'93 K3500, 454 TBI, poor drivability

[Idaho83815]

Yes the ground wire is at the T stat housing. It grounds thru the 2 bolts that hold down the T stat housing.

Thanks udidwjat.
And for those suggesting the fuel pressure test, I have a loaner on reserve as soon as the previous user returns it….

 

[Sabinoerc]

Thanks udidwjat.
And for those suggesting the fuel pressure test, I have a loaner on reserve as soon as the previous user returns it….

Check pressure after the fuel filter. On my 93 5.7l, you can release a clamp on the fuel line going up to the TBI and get enough space to put in the test adapter. Also, blip the throttle while reading and make sure it’s solid. My problem was the pressure read ~9.5 at idle but when blipping throttle it would drop a bit. Replaced pump and got a solid 13psi and my drivability problems went away.
Those serious about performance/tuning install a fuel pressure gage by the TBI to make it easier to monitor.

As I said, not saying this is your drivability problem but if you’ve never checked and it’s this old chances are it’s marginal and won’t be a waste of time to verify in any event. Good luck!

 

[Idaho83815]

Check pressure after the fuel filter. On my 93 5.7l, you can release a clamp on the fuel line going up to the TBI and get enough space to put in the test adapter. Also, blip the throttle while reading and make sure it’s solid. My problem was the pressure read ~9.5 at idle but when blipping throttle it would drop a bit. Replaced pump and got a solid 13psi and my drivability problems went away.
Those serious about performance/tuning install a fuel pressure gage by the TBI to make it easier to monitor.

As I said, not saying this is your drivability problem but if you’ve never checked and it’s this old chances are it’s marginal and won’t be a waste of time to verify in any event. Good luck!

Thanks Sabinoerc. I have seen/read where most folks test on pump side of filter, I suppose I can try on both sides to see if any difference. One test fixture I saw online actually replaced the filter with the test fixture…. Hmmm?.

 

[Schurkey]

Fuel pressure test adapter that replaces fuel filter:
www.amazon.com/Tool-Aid-37650-General-Adapter/dp/B0002SR544/ref=sr_1_1?

I've had to use that style on vans, where there's no clearance for the kind that goes in-line at the rear of the throttle body.

Overall, not my first choice--but needed in some applications.

Be sure you buy the kind that has a Schrader valve that fits your fuel pressure testing hose. There's at least two sizes of Schrader valves, and OTC and others use a quick-connect style. It really sucks when you spend the money for an adapter...and then your pressure tester won't connect to it.

 

[Idaho83815]

I got a great loaner fuel pressure test rig in the loaner pool at O’reillys. It has the GM TBI capability at the filter with the fitting in the bottom right of the attached photo. They just got this kit in and it is new. Glad it is a loaner as it is $150 if I don’t return it.

 

[Schurkey]

I got a great loaner fuel pressure test rig in the loaner pool at O’reillys. It has the GM TBI capability at the filter with the fitting in the bottom right of the attached photo.

I looked all over Amazon for a photo of that style TBI fuel pressure adapter, and couldn't find one. Too lazy to take a picture of the one I own.

For the record, that adapter is the style that goes between the fuel hose and the throttle body. It's not the kind that goes in place of the fuel filter. Much better, in my opinion, since it measures pressure AFTER the filter, so if the filter is plugged...it'll show up on the pressure gauge.

Some folks have complained about that style of adapter leaking. Must have had damaged O-rings inside.

 

[Idaho83815]

I looked all over Amazon for a photo of that style TBI fuel pressure adapter, and couldn't find one. Too lazy to take a picture of the one I own.

For the record, that adapter is the style that goes between the fuel hose and the throttle body. It's not the kind that goes in place of the fuel filter. Much better, in my opinion, since it measures pressure AFTER the filter, so if the filter is plugged...it'll show up on the pressure gauge.

Some folks have complained about that style of adapter leaking. Must have had damaged O-rings inside.

Well I will have to verify specs but this 93 7.4 with TBI should have 11-13 PSI. I am only getting 8 psi on key and 11-12 at idle. That gauge in the loaner kit is a 100 psi gauge so it may be a few percentages off at such low psi, but it is probably pretty close. I primed it multiple times and it did not get above 8 or 9 until I started it. This vintage also releases pressure when key off or after prime from what I understand. And it did just that. On initial prime it might have only hit 6 then started dropping.
Feel like the pump is weak
Also those fittings are good on paper but leak a bit. No matter how tight. Also it was brand new with new o rings. Leak seemed to be where it swivels, not on either end or at schreider valve. Don’t think the small leak affected my reading much but….?

 

[Schurkey]

Verify fuel pump voltage on the supply wire, as close to the pump as practical. Check for voltage on the ground wire, as close to the pump as practical.

BOTH tests done with the pump RUNNING. Preferably with the engine running, and alternator charging.

 

[Idaho83815]

Verify fuel pump voltage on the supply wire, as close to the pump as practical. Check for voltage on the ground wire, as close to the pump as practical.

BOTH tests done with the pump RUNNING. Preferably with the engine running, and alternator charging.

I think I smell what you are standing in..... good ideas, thanks.
Now how to get close to the pump and find the right wires.. I will have to do some looking around under there.

 

[Schurkey]

Usually, the access point for the supply is the rearmost connector before the harness goes up 'n' over the tank. The ground is usually a separate, single wire with a screw connecting it to the frame.

I built a one-foot "extension" harness using Weatherpack connectors and pins, and some plain ol red 14-gauge wire. I can poke holes in the extension harness for test probes and not have to worry about the vehicle harness corroding.

Don't have a good photo of it. Just barely visible at the bottom of this picture. There's no good access to the pump ground wire except at the frame, unless you take the bed off..

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