Duramax Diesels Forum Truck of the Week
  #1  
Old 05-19-2019, 07:04 PM
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Default Think Im in Limp Mode

Hey guys, I think my lift pump (Raptor 100) died...
I was driving down the flat highway cruising at 70. It felt like it was losing power so I gave it more throttle and it still felt like it was slowing down. The trans tried to downshift into 4th once I was down to 50ish but it just kept staying in 5th. Then all of a sudden the trans started to downshift 1 gear at a time (on its own) and brought me to a stop (while I was still on the throttle trying to get somewhere safer to pull over). When I was pulled over I didn't see anything leaking or overheating or out of the normal.... I shut the truck off and gave it a few minutes (in 90* weather). The truck started right up no problem so I tried to drive and it wouldn't shift or go over 11 mph in 1st gear..... Since I don't have AC and my wife and baby were with and we were now running late for my brother's wedding, I kept going.... 11 mph down the shoulder of the highway! About 2 miles later it shifted into 2nd gear and we were cruising at 22 mph.... 9 miles later I took the 1st exit and pulled into the AutoZone. I had someone scan the OB2 and it came back with no codes. My lift pump started making a loud whining about 100 miles ago and now its not making any noise at all... so I think it's dead. My truck is all stock with fairly new injectors. From my research it seems this is common to throw the trans into limp mode with low fuel pressure.

So I'm planning on buying a new lift pump and changing both fuel filters before I do anything. I know most people are going to tell me that Raptor lift pumps suck and that I need a Fass setup for $800 but I already have a Raptor 100 mounted and wired up with new fuel lines, so I don't really want a whole new setup. Plus they come with a 4 yr warranty so that will be good enough for me.
What I'm wondering is if I can upgrade to a Raptor 150 which is a "universal" lift pump and have it be a direct replacement for me??

Also, does anyone have other thoughts or questions on my situation?
Anyone think I might have a problem with something other than the lift pump causing low fuel pressure?

Truck has just under 140k miles.... Let me know your thoughts.

Thanks!
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Old 05-19-2019, 08:08 PM
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Is your truck tuned? Should have thrown a code if it did go into limp mode. But the way you describe it, it does sound like limp mode

If I'm not mistaken, the raptor lift pump are flow through design so even with it not running the fuel should still be able to get pulled through.

I would not say the raptor lift pump is bad buy any means. I have a raptor 150 or maybe the 160 on my truck. Baught it used about years ago and it is still running strong today
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Old 05-20-2019, 05:48 AM
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I have an Edge tuner and I've never had it on anything besides the #1 setting (Fuel Economy). I've read a lot of other posts where others were in limp mode with no engine codes, what's the deal with that?
I haven't tried to disconnect the batteries and see if it fixes itself yet... but I know for a fact that the lift pump is dead so I want to replace that before I do anything, and the filters are due for a change anyways too so I want to start there.
Anybody have any other thoughts or ideas?
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Old 05-20-2019, 06:04 AM
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Have you tried to bypass the pump and run it?

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Old 05-20-2019, 10:34 AM
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Quote:
Originally Posted by ironmax View Post
Have you tried to bypass the pump and run it?

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I haven't. I don't have a way to connect the lines to each other.

I was just looking under the truck and the fuel line that goes from the tank to the Raptor looks factory and its dry rotted and it feels very weak...
I read someone else's post about the fuel line collapsing. Maybe that happened and burned out the lift pump for good?? Idk, that Raptor has been screaming for a few weeks now, I knew it was only a matter of time before it died!

The guy at Raptor tech support just told me that the Raptor 150 will be a direct swap and shouldn't need different fittings or anything so I'm probably going to go with that, it's only $244+$25 shipping. That's still $8 less than the Raptor 100 on Amazon with Prime!
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Old 05-20-2019, 11:05 AM
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Did you take the edge out and see? What you are describing is not limp mode. Trans limp is 3rd gear and reverse only, engine limp is 2k rpm limit.

It sounds like you could have lost fuel pressure and it just simply can't make enough power to move the truck. I would have expected a code if it is running that poor.
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Old 05-20-2019, 07:53 PM
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Quote:
Originally Posted by snowman22 View Post
Did you take the edge out and see? What you are describing is not limp mode. Trans limp is 3rd gear and reverse only, engine limp is 2k rpm limit.

It sounds like you could have lost fuel pressure and it just simply can't make enough power to move the truck. I would have expected a code if it is running that poor.
Engine limp and Transmission limp are two different things. Transmission limp locks it into 3rd for forward. Engine limp restricts fuel MM3. Both happen for different reasons.

If it is still shifting through the gears but requires pushing the throttle all the way down and it doesn't seem to want to get up to speed or is restricted to a low MPH then it is likely engine limp home mode
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Old 05-21-2019, 09:23 AM
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Quote:
Originally Posted by snowman22 View Post
Did you take the edge out and see? What you are describing is not limp mode. Trans limp is 3rd gear and reverse only, engine limp is 2k rpm limit.
It sounds like you could have lost fuel pressure and it just simply can't make enough power to move the truck. I would have expected a code if it is running that poor.
I didn't uninstall the Edge tuner and try it. I will do that if nothing changes after I install the new lift pump, water separator, CAT filter and reconnect the batteries.
I usually drive in tow mode so that the trans stays in current gear a little longer while accelerating and so that it downshifts to help slow me down while on the brakes. When I was on the highway, it kept me in 5th gear and wouldn't downshift until I was going really slow and it just started down shifting and slowing me down without touching the brake pedal.... that's what makes me think it's a trans thing.
I think it lost fuel pressure too when the lift pump died. The truck felt like it was losing momentum going up a slight hill.... but the road was flat! It just kept driving 50-60 mph no matter how much throttle I gave it, for about a mile or so, then it started down shifting on it's own and I didn't have a choice but to pull over (no brake pedal).
I can't believe there is no CEL code after *something* made it slow down and bring me to a stop... it was creepy like someone else was controlling it. The engine didn't make any weird noises and anything out of the ordinary besides the slight power lose, at the same time, the gauge said I was still making 7-10# boost depending how much throttle I was giving it to keep going, which was normal for that road and that amount of throttle. Maybe I was making too much boost for the low fuel pressure and it lean'd out??
When I was restricted to 1st and then 2nd gear, the engine sounded and ran good, like normal. I was getting a steady 47-49 mpg @22mph in 2nd gear for 9 miles down to the exit.
When the tow truck got there, I drove right up onto it no problem, drove up my parent's driveway no problem, reversed no problem.... Let the engine cool down, it started right up no problem.
It's been parked there since. I'll be back over when the lift pump shows up.

Quote:
Originally Posted by 2004LB7 View Post
Engine limp and Transmission limp are two different things. Transmission limp locks it into 3rd for forward. Engine limp restricts fuel MM3. Both happen for different reasons.
If it is still shifting through the gears but requires pushing the throttle all the way down and it doesn't seem to want to get up to speed or is restricted to a low MPH then it is likely engine limp home mode
It wasn't shifting thru gears, it was stuck in 1st gear for like 2 miles @11mph then it finally went into 2nd gear to 22mph... never 3rd ect. When I pushed the throttle down, I didn't go any faster or go into another gear, it just hovered where it was... when I was doing 70 down the highway, it was the same thing, more throttle but no more power.
So I guess I need to look up Limp Home Mode... never heard of that.
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Old 05-26-2019, 06:43 AM
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So this weekend I installed the new Raptor 150, Baldwin water separator (with a hand-made mount for the housing) and a CAT fuel filter on the engine..... No more Limp Home Mode and the truck feels to have a little more power. Although it also seem to have a slight hiccup while accelerating every now n then, never did that before.. Maybe air still in the system?
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Old 05-26-2019, 08:48 AM
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Air is purged quite quickly when the engine is running. Unlikely that unless you have a leak on the suction side of the pump causing it to draw in air all the time. A piece of clear tubing will show you if it is

It may be possible the hesitation is in the transmission as it may be adapting to the new power from what it was at when in limp home mode
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Old 05-27-2019, 07:14 AM
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I assumed the air would be purged on its own. The hesitation isn't crazy but slightly noticeable. I definitely made sure there was no air/fuel leaks. How long do you think it will take the trans to "relearn" what it needs too?
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Old 05-27-2019, 08:52 AM
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I've noticed the LB7's won't actually go into limp state on low fuel pressure, just drops power. Worked on one the other day that CP3 wouldn't make 12k psi, but never set any codes with stock tune. Not first time I've noticed that one..

Trans is gonna need to relearn once power back up to normal.
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Old 05-27-2019, 09:48 AM
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My 04 LB7 certainly did. Got annoyed with it and had to tune it out. Would rather it drop a little power then go into crawl mode

Not sure how the 03s respond to low pressure as I've never played with one
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Old 05-28-2019, 06:27 AM
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The hesitation hiccups seem to be getting less and less as I drive, but the trans is acting weird now too. Yesterday I was going down a slight hill and when I touched the brakes it downshifted a gear, like normal... but then it downshifted another gear right away and sent the RPMs screaming up to like 3500. I thought it was going to blow the engine! Has anybody had that happen before? It wasn't a very steep hill and I wasn't on the brakes very hard... I was towing a small 5x8 trailer with my kid's dirtbikes, but that's only an extra 1,000 lbs or so, I doubt the truck even knew it was there.
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Old 05-28-2019, 08:42 AM
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Where you in T/H?
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