter... that same plug is nearby the ground, but inside the frame.


Make sure the ECM B fuse isnt blown.

 

[Matt Buffington]

Hello! Okay, found the fuel pump ground disconnected!!! However, for the life of me, I cannot figure out where it was connected....and I'd rather not drill another hole in the frame. There must be a logical place for this to go. Any ideas?

 

[Matt Buffington]

Update! I found the fuel pump ground disconnected. Reattached to frame rail, and still nothing. Measured 6.25 v on power line to fuel pump (measured at connector near fuel tank). Replayed fuel pump relay, no difference. It’s raining and dark out now, but tomorrow, I plan to attack this again. Any suggestions on how to proceed? I’m losing voltage somewhere, it seems, between the battery and the fuel pump. Just gotta figure out where!

 

[Vbb199]

Make sure theres not a signal break at the oil pressure sender at the back of the block (sbc), or towards the front , down low by the timing cover(bbc), its 3 wires.

If all ok there, check the wire that comes from the fuse block/ union right by the booster....

Your burb will have a pink... and... purple? Grey? (Idr) wite that leads to the pump.... its maybe 14 gauge wire...

It'll be right next to bigger wires that are about 10-8 gauge....

Make sure theres no breaks leading from the fuse block to the pump....

Forgive me on spelling , been drinking hahahahaha

 

[Vbb199]

We just had this issue on a friends 89 k5... he was getting 8.5v to the pump, it was a total wiring nightmare, but inspect your wires at the oil pressure sender for a break, the ecm doesnt like less than 12v at the oil sender........


VERIFY you have 12v on the fuel pump relay's bulk head..... its the orange wire.. Check from orange wire to battery ground


Veryify the bulkhead has a solid ground (pin black on multimeter with battery positive)


With key on, you will see 12v on the GREEN wire.

If all checks out ok, plug the bulkhead back in, backprobe the grey or tan wire on the relay harness, make sure 12v is going to it with KEY ON.

 

[Matt Buffington]

Thanks buddy; I’ll get into this today and let you know what I find. Has to be some little break or something, as it happened all of a sudden. Oh, and you know what? When I first got under the hood to check out why the truck wouldn’t start, I noticed the fuel pump relay ‘dangling’ and not in its mounting spot. Coincidence maybe?

 

[Matt Buffington]

Update: 12v on orange wire, check! Ground solid: check! Green wire....zero. Should it only read 12v at initial key on/cranking? I was alone and could get to the engine bay fast enough to tell....

checked wiring to oil pressure switch and all looks good; nothing corroded/broken/disconnected. Also checked wiring as best I could along frame rail; all nicely bundled in the plastic looming tubes and while older looking, nothing torn/weird/corroded.

thunderstorm rolling through now so I had to pack up the shop!

 

[Vbb199]

Update: 12v on orange wire, check! Ground solid: check! Green wire....zero. Should it only read 12v at initial key on/cranking? I was alone and could get to the engine bay fast enough to tell....

checked wiring to oil pressure switch and all looks good; nothing corroded/broken/disconnected. Also checked wiring as best I could along frame rail; all nicely bundled in the plastic looming tubes and while older looking, nothing torn/weird/corroded.

thunderstorm rolling through now so I had to pack up the shop!

It should have it on i think momentarily, as the pump primes, then when the truck is running, it stays on.

The green wire is the trigger wire for the relay.


Is your multimeter leads not long to crank the truck while looking at the display?

Set the meter on the hood while its open, pin the black om the bulkhead, and the red lead on the green wire


Did you backprobe the grey/tan wire with the relay plugged in also? That grey/tan wire is pump signal.

 

[Vbb199]

Here.

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[Matt Buffington]

Next update:

green wire at relay 12v when switched on, so that checks.

rechecked power at back to pump; pink wire is 8.5v, white/grey is 12v for about three seconds, then off....

so...i think I was not probing the connectors by the pump correctly before, as I had my wife cycle the truck on and off so I could watch the numbers. I was also not getting the probes in correctly, I think...

Observations? Perhaps it actually is a bad fuel pump?

 

[Matt Buffington]

Another follow up: I wired 12v directly to the fuel pump at the rear of the truck...no whir, no response at all. Looks its been a shot fuel pump the entire time.

 

[Vbb199]

Another follow up: I wired 12v directly to the fuel pump at the rear of the truck...no whir, no response at all. Looks its been a shot fuel pump the entire time.

Welll, thats kinda how it goes lol, spend all that time diagnosing and its dead

You either just pull it for suspicion, or diagnose to find its dead, idk why its like that, but it is.

I will tell you, we just did the same thing, ran a 12v source directly to it
.... no go...


Pulled the entire sending unit, fiddled with connections on it, tried the whole setup again at the battery.... it wotked....


Those ****** parts store "reman" kits suck.

 

[Vbb199]

Fun factoid:


If the rubber hose on the sending unit to the pump has plastic black clamps, its OEM, if its metal hose clamps.... its aftermarket

 

[1985c20]

Fun factoid:


If the rubber hose on the sending unit to the pump has plastic black clamps, its OEM, if its metal hose clamps.... its aftermarket

Not necessarily. Look on Rock auto some have plastic clamps some have metal. The cheap ass airtex, plastic. The “gm genuine” has metal. Guess it’s whatever they feel like