LB7: Low rail pressure, sometimes??

[snowman]

I got a friend of mine with a 02. it has mild mods,and full trans, all the usually stuff. But he was at the track in october with a 100hp tune in it, and went 14.6. Well he wented a bit more so our guy put in a tune that he just made, but haden't tested it yet. It was suppose to be built off a 300hp tune, and suppose to fuel quite aggressivly. He did another run with the truck and only got a 14.2, and NO smoke, none what so ever. So we checked rail pressure and at full throttle it was only 15k psi. But now when he drives down the road it will go to 30K??? then next time it's only 15K?? Any help at all would be greatly appreciated!!! Oh and the tuner said we probably have to change the CP3???

 

[Duramax963]

Bump... Need this info as well.

 

[JoshH]

Find a new tuner. Someone who knows what they're doing doesn't make tunes that are "built off a 300hp tune."

 

[TQMONSTR]

Find a new tuner. Someone who knows what they're doing doesn't make tunes that are "built off a 300hp tune."

pretty sure his tuner knows exactly what he's doing.

 

[JoshH]

pretty sure his tuner knows exactly what he's doing.

Since I don't know who his tuner is, I can't say, but the way the post is worded, it sounds like he is making his tunes by modifying a known tune (I'm assuming someone else's). Also, most tuners know how to diagnose a problem. If the tuner is so knowledgeable, why is his diagnosis being second guessed? There wasn't much info in the original post to go off of, but you seem to know what is going on. Why don't you fill in the blanks?

 

[TQMONSTR]

tuner writes all his own tunes. truck was never brought to the tuner to find the problem but he was asked what he thought the problem might be. I am guessing a second opinion is being asked on this forum so that the truck owner doesn't drop 1000 bucks on a cp3 if it's not needed.

 

[JoshH]

So are you the tuner?

 

[TQMONSTR]

nope. but he is a good friend of mine and my truck runs great.

 

[Chevy1925]

Plain and simple, he needs to go back to the tuner.

 

[Mike_S]

Plain and simple, he needs to go back to the tuner.

I agree. Only thing changed was the tune, right? Stands to reason either the tune is poorly written or the truck cannot handle the pulse its commanding. Either way, you need to get the other tune put back in or have this one fixed. IMO.

 

[Mike_S]

nope. but he is a good friend of mine and my truck runs great.

I have a good friend who is a good tuner, and my truck runs great too...but his tuning isnt in my truck. :thumb:

 

[snowman]

just to fill in some of the blanks. the old tune bearly smoked at all as well, and with this tune it smoked less. Weird??? i'm a diesel tech, but still learning on these trucks, so help is greatly appreciated. Maybe i'll swing in one day at the tuner's place and have a chat with him.

 

[adrenaholc1]

I have had the same problem for seven months now even as bad as low rp at the dragstrip and 500psi under on the way home. I just haven't had time to research the fuel system further and troubleshoot. Trans build needs to happen next anyways. Seems like there has been a handfull of people on here with the same problem but they fail to follow up on their threads or sell there trucks. Does that lb7 have a modded pump?

 

[JoshH]

just to fill in some of the blanks. the old tune bearly smoked at all as well, and with this tune it smoked less. Weird??? i'm a diesel tech, but still learning on these trucks, so help is greatly appreciated. Maybe i'll swing in one day at the tuner's place and have a chat with him.

That doesn't really help us diagnose the problem. A bad CP3 will be bad with either tune, as will injectors. We really need to see a log of both tunes to see what kind of pulse width and pressures it is commanding an holding. Pretty much every bad CP3 I've seen was having rail pressure problems even with a stock tune.

 

[LWATSON]

I've only had experience with one dmax with a bad CP3 it was an 01 and it would 1093 with the stock tune.

 

[snowman]

we just put in new injectors, and as far as i know that's the original injection pump. and the truck has 384,000kms. Plus i just found out that it's had gas run through it in the past. This is the trucks 4th set of injectors.

 

[Dallas S]

Find a new tuner. Someone who knows what they're doing doesn't make tunes that are "built off a 300hp tune."

Im guessing the tuner is Brian Wall. fairly big lsx efi tuner and FJO. some interesting dmax tunes in the past however. take the truck back to brian and let him have a crack at it. ive only had 2 bad cp3's but could get them both to1093 on stock pw and mpa. if it is a cp3 and the guy doesnt want to dish out a grand, I have a known good cp3 in winnipeg.

also, maybe i missed this, but what if any lift pump is he running?

 

[snowman]

yes it's brian and he has a brand new BD lift pump, i just helped him install about 12,000 worth of parts into this thing in july.

hey did you build up allens D-max? and brents?

 

[TQMONSTR]

Im guessing the tuner is Brian Wall. fairly big lsx efi tuner and FJO. some interesting dmax tunes in the past however. take the truck back to brian and let him have a crack at it. ive only had 2 bad cp3's but could get them both to1093 on stock pw and mpa. if it is a cp3 and the guy doesnt want to dish out a grand, I have a known good cp3 in winnipeg.

also, maybe i missed this, but what if any lift pump is he running?

what is so interesting about them?

 

[Dallas S]

unique pulsewidth tables for one. can tell it was a quick job. 120mpa/70mm3 and down, adjusted by certian % no blending. 120mpa and 4200pw? just sucks paying $1000 for a "custom" tune...

I have seen first hand the work come from walls. i could have just gotten the worst of the worst, and every time it is "employees screw up". just a bad taste in my mouth.

snow, Brian built Al's truck when Harry H owned it. a 7000 dollar allison... and a tune. Brents truck I tuned. Harrys new truck I built.