Reduced engine power no CEL or codes

VEfreak

Member
Oct 18, 2018
52
1
8
Hey guys,

I recently started having trouble with my LBZ. At random times the "Reduced Engine Power" message will display with no CEL and no codes (I carry a foseal reader with me). This message will clear if I turn the truck off for a while and let it sit. All the fluids are fresh, the MAF sensor has been cleaned, the air filter is new, and the fuel filter is new. The truck is almost bone stock. When I changed the fuel filter recently I installed an adapter and donaldson fuel filter. I don't think this problem started until after I did that, so I'm considering moving back to a stock fuel filter set-up and seeing if that helps.

Is there anything else I should be trying to fix this problem? It comes at an unfortunate time because I was hoping to sell this truck soon.
 

Pure Diesel

Member
Apr 22, 2008
761
17
18
So. Cal.
How many miles on the truck? Have you checked to see if the fuel filter primer button is hard after this happens? If it isn't, try pushing the button and see if it gets hard. If it doesn't you might look into rebuilding or replacing the filter head. Check the rubber hoses too.
 

VEfreak

Member
Oct 18, 2018
52
1
8
Thanks guys. I'll try those things and report back. The truck has 360K miles btw and runs great. I put a ton of work into rehabbing it and giving it the TLC it needs after I bought it off a contractor at 340K miles.
 

VEfreak

Member
Oct 18, 2018
52
1
8
How many miles on the truck? Have you checked to see if the fuel filter primer button is hard after this happens? If it isn't, try pushing the button and see if it gets hard. If it doesn't you might look into rebuilding or replacing the filter head. Check the rubber hoses too.
Just to be sure, trying pushing the fuel filter primer button while the engine is running? Or not running? Do you think the problem is that it might be drawing in air?
 

Pure Diesel

Member
Apr 22, 2008
761
17
18
So. Cal.
Before you start the truck check the primer. If soft pump it till hard. Start truck and drive around. Turn the truck off and let it sit for a while. Check it to see if its hard again. It doesn't stay hard you've probably got a leak.
Does the truck start right away?
Look for any signs of Diesel around the head.
Put a factory filter back on it if any of the conditions still persist.

Sent from my LM-G820 using Tapatalk
 

2004LB7

Well-known member
Dec 15, 2010
3,297
202
63
Norcal
Before you start the truck check the primer. If soft pump it till hard. Start truck and drive around. Turn the truck off and let it sit for a while. Check it to see if its hard again. It doesn't stay hard you've probably got a leak.
Does the truck start right away?
Look for any signs of Diesel around the head.
Put a factory filter back on it if any of the conditions still persist.

Sent from my LM-G820 using Tapatalk
primer will always go soft after running the engine. just the nature of pulling fuel through the filter will reduce the pressure in the line. it does run with negative pressure unless he has a lift pump, in that case primer will tell him little
 

Pure Diesel

Member
Apr 22, 2008
761
17
18
So. Cal.
primer will always go soft after running the engine. just the nature of pulling fuel through the filter will reduce the pressure in the line. it does run with negative pressure unless he has a lift pump, in that case primer will tell him little
You are correct on that. I've had a lift pump on my truck for 200k miles. When my filter head(LB7) started to leak, the button would totally go soft and barely get hard after pumping. Truck would start but not as quick. The lift pump made it start a lot quicker but would still go soft after shutting off the engine.
After transplant
When the LBZ head started leaking, it would be firm but not hard. Its weird how the two acted differently.

Sent from my LM-G820 using Tapatalk
 

VEfreak

Member
Oct 18, 2018
52
1
8
Truck has always fired right away. Never had a problem with that. The primer can be pumped until it gets hard, so maybe that will help the problem. This problem can happen at any time, not just on start up. I got the reduce engine power message the other day while waiting at a stop light. I can recall if I've ever had it happen while under load. I'll probably swap back to stock filter if this persists because I'm taking the family on a road trip over the grapevine and this is our big vacationing vehicle. I don't have any kind of after market lift pump. Is there a fuel pump in the tank or in-line to the fuel filter? Could that be failing. I'd be a little troubled if it was the next donaldson fuel filter setup since that is a common upgrade for these trucks.
 

VEfreak

Member
Oct 18, 2018
52
1
8
Thanks. I started the truck. It fired right up but the REP message was displayed immediately. I turned the truck off and pumped the primer. Started the truck back up and the message was gone, but then REP came back while backing out of my driveway. I went to home depot anyways because I can deal with it on short trips. Not rev limited to 2000 rpm, but it's gutless if you try to bury your foot in the accelerator. When I left home depot the REP message was gone and didn't come back during the trip home. I stopped at autozone anyways and got a fuel filter head reseal kit.
 

Leadfoot

Needs Bigger Tires!
Dec 27, 2006
838
10
18
45
Western MA
www.matpa.org
Hey guys,

I recently started having trouble with my LBZ. At random times the "Reduced Engine Power" message will display with no CEL and no codes (I carry a foseal reader with me). This message will clear if I turn the truck off for a while and let it sit. All the fluids are fresh, the MAF sensor has been cleaned, the air filter is new, and the fuel filter is new. The truck is almost bone stock. When I changed the fuel filter recently I installed an adapter and donaldson fuel filter. I don't think this problem started until after I did that, so I'm considering moving back to a stock fuel filter set-up and seeing if that helps.

Is there anything else I should be trying to fix this problem? It comes at an unfortunate time because I was hoping to sell this truck soon.

MAF sensor cleaned with what? I remember YEARS ago people cleaning MAF sensors if they got oiled (from some of the old K&N or knockoffs) and it didn't go well. Sometimes the sensor was damaged while cleaning and didn't show up right away. Not saying that is your issue, but as soon as I saw it, it reminded me of people having issues...
 

VEfreak

Member
Oct 18, 2018
52
1
8
I used an aerosol MAF cleaner, not brake cleaner or anything like that. I can't find the can right now so I can't tell you exactly what it was. MAF would probably trigger the CEL, no?
 

VEfreak

Member
Oct 18, 2018
52
1
8
Did the rebuild today. The old o-rings were work hardened and not pliable anymore. One of the primer seals had gone flat or dead, the best way I can describe it. Took the truck out and put her through her paces. No REP message but that doesn't mean much. I've gone days and weeks without seeing REP. Still, it's something. My trip up to Northern California this weekend will be the true test. Thanks for the help fellas, I'll report back if the problem rears its ugly head again.
 
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VEfreak

Member
Oct 18, 2018
52
1
8
Whelp, the problem decided to pop back up almost immediately when we were leaving San Diego. Went over the grapevine with surprising ease. I think I remember reading that REP will happen when the accelerator pedal starts failing. Does this sound familiar to anyone?

I can also make REP go away by priming my pump until the primer button is hard. Is my fuel pump maybe having a difficult time drawing in enough fuel? Maybe this is why the problem popped up only after I installed the Donaldson fuel filter. Maybe a lift pump setup would solve these problems?
 

2004LB7

Well-known member
Dec 15, 2010
3,297
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Norcal
lift pumps are never a bad idea. certainly makes priming and keeping the air out of the lines easier
 

VEfreak

Member
Oct 18, 2018
52
1
8
lift pumps are never a bad idea. certainly makes priming and keeping the air out of the lines easier
I feel the same way, but I don’t want to make such an expensive upgrade to a truck that I plan on selling in the near future. On the other hand there’s no way I’m selling this truck until I’m 100% sure the root cause of the problem has been identified and corrected. I really wish this thing was throwing a damn code and would just telling me what is wrong.

If anyone else has some ideas for things to try and test just keep em coming. Thanks in advance!
 

VEfreak

Member
Oct 18, 2018
52
1
8
I’m in Modesto right now. Getting a new front passenger side tire put on tomorrow (Saturday), and then I’ll be in foresthill all Sunday.
Thursday was a bit of a disaster. Something punctured my front passenger tire badly in between Burbank and Santa Clarita at around 7PM. Couldn’t get the spare out and mounted until around 10PM because the spare tire mechanism was wrecked from lack of maintenance. Didn’t get to Modesto until almost 3AM despite leaving San Diego at 3:30PM the previous day.
 

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