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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 2721
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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2007 GMC C6500, vin#1GDJ6C13X7F412782. Truck came in for

Customer Question

2007 GMC C6500, vin#1GDJ6C13X7F412782. Truck came in for brake issue, I pulled the truck in the bay (running fine) replaced the hydroboost on the brakes, went to test drive and the rpm will only go up to 1000 rpm and stay there. Let off the pedal and it goes to 700 rpm. Scan the ecm, no codes present. Replace the accelerator pedal assembly, no change. Check fuel pressure, its normal, check all wiring seems good, remove every electronic connection I can find, spray deoxit and di-electric grease on them and re-install. check air filter, its clean. Bypass all fuel filters run a direct line with pump, no change. No de-rate light or any engine light at all, they have never had the issue before, I removed the new hdroboost assembly and installed the old one, no change. I checked to see if the brake lights were actuating at the same time the pedal was pushed, it is. I am not sure where to go from here.
Submitted: 2 months ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 2 months ago.

My name is ***** ***** I will assist you with your question today.

I'm an independent contractor and I don't get paid until a rating has been made.

We can go back and forth as needed to help resolve the issue you are having. Follow ups are free after ratings, so rate in confidence!

That sounds typical for the Isuzu when it goes into limp mode. You won;t always get a dash code, but if you connect the scanner, you will have a limp mode code. Question is, what set it? The bad part, once that code is set, you have to use the scanner to reset it. It is what we call a "hard set" code in that it won;t clear on its own. Yu may have had another issue that you did get resolved and the code tripped, It can also trip sometimes when removing the batteries to do work and the ECM gets a jolt.

If you have a IDSS or GM Tech2, use one of them to reset the code. Sometimes the Snap On Modis can reset it, but I've also heard some people say that you can;t reset a lot of the Isuzu limp codes with it.

I wish it was easier to do for this to reset the code.


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Customer: replied 2 months ago.
Had gm come out and hook up gm tech 2, had a foot pedal code, I'm guessing where we replaced the foot pedal but no other codes present.
Expert:  Jeremy replied 2 months ago.

The foot pedal sensor can keep the truck from revving, but it didn't resolve it? Am I understanding correctly?

Customer: replied 2 months ago.
I am uncertain if the foot pedal will cause the issue, it was the only thing I could think might cause the issue so I tried it. I put a new one in, and it made no change
Expert:  Jeremy replied 2 months ago.

I understand now.

An no dpf light at all?

Here is the down side. It can be anything on the engine. It can be the flow valve, bad fuel, fuel filter suction from the tank, fuel filter housing. The list goes on. I will add a symptom chart to go through that should hopefully assist.

There isn't anything on the hydroboost or that job that would cause the limp mode like that. That is why I am so thrown off. I'm certain it has you thrown off as well.

CLICK HERE for the symptom chart with no codes.

Customer: replied 2 months ago.
There is no dpf on this unit
Expert:  Jeremy replied 2 months ago.

So you have the early 07 then. That narrows that whole system out of the equation.

That leaves flow valve, rail sensor, fuel contamination, etc.

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