1996 c3500 7.4 liter p0151

Question about oxygen sensor wiring diagram at the pcm it says high signal and low signal? What is it supposed to read. It’s stuck dead rich unplugged it went to 450 mv on scanner. But replaced still stuck dead lean

High is the signal and the low is the ground. I think 450 mv on signal and 0 on ground. Verify on the driver bank to be sure.

If you plug in a cold sensor this 450 mV should not budge because a cold sensor is basically an open circuit. If it goes low as soon as you plug it in, then the o2 sensor's bad.

Verify the o2 sensor and make sure it reads OL or something in the megaohms range on the two wires that's not the same color.

imgur.com/a/rKmx2Pz

Yea it’s in a horrible place the connector is, the sensor is new but I’m going to recheck everything tomorrow. Thanks for the flowchart.

Well I was able to get to the sensor today. I have a ground for the oxygen sensor heater. Now here’s where it’s weird pin b I get 12 volts key on. Lights test light. That’s the blue wire on a standard o2 sensor. 2 black wires a white and blue. I unplug it goes to 450 mv can drive it low with test light but can’t drive it passed 450 with test light

So I’m trying to get the heater circuit figured out then go from tgere

wrote:

pin b I get 12 volts key on. Lights test light. That’s the blue wire on a standard o2 sensor. 2 black wires a white and blue. I unplug it goes to 450 mv can drive it low with test light but can’t drive it passed 450 with test light

That is weird. That current can't possibly be coming out of the computer, let alone that signal wire going to the computer. Leave the test light in place and trace where the current is coming from with an amp clamp.

I always remember the signal wire like this: what color are bruises? black and blue lol.

Simplest explanation: the current's coming from heater circuit and this short's going away when you unplug the sensor. I'd say remove the fuse, but unfortunately that fuse also powers the computer.

I unplug it goes to 450 mv can drive it low with test light but can’t drive it passed 450 with test light

I'm going to chalk that one up to scanner problem. If you can pull it down and pid changes that's good enough for wire integrity for now but don't let this stop you from trying to pull driver's side signal high with a test light.

Yea it’s an old trashy rollback. The wiring harness is butchered. They said they ran new wires to the o2 sensor. They used speaker wires they said.the connector is melted so I cut it off and spliced the heater circuit correctly. So do you think the o2 signal low would light a test light? I’m going to compare to the other bank and go from there. I

o2 sensor will use sensor low when it goes lean so if you jump the 450 mV to sensor low and it goes low, then that sensor ground's just fine. If you want to hit it with a 200ish mA load I think it can handle that although I don't think sensor grounds are meant for anything but sensor low reference so just very low micro amps.

So I required the oxygen sensor it now reacts to brake clean in the intake. Now it has low power. Checking fuel pressure at idle it’s 49 psi but if I pinch off return line to pump it goes to 65 psi. Would this be a faulty regulator? It bleeds off from 50 to 20 in 3 minutes. Can’t do an injector balance test until I pull the intake if it heads that way. It has an extended crank cold 10 seconds or so

Also has no converters it’s been straight piped

Service info says good is 56-63 psi so your 49 psi is slightly low.

A stuck open regulator or bad pump can cause that low pressure. You can rule out the regulator by revving the engine and see if the pressure falls. A drop in pressure means a weak pump - basically, insufficient fuel volume that can't keep pace with more injector openings. You can rule out the fuel pump if it passes the dead-head test by checking its max pressure output by either pinching off the return line or fitting a pressure gauge at the filter with no Tee. Is 65 psi a good number for that pump with the return blocked? I don't know, but it sure seems low. I would have expected much more than 65 psi which again makes me suspect the pump is weak.

As for the extended crank to when it finally fires up, does that correlate to fuel pressure finally reaching 49 psi, or does it still crank-no-start even after that pressure is reached? 

Slow build up in pressure to engine start? Yes/no?
Pressure fall when you snap the engine? Yes/no?
Only 65 psi when you dead-head the pump? Likely bad
Pressure fall with the key off in 3 minutes? Maybe bad?
Seems like signs of a worn pump to me.

Wait, did I just say 49 psi is low?
Guess what, 49 psi was fine for this 2010 C3500HD 7.4L on the Premium page !
There's too many variables to consider lol.

The video did not work. But forgot to mention that wide open throttle it drops to 44 psi. So definitely seems like a pump now because I’d figure dead heading the pump would do at least 80 psi instead of 65

Oops forgot to include link to video: www.scannerdanner.com/scannerdanner-prem...l-c3500hd-truck.html

Yes that drop on snap is bad. If you do the dead-head test at the filter and find it's low there, and the pressure is leaking down and the power and ground is good, then I'd put a pump and filter on this.

Would a wide open throttle in drive down the road be the same as a snap throttle in park? But yes it dropped to 43 when accelerating from a stop. I voltage dropped the power and ground it was below a half volt on both.

Do you have experience what the pressure should be on the older chevy vortecs dead heading the pump

wrote:

Do you have experience what the pressure should be on the older chevy vortecs dead heading the pump

No I do not but I expect around 80 psi:  chevroletforum.com/forum/general-tech-10...ions-454-spfi-52459/

I’m calling a weak fuel pump

Got new pump in. Redid the dead head test goes to 86 psi instead of 65. Still has a lack of power.