NEED HELP!

[MasonScott]

Just recently picked up a 1987 GMC R1500 350 with TBI, I originally got the truck and it ran perfectly. Drove it an hour home. Few days later it started acting funny. Started idling funny, stumbling at a stop in gear. Almost like it was gonna fall on its face and than it’d pick itself up. It even died a couple times putting in reverse backing out of a spot if I didn’t give gas right away. Tried checking for vacuum leaks. Couldn’t find any. Replaced/rebuilt all new seals on tbi, even new injectors. Chamged vacuum modulator on the trans. The IAC and the TPS. but now it’s thudding going down the road and won’t even get up to speed.does anyone have any suggestions? I was thinking fuel pump?

 

[Vbb199]

Unplug the map sensor, take it for a drive.
If it runs fine, it's the fuel pump.


Map sensor is a rectangular box to the left of the tbi.
1 plug, and 1 vaccuum line.
Just unhook the electrical plug.

 

[Vbb199]

Unplugging the map causes it run in a really really rich state, essentially dumping fuel in the barrels, so don't go driving around like that forever, it's just kinda a "test" to see if the fuel pump is dying or not.

Ideal fuel pressure if you have a gauge is 12-15 on the tbi.

 

[Rusty Nail]

Different gas.
Ran great until the first time you filled it up?

 

[MasonScott]

Unplug the map sensor, take it for a drive.
If it runs fine, it's the fuel pump.


Map sensor is a rectangular box to the left of the tbi.
1 plug, and 1 vaccuum line.
Just unhook the electrical plug.

Unplugging the map causes it run in a really really rich state, essentially dumping fuel in the barrels, so don't go driving around like that forever, it's just kinda a "test" to see if the fuel pump is dying or not.

Ideal fuel pressure if you have a gauge is 12-15 on the tbi.

Okay awesome, I appreciate the response. I’ll give it a shot!

 

[MasonScott]

Different gas.
Ran great until the first time you filled it up?

It had the same gas when it started. I thought maybe it was the gas so I ran all the old gas out and than filled it back up and it didn’t help.

 

[Vbb199]

It had the same gas when it started. I thought maybe it was the gas so I ran all the old gas out and than filled it back up and it didn’t help.

When you say "ran all the old gas out" does that mean purging it all from the tank? Or driving the truck until it quit running?

Because my next concern if your answer is the latter is it could have a deteriorated screen on the bottom of the fuel pump in the tank and you sucked up a bunch of crap into the fuel system, or just the fuel filter is clogged up.

 

[Vbb199]

A clogged filter can deceive you.
But I suggest starting with just unplugging the map and seeing what happens. It's free!

If the condition doesn't change, see what kinda fuel pressure you're getting AFTER the fuel filter
(filter is pass side, inside of frame rail, middle of truck almost)

If pressure is low, check before fuel filter, if pressure is between 12-15, then it's a clogged filter. If pressure is still just fine at 12-15, then we move onto bigger things, like ignition troubles. Lol

But this story sounds like a very familiar story of a dying fuel pump.

 

[MasonScott]

When you say "ran all the old gas out" does that mean purging it all from the tank? Or driving the truck until it quit running?

Because my next concern if your answer is the latter is it could have a deteriorated screen on the bottom of the fuel pump in the tank and you sucked up a bunch of crap into the fuel system, or just the fuel filter is clogged up.

So I guess i didn’t really run it all out, I pretty much ran it all out but not out to the point where there wasn’t anymore and the truck died, I just drove it to where all the fuel was pretty much gone and added new fuel. But I changed the fuel filter as well. Havnt checked fuel pressure though.

 

[Vbb199]

Those fittings at the filter are 14mm x 1.5 OR 16mm x 1.5, can't remember which is which.

 

[Vbb199]

So I guess i didn’t really run it all out, I pretty much ran it all out but not out to the point where there wasn’t anymore and the truck died, I just drove it to where all the fuel was pretty much gone and added new fuel. But I changed the fuel filter as well. Havnt checked fuel pressure though.

Oh ok.
Then we can probably cross off the fuel filter as being an issue.

 

[Vbb199]

Those fittings at the filter are 14mm x 1.5 OR 16mm x 1.5, can't remember which is which.

Admittedly, I kinda cheat on checking pressure... I have a piece of old hard line from Another fuel injected truck that I can screw into the fuel filter, and just tap onto that with a rubber hose and my low Pressure fuel gauge.

 

[Crispy]

I cheated on checking fuel pressure too. Search for a TBI fuel pressure adapter on the google machine. Well worth it IMO.

Thats not my pic btw, just something for reference.

You must be registered for see images attach

 

[MasonScott]

So update: I ordered that fuel pressure adapter. Got a fuel gauge, Hooked up the adapter and tried firing it up and the truck wouldn’t start. So I figured maybe the pump is fried. So I took the adapter off and hooked up the lines and it fired right up. So I replaced fuel pump in the right tank, the one I’d been using. So when I was replacing it, after I got it installed I have a loose wire, coming from the top of the fuel sending assembly that sits in the top of the tank. I wasn’t sure where it came from so I grounded it to the frame. The truck fired up with the adapter on. So that was good, got the 12psi I needed. Still sounded kinda rough. I took it for a drive, ran but not great. Went to turn my lights on, whole truck turned off. Checked fuses, were all good, thinking maybe relay? Or breaker of some sort. Maybe that wire isn’t there for a ground?

 

[Bextreme04]

You need to troubleshoot and isolate the problem. Fuel, Air, Spark. If you turned on the headlights and your engine died, I would suspect bad grounds. Check all of them, especially battery to frame, frame to alternator bracket, back of passenger side head to firewall, and the two headlight harness grounds on the core support. if the TBI fuel pump harnesses are similar to most, I think you should have a power and ground for the pump, a signal wire to the sender, and a ground for the sender to the frame.