I get alot of PM's about P1093's, so I wrote down what I know from the last 6 months worth:

Basics -

Your engine is a Common Rail fuel injection design. This means that there is high pressure fuel at the injectors anytime the engine is running. On older designs, the fuel pump pushed fuel to each injector in sequence, and the pump controlled the timing, not the injector. With ours, the injectors are operated electronically by the ECM via the injector control, and are independent of the pump. The pump does nothing but make pressure for when the injector demands fuel.

Our fuel system has the following main parts:

ECM - Engine control module, aka PCM, aka the computer.
Tank
Filter assy
CP3 High Pressure Fuel Pump - This has 2 sides, a low pressure side, and a high pressure side. The low pressure feeds the high pressure.
CP3 solenoid - This controls the pressure going to the injectors by diverting fuel from the low side to the high side.
Fuel rails - holds pressured fuel for the injectors. There are two, drivers side and passenger side.
Fuel pressure sensor - on the passenger side rail.
Injectors
Return line - When the injectors fire, they must release some fuel so the plunger can move. The CP3 also needs to release fuel to operate the low pressure side. This no-pressure fuel goes back into the tank after being cooled in a fuel cooler near the tank.
Fuel pressure relief valve - In case something goes wrong with the CP3 solenoid, this valve will bleed off excessive pressure. It is on the back end of the driver's side fuel rail.

When the engine is running normally, as you drive the ECM commands the CP3 to make a certain fuel pressure to feed the injectors. Different driving conditions require different commanded pressure. The ECM checks the pressure sensor to verify the CP3 has made the desired pressure. This is the heart of the P1093 problem. If the engine gets readings that say the rail pressure is too low, and gets them enough times, it assumes something is wrong with the fuel system, and puts the truck into LIMP HOME mode, which in this case is a 2000rpm engine RPM limit.

With a stock truck, with stock tuning, this can be caused by various reasons:
Dirty fuel filter
Fuel line restriction
Pin holes in the fuel feed system/bad fuel filter seal
Leaks in fuel rails
Bad CP3 pump or solenoid
Faulty Pressure Relief Valve

A stock truck should not throw P1093's with a clean filter. First change your filter, and if that doesn't fix it, see your dealer.

To see if the Pressure Relief Valve is faulty, here is how you troubleshoot it:

On the driver's side fuel rail near the back end, there is a 3" long rubber tube. It is the drain tube for the relief valve and it goes into the fuel return block, and back to the fuel tank. Pull off the end that goes to the rail, and plug the hose so other return fuel does not spill. This is no-pressure fuel. On the exposed nipple that you pulled the line off of, put about 3' of fuel hose on it and hose clamp it on. Put the other end of hose into a 1 qt bottle, and attach it to a safe place away from exhaust, and as low as you can.

Drive around with this setup and check the bottle. There should be no fuel collected in it. If there is, and your truck is stock, the driver's side fuel rail assy needs to be replaced. GM does not sell the relief valve separately.

When we started getting P1093's in great numbers is when the bigger tuners for the LLY came out. First we thought it was fuel starvation, and put lift pumps on, and low-restriction fuel feed systems. While some people reported success doing this, most only saw a reduction in P1093's, not elimination.

We now know what causes the Type II high performance P1093, and it is actually high pressure, not low pressure. Apparently the big tunes on the LLY are causing a brief pressure spike which trips the fuel pressure relief valve, which in turn dumps most the pressure out of the rail, and back into the fuel tank. The ECM sees the low pressure, and can set a P1093.

You can be dumping fuel from the return valve without setting a P1093. Apparently it must do it enough times before it sets it.

Some owners have reported LIMP HOME mode without setting a light. Personally, I've never witnessed it. Need to collect more data in this area.

Why don't you set P1093's when racing? The high demand for fuel when racing, seems to keep the pressure spikes low enough to avoid popping the relief valve. Or perhaps the P1093 test is more lenient when the engine is at WOT.

What conditions set the most P1093's - Big programs or stacks. Lifting the throttle quickly and reapplying it, like in a passing situation. Old fuel filter. Low fuel level.

How do you get rid of a P1093? - With a Predator, PPE, Quad, Bully Dog, Tech II, AutoTap, EASE, Actron, etc you can go to a diagnostic menu (even while driving) and erase DTC's (diagnostic trouble codes). Or you can pull over, shut off the engine. Restart the engine 3 times, allowing it to run long enough to go through the startup cycle.

What can I do to fix it? Run a smaller tune, or increase the pop pressure of the Fuel Relief Valve. A tutorial to modify the valve will be posted soon.

Great information, Thanks for posting. I am trying to buy a duramax right now, accumulating info on what to do to make it pull better. BTW... I also own a electron blue 02 z, mti 427, it sees the drag strip, road course and High speed road rally events. Would be cool to compare notes.

Mcrat: THANKS!! alot of good info ,and understandable , I had my first experience with this , and it gives me alot to start with to avoid the dealer hassles.

Mine throws the P1093's and goes into limp and I have never seen a Check Engine Light...

will dual pumps fix the bursting//limp with 1093 when running a stack or do i need to shim the pressure relief valve also??

will dual pumps fix the bursting//limp with 1093 when running a stack or do i need to shim the pressure relief valve also??

Dual pumps will not fix the 1093 issue

I have continually seen 1093 with the hypertech and a fresh filter. ?????????????????? I just erase it and go on my merry way. Now do I have to worry about limp setting in at any givin time? I just put 3300 miles on it and a lot of fuel within the last week and I had no problems at all other than an occassional harsher than normal shift.

thanks McRat :)

I threw the p1093 code yesterday, I was trying to pass a slow car, level 5 j/a, low fuel light came on, then all of a sudden sounded like an air pop, hesitation, limp mode, check engine light. Made it to the fuel station, 25.5 gallons later, check engine light still on, limp mode gone. Went to my parents house, started back up, check engine light off, but still has p1093 code stored in computer, because I checked it with my OBD II code reader.

Nice write up McRat, Thanks. I love learning about what makes these things tick(or not tick). I'll be patiently waiting to hear about the fix. Thanks for the sharing of all the great info.:ro)

Rob

Thanks for the info. I tow a lot and this has been happen'n frequintly since I installed the edge e-z, I use it on level one! FYI, never a code, I stop the motor, give the prime pump a couple of shots then with one restart she's ok! Oh well, I just orded a lift pump for the aux fuel tank, at least I'll be able to spend some quality time with the ole girl install'n it!

I just got a 1093 for the first time.
Was going a steady 70mph at the time though? I had romped the throttle 30minutes and a shutdown earlier, but didn't go in to limp at the time.

You said you can reset the codes while rolling... will it come out of limp without a restart that way then too?

Thanks for the great info...!

You said you can reset the codes while rolling... will it come out of limp without a restart that way then too?

Yes, with the Predator. He has gotten quite good at it :D

Yes, with the Predator. He has gotten quite good at it :D:cool2:



I also threw a P0401 (EGR flow, insufficient detected) & 0404 (EGR Circuit range / performance) at the same time... related at all?

:cool2:



I also threw a P0401 (EGR flow, insufficient detected) & 0404 (EGR Circuit range / performance) at the same time... related at all?

I don't know:( I will have him read this when he gets home :cool:

The EGR codes are usually caused by either insufficient backpressure in the exhaust system, or a hiccup in "harness box" style tuners.

For the backpressure, use a finger stick to fix. For the harness box, try pulling on the pins with needle nose pliers.

Please don't just diassemble these valves and shim if you don't know the cause of your 1093. The catch can has been explained several times to verify your relief valve poping.
I disassembled several more today all brand new. Some bosch replacements and some GM fuel rails.I was amazed the extreme variance in these valves.I measure the spring bore depth to the inner shelf plus the measurement of the seat depth to the washer(the one with the 3 holes) to come up with the spring assembled height.Most valves are .780 + - The new valves from Bosch measured only .750 depth. This makes the assembled height .030 less than most.Here's the kicker, Most springs are 38-40# @.780. These springs were only 32# @ .750. Thats 6-8 # less and .030 shorter assembled height.It took over .060 to get the seat pressure to 38# pressure at the proper assembled height.(.750 on this valve)
Once I got the seat pressures close, the next .010 increments matched very closely to the spring I took out of a valve I replaced that was poping on a STOCK TRUCK.This spring in the other valves was under 15# @.780 assembled height.The bore depth on the GM SPO valves were.770-.784 and for comparison sake 38-41#@.750
Just don't shim these valves just because you can. At least try to get measurements so there is some method to your madness.I think I found another INDEPENDANT testing facility that will test these if I send them.I don't have time to go myself.

Please use this information at your own risk. Don't get stupid.):h
The relief valve torque is in German. It's goodntight
Tim