Manager Forum

Customer complaint of intermittent rough running, no power, misfire, backfiring. Codes were p0191, p2635 and right and left bank lean. I scoped the fuel pump and it looked good. Fuel pressure on a manual gauge matches scan tool data and it is up to spec except under hard accel on the scan tool. I havent watched it on the manual. I would say its most likely the fuel pump except the PID fuel pressure sensor volts is 3.6V plugged or unplugged. I measure reference voltage at the sensor and it was 5v. The actual fuel pressure follows the desired pretty close until you really get down on it and then the actual may get as low as 17psi with 58 desired.

Sounds like a classic low volume problem except for that one line of data. I have even put in a jumper between the reference and signal wires and got no change except for the actual pressure went to 98 psi. Should I forget about the 3.6v and get a pump or what. I cant figure out what that line of data represents.

This truck is still under warranty but this customer spends big bucks here and has no confidence in the dealer and thinks I am a genius.

Oh, and I can't duplicate the rough running and all of that. Runs decent, but I can tell the power is a little bit low when you really call on it.

since the pumps on these gm's are a fail point i would explain to the customr what you've found and even if the pump isn't today's problem it's going to fail and leave him stranded. now in reading your post i get the feeling that this truck has had a few other issues and your customer is sick and tired of bringing it to the dealer. if this is the case i'd be real carefull. there may come a time where he may not be so nicey nice and may take out his frustration on you. i've had situations like this a few times and have strongly suggested that they take their vehicle to another dealer. an example of an issue would be the engine or transmission blows, it goes to the dealer and it gets bounced because of some work that you had done on it that should have been done as a warranty repair. not that the dealers do this . at least in my area some dealers will try to bounce a warranty claim because the customer didn't have the air pressure adjusted at the dealership or have after market wiper blades on it.

That data line tells the PCM what the fuel pressure is. In turn, the PCM delivers that signal to the Fuel Pump Control Module which controls the fuel pressure on the pump. That PID should change and follow fuel pressure. If it doesn't, I'd be looking at why before putting in a fuel pump.

Which engine is this and how many miles on it? Is this a 1500? 2500? 3500?

No this truck has never had any problems and this customer would never treat me that way. We have been close friends for a very long time and he has paid me to do work on his vehicles before even if he could have gotten it covered. Its just the type of person he is.

The truck is a 1500 with a 5.3 VIN 3. 57,000 miles. That's whats bugging me. The pcm picks up the fuel pressure change accurately, but that line of data never changes. I'm not sure what that line of data represents. It doesn't match the reference voltage or the signal voltage. I know this because I can put 5V to the signal wire or unplug the sensor completely and get the same thing. Shopkey should have a dictionary of PIDS for each vehicle. It would really help on some of these. My modis sometimes has an option for that but not on this vehicle.

Interesting. The PO191 clearly shows the PCM is not seeing the change. I'd ignore P2635 for now because it can't set with a PO191. Have you measure the circuit right at the PCM? If it's changing there and the PCM still does not recognize it, then there may be an issue right at the connector or internally.

I haven't done that, but that was the reason I put the manual fuel pressure guage on it. It reads right on what the scan tool says the fpcm is seeing. Even with 0 psi on the guage and the fuel pressure PID that line still says 3.6v. Still 3.6v when I jump between the signal and reference wires which applies 5v (I measured it) and makes the fuel pressure PID read 98 psi. Can that PID reading be skewed by the time it gets to the scanner?

Sure. Partial short to power or ground, or shorted to another wire. Green death in a wire somewhere.

But because this is so new, it might just be a PCM.

Do you know the volume specs for this pump? I think I'm gonna find the signal wire coming out of the fpcm and cut it an see if it changes anything. I'm just hung up about it because the displayed pressure reading changes but the voltage doesn't. The good thing about it is that it has a separate controller for the fuel pump. It's only $130.

OK, after much digging on Shopkey I did find a dictionary. It says the voltage should be 4.9 to 5.1v. That tells me its reading reference voltage. I know that it measures 5v at the sensor. I will cut the reference wire and see what happens.

Have you scoped the wire, it sems poss that the signal might be a frequency type signal and the voltage wouldn't change. Just a wild speculation as I have not worked on one of these systems yet...

i'm a bit confused here. are we talking about the reference voltage feed or the sensor wire? what happens when the pump is unplugged? does the pcm read voltage or resistance.

I replaced the module and the voltage still didn't come up on the scan tool). 5V to the sensor. I think I'm gonna chalk that one up to a problem in the interpretation between the scan tool and the vehicle because the pressure reading is spot on with the manual guage. The dropping out and backfiring was a problem in the connection between the harness and the FPCM.