Lb7 long cranking to start

Mnesset09

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Jan 3, 2020
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I have a 2003 lb7 with 130k on it. Never any problems with the truck until now. I changed my primary fuel filter two weeks ago which is the CAT filter conversion with a lift pump behind it and it primed up and started normal and ran great for a few days but start after start it started getting longer until the one time after driving 5 hours I shut it off to have it not start. Primer was rock solid but still wouldn’t start without starting fluid. The other night I took Apart my fuel filter housing and everything looked fine so I Lubed up the Orin’s put a new filter on it and to my amazement it fired right up like it should. Ran great for a day But now each start gets longer until it finally doesn’t start. I know I’ll get the “your injectors are bad” talk but I honestly do not think that is the issue. When the truck is up and running it never starves for fuel or has any surging. Starts better when the engine is cooled down. Any suggestions on what to try would be greatly appreciated
 

TheBac

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Apr 19, 2008
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No more "I think"...

Time to get a scanner and start checking balance rates, main rate and desired/actual fuel pressures.
 

2004LB7

Well-known member
Dec 15, 2010
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Fuel when hot is thin. Thin fuel leaks past injector returns and pump pistons easier. Thus making it harder to build pressure

Do the easy stuff first like return rate test on cp3 and each injector bank. Don't forget the pressure relief valve too
 

Mnesset09

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Jan 3, 2020
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So over the last few weeks I have replaced the fuel filter head. Put a ppei fuel pressure relief valve in. Checked my return rates which was in spec. Checked balance rates on injectors all good. I’m going to throw a fuel pressure regulator at it and see what that does.
 

KyleC4

Tech
Dec 30, 2016
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And what is actual vs desired fuel rail pressure while cranking when it doesn’t want to start? Also make sure engine is spinning enough rpm to make good fuel pressure.
 

Bdsankey

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So over the last few weeks I have replaced the fuel filter head. Put a ppei fuel pressure relief valve in. Checked my return rates which was in spec. Checked balance rates on injectors all good. I’m going to throw a fuel pressure regulator at it and see what that does.
Before you start just chucking parts at it you could really benefit from a proper diagnosis.

What is desired rail pressure vs actual while cranking? What is actual rail pressure while cranking? What RPM does it crank at? What is FPR commanded percentage or current?
 

DAVe3283

Heavy & Slow
Sep 3, 2009
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ReaperLegion.net
X2, get a scan tool on there and do some basic diagnostics.

The FPR can theoretically cause a hard/no start condition, but that is not normal. In fact, I have never heard of that happening. If it can't build pressure to start, there are only a few causes: bad CP3, bad injectors, air in fuel, or fuel leak.

Did you do the return rate test when the truck was hot, or cold? Since it starts fine when cold, doing the return test cold won't really tell you much. And while a return test can tell you for sure that a bank of injectors or the CP3 is bad, it doesn't necessarily catch a weak CP3. In other words, an out of spec return rate test proves a failure, but an in-spec return test doesn't prove good parts.
 

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