How to tell if its cp3 or injectors

[onebaddmaxxx]

I have a thread a few threads down about my lb7 I just got and its hard start issues. Basically everything iv covered is the truck has an airdog that has 12psi at all times (checked with manual gauge at test port) I have ran new hose with pushlock from the tank to the airdog, from the airdog to FICM, and to the cp3 to a 1/2" fitting. The fuel filter head is deleted. As soon as you shit the key off, The MPA instantly drops to .9-1.0 and fluctiates between them. Gm says that it should be between 1.4-2.0. I logged another trucks and probably takes 20 seconds to come to 1.8mpa and stays there. As soon as you shut the truck off and listen to the engine, you can here bubbles on the top end for about 5 seconds then stops. Reason to believe a cp3 or injector might be dumping off. When I bought the truck It was like this.

When the truck sits overnight and I go to start it in the morning, I plug my v2 in and its still at 1.0MPA. Doesnt matter if you cycle the glowplugs or not (I just replaced them with new ones yesterday) It will crank at 180-200rpm and the rail mpa sits at 7.0 and will not build above it cranking. I cranked 4 times this morning for 15 seconds at a time. It instantly builds up to 7.0 then stops there. THe boy before me used ether to start it and after you got it running the first time of the day, it would start on its own build cranks for 5-7 seconds to start. There is absolutely NO i mean ZERO smoke of any kind. Balance rates are good. He said the injectors and cp3 were replaced 14k miles ago but who knows and I know new parts arent always good. What does everyone think I should do. Return rate test to see if one side returns more than another? How can you see if the cp3 is bad? Once the truck runs, it runs PHENOMENAL and I have a 3100us tune in it and it hold 160mpa perfectly. Never blows any kinda white smoke on start up or anything. Just looking to diagnose my issue before I start slapping parts on it

The balance rates are all within -1.0 - +1.0 and I have ZERO fuel in the oil and the level is not rising at all (iv put about 1000 miles on it). I just leave it plugged in at nights and it starts easier. Any help is appreciated!

 

[THEFERMANATOR]

What is the fuel rate when you check balance rates? Stock injecotrs or oversized? I think cranking speed is supposed to be 250-275 RPM's, but mine cranks over at close to yours and still pops off quick(I have smaller batteries due to my swap). A return rate test if done properly will tell the story. When you do it you are checking 3 returns, not just the 2 injector banks. You are also measuring the return rate of the CP3 during the test. Until you reach roughly 10 MPA, it will not start. You need roughly 1500 PSI(10 MPA) to get the pressure differential needed to make the injecotrs pop and inject fuel.

 

[plowboy_lbz]

Sucking Air somewhere?

 

[onebaddmaxxx]

Unless an injector is sticking open when I shut it off and it pulls fuel up in through the injector then I dont see how. All the supply side from the tank to the cp3 are new. and it did it prior to that.

Ferm-not sure i wouldnt be able to look at what the fuel rate is until I get off work. They are stock injectors

 

[THEFERMANATOR]

Do a return rate test. If you don't have the test set, then yo ucan do the poor mans cheap check to isolate if it's injectors or CP3. Disconnect the return line at the rear of the engine and cap it off. Next you will need 2 containers to measure returned fuel in, and 2 sections of 1/4" fuel line about 2-3 feet long. Disconenct the line that conencts the CP3 to the return line, connect one section of line to the CP3 return fitting, and the other end to return line. Disconnect the FICM and the FCA, put the lines into seperate containers, and crank it over until you get fuel out of one of the lines. Then crank it over for 15 seconds and measure how much fuel is returned into each container. Ideally you want to have less than 10 ML returned for all 8 injectors and popoff, and less than 1 once for the CP3. GM specs call for less than 2 onces from the CP3(35ML I think it is), and 30 CC's from the injectors(4 per injector, but 30 total. Yes the math don't work out right.). And the popoff shouldn't return anything.

This check will point you in the direction you need to go next.

 

[brewmax]

should the return check always be performed with the engine cranking over with the starter and without it running? If so how do you do this on an LMM? where is the ficm?

 

[THEFERMANATOR]

should the return check always be performed with the engine cranking over with the starter and without it running? If so how do you do this on an LMM? where is the ficm?

Yes, you can only do a return rate test with the injectors not being triggerred. Not sure, but I believe you pull the ignition 1 relay on an LBZ/LMM during the test. If the engien is running, then the injectors are going to return fuel(they do every time they are fired).

 

[x MadMAX DIESEL]

I think I had to pull the main bale connectors on mine. Iirc I couldn't find a fuse that would allow crank without starting

 

[brewmax]

Ok thanks I'm gonna try to get back on it later today.


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