LLY: Still having Rail pressure problems

[Cougar281]

I got my KD Twin lift pump kit installed a few weeks ago, and am still having rail pressure problems. It's better than it was (Used to drop to about 60MPa when commanding 159, now drops to 125 or so).

At Eric's SF last weekend, I had my truck on the dyno and put down 446/820 with the PPE 330 tune. I had Dunbar log my runs, and found that then he went to 100% TP, the rail pressure dropped about 5200PSI almos right away (after about a quarter of a second), and over about 2.5 seconds, built back up to 20719, dropped back to 18760, then built back up to 19674 until it spiked when he let off the throttle.

I found it won't hold pressure with CIT1991's Economy+120 tune either.

When logging, I noticed that while rail pressure is building back up, the FRPACOM mA value remains lower than the FRPADES mA value while dropping until I let off the throttle.

I've got the PPE race valve and KD twin pumps (which I've verified are making over 7.5 PSI). I've attached a short log from one of my "pulls" with the "economy" tune.

Any suggestions what I should look for? I've checked the race valve (It's on SUPER tight), but I'm not sure what else to look at.

Thanks

 

[duramaximizer]

It has to be in your tune, because what you show us is the desired fuel rail pressure, not the actual. If you could show us what the throttle positioning sensor was reading, it would be a big help. It looks to me like he let off the throttle.

 

[Cougar281]

It has to be in your tune, because what you show us is the desired fuel rail pressure, not the actual. If you could show us what the throttle positioning sensor was reading, it would be a big help. It looks to me like he let off the throttle.

Everything's in that log file (The Engine PIDs I was logging are: Actual & Desired Rail Pressure, Desired & Commanded Regulator Current, Commanded Regulator flow, RPM, TP, MAP, BARO2, VSS). I doubt it's in the tune, because a) the commanded pressure is 23,061 the entire time I was at 100% TP and it seems for some reason the CP3 wasn't able to deliver and b) the plain, unmodified PPE tune is doing the same thing (to a larger degree), and I doubt every truck with big PPE tunes in it has this problem...

 

[duramaximizer]

Sorry for some reason the log showed up different earlier when I loaded it and looked at it. Does it shift there? Defuel? Fuel Pressure regulator going bad? Fuel filter, a bad fuel line some place sucking shut for some reason? Some junk in your tank? IDK just asking.

 

[Cougar281]

Nope, no shift... It's a 4th gear WOT run (5th Locked out, and defuel is disabled in the TCM), Fuel filter was changed two or three weeks ago and I verified that my lift pumps where operating properly; The Pressure regulator is something I was wondering if could be going bad, but I don't know what to look for or how to test it...

 

[slowlmm]

How did you verify your lift pump psi ?

 

[Cougar281]

How did you verify your lift pump psi ?

I used a KD Gauge when I was up at Eric's this past weekend.

 

[WICKED DIESELS]

A CP3 will get weak with added miles. Do you have the high flow rail fittings? It could be as simple as a bad FCA (fuel pressure regulator), might be that you are commanding more fuel than the CP3 can deliver.

 

[McRat]

Normally when I tune an engine, peak HP occurs with about 10% rail pressure drop. ie- if you were programmed at 23k PSI, you should be about 20.5k PSI at WOT 5th gear going through the traps.

A lot of folk have "fixed" excessive rail pressure drop with a ported FPR (fuel pressure regulator).

If you want to try that, I have WD PRV's in stock right now.

 

[Cougar281]

Well, at first I thought maybe it wasn't flowing enough fuel for the 330HP tune, but since it does the same thing to a lesser degree with a MUCH smaller tune, I'm thinking something else is up. I don't to just throw parts at it and hope something fixes it, though. I'd like to try to pinpoint the issue. Anyone know what a LLY with the PPE 330 (HOT+2 ET tuner) should dyno near?

 

[Kat]

Well, at first I thought maybe it wasn't flowing enough fuel for the 330HP tune, but since it does the same thing to a lesser degree with a MUCH smaller tune, I'm thinking something else is up. I don't to just throw parts at it and hope something fixes it, though. I'd like to try to pinpoint the issue. Anyone know what a LLY with the PPE 330 (HOT+2 ET tuner) should dyno near?

Should dyno +/- 500 rwhp

 

[JoshH]

What log is this? The lowest I see the rail drop is 20815 on a commanded 23061. I'm gonna guess pressure is up and then drops because it isn't commanding full fuel, but once it gets up to full fuel, it pulls the pressure down. Do you have a log with fuel quantity or pulse so we can see exactly why it is dropping?

 

[Cougar281]

The log attached is from the smaller tune... My dyno run with the PPE tune the numbers went much lower. Attached here is my log of the dyno run.

 

[JoshH]

I'm not sure what kind of pulse LLYs usually run, but 3350 sounds like a pretty big shot of fuel. I wouldn't be surprised if you are just running too much fuel for a stock CP3 to keep up with.

 

[Cougar281]

Could be with the PPE330, but the smaller tune (the first log file) should be able to hold pressure... I updated the log profile I was using and when I get a chance to do a few WOT 4th gear runs, I'll get another log of the smaller tune with the pulsewidth.

 

[LBZ]

Fuel Filter?
What was your lift pump pressure at WOT? 7.5 at idle may drop below 0 at WOT if there is an issue with the pumps. Obviously, you want a positive pressure against the CP3 at WOT.
Pumps in series help with pressure, but don't help much with volume. For more volume, plumbing two pumps in parallel is the reccomended set-up by Carter fuel pumps and I agree.
I will be testing this theory in the near future. :cool2:

Maybe try doing a log of a stock tune as well for reference.

 

[Cougar281]

Maybe try doing a log of a stock tune as well for reference.

Good point. As much as I hate to go back to stock......

The fuel filter is about three weeks old, but I got the same/similar results when the filter was fresh right after I installed the pumps.

I don't know what the pressure was WOT, but at idle it was in excess of 7.5 (pegged the gauge). Supposedly, with the twin pumps, it should be 8-9.5 or so. Also supposedly, these pumps are supposed to be high volume/low pressure.

 

[sweetdiesel]

you mentioned that you logged the MA of your regulator GOOD CALL

your regulator is not good....best way to fix it is get a new or modded one from Pat or GM
quick fix would be adjust b1010 if your lly or lb7,But it only slightly fixes the issue
just try bring it as close as possible actual MA vs desired MA

 

[Cougar281]

you mentioned that you logged the MA of your regulator GOOD CALL

your regulator is not good....best way to fix it is get a new or modded one from Pat or GM
quick fix would be adjust b1010 if your lly or lb7,But it only slightly fixes the issue
just try bring it as close as possible actual MA vs desired MA

Ok, so based on the logs, you find the regulator is bad? As a note, I logged the same PIDs and under the same circumstances (4th gear ODLO, WOT) and it held rail pressure. I'm guessing the stock tune isn't stressing it much... On my way home I'm going to log the Predator 85HP tune...

For a stock regulator, is there any difference between a LBZ and LMM? Either way (stock or modded), I'm going to have to live with it for a bit and save my pennies.

 

[sweetdiesel]

Ok, so based on the logs, you find the regulator is bad? As a note, I logged the same PIDs and under the same circumstances (4th gear ODLO, WOT) and it held rail pressure. I'm guessing the stock tune isn't stressing it much... On my way home I'm going to log the Predator 85HP tune...

For a stock regulator, is there any difference between a LBZ and LMM? Either way (stock or modded), I'm going to have to live with it for a bit and save my pennies.

best thing to do is put in a stock tune and see if you can match up the MA run it hard if you cant maintain then its done

how does the truck idle?

its possible that you just have a crappy filter, bad fuel obstruction of some sort

a friend of mine had a truck with only 3000 miles and a bad fprv it was due to bad fuel and ended up sending crap to the valve