More Tbi fuel pressure..1993 5.7....

[scott2093]

I started to hijack a thread but I'm going to be getting into the weeds it looks like so wanted to start another separate thread. Hopefully I can finalize it with results as we apparently have enough cliffhangers around to satiate my appetite for those.

From the other thread

Starting some troubleshooting on my 93 350tbi. About 4 months ago I installed a Delphi FE0114 for reference....
Checking fuel pressure today I'm getting 14psi. Stock spring afaik....

But when I depress the pressure relief button or whatever it's called on the gauge, the fuel pressure drops to 8.
Please tell me the pump isn't the only suspect.
I'm going to start another thread most likely but wanted to ask because I remember Schurkey saying that depressing the button was a way to simulate a load and I'm seeing this behavior.

Pushing the pressure relief button on the fuel pressure tester causes additional fuel to flow, simulating high-load engine demand.

I suppose different fuel pressure gauge assemblies could flow various amounts through the pressure relief. So MAYBE the pressure relief on your gauge flows a huge amount and the pump can't keep up.

But I doubt it.

I'd be checking the voltage and amperage as close to the pump as practical, probably the most-rearward connector before the harness goes up 'n' over the tank. (The amperage can be tested anywhere in the circuit, but the voltage has to be tested close to the pump.) The pump MUST be running to test the voltage and amperage.

How old is the fuel filter?

Will do. But scanner showing 13.6v at fuel pump now that truck is running......
My truck randomly died while idling in driveway trying to get my new aldl cable working with my phone. It actually wouldn't start for a bit too. Just cranked... then started working. When I put the scanner on to start looking at stuff I noticed the fuel pump voltage acting flakey when cranking.....drop to 8 volts...showing 10v koeo.... not normal.....

I was paranoid the aldl interface messed something up but I'm guessing just an odd coincidence since truck died after with my normal scanner........
truck not acting up now apart from me noticing the fuel pressure drop. I'm just getting started on looking at things. This issue is going to catch me off guard if I don't get it sorted....

Old enough I guess.....maybe 6 months or more...would have to check......and I never changed it after putting new fuel pump in 4 months ago....

The scan tool CANNOT "show 13.6V at the fuel pump" because there's no voltage sensor at the fuel pump.

The voltage reading is from significantly further forward, where the harness ties into the computer.

You MUST measure voltage as close to the fuel pump as practical, and you can't do that with a scan tool.


Yup, you need to do some tracing and examination of the wire harness with a multimeter. Burnt contacts on the fuel pump relay are somewhat common, and I suppose the same for the contacts in the oil-pressure switch. TEST BEFORE REPLACING

Thank you....
That does make me think more about how I was seeing my oil pressure gauge in the dash acting a bit funny and moving 5 or so psi back and forth...maybe more..... I couldn't see the movement on my mechanical gauge but I never bled the air in that line so I'm not clear how sensitive it is to movement.. It was dramatic enough on the dash to take notice though....
Will investigate more tomorrow... God willing of course...

There is ZERO point to "bleeding air" from oil pressure gauge tubing. It will re-fill with air at every engine shut-down.

The biggest advantage to plumbing the oil pressure gauge with copper tubing, is that they can't see the air inside, and therefore it doesn't bother folks.

 

[scott2093]

Just going to leave some notes....
obviously comments are welcome

I'm getting .04ohms between the fuel pump harness ground that bolts to the frame to battery negative..

not sure why my harness connectors at the fuel tank have 3 wires and my schematic shows 2?
I have a purple wire for B....///edit--guess the wire is pink(what it says in manual) ??sure looks purple...fuel sender circuit for fuel gauge at dash is #30...doh

getting 13.5vdc at fuel pump connector with engine running...kinda low koeo...11.5 ish til it goes off.....weird....
.7r resistance from fuel pump connector to pcm..... circuit 120 gray....

 

[scott2093]

I get 3.9A draw on circuit 120 with truck running..... stays there even when losing fuel pressure by depressing gauge relief button.....
Could take a drive with gauge but I'm afraid it may act up and leave me stranded.....

Note too that oil pressure switch is newish GM...maybe 3 months old.... was less accurate and jumpier than the GM one it replaced...Amazon....?
fuel relay plug contacts looks super clean...haven't opened the relay itself yet.....

man that relay connector looks like it costs a couple of bucks,,,,

 

[scott2093]

The back story that lead me on this chase of whatever.....

I received my new Boosted ALDL cable and was trying to get it working with ALDL Droid and with TunerPro...
That's another story I'm still trying to figure out...

While I was idling the truck trying to get things to respond with ALDL droid, the truck just died... Not electrical power, just shut off...like no fuel or spark.....
Truck started right up after.
Did this a couple of more times. I thought it has to be something with the cable obviously.....
Put my real scanner on it and was looking at things, nothing odd, no codes, then truck died again......

Went to start it again and all it would do is crank and crank...Came back to it the next morning and same thing...
Didn't have a spark tester, ordered one..be here tomorrow, ...

battery good....
I checked the ignition coil for 12v...good
Checked for the ignition coil's switching signal and it was good...also, the truck fired up at the same time... coincidence?
Truck has fired up and idled fine since but I obviously want to figure out what is going on.

So I checked fuel pressure and that's when I see it dropping 6psi ish when pressing the pressure relief button on the gauge.... It's a vortec pump so you'd think it wouldn't lose pressure? It stays low and doesn't recover until I release the button.... Really not a terrible amount of fuel coming out so I can't see it being that much of a draw on the pump.....

I ordered a new fuel filter...It's weird how the truck just died...no sputtering...nothing...just a complete stop of running in a split second......

Maybe I'll throw the aldl cable back on and see if it acts up...But my regular scanner was hooked up when it last died so that wouldn't make sense....
Coincidence for sure but I need to figure out what the heck.....

 

[scott2093]

how are you guys tapping into wires? I'm creating a lot of work for myself destroying mine.... I tried real small nails...tacks worked ok....but ended up trying to strip some areas to put clip leads on that I have a stash of to get some length going.........
Those stabby things I see work? Seems like the needle needs to be pretty small and tough to get through some of these jackets.....

haven't had success with liquid tape in past so...

 

[scott2093]

Just measured circuit 120 at the fuel pump relay...Weird erratic behavior until it stabilized at 14.2v with engine running.... was 8, then 10, then 7, 11,12---
not sure if that's normal ....?

Going to check that circuit at oil pressure switch tomorrow...I can't get a good piece of it an am over it...Really need to find a better way to probe... going to Google in a few.....

 

[Schurkey]

I'm getting .04ohms between the fuel pump harness ground that bolts to the frame to battery negative.

Almost but not quite meaningless. A voltage-drop test would tell you much more. A single strand of wire out of a multi-strand cable can provide continuity.

not sure why my harness connectors at the fuel tank have 3 wires and my schematic shows 2?

I don't know why your schematic only shows two.

I'd have expected three wires in a molded-plastic connector, plus another wire grounding at the frame rail.

Two wires, fuel level sending unit power and return.
Two wires, fuel pump power, and the fuel pump ground is the single wire attached to the frame.

At least, that's how my '88 is set-up.

You must be registered for see images attach

The little bit of red wire at the very bottom of the photo, at the molded-plastic connector, is my "test harness". I built a one-foot-long, three wire temporary harness using generic wire, and Weatherpack connector pins, seals, and the connector bodies. I can pierce and probe my temporary harness without fear of damaging the vehicle harness.

getting 13.5vdc at fuel pump connector with engine running...kinda low koeo...11.5 ish til it goes off.....weird....
.7r resistance from fuel pump connector to pcm..... circuit 120 gray....

"Resistance" is unimportant.

Your voltage measurements seem fine to me--provided the pump was running during the measurement. Since you say the engine was running, I guess the pump was, too.

However, you didn't check for voltage on the ground wire. A poor ground can disable the pump just as easily as poor connections on the supply side.

I get 3.9A draw on circuit 120 with truck running..... stays there even when losing fuel pressure by depressing gauge relief button.

If 120 is the fuel pump power wire, 3.9 seems a bit low. I'd have expected more like 5.5--6.5 amperes.

Note too that oil pressure switch is newish GM...maybe 3 months old.... was less accurate and jumpier than the GM one it replaced...Amazon....?
fuel relay plug contacts looks super clean...haven't opened the relay itself yet.

Worth checking out.

the truck just died... Not electrical power, just shut off...like no fuel or spark.....
Truck started right up after.
Did this a couple of more times. I thought it has to be something with the cable obviously.....
Put my real scanner on it and was looking at things, nothing odd, no codes, then truck died again...

...I checked fuel pressure and that's when I see it dropping 6psi ish when pressing the pressure relief button on the gauge.... It's a vortec pump so you'd think it wouldn't lose pressure? It stays low and doesn't recover until I release the button.... Really not a terrible amount of fuel coming out so I can't see it being that much of a draw on the pump.

My guts say the pump is weak, or the filter sock is getting plugged. Either way, the tank is getting opened-up.

 

[scott2093]

I don't know why your schematic only shows two.

I'd have expected three wires in a molded-plastic connector, plus another wire grounding at the frame rail.

Yes the manual decided to save space in that diagram I'm guessing. The missing wire was for the fuel gauge... I didn't bother to look for the drawing it may be on..weird...yours looks purple to me as well but manual says pink....

If 120 is the fuel pump power wire, 3.9 seems a bit low. I'd have expected more like 5.5--6.5 amperes.

hmm..yes circuit 120 is power wire afaik,,,,I'll double check reading to make sure.... I grabbed the wire with a little clamp on meter just under driver side floorboard where all wires come from pump and rear of truck...Should I splice wire instead?? Need to make sure I can ...

My guts say the pump is weak, or the filter sock is getting plugged. Either way, the tank is getting opened-up

that sucks... pump is only a few months old...tank was super clean.... Fuel filter won't be a hail mary? Getting one tomorrow....or would I have low pressure all the time and not just under load if it were acting up???

Any idea why I might get jumping voltage at fuel pump relay circuit 120 until it stabilizes? I still need to look at contacts...

 

[scott2093]

relay contacts looking a bit crusty.... totally white on back side of those two?

 

[Schurkey]

circuit 120 is power wire afaik

OK.

I'll double check reading to make sure.... I grabbed the wire with a little clamp on meter just under driver side floorboard

Should be OK.

pump is only a few months old...tank was super clean.

One tank of contaminated fuel can screw-up a pump...or plug a filter.

But having a new-ish pump does make failure less likely.

Fuel filter won't be a hail mary? Getting one tomorrow....or would I have low pressure all the time and not just under load if it were acting up?

It "could" be the problem. Filter partially-plugged, allows enough fuel flow for idle and low-power operation; won't flow enough for high-demand operation, so the pressure drops.

Try it. If the new filter doesn't improve things, put the "old" filter back in the box, set it on a shelf, and use that for your next replacement in a year or three.

Any idea why I might get jumping voltage at fuel pump relay circuit 120 until it stabilizes? I still need to look at contacts...

The relay contacts that tend to burn are the ones INSIDE the relay, not the blade connectors on the bottom--although they can oxidize, or have loose female connectors in the fuse/relay box.

There's a dozen things that could cause jumpy voltage, from poor alternator regulation to intermittent connections.