98 K2500 Fuel pump - brake pedal issue

[Schim]

Recently did a engine swap on a 98 k2500 with the vortec 350 with automatic. Truck came in with a locked up motor so we don't know if this problem existed before hand but owner doesn't recall anything like this. Year of the donor truck was also 98. Truck starts and runs but as soon as you press the brake pedal more than half way down it will shut off. If you lightly press the brake pedal you can stop the truck with the brakes and it will stay running. If you press the pedal more than half way to where it starts to shut off you can let off the brake pedal and it will stay running. It will idle and run all day long fine with good oil pressure. We pulled all the abs and brake fuses\relays and the problem still persists. No codes no check engine light. It seems like the brake switch is either cutting power to fuel pump or injectors. We also swapped throttle bodies from one to the other with no difference. Anyone ever had this problem?

 

[east302]

Normally I’d say it’s a vacuum leak with the brake booster. But with it being a 3/4-ton, do you have vacuum or hydroboost brakes?

 

[Schim]

Normally I’d say it’s a vacuum leak with the brake booster. But with it being a 3/4-ton, do you have vacuum or hydroboost brakes?

I should've mentioned that we unhooked the vacuum line and plugged it with our thumb. After pressing the brake pedal down it still shut the truck off so we are thinking that the vacuum side of things aren't causing this issue.

 

[east302]

I should've mentioned that we unhooked the vacuum line and plugged it with our thumb. After pressing the brake pedal down it still shut the truck off so we are thinking that the vacuum side of things aren't causing this issue.

It does it in park as well, correct? Thinking of maybe TCC here if it’s only when driving.

Does it do it (or is idle impacted) with the lights and/or hazards on?

Is the battery good and alternator charging?

You mentioned pulling the fuse (fuse 1, stop/hazard?). What about disconnecting the brake switch?

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[Schim]

It does it in park as well, correct? Thinking of maybe TCC here if it’s only when driving.

Does it do it (or is idle impacted) with the lights and/or hazards on?

Is the battery good and alternator charging?

You mentioned pulling the fuse (fuse 1, stop/hazard?). What about disconnecting the brake switch?

Yes all abs, brake fuses and relays, brake pedal switch, and even the wiring that feeds the tail lights\reverse lights were all unhooked during trouble shooting. The problem never changes no matter what we leave plugged in or not plugged in. Is there anything safety related that could cause this? Or maybe the BCM or ECM?

 

[Schim]

Didn't see all of your message at first. It does it in park and or driving, it doesn't matter.
During our troubleshooting\research we had read about voltage issues so we turned on all the lights, blower, anything we could to draw power. It didn't matter it still acts the same.
Battery is 2 years old, not totally sure on the alternator, haven't checked it. But we've been cranking the heck out of it and giving it run time and haven't had to charge the battery yet.

 

[stutaeng]

Well, that sounds really strange, but being a new, old truck, you don't know the history. People do crazy crap modifying or trying to fix stuff.

I would say look for shorted wires and stuff like that. Does it have a SES light? The brake switch is a on/off switch. There's no "halfway" mark, if you know what I mean.

Hook up a fuel gauge and see if you lose fuel pressure before stalling. Also, the hydroboost brake system works off the power steering pump. Maybe PS pump is causing the stall? I can't see this happening, though. Brakes work by vacuum created by PS pump. Maybe I'm wrong?

But you mentioned vacuum hose, you I guess you don't have hydroboost? Some K2500 had this, depending on options.

 

[Schim]

Well, that sounds really strange, but being a new, old truck, you don't know the history. People do crazy crap modifying or trying to fix stuff.

I would say look for shorted wires and stuff like that. Does it have a SES light? The brake switch is a on/off switch. There's no "halfway" mark, if you know what I mean.

Hook up a fuel gauge and see if you lose fuel pressure before stalling. Also, the hydroboost brake system works off the power steering pump. Maybe PS pump is causing the stall? I can't see this happening, though. Brakes work by vacuum created by PS pump. Maybe I'm wrong?

But you mentioned vacuum hose, you I guess you don't have hydroboost? Some K2500 had this, depending on options.

No SES light, but I don't know about any stored codes. I know what you mean by on\off. We even unplugged the brake switch and the symptoms stayed the same. That leads me to believe that the brake switch is probably ok but the wiring going to it maybe the culprit... IDK, am about to give up on it.
We can hear the fuel pump running until the brake pedal is pushed in.
I don't believe it has hydroboost but I can't say for sure. I think we will have to hook up a fuel gauge just to see what is exactly happening.

 

[east302]

Yeah, if you have the brake booster, then it’s not a hydroboost setup. If unplugging the brake switch connector didn’t resolve it, that should rule out anything in the steering column (shorting our ignition switch). The brake switch output goes through the mess of wires included with the turn signal switch and ignition switch.

Maybe the brake pedal itself is hitting some (aftermarket?) wire and shorting something out?

 

[stutaeng]

You said the you can hear the pump until the brake is pressed? This is with the brake light switch wire removed? That doesn't make sense... unless the pedal is shorting some wire, causing fuel pump to stop running. Seems like it is a short it would blow some fuse, no?

Try removing the belt to see if the symptoms remain...