P0168 code on c5500

[coors1436]

Hello, I am currently working on a 07 lly c5500 that has an issue i can't seem to work out. Truck came in with p0087 code and obvious rail pressure concerns. Diagnosis of low pressure yeilded a new injection pump, and one injector that had a high return rate. After replacement of those parts as well as a FPRV shim kit i no longer had rail pressure concerns and no more limp mode. During a test drive with trailer on warm (90) day, I was suprised with a reduced engine power message on IPC with no rail pressure problem. Only DTC setting is a P0168 fuel overtemperature, which according to TAC will not cause a reduced engine power light. So i looked at the snapshot i took at time of REP light and the DTC P0168 set at the same moment. Fuel temperature was at 239 during this frame and it states in the DTC conditions for setting that it must see 252 for at least 5 seconds to set the P0168 code, weird. I then attempted to turn that DTC off using EFI, I also allowed more rail pressure per fuel temp. After tuning DTC continued to cause REP light and dinger to come on although DTC was invisable to a TECH 2, it could still be read with the V2. I then tried a fresh GM tune, and allowed it to ramp down the rail pressure to control fuel temp. That test drive it would only get to 235 before being held back by stock tune, but yet the P0168 still set at that temp, Any Ideas as to why I can not shut that code off and why it is setting below the stated parameter? Any Ideas would be considered and appreciated, thanx

[Motorzane]

We had a 07 C5500 with a LLY 6 months ago with the exact symptoms. We wound up replacing all 8 injectors. One had 5ml of return and the others had between 3-4.5ml. We sent them all out to be tested and the injection shop wanted to know how much Bio Diesel was being ran. After replacing all 8 injectors finally the issue. Make sure you program the flow rates with the Tec2 in the ECM and the GPCM.

[coors1436]

I agree 100%, i thought it over after I had an Lmm come in that had all 8 sticks brutally fail gm's 18 ML spec at 140 mpa, After replacement of those injectors, fuel temp dropped nearly 100 degrees, even on an almost empty tank. thanks for confirming my suspicion. The fuel cools by the time you get that test set up and i think the return values change to a much lower value. I still wonder why it sets it below the spec and the efi live tune can't shut it off, there is more to that os than what you can see with efi live, and the gm scan tools for that matter. Thanx again for taking the time to give a guy a second opinion sometimes that's all it takes

[roleary44]

Hello, I have a 08 c5500 with a lmm and I'm having one heck of a time with hot fuel and 0087 (235 deg). My truck works 6 days a week so it needs to run correct. I have a DPF and EGR delete, full turbo back exhaust, efi live tow tune and ct insight. If I run a injector balance #5 shows +2 and the rest are in the minus, so I'm thinking that might be a start. All the injectors were replaced at 85,000 miles and the truck broke a crank at 135,000 so I've put plenty of $$$ in this truck. Currently we have 202,000 on it. Should I spend the money on more injectors and cp3 pump or just trade it for a Dodge?

[Motorzane]

If you getting high fuel temps, then your injectors are returning too much fuel back to the tank. If your balance rates are all close to each other, then it's more likely that all injectors are returning too much fuel. Spec on the Balance Rate is +4 to -4 in park or neutral and +7 to -7 in gear