Defintley on my list of next mods../ trans work/Better tuning/ some type of fuel mod.-thanks for the info.

 

OK, to clear up the misinformation.

Some credit me with the "shim mod" for the LLY, but the truth is it was a group effort by the LLY racers, I just figured out how to shim the valve without hurting it first.

Here's the scoop. We run a high pressure common rail fuel system that is programmed for a peak of 23,000 PSI fuel pressure. The CP3 high pressure pump is capable of going up past 26,000 PSI. There is a pressure relief valve that will open when pressure exceeds about 25,000 PSI. When it opens, it dumps all the fuel pressure and the rail pressure sensor reports this to the ECM, which assumes that there is a massive fuel leak. It throws a P1093 code, and limits RPM to 2,000. As is stated above, this can be a serious safety hazard in some circumstances, passing on a 2-lane road comes to mind.

To make matters worse, the quality control on the PRV's was not good. Some pop at just over 23,000 PSI, and will P1093 with stock tuning. There is a GM TSB on it. By increasing the pop off pressure, you fix the problem. Tunes over 75rwhp gain tend to P1093 because they are shooting in over 1/3rd more fuel, so when you lift the throttle, it "spikes" the pressure more than it would with stock tuning.

Why did GM put in a PRV? It has to do with how the CP3 pump works. With no power supplied to the CP3, it goes full blast, with power applied to it, it cuts the fuel. So if the wire gets cut or solenoid fails, the CP3 goes max pressure all the time, even at idle. By having a PRV, it will dump the fuel and throw a code for service.

So far, it appears that shimming the PRV has no ill effects, and many trucks are now over 50,000mi with no problems, literally HUNDREDS of trucks.

By itself, shimming is not a power adder. But it does allow you to run more powerful tuning than the stock PRV would support.

PPE sells shim kits with instructions, as well as Race Valves that never pop off. www.pacificp.com

I can write custom tuning that removes the 2000rpm limit if you 1093, and has a very smooth fuel curve that resists popping the valve. But for racers I still recommend shimming.

 

Nice explanation McRat!

 

Btw, whats the difference between the Race Fuel Valve and the Shim kit?

 

I agree! You got to post here more often!

 

I removed the 2k limit on mine, so when it 1093ed I would still beable to drive. I was a little concerned when I did it though. Is there any problems that could result from driving with the 1093 active and still being able to go over 2k?

 

Difference between a race valve and shimming, is that shimming correctly will make the valve pop at about 28k PSI, a race valve is blocked entirely.

No problems that I have observed on dozens of truck by stopping P1093's through shimming, blocking, or reprogramming.

If you play with the fuel tables, you can reduce or stop the P1093's.

For the paranoid folk, you can replace your LLY PRV with a LBZ PRV, which already has a higher pop pressure.

 

Thanks!

 

Is the P1093 what causes limp mode?
I was looking around in HP Tuners on a LLY tune and found there is a Limp Mode and Engine Protection Modes 1-4. I've been trying to find out what causes each.

What causes each mode and can they be safely changed?

Thanks