Lbz fuel pressure sensor data

[whytry]

Can anyone tell me the limits of the Lbz fuel pressure sensor? I have an Lbz that will not start. Upon diagnostics I found the rail pressure is reading a steady 71psi when ignition is on and when cranking....It doesn't move at all...When the sensor is unplugged it reads 29k~...I am thinking I have a bad sensor ...Not showing pressure and not allowing the ECM to fire the injectors? Most all other Duramax's show 200-250psi when sitting with engine off

 

[IOWA LLY]

I'm pretty sure the ECM will still fire the injectors. I'm betting you have other problems.


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[MarkBroviak]

I've seen anywhere from 0-270psi with the key on engine off. The ecm won't fire the injectors until the actual rail pressure jumps past 1200psi so you need to figure out why it isn't showing any pressure cause it appears that you are good on the wiring for the sensor. Crack a line loose and see if you have fuel pumping while cranking.

 

[Awenta]

Return rate test time. Or a dead cp3


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[2004LB7]

ECM will not open injectors until it sees 10 mpa or 1450 psi

I have gotten my truck to start when it had bad injectors by unplugging the pressure sensor tricking it into thinking it had max pressure while cranking

 

[whytry]

Thanks guys... The truck has a new cp3... I have unplugged the sensor and showed the ECM 29k~ it still wont fire..The sensor reading still has me baffled I never seen a rail pressure ready so steady normally they bounce around or "jitter" I mean it don't move off of 71 psi
But there again if I do indeed have rail pressure why wouldn't it start with the sensor unplugged? I suppose I will crack a line and see whats there...

 

[whytry]

I guess what's really hard to believe is that there is that big of a leak in the fuel system that won't allow any amount of pressure to be built. Specially with a new cp3

 

[DAVe3283]

I guess what's really hard to believe is that there is that big of a leak in the fuel system that won't allow any amount of pressure to be built. Specially with a new cp3

^^ Actually, if you just put a CP3 in, a no-start isn't that surprising. It either hasn't primed yet (still has air on the suction side), or you got a bad pump. Wouldn't be the first time, sadly.

 

[whytry]

Well I had the same symptoms before the new pump...As a matter of fact the same exact symptoms ...I have put many fuel systems together and don't feel it's a priming issue. I need to come up with a way to test rail pressure there can't be any way that there is only 71psi in that rail and also why is it that rock steady. Those sensors are so delicate there bounce around. Even with the ignition off this one don't. I appreciate all the input.

 

[Frankin_truck]

My guess Is you have a massive leak. Never seen it happen to me but you could have a hung open injector?

 

[whytry]

I suspose I can check returns on the injectors while cranking. I don't think there is a published spec but I would imagine it should be near zero ...I've done many test at idle while commanding 17k

 

[2004LB7]

Have you tried disconnecting on of the high pressure lines to see if you are getting any fuel out of the CP3?

 

[whytry]

My guess Is you have a massive leak. Never seen it happen to me but you could have a hung open injector?

I'll pull glow plugs and see if there is a bunch of fuel in one cylinder

 

[whytry]

Have you tried disconnecting on of the high pressure lines to see if you are getting any fuel out of the CP3?

Not while cranking but I have opened injector lines after cranking and there was fuel there ...I don't like guessing/ or cracking high pressure lines but I am at that point I guess

 

[mebob2001]

I had one as a no start 3 weeks ago. Lbz. That had 5 injectors returning to much and it wouldn't build past 400. Do a good visual inspection of fuel system no kinked lines. It's not sucking air can be done with a clear plastic hose.
But I would test FPRV first then focus injector return. If u put a pump in and zero change. Then u know it wasn't the pump. U can try just removing the return lines from tops of the injectors and just crank the truck and watch. 3ml of fuel isn't very much. It's usally obvious which ones are really bad. Spec is 3 to 5 ml depending on a piece rating of fuel.... but ya anything over 3 to 4 range no good.



But if u have access to service information or all data under high pressure side diagnoses. U can find it under p0087 code. U will see engine will start testing and engine will not start. That same truck I put he 5 in it got it to start no fuel knock at idle but command rail pressure up to 12k and it would fuel knock
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[whytry]

I had one as a no start 3 weeks ago. Lbz. That had 5 injectors returning to much and it wouldn't build past 400. Do a good visual inspection of fuel system no kinked lines. It's not sucking air can be done with a clear plastic hose.
But I would test FPRV first then focus injector return. If u put a pump in and zero change. Then u know it wasn't the pump. U can try just removing the return lines from tops of the injectors and just crank the truck and watch. 3ml of fuel isn't very much. It's usally obvious which ones are really bad. Spec is 3 to 5 ml depending on a piece rating of fuel.... but ya anything over 3 to 4 range no good

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In your experience would 3-4 ml cranking cause a no start condition? And cranking how long 10seconds? I would think cranking it's gonna make 3-4k rail under normal conditions so and fuel returning wouldn't be good

 

[mebob2001]

In your experience would 3-4 ml cranking cause a no start condition? And cranking how long 10seconds? I would think cranking it's gonna make 3-4k rail under normal conditions so and fuel returning wouldn't be good

Sorry I should have said that sorry have a shit ton on the go right now. 6 hours sleep in the last 72 hours. But 3 to 4 ml in 15 seconds of cranking.
Well for example the one I just had 3 weeks ago the total amount returning off the 5 injectors was 19ml (3 injectors) on passengers and 11ml(2 injectors) on drivers. I added them up. But u could also check total return on one bank. Find the end of the return lines on drivers side is behind the GPCM. And on passengers it's at the front of the engine under the intake to you bridge. Right in front of egr valve and beside the fuel lines. If u check the total return per bank just use 16 ml as a guideline anything more then test them one at a time.

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[whytry]

OK update I have preformed a injector return rate test on both banks . I have cranked for 15 seconds I preformed the test with the sensor plugged in and unplugged. I didn't even get a drop of fuel from the return. I also checked the return loop at the relief valve. Also not even a drop. Still no reading from the fuel rail sensor. Still displays 71psi any body know what pids to log to show if the injector is getting the signal to fire? Also is there a test I can preform on the fuel rail sensor? Like with a multimeter. Still completely baffled to why the scan tool reads 71psi all the time ...That damn Reading don't even flinch

 

[IOWA LLY]

Sounds like your CP3 pump isn't pumping any fuel.

 

[Dvierra86]

If you unplug the frp sensor it would go one way. Then short the 5v ref and the signal together using 2 amp jumper and the frp gauge so change to the max opposite number


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