Incorrect fuel pressure Key on and Engine Running

[Matthew Jeschke]

Dropped the tank again. I'm loosing track of how many times I've taken the tank off at this point. I took all the lines apart. Blew air down them... No obstuctions what so ever. I even took bypass tube off rail and blew air through it in every way conceivable. I took the test port valve out and did the same. I sprayed all the orings with pblaster and put it back together. Pressurized the system and checked for leaks again and again. I also visually inspected all the lines... no kinks or damage anywhere. Same result from test 48PSI, when is supposed to be 58 PSI to 62 PSI. I am pretty sure the harbor freight kit includes the test fitting for checking pump pressure / unregulated pressure. I'm going to buy it tomorrow and see what results I get. That aside, lines, rail and regulator are working as expected.

 

[Matthew Jeschke]

I'm going to see if maybe I have a poor ground that's leading to reduced current to the pump. Pulled this out of the FSM. But cannot find where G102 or G403 are located. Anybody know?

 

[Fless]

This is what the upfitters guide shows, the fuel pump ground on the body mount underneath the driver's door hinge:

Zoomable PDF attached.

Presumably a high number ground (like G403) would be near the rear of the vehicle, but I'm not sure of that. Alldata would likely show them.

 

[Matthew Jeschke]

@Fless thanks!!! I dug through AllData for quite a while but couldn't seem to bring up those two grounds. I internet searched them, but seems you're picture is much better than what I found on the internet. Very helpful.

 

[nonickatall]

This is normal if your hose is lower than the level of your tank. Then there is this effect.

This is how tanks used to be emptied, the hose sucked in petrol, and the hose hung lower than the tank was full.

 

[Matthew Jeschke]

Oh, that makes since. Hurt like heck though getting my arm soaked in fuel.

On another note, Delphi got back with me. They said I need to do a voltage drop test... from relay to pump, and pump to ground. Guess I need to drop the tank... again. Sigh. At least far enough to fit some probes in the lines to the pump.

I tried test on my Corvette. It was within spec. I did notice one thing though. The pressure PEAKS in the test range. It doesn't STAY in the test range. When the pump cycles on it hits within the test range but once it shuts off it drops below. I assume that's proper function. The FSM wasn't too clear about that.

I've attached troubleshooting guides from Delphi.

Test Measurements:

5 AMPS draw at 12.1V for pump

 

[strutaeng]

@Matthew Jeschke My 99 Silverado has started having an issue where it will crank a while and kinda struggles to start.

This morning I finally put a fuel pressure gauge on it and got 54 psi at prime and like 48 psi at 2,000 RPM.

I don't believe I have a fuel delivery problem...

I guess I'm not seeing how you determined that your fuel pump is bad? Did I miss that? You have a return style setup, right?

 

[Matthew Jeschke]

I have a return style setup. My Key On Ignition off pressure is 48PSI. 7 below spec which is 55 to 62 PSI.

I noticed, fiddling with my Corvette that the gauge will PEAK in the 55 to 62 PSI and settle down a bit lower, possibly even touch below that range. I'd venture you're pump is in great shape if you get 54 PSI during that test. Mine falls to 40PSI when engine is rev'd.

I have a new pump from Delphi (warranty replacement). Original pump I'm quite sure was bad regardless of pressure because the line set bled off quickly after engine & pump were shut off. It'd bleed down to like 35 PSI in a matter of minutes. This new one hold pressure in the lines even as much at 15 minutes later non has bled down.

Delphi tech support has me checking circuitry. They said to do voltage drop tests. I'm going to do that now. One across the whole line for starters then break into the positive side then negative side if necessary. Current draw, patching my amp meter in in place of relay was 5AMPS. I think maybe supposed to be 6 AMPS but battery charge was low. Lots of stuff I'm checking... Will update here as I go through whole process. I should have tested the voltage drops when I had the tank dropped. Now I have to drop it again *sigh* all be it only part of the way to access top of pump.

 

[Matthew Jeschke]

There is a connector, C152A (also C152? connector on wiring diagram but not listed in FSM)... I cannot locate it? Anybody know where it is?

 

[Matthew Jeschke]

Not a single electrical fault. Positive side voltage drop 0.6mV negative side voltage drop 115mV tested against all three chassis grounds on driver side.

I opened up the Harbor Freight fuel pressure test kit. Patched in on input side of fuel rail... 58 PSI. It's supposed to be a minimum of 62 PSI. I'm going to see if I can disconnect front of tank and patch in pressure gauge there to see what pressure I get. Maybe there is an obstruction despite I blew out the lines??? Long shot but I'll see.

 

[Fless]

I would think that Alldata would have that. Found this, though (#3 on diagram):

 

[Matthew Jeschke]

I'm almost CERTAIN this replacement pump is no good too *sigh*. RockAuto hasn't processed my warranty yet... but I want to find out if I can get my money back. There's a couple more tests I can do but pretty sure my line isn't blocked. If it was I'd assume it would pressurize much more inconsistently and getting even 55PSI would be tricky. I think they're not building the pumps to spec... Likely they shifted production to China, India, or some other place other than western world where we care about quality and they're lumping defects in with good pumps.

 

[Matthew Jeschke]

I would think that Alldata would have that. Found this, though (#3 on diagram):

View attachment 431797

Thanks! I ended up testing and found out there's no electrical issues in my fuel pump circuit. I maybe dense... I cannot figure out a great way to search AllData (at least the old version). I search for connector numbers and nothing comes up. Then I went to connectors by number and it didn't list C152. Thanks for finding that!

 

[Fless]

Be careful; that diagram doesn't look right if your PCM connectors are like mine. I have two long ones, not 4 shorter ones. YMMV

 

[Matthew Jeschke]

I'm looking at the fuel pressure spec for the Flex Fuel L59 engine in my Chilton's manual. It seems I'm getting the KOEO pressure the Flex Fuel system has. My system is a gasoline system with injectors are from the flex fuel system (poor man's high flow injector upgrade). I had assumed the injectors (when closed) would have no affect on pressure. Am I wrong?

 

[strutaeng]

There is a connector, C152A (also C152? connector on wiring diagram but not listed in FSM)... I cannot locate it? Anybody know where it is?


View attachment 431792
View attachment 431793

I believe these are inside the fuse box. Pop the the little tab and the fuse panel swings up/out.

I have a return style setup. My Key On Ignition off pressure is 48PSI. 7 below spec which is 55 to 62 PSI.

I noticed, fiddling with my Corvette that the gauge will PEAK in the 55 to 62 PSI and settle down a bit lower, possibly even touch below that range. I'd venture you're pump is in great shape if you get 54 PSI during that test. Mine falls to 40PSI when engine is rev'd.

I have a new pump from Delphi (warranty replacement). Original pump I'm quite sure was bad regardless of pressure because the line set bled off quickly after engine & pump were shut off. It'd bleed down to like 35 PSI in a matter of minutes. This new one hold pressure in the lines even as much at 15 minutes later non has bled down.

Delphi tech support has me checking circuitry. They said to do voltage drop tests. I'm going to do that now. One across the whole line for starters then break into the positive side then negative side if necessary. Current draw, patching my amp meter in in place of relay was 5AMPS. I think maybe supposed to be 6 AMPS but battery charge was low. Lots of stuff I'm checking... Will update here as I go through whole process. I should have tested the voltage drops when I had the tank dropped. Now I have to drop it again *sigh* all be it only part of the way to access top of pump.

Interesting. Yeah, definitely should not be dropping pressure that much. To determine if it's an injector or the checkball, temporarily plug the return fuel line at the rail. I did this once with a vacuum cap and hose clamp. Prime the system and the pump will deliver maximum pressure (80 psi on mine IIRC). If an injector is leaking, it will drop pressure. If pressure doesn't drop, then you know it's not injectors and most likely the pump checkball.

BTW, my engine is actually an L59 with flex fuel injectors. Although it's running on an LM7 tune.

 

[Matthew Jeschke]

Good idea

 

[strutaeng]

Good idea

Or just pop the fuel rails, prime the system and let it sit for a bit. If one is leaking down, it will be pretty obvious. Put some shop towels down in case there's fuel coming out of them.

 

[Matthew Jeschke]

My old ones leaked... Yeah leaking injectors is on my list of things to check. I figured I'd pull up the rail and put shop towel under the injectors to see if there's any drips.

I was smelling the oil to see if there was any gas smell in it from leaking injectors. I'd imagine after shutdown the gas would leak out and eventually down into the crank case. I don't smell anything but maybe it's not enough gas.

I wish I'd built a fuel injector test setup out of an old rail but didn't think about it till after my project was done. I should probably still do that.

 

[Matthew Jeschke]

Or just pop the fuel rails, prime the system and let it sit for a bit. If one is leaking down, it will be pretty obvious. Put some shop towels down in case there's fuel coming out of them.

So I did what you said. I found a way to disconnect and cap the return line. I put on a test gauge.

It got up to 80psi priming the pump w/ return blocked off.
The Pressure bled down quickly but I couldn't see any leaks in the line set.

I then put the return back on, and took off the rail (raised it above the intake). I put paper towels under each injector.

Not a single drip of fuel under any of the injectors.
Now pump only primes to KOEO around 45PSI.

I'm studying this...

It seems the drop in pressure from 85 would be correct. It should have a backup relief valve in the pump itself.