Loosing boost while pulling

[I_B_RAGZ]

Hey guys new to the page but have pulled info off it before for little things here and there.. ill get to the point, my truck looses about 7-10 psi while pulling my 40’ trailer only on a hard pull up a hill. Thing is that it will drop the psi but after a little bit it comes back. I run about 25-28 while pulling and it will all of a sudden drop to 18-20 then come back and go away periodically while pulling the hill.. running a mechanical boost gauge of that helps at all... thanks

 

[DAVe3283]

Just the gauge jumping up and down, or can you feel the power go up and down too? That should be A pretty substantial change in power.

Unclear from your signature what tuning is on the truck. Could be going in and out of a silent limp mode. Might be good to get a log and post it here. Things to watch would be main fuel rate, desired and actual fuel pressure, and main pulse width.

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[I_B_RAGZ]

Ok thanks, and yes huge loss of power. I’m pretty sure it’s the stock tune seeing how I bought it from a dealer 7 years ago and I haven’t done anything to it besides the exhaust.. this am I took my trailer down to our shop and the mechanic put his snap on comp on it while I towed the cp3 was pumping around 24,500 and when I lost boost I lost around 7,000 at my fuel rail only.. I’ve seen some threads on here about a relief valve?? People plug them or shun them.. any pros or cons to plugging it??

 

[Ron Nielson]

Clean both the MAF and the MAP. MAP senses boost. Also check to be sure you have a clean 5V feed to the sensor. Also check for any bare sensor wires/shorting and that you have a good ground. If your MAP provided bad info (lower than actual), both boost and fuel PSI would be decreased by the ECM.

 

[DAVe3283]

You need to log both commanded and actual fuel pressure. Losing 7k PSI isn't a problem if it was commanded to do so. If the tune says to reduce pressure, removing the relief valve won't fix anything.

The LB7 trucks rarely have the relief valve issues the LLY/LBZ had, so that is probably not your problem.

Go log it again, and watch the commanded rail pressure PID as well. You can also look at the MAF and MAP PIDs, and cleaning the MAF isn't a bad idea. Make sure to use MAF Cleaner fluid, not anything else. I don't know how you'd clean your MAP sensor safely...

 

[I_B_RAGZ]

You need to log both commanded and actual fuel pressure. Losing 7k PSI isn't a problem if it was commanded to do so. If the tune says to reduce pressure, removing the relief valve won't fix anything.

The LB7 trucks rarely have the relief valve issues the LLY/LBZ had, so that is probably not your problem.

Go log it again, and watch the commanded rail pressure PID as well. You can also look at the MAF and MAP PIDs, and cleaning the MAF isn't a bad idea. Make sure to use MAF Cleaner fluid, not anything else. I don't know how you'd clean your MAP sensor safely...

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[I_B_RAGZ]

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What does PID mean? And I have both brand new AC Delco map and maf sensors in the truck. And good to know about the LB7 not usually having the issue. I believe the desired was still up at 24 while the actual is what dropped. He said his computer will retain the info and he will bring it up when so I’ll get that info on here as soon as I can thanks for your help guys! I did injectors about a year ago hoping this problem would go away with new injectors, fuel lines, filters etc. but no luck..


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[DAVe3283]

PID is the name for a Parameter IDentifier, and is a thing you can log. Desired fuel pressure, for example, is a PID.

If the desired pressure is staying high but actual falls, that sounds like a mechanical issue, unfortunately. Get the data and we'll see what we can figure out.

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[I_B_RAGZ]

PID is the name for a Parameter IDentifier, and is a thing you can log. Desired fuel pressure, for example, is a PID.

If the desired pressure is staying high but actual falls, that sounds like a mechanical issue, unfortunately. Get the data and we'll see what we can figure out.

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Ok sounds good man thanks a bunch!


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[clrussell]

Sounds like a weak cp3 if the injectors are fairly fresh.

I’ve seen it a bunch on 01-02 trucks

 

[I_B_RAGZ]

If the cp3 was weak or going bad do you think it would do it with/out towing anything?? When I tow my jeep on my flat bed it doesn’t do or with my boat, only with a heavy load behind it

 

[clrussell]

If the cp3 was weak or going bad do you think it would do it with/out towing anything?? When I tow my jeep on my flat bed it doesn’t do or with my boat, only with a heavy load behind it

Heavier load= hotter fuel. Also might be doing it all the time just don’t notice it because of load. You need to watch actual fuel pressure vs commanded fuel pressure.

Might be a completly different issue

 

[fast03]

I am not remembering right? 19-21psi stock peak boost on LB7 with stock tuning. If the OP is getting 28psi on full boost he is tuned. I don't think the PPE boost valve will give the boost unless you have the fuel.
Maybe the gm safety tuning is shutting down fuel when exhaust temps climb with the heavy load climbing a hill.

 

[DAVe3283]

Good point Steve. 28 PSI does typically require tuning, the PPE boost valve on its own won't do much. But that would just increase stress on the CP3, so that is still suspect. Though the solution could be as simple as turn the tune down if it is tuned.

OP, it sounds like you bought your 2002 in 2011. It is possible, even likely, that a previous owner put a tune on it. Tuning was big business even that far back. I bought my 2003 in 2008 and it had a Edge on it, and I put EFILive on it the next year.

Have your friend with the scanner read the main injector pulsewidth PID at full throttle, as well as main injection timing. With the fuel pressure (desired), pulsewidth, and timing, we can tell you if it is a stock tune or not.

 

[2004LB7]

I am not remembering right? 19-21psi stock peak boost on LB7 with stock tuning. If the OP is getting 28psi on full boost he is tuned. I don't think the PPE boost valve will give the boost unless you have the fuel.
Maybe the gm safety tuning is shutting down fuel when exhaust temps climb with the heavy load climbing a hill.

I dont believe the LB7 has any exhaust temp sensor. It has no way of knowing if it is too high.

More then likely it is fuel pressure related. But could potentially be boost related too if the desired boost wasn't corrected for.

Both of these should trip a CEL but because it didn't leads me to believe that it is definitely tuned

 

[LBZ]

Maybe a fuel or other temp limiter?? If it comes and goes it doesn’t seem like it’s a hard part.

 

[I_B_RAGZ]

It used to throw the CEL light as an overboost code when it would come back, it would spike close to 30 psi and level down to 28 ish. But the new map sensor I believe took care of that.. I’m gonna try and get those numbers Dave is asking for today so hopefully I’ll kno more this afternoon. As of the “tuning” issue I’m not sure if it’s tuned or not all I’ve ever done to it was check and clear CEL.. was told by a buddy he thinks maybe a fin kit in the stock turbo housing but not 100% sure cause I’ve never had the urge to take it apart to check.. and for the boost leak i made an adapter to hook my air comp up with a gauge to check for leaks and all that checked out. Thanks again for all the help so far.

On another note would it be worth while to plug or shim that relief valve? Or would it hurt it not being able to return that fuel?? Towing my trailer this weekend to a hunting trip so wondering if I should throw that in and see.. thoughts???


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[fast03]

My LB7 never had a fuel rail issue and I was running 44lbs boost 600HP at the wheels.
so I would say not to worry about the shim.

 

[fast03]

I dont believe the LB7 has any exhaust temp sensor. It has no way of knowing if it is too high.

More then likely it is fuel pressure related. But could potentially be boost related too if the desired boost wasn't corrected for.

Both of these should trip a CEL but because it didn't leads me to believe that it is definitely tuned

Matter of fact, you are correct. No Exhaust temp probe. maybe some other sensor though since it is intermittent, while towing heavy,

OP: try dropping a gear sooner on long climbs to keep temps in check.
run 4th all the way up the grades at about 2200-2400 rpm and see if you still lose power.
Something is telling it to cut fuel.

 

[DAVe3283]

You're just as likely to cause the relief valve to leak as you are to fix it with a shim. It has to hold 24,000 PSI. LB7s aren't known to have them fail, so I don't think you'd see any benefit to messing with it. And there is the possibility you create the very issue you are trying to avoid. I am running the stock relief valve on my built motor that sees 27,000 PSI and it doesn't leak or pop open.

If the ECU is setting the overboost code, it can go into a limp mode, and that would reduce the power output. You might want to remove the PPE boost valve before your trip, as that might solve all your problems.

The Duramax is a complex engine with a complex engine management system. Adding hard parts without a tune will at best do nothing, or cause problems. So if the truck is stock, that boost valve might be your problem. You always need the hard parts and tune to match on a Duramax to get good results.