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-   -   Long cranking time when engine hot (https://www.dieselplace.com/forum/63-gm-diesel-engines/40-duramax-second-generation-2004-5-2005-lly/974648-long-cranking-time-when-engine-hot.html)

zr2yz125 03-19-2019 02:40 PM

Long cranking time when engine hot
I'm getting an intermittent problem where the truck needs to crank longer than usual to start. Truck would usually start within 1 second of cranking, now it is taking up to 5 seconds of cranking.

Problem is more pronounced when engine is warm. I can drive the truck, shut it off for 10 seconds and then go to re-start and have the long cranking issue - but it does not happen all the time (don't you love intermittent problems).

90,000miles on truck, no chip/programmers. Fuel filter changed at last oil change. Batteries are about 4 years old, but I was ruling out the batteries because it starts after 5 seconds of cranking - a bad battery would not do this.

I'm starting to set up my OBD2 data logger to monitor FRP during crank. What does the rail pressure need to be start the truck?

I've seen the posts about the Fuel Filter Housing o-rings and rebuild. I'm leaning away from that as the problem is more pronounced when engine is warm, and less so when the engine is cold.

With no codes and the truck running great once its started, I'm not too sure where to start troubleshooting.

Thanks in advance!

gbhrps 03-19-2019 09:16 PM


At this age and mileage, I have now had 3 Duramax's that have had similar symptoms with long cranking times that were puzzling to diagnose the cause .... and each one came down to the same fix, even though each one had slightly different signs.

Rebuild your fuel filter head. You can buy the seals online for $30 or so on eBay, and YouTube has videos showing how to do the rebuild so that you get things right. Its not hard to do, its inexpensive, and the only special tools you'll require is the proper sized Torx screw driver ( why you don't already have a set in your toolbox is a sin anyway).

Video Link:

Now, while I won't put big money on this being your fix, I would be willing to put $20 on it, ... if we were friends who lived in the same area. This was the fix for my first 2004 2500 LBZ, my present 2005 LLY 3500, and a neighbour's 2005 2500 LLY. Good Luck!

Rvn4me 03-20-2019 09:21 AM


Originally Posted by zr2yz125 (Post 9807202)
What does the rail pressure need to be start the truck?

It takes about 2500 PSI rail pressure for the injectors to deliver fuel and the engine to start.
Just started?
That condition is usually caused by leaky injectors, that will not build up the rail pressure to fire the injectors. The reason it starts cold OK, is the fuel is more dense, tolerances aretighter, and does not leak as much.
Excessive leakage from the injectors usually results in a starting issue, which could occur hot or cold, but usually occurs hot because the fuel is thinner when hot. However, excessive leakage from the injectors can also cause a DTC P0087, P0093 or a P1093 to set. When using the scan tool to increase rail pressure at idle, if you can’t get to 21,000 PSI then the injectors are usually bad.
You can also use balance rates to help determine if you have any bad injectors. If an injector is leaking excessively into the return the balance rates are often at the edge of specification. Injectors that have a poor cylinder power contribution or a noise or smoke change when canceled will also need to be replaced and are likely to cause low rail pressure during cranking.
Balance Rates, when checked hot in drive after a hard test drive, should indicate bad injectors. Any injectors that are more than +/- 6 are a possible cause for rough run.
You can check the fuel return rate by removing the rail return line on each rail, and measure the amount of fuel in 15 min of cranking.
Engine at normal operating temperature, 181-189 deg. F.
Remove return hoses from one bank of injectors.
Use adapters to run hoses off of injector returns in to individual graduated containers.
Run engine until fuel flows from all four injector return hoses. Then with the engine at idle, command rail pressure to 17,400 psi with the scan tool. Place the hoses in the graduated containers for 30 seconds.
Turn off ignition and record measurements.
Repeat above steps on the other bank of injectors.
Add up all recorded measurements to determine total injector return volume. If the return volume is less than 144 ml for all 8 injectors, refer to the high pressure pump test. If the return volume is greater than 72 ml per bank, replace any individual injector with return volume greater than 18 ml.
What are your balance rates?
Google (Duramax lly hot start problem) lots of pointers.

zr2yz125 03-22-2019 08:49 AM

Thank you for the replies. I was searching "long crank" not "hot start problem"

...and RVN, did you mean 15 seconds (not minutes) of cranking? I dont want to add a starter to list as well...hahaha!

zr2yz125 03-29-2019 11:53 AM

Follow up - Fuel system was losing prime. Re built fuel filter housing and used the GM "upper engine cleaner" while I was at it. So far so good.

As a side note, truck seems to be getting better fuel economy now. Not to start a major debate here, just posting my observations...

Thank you all for your help!

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