My L29 Laundry List

[Tom P]

So I've been working on my K3500 for about 9 months now and its been a problem child since day one. I've worked through a myriad of troubles thanks those of you who are familiar with this engine but like the the little dutch boy, every time I stick my finger in the dam, another hole opens up and now I'm treading water.

So the usual L29 issues have been addressed:
Fuel Injectors (replaced twice, long story, currently have 6 hole Multecs)
Fuel Pressure Regulator (delphi)
Fuel Filter
Ignition Coil
Spark Plugs, wires, cap, and rotor were all replaced in January by P/O (have receipts)
Fuel Pump / sender unit replaced in January as well (also have receipts)
New muffler back exhaust (Jones max Flow + 3" tail pipe)
Tuned PCM
Air filter and regular servicing as well
Compression test done: All pistons around 150 except #5 is closer to 140

What I know: According to the fuel pressure gauge I rented from the auto parts store, the truck makes 30psi of fuel pressure key on, truck running. From 0 if you turn the key on, not running, it builds about 10psi, and if you cycle the key one or two more times you can build up to 20, not running. No obvious leak down detected in the short time I had the pressure gauge. The truck starts hard after sitting over night and occasionally idles like crap and has almost died on me slowing down to turn a corner or other similar situation. Tail pipe smells like raw gas and my fuel mileage is in the *******. This problem also started suddenly. I installed the exhaust in early October to compliment my tune, and the truck was really starting to impress me. Mileage and power were both up. Then one morning after sitting over the weekend in late October it just didn't have much will to start or live. Been babying it ever since. It still seems to snort if I put my foot on it. Low end up to 3500 rpm is still ok. Beyond that its a joke.

I know the fuel pressure is my number one priority and I hope to address it Saturday when I have some time to do so. Getting to the sender is not exactly an easy job from what I can tell and I'll probably have to drop the tank. The sender is less than a year old and I really want to doubt that its the issue here because its less than a year old, and from what I've read is that when they go, they go. No slow death. My one hypothesis to test is either a bad ground, or bad connection on the fuel pump power wire. and besides that I'm at a loss. If anyone has any other ideas to throw in the hat let me know.

 

[someotherguy]

Try another gauge first to be sure, especially since you're using a rental/borrow gauge; people treat those tools like **** and it doesn't take a gauge being dropped many times before it is no longer reliable.

30psi is definitely not even close to enough to operate that engine.

If your fuel pump ground was poor I don't think you would get any action out of it at all, but you could certainly check it first. You can do that easily from under the truck. The pump grounds right there at the frame near the sending unit.

Richard

 

[Tom P]

Try another gauge first to be sure, especially since you're using a rental/borrow gauge; people treat those tools like **** and it doesn't take a gauge being dropped many times before it is no longer reliable.

I thought about that.. however none of my other local parts stores rents a gauge for me to compare it to. I can buy a Bosch gauge for 45 bucks, but my truck budget is shrinking by the day, and I don't want to spend more money on a diagnostic tool I'll use once in the next 5 years.

The reason I suspect a voltage issue is a hypothesis one of my co workers came up with. At ~12.5v (key on not running) the pump was only able to produce 20psi after a couple of cycles, but only 10psi after one cycle. Apparently the fuel pump cycles for a period of time before it shuts off and not until a pre-determined PSI has been reached as I previously thought. At ~14V (engine running) the pump was able to generate more pressure because the pump was spinning faster due to higher voltage. And I'm talking engine voltage here because I have not yet been able to jack the truck up and get underneath it yet. So if we have bad voltage going to the pump, say by the time it gets there its only 8 or 9 volts when the key is on. The pump wont spin like it should under normal circumstances.

Oh and I know how obnoxiously loud these fuel pumps are... I can barely hear mine any more when I turn the key on. Is there any way to temporarily disable the warning buzzer on these trucks? Its impeding my ability to hear things clearly.

 

[someotherguy]

On the TBI era trucks the buzzer was simply plugged in at the convenience center under the dash, driver side foot area. Probably the same on yours.

Another area to check would be the fuel pump relay and its socket under the hood; the Vortec era trucks are a little bad about roasting the relay. Personally I think it's marginally spec'd so once your pump gets old or the circuit has some resistance to it, it pushes that relay/socket over the edge.

Richard

 

[Tom P]

We checked the relay. Its fine.

 

[AllThrottleNoBottle]

So I've been working on my K3500 for about 9 months now and its been a problem child since day one. I've worked through a myriad of troubles thanks those of you who are familiar with this engine but like the the little dutch boy, every time I stick my finger in the dam, another hole opens up and now I'm treading water.

So the usual L29 issues have been addressed:
Fuel Injectors (replaced twice, long story, currently have 6 hole Multecs)
Fuel Pressure Regulator (delphi)
Fuel Filter
Ignition Coil
Spark Plugs, wires, cap, and rotor were all replaced in January by P/O (have receipts)
Fuel Pump / sender unit replaced in January as well (also have receipts)
New muffler back exhaust (Jones max Flow + 3" tail pipe)
Tuned PCM
Air filter and regular servicing as well
Compression test done: All pistons around 150 except #5 is closer to 140

What I know: According to the fuel pressure gauge I rented from the auto parts store, the truck makes 30psi of fuel pressure key on, truck running. From 0 if you turn the key on, not running, it builds about 10psi, and if you cycle the key one or two more times you can build up to 20, not running. No obvious leak down detected in the short time I had the pressure gauge. The truck starts hard after sitting over night and occasionally idles like crap and has almost died on me slowing down to turn a corner or other similar situation. Tail pipe smells like raw gas and my fuel mileage is in the *******. This problem also started suddenly. I installed the exhaust in early October to compliment my tune, and the truck was really starting to impress me. Mileage and power were both up. Then one morning after sitting over the weekend in late October it just didn't have much will to start or live. Been babying it ever since. It still seems to snort if I put my foot on it. Low end up to 3500 rpm is still ok. Beyond that its a joke.

I know the fuel pressure is my number one priority and I hope to address it Saturday when I have some time to do so. Getting to the sender is not exactly an easy job from what I can tell and I'll probably have to drop the tank. The sender is less than a year old and I really want to doubt that its the issue here because its less than a year old, and from what I've read is that when they go, they go. No slow death. My one hypothesis to test is either a bad ground, or bad connection on the fuel pump power wire. and besides that I'm at a loss. If anyone has any other ideas to throw in the hat let me know.

I would say you need to somehow beg/borrow/steal a reliable fuel pressure gauge and verify the pressure is that low. Or, based on your symptoms, you can go ahead and assume it's that low and replace the pump. You said it was recently replaced, but do you know what brand pump was used? All the aftermarket (Airtex, BW, Uncle Bob's Special, etc.) suck ass (instead of fuel). Only use a Delphi or OEM AC on these trucks (any GM for that matter). Myself, I would suspect the pump as you say it'll build a little more pressure with more key cycles...it should be up to pressure in a few seconds of 12V applied, period. (With my luck, I would also suspect the pump as it's a total pain in the ass to drain/drop the tank!)

Just my $0.02, usually worth less...

Rich

 

[Tom P]

This was the part number of the pump used.

http://www.jegs.com/i/Derale/259/USEP3962M/10002/-1

 

[Beeffajitas]

Hate to say it, but just because it's new, doesn't mean it's not bad. Everything is junk these days. I vote for fuel pump.

 

[Tom P]

The fuel tank is very close to empty, and I'm trying to keep it that way for saturday. We have a platform/transmission jack that should assist in lowering it. I think I'm just going to drop the tank regardless, maybe not all the way out of the truck, and check things out with my multimeter. Been scouring the GM Repair manual for other things to check and I've got a few other things to test to diagnose the issue as well. I'll keep everyone updated

 

[Kacy]

Did you notice fuel pressure drop with the key off? If you put a stock replacement regulator on the truck how do you know it hasn't gone bad again?