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Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!

1996 c3500 7.4 liter p0151

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4 years 1 month ago #50751 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
Dang. :( How's pressure when you snap it? Front O2s indicating rich at WOT?

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  • Cheryl
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4 years 1 month ago #50753 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
The fuel pressure bleeds off from 52 key on engine off to 15 in 3 minutes. Then it’s hard to start. So I’m going to have them pull the intake manifold and see if they can look for a leaking injector or regulator. As the owner says it only has 330,000 miles on it. The pressure still seems low at idle it’s 52 spec says 56-60 I think. So when the bleed off problem is fixed guess we will go from there

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4 years 1 month ago #50754 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Do you use a scan tool to check timing on that model? Or is it to old.

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4 years 1 month ago #50756 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
I don't know.

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4 years 1 month ago #50759 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
I’ll see what all data has on it. After they call when they get the bleed down issue fixed.

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4 years 1 month ago #51140 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Fuel pressure regulator was leaking. Installed new one found 4 injectors dribbling with the system primed. So going to put injectors in

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4 years 1 month ago #51430 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Well we put a new set of injectors in.6 of 8 we’re dribbling down manifold. Put it back together still low power. Fuel pressure regulator was leaking had fuel where vacuum hose goes. Pump and filter new. Still shows bank 1 lean plus20%

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4 years 1 month ago #51439 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Well went to the computer and cut both oxygen sensor wires bank 1 sensor 1 reads 450 mv like I’d expect on a gm. But bank 2 sensor 1 reads 0.00mv. Checked pin terminals at connector good drag. Is this looking like a bad pcm?

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4 years 1 month ago #51454 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
It would seem so :(

As for the bank 1 running lean, maybe it's an internal vacuum leak? Try blocking off the PCV and the breather hose and see if pressure builds in the dipstick.

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4 years 1 month ago - 4 years 1 month ago #51502 by VegasJAK
Replied by VegasJAK on topic 1996 c3500 7.4 liter p0151
If you're testing for an internal vacuum leak by using this technique you want to note any vacuum in the dipstick tube. Pressure is a normal by product of the engine running any vacuum would be from a bad intake manifold gasket which would cause a lean condition in a single bank. A severe leak will have vacuum at the valve cover if you pull the PCV. You may have to use a vacuum gauge in the dipstick tube to discover a small leak, if there is one.
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
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Last edit: 4 years 1 month ago by VegasJAK.
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4 years 1 month ago #51503 by VegasJAK
Replied by VegasJAK on topic 1996 c3500 7.4 liter p0151
Respectfully, you're mixing bias voltage and signal voltage. The bias voltage of 450mv is a diagnostic signal from the PCM. It lives on the signal wire but is a very weak signal and does not interfere with the 02 signal. Tested with KOEO, you can pull down the bias voltage but you cannot drive it up over 450mv. You want to see the 450mv on the scantool when you unplug the sensor. Shows wire integrity when you pull it down. Conversely, an 02 signal voltage (100 - 800mv) can be driven lean by a vacuum leak and rich by introducing propane. Fluttering the throttle will do the same and WOT will produce a rich signal. All on a working 02.
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
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4 years 1 month ago #51507 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
I cut the o2 high and o2 low at the pcm on both sensors one reads 450 mv on the scan tool one does not. If you have a wiring diagram you can see what I’m talking about. That’s why I’m thinking pcm

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4 years 1 month ago #51508 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151

cut both oxygen sensor wires bank 1 sensor 1 reads 450 mv like I’d expect on a gm. But bank 2 sensor 1 reads 0.00mv
Jump the 5V reference into bank 2 sensor 1, if the car dies then yes that signal wire's shorted to ground internally. Or use a test light but the 5v ref is more exciting :)

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4 years 1 month ago #51510 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
So jump the 5 volt reference into which wire? The high or the low? You talking about pcm side not harness side? Because I’ve got the harness side eliminated on both banks except for 3 inches coming out of the pcm

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4 years 1 month ago #51511 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
In to the high, yes PCM side.

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4 years 1 month ago #51512 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Just tried it b2 doesn’t move

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4 years 1 month ago #51513 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
Try putting Bank 1 sensor 1 (high) into Bank 2 sensor 1 (high). Does Bank 1 sensor stay at 450 mV?

If bank 1 sensor 1's 450mV drops to 0, then bank 2 sensor 1 (high) is shorted internally.

If nothing changes, and bank 2 sensor 1 is still showing 0, well, the PCM is just faulty, at least on Bank 2 sensor 1. Because it's not reading anything lol.

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4 years 1 month ago #51514 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
Also, you've confirmed with a meter right? Nothing's coming out of bank 2 sensor 1?

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4 years 1 month ago #51515 by Cheryl
Replied by Cheryl on topic 1996 c3500 7.4 liter p0151
Swapped both highs and lows same results. Bank 2 always stays at 0

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4 years 1 month ago #51516 by jreardon
Replied by jreardon on topic 1996 c3500 7.4 liter p0151
Give it a good thump and a shake and if that fails, then yes that circuit's dead.

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