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Discussion Starter · #1 ·
I just bought a 2013 LML truck with 176k miles on it. When I bought it, it was all stock except tires and wheels. I noticed that sometimes it required extra cranking time to start up. Did some research and found that sometimes it could be due to a bad fuel filter vacuum sensor, or filter head.

Fast forward to today, I now have an airdog FP-100, fuel filter delete "bowl" on a brand new filter head, and plugged the fuel filter vacuum port.

No leaks anywhere. Sometimes the truck starts right up, usually when cold. Other times it takes a bit longer, but sometimes it won't start at all. Even after multiple extended cranking tries. Then If I leave the truck sit for 20 minutes or so, and come back to it, it'll start up after an extended crank.

Can anyone tell me what this might mean?? Getting very frustrated, as it seems to be a crap shoot on if my truck will ever start or not.

Once the truck starts, it runs perfectly, no codes, and has plenty of power!
 

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Discussion Starter · #2 ·
So i have been reading almost non-stop for the past day on stuff similar to this. I found a thread about the FPR2 (driver side front fuel rail) giving problems, possibly stuck open and not letting the rails build up enough pressure to start. So I ordered a new FPR2. In the meantime, I did some testing, just not sure what to make of the results.

I tested the voltage at the FPR2 connector. With the truck off, its at 0.00 volts. With the truck turned to Run (not cranking or starting), it reads about 0.49 volts. When cranking, the voltage starts to rise, and when the truck is running, it is at about 1.80 - 1.90 volts. Mini Maxx is reading 5-6ksi fuel pressure at idle, and during cranking increases gradually, reading at about 1.8ksi as the truck fires up.

This is on a cold start. I tried this test 3 times to make sure i got consistent results, and the truck started up perfectly fine every time! I will drive the truck to warm it up, and then see how it performs, if/when I run into the no-start problem again. i keep a DMM in my truck, so will be able to check voltages on the go.
 

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Discussion Starter · #3 · (Edited)
A bit more info....

While at operating temp and engine running, the fpr2 is getting about 2.1 volts. I guess this is normal, because I was idling around 4.5 - 5 ksi of fuel rail pressure.

While at operating temp, engine off, truck decides not to start again. I monitor voltage while cranking. At first you see 4 volts, but then goes down to 3.1 volts, and a fuel rail pressure of 1.6 - 1.7 ksi....just barely too low to start, since I read the truck needs 1.8 ksi

What gives? Why is the ECU not delivering enough voltage to this fpr? Or is there something wrong with the fpr where the proper voltage is still not allowing enough pressure? Hopefully a new part will fix it....

Battery voltage (not running) is at 12.41 volts, so thats good.
 

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Discussion Starter · #4 ·
So I happened to meet a diesel mechanic yesterday who told me a possible cause for the hard start: warm fuel is thinner than cold fuel, therefore harder to build pressure.

This would make sense as to why when the truck is cold it starts fine, but when its warmed up it takes longer to build up pressure to start.

I ordered a bunch of parts yesterday to add a fuel cooler to the truck, hopefully that helps!
 

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Discussion Starter · #5 ·
I replaced the CP4 FPR1 with the Exergy system saver FPR, and also replaced the driver side fuel rail FPR2 with a new OEM one. Also deleted the truck today, and tuned with mini maxx. For some reason the truck will not start, and is building even less rail pressure now, about 1 ksi while cranking. When I removed the originals, these are what FPR1 and 2 looked like:

Planer Automotive tire Azure Bicycle part Rim


Automotive tire Household hardware Gas Engineering Metal


Automotive tire Motor vehicle Camera lens Automotive lighting Rim


Household hardware Gas Fastener Auto part Nut


Automotive lighting Automotive tire Motor vehicle Liquid Automotive design


What could be going on? I dont think its the CP4 yet honestly, because it did start up once and run perfectly with great idle rail pressures! Now back to low rail pressure...
 

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My guess is that if you start pulling things back apart you'll find more metal shavings through your fuel system
 

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Discussion Starter · #7 ·
do you have any guess as to why the cranking rail pressure was affected after new FPRs installed? Should I put the old FPR2 back in to see if it will start with that one?
 

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do you have any guess as to why the cranking rail pressure was affected after new FPRs installed? Should I put the old FPR2 back in to see if it will start with that one?
Again, it very well could be metal shavings causing issues within your fuel system.
That stuff is like fine grains of sand/silica, floating around in your fuel system right now.

I would not reinstall the old contaminated parts
 

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Discussion Starter · #9 ·
well i cleaned off the original FPR2, reinstalled it, and same thing. Do you think the updated tune, for some reason, changed the voltage signal going to FPR2 which changed the pressure it's set to?
 

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well i cleaned off the original FPR2, reinstalled it, and same thing. Do you think the updated tune, for some reason, changed the voltage signal going to FPR2 which changed the pressure it's set to?
To test that theory you'll need to put it back to stock tune
 

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Discussion Starter · #11 ·
I know that with a stock tune and modified EGR/exhaust, the truck is basically in limp mode, but would it still be able to start and idle? I wouldnt want to damage the engine by using a stock tune to start the engine.
 

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I know that with a stock tune and modified EGR/exhaust, the truck is basically in limp mode, but would it still be able to start and idle? I wouldnt want to damage the engine by using a stock tune to start the engine.
The OEM stock programming from GM should not cause issues. It should start and idle
 

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Discussion Starter · #13 ·
Well I put the tune back to stock and no dice, still only about 1ksi rail pressure while cranking. I assume this leaves only one thing - bad CP4 pump?
 

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Well I put the tune back to stock and no dice, still only about 1ksi rail pressure while cranking. I assume this leaves only one thing - bad CP4 pump?
You need 4-6k for start and idle. Double check all fuel lines for any kinks or leaks .
Before you replace the pump I would go back through the fuel system and clean things...
ie ..drop clean the tank, fresh fuel, filters, etc

The more you clean the less likelihood of another parts failure.. Those metal shards left behind will flow through the system and tear up stuff..
 

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Discussion Starter · #15 ·
i read somewhere that 1800 psi is needed to begin firing the injectors while cranking, and is consistent with what I saw when I first started this thread. Then once the engine IS idling, it was between 4 - 6ksi, yes.

I went ahead and ordered the CP3 conversion from S&S, since it seems it will have to be done soon anyways, and what better time than when the truck is already down for the time being. Upon removal of the CP4, I will remove fuel system items and inspect/clean as necessary. And even though the lift pump and filters are brand new, I will go ahead and replace those filters before first start. Wish me luck!
 

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Discussion Starter · #16 ·
I went ahead and ordered an S&S CP3 conversion. I got down to pump removal and wouldnt you know it, it seems someone already replaced the CP4 on this truck. This current pump is definitely a reman pump from Diamond Advantage Diesel! Interesting....
 

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Discussion Starter · #17 ·
after the CP3 conversion, the truck seems to start up and run just fine! Hallelujah! I have yet to take the truck on any extended trips to get it up to operating temps, but it has gotten pretty warm around the neighborhood and will still start no problem when warm. Will update after some long hot drives!

I also ordered a bunch of parts to install a fuel cooler, which I may try to do this weekend sometime. Just to help the fuel system even more.
 
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