Vortec 5.7 fuel trims positive, running rich, fuel pressure issue?

[mistaake]

97 SUburban 5.7, 214k miles, running pretty rich (but NO codes), fuel trims are very much positive. LTFT is 20-25% on both banks. Even if I reset the codes it goes back to that almost right away.

I have tested it for vacuum leaks and took it to the shop and they found no vacuum leaks either and generally said nothing is wrong with it.

I thought the MAF was bad so I replaced that. Also cleaned EGR valve because I thought maybe it was sticking open a little causing a vacuum leak since earlier this week I had a P0400 code once, but it never came back. That seems to have made my hesitation off idle especially on a cold engine much less noticeable but it didn't fix the main issue.

upstream O2 sensors while driving:

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I don't get why they are different, I'm wondering if that is because it's running rich or is the O2 sensor the problem?

I don't know why I bothered testing fuel pressure when the truck starts up right away and runs but this is the result. Based on this, I think I might have more than one problem. The fuel pressure is not consistently 60+ and it bleeds down very quickly. I would suspect the fuel pressure regulator is bad and leaking gas into the intake, but I replaced that a few thousand miles ago and also I would expect the fuel trims to be negative if it was leaking gas into the engine, right?

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Thank you for any input...

 

[lessersivad]

I may be WAAAYYYY off but,

What is the coolant temp reading on the scan tool?

I ran across 3 different GM vehicles in 2 days with no codes and running rich. ALL off them had the same reading of -40° on the scan tool. (Must be the 'default'). Changed the sensors on all 3 vehicles and no more rich conditions.

Hope this helps.

 

[mistaake]

I may be WAAAYYYY off but,

What is the coolant temp reading on the scan tool?

I ran across 3 different GM vehicles in 2 days with no codes and running rich. ALL off them had the same reading of -40° on the scan tool. (Must be the 'default'). Changed the sensors on all 3 vehicles and no more rich conditions.

Hope this helps.

My sensor seems to read normally, about 190f when warmed up.

 

[lessersivad]

My sensor seems to read normally, about 190f when warmed up.

That sounds like good numbers. Just threw the CTS out there because that's something not usually thought about.

 

[lessersivad]

IF the wiring was long enough you could simply unplug the sensors and swap the connectors to each other to see if the problem shows up on the opposite bank.

Unfortunately I don't think there is enough 'slack to perform such a move.

 

[mistaake]

That sounds like good numbers. Just threw the CTS out there because that's something not usually thought about.

Hey I mean it's totally the symptoms of a bad cts so it was a good call. I can get a new one for $4 but since the reading is good I take it that's a waste of time and money?

 

[lessersivad]

Hey I mean it's totally the symptoms of a bad cts so it was a good call. I can get a new one for $4 but since the reading is good I take it that's a waste of time and money?

Yea, if the numbers are 'normal' I wouldn't even load a small round in the 'parts cannon' if it's not reading bad.

 

[east302]

What is the fuel pressure with ignition on/engine off?

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[pro17]

FYI positive fuel trims means you're running lean (adding fuel to compensate)

 

[lessersivad]

FYI positive fuel trims means you're running lean (adding fuel to compensate)

DOH!!! I KNEW that too. Don't know what I was thinkin' with the coolant temp suggestion.

Thanks for the reminder.