Bad fuel pump?

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VIKING_MECHANIC

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Alright, so for the last few weeks, I've been having issues with my C1500 running. I can feel it shuddering, sometimes I can barely feel it and other times It feels like the entire truck is shaking.

So I look at the live data to see what is going on and this is what I see:
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Okay, so one bank is running lean and other is running rich. Got it but not really helping me figure this out. I ended up replacing the MAF because I will sometime get a "no signal/low voltage" code. Usually cleaning the MAF and the codes go away. I figured it was time to replace it because I was thinking the ECU wasn't getting the correct information and was adjusting the air/fuel accordingly. I replace it about a week ago and I'm still having this issue.

I got home from work today, and checked the fuel pressure. From what I understand, the operating range for these Vortec engines is 62-66PSI. I turned the key on and the system primed to 60PSI. I started the engine and it fell to 50PSI and held there while running. I tried the test a few different times and every time it would prime to 60 and drop to 50 when running.

Do these pumps show any sign of early failure? I don't think this is because of a clogged filter/strainer or a restriction in the line as it happens rather randomly. I have noticed though it will seem to favor to do this in the morning when the air is cool and dense. And with the engine running both lean and rich at the same time, I'm at a loss on that.

What's you guys' take on this?
 

stutaeng

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Most likely not a fuel pump problem if it primes to 60 psi. You don't have CEL lean/rich codes?

What do the fuel trim numbers do when you rev up the engine? Could be a combination of fuel pressure regular bad (leaking internally into intake) and/or injector(s) stuck open on one side and stuck closed on the other side? I'm just guessing here...need to test injectors.


But since it could be either of those and the injectors come with a new FPR, that's likely what would fix your problem. Need to check 'em though before you condemn them.
 
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VIKING_MECHANIC

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I don't have any codes present, at least none that my reader will display.

When I rev the engine, side 1 will increase and side 2 will decrease. And it's quite a bit too.

I didn't really think about the injectors. Although I had them and the intake cleaned when the head gaskets blew a few months ago.

I may have to take the truck somewhere that has a scanner to see more detailed info.

But shouldn't the psi be no less then 62 when running? I read somewhere that anything less, the engine will run but may have issues.
 

stutaeng

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I don't have any codes present, at least none that my reader will display.

When I rev the engine, side 1 will increase and side 2 will decrease. And it's quite a bit too.

I didn't really think about the injectors. Although I had them and the intake cleaned when the head gaskets blew a few months ago.

I may have to take the truck somewhere that has a scanner to see more detailed info.

But shouldn't the psi be no less then 62 when running? I read somewhere that anything less, the engine will run but may have issues.

Usually a lean condition (negative fuel trims) that increase (become more positive) as you increase the RPM is caused by a vacuum leak, but that doesn't seem to be the case on your bank 2... I can't remember why the rich side would become richer at higher RPM...That's really strange.

Even more strange you don't CEL, the fuel trim with 50 is way outside of what the ECU can compensate for, so that's why the engine is running poorly.

BTW, why are you in open loop? Should be closed loop! That may be the real problem?!

The reason the fuel pressure is higher at rest is because the only force acting on the fuel pressure regulator is the spring stiffness. When you start the engine, there is in addition engine vacuum that "helps" the spring stiffness and the more fuel is allowed to return to the return line, so fuel pressure drops. The pressure varies based on RPM (engine vacuum varies with RPM.)

I've posted a few YT videos on the mechanics and diagnosis of the these fuel systems. Let me see if I can find them.
 

stutaeng

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VIKING_MECHANIC

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Usually a lean condition (negative fuel trims) that increase (become more positive) as you increase the RPM is caused by a vacuum leak, but that doesn't seem to be the case on your bank 2... I can't remember why the rich side would become richer at higher RPM...That's really strange.

Even more strange you don't CEL, the fuel trim with 50 is way outside of what the ECU can compensate for, so that's why the engine is running poorly.

BTW, why are you in open loop? Should be closed loop! That may be the real problem?!

The reason the fuel pressure is higher at rest is because the only force acting on the fuel pressure regulator is the spring stiffness. When you start the engine, there is in addition engine vacuum that "helps" the spring stiffness and the more fuel is allowed to return to the return line, so fuel pressure drops. The pressure varies based on RPM (engine vacuum varies with RPM.)

I've posted a few YT videos on the mechanics and diagnosis of the these fuel systems. Let me see if I can find them.
What's the difference between open and closed loop?

Another thing I notice, when this does happen, if I put the truck in neutral for a few seconds then back into drive, it will smooth out and the positive 50 value goes away. Although, it's only 50/50 chance that happens when the engine does this.
 

stutaeng

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Open loop is when the engine is cold, and the ECU is not relying on data from sensors for fuel trims...as soon as the engine warms up to a set temperature is when it goes into closed loop and ECU is taking data from sensors. Can you look at your O2 sensor waveforms? Make sure they look like they are reporting correctly...
 
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VIKING_MECHANIC

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Open loop is when the engine is cold, and the ECU is not relying on data from sensors for fuel trims...as soon as the engine warms up to a set temperature is when it goes into closed loop and ECU is taking data from sensors. Can you look at your O2 sensor waveforms? Make sure they look seems like they are reporting correctly...
Sooo my O2 sensors have been disconnected for at least 6 years. I did some work on the exhaust, and never got around to reconnecting them.

Weirdly, I've never had any issues like this over the last few years until now.
 
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