Vortec 454/4L80e HD swapped 1999 Tahoe OBS

[bigfootchiro]

So I sprayed brake cleaner around the intake and didn’t see any change in STFT. However, when I unplugged the low pressure AC switch, Bank 1 barely changed, but Bank 2 changed drastically. Also, after driving around with the switch unplugged, it drove perfect! Ran it about 5 miles with no hesitation. Gotta trace back the wiring schematics to see how this is possible!

 

[bigfootchiro]

Alright fellas, just when I think it’s fine and dandy, it’s not. This Hoe is temperamental. First picture is after I pressed the AC button On, the STFT on Bank 2 levels out. Second picture is when I turn it off and it shoots back up.

Bad ground? How the fugg does this affect Bank 2 only? Only thing I can think of is something in the harness. Thoughts? Any and all are appreciated!

 

[Sean James]

It COULD be that the drag from the compressor is just lowering the rpm for a second an creating a load, making the stft numbers more obvious. Your not showing the rpm's when the trims change so it's hard to tell. I know you have used carb cleaner, but you might want to try propane. Rig up a small tube, really small, with a rubber hose or a smashed/squished metal brake line hooked up to the propane bottle. Turn on the propane ever so slightly into the air filter intake and watch fuel trims. Then feed some propane around intake manifold on each bank, working from front to rear. Watch stft's a/c off, and a/c on. It works better than carb cleaner usually, and is less messy and damaging to plastic and rubber parts. It has to be vacuum issue somewhere. Hoses, vents, egr, fuel tank charcoal evap canister or solenoids, power brake booster, etc. If your hell bent on harness or electronics, bank 2 could be any one of 4 injectors or coils on that bank. Get back to basics, spark, fuel, then vacuum. Keep giving that propane to anything that can suck in, you'll find it.

 

[bigfootchiro]

It COULD be that the drag from the compressor is just lowering the rpm for a second an creating a load, making the stft numbers more obvious. Your not showing the rpm's when the trims change so it's hard to tell.

Thank you for the advice! I will definitely give it a shot. The RPMs were at idle in Park. When compressor turns on, the RPMs don’t fluctuate much. Even so, Bank 1 doesn’t fluctuate on STFT, only Bank 2.

 

[Sean James]

Is it throwing a P0174 code? I'm assuming it is. And if your scanner has freeze data, bring the rpm's up to 1500-2000 and try to hold it there and unplug the low pressure switch for a minute and then plug it back in, does the stft still run lean? Could even be bank 2 o2 sensor. You can reverse the o2 sensors and see if the issue moves to bank 1. That would prove that it is the o2 sensor, but usually not the case, but it's very possible. I know, pain in the ass, but the front o2 sensors are the same and reversable and an easy way to eliminate them as the cause as well. All these checks are easier than tracing down a harness issue, although there are many guys on here that probably know right where to go to fix your issue, they just haven't seen your post yet. Just remember, it's a challenge, not a problem, you'll find it.

 

[bigfootchiro]

Is it throwing a P0174 code? I'm assuming it is. And if your scanner has freeze data, bring the rpm's up to 1500-2000 and try to hold it there and unplug the low pressure switch for a minute and then plug it back in, does the stft still run lean? Could even be bank 2 o2 sensor. You can reverse the o2 sensors and see if the issue moves to bank 1. That would prove that it is the o2 sensor, but usually not the case, but it's very possible. I know, pain in the ass, but the front o2 sensors are the same and reversable and an easy way to eliminate them as the cause as well. All these checks are easier than tracing down a harness issue, although there are many guys on here that probably know right where to go to fix your issue, they just haven't seen your post yet. Just remember, it's a challenge, not a problem, you'll find it.

No codes other than post cat O2 sensors (tech 2 data), because they aren’t hooked up on either bank. I will try reversing the sensors, though, and see if it moves from side to side. I will definitely try bringing up the RPMs and unplugging/plugging the switch. Looks like I’m gonna be busy Thursday morning/the weekend!

 

[Sean James]

O.K. why would an o2 sensor have anything to do with the A/C switch being on or off. That makes no sense, fuel trims should fluxuate normally at idle or rpm weather or not the a/c is on or off, the only difference being the drag and the pcm adding more fuel to bring up the idle speed. The only variable is fuel lean on bank 2, and a/c load on the engine. It doesn't make sense the more I think about it. This is a tough one, are the stft changes immediate, or do they take a few minutes to change. Is the engine warm, hot or cold or does it matter, that makes no sense either as temp issues would affect both banks. I'll talk to my boss later today, the old man knows everything.

 

[bigfootchiro]

O.K. why would an o2 sensor have anything to do with the A/C switch being on or off. That makes no sense, fuel trims should fluxuate normally at idle or rpm weather or not the a/c is on or off, the only difference being the drag and the pcm adding more fuel to bring up the idle speed. The only variable is fuel lean on bank 2, and a/c load on the engine. It doesn't make sense the more I think about it. This is a tough one, are the stft changes immediate, or do they take a few minutes to change. Is the engine warm, hot or cold or does it matter, that makes no sense either as temp issues would affect both banks. I'll talk to my boss later today, the old man knows everything.

They are immediate changes, and doesn’t matter is warm or cold. Bank 2 is the only one that fluctuates drastically in comparison to Bank 1, and in relation to the low pressure AC on the accumulator. This is what leads me to think it’s potentially grounding out or shorting out somewhere in the harness, but must be pretty close by to be Bank 2. You’ve been a big help so far, and can’t wait to hear what you’ve got to say later.

 

[Sean James]

Yea, your post cats are not an issue. But I don't know if I would bother switching pre cat o2's either, s1b2 seems to be working because it is changing values when you flip the a/c. Are the converters gutted/gone? That shouldn't matter either. It has to be like your first instinct...somehow the a/c wiring is jacking with bank 2 injectors making 1 or more quit or run lean, or activating a vacuum solenoid causing a lean on bank 2 only. Which vacuum lines run to bank 2? Fuel tank evap? Is EGR tied into a/c? Does a/c somehow interact with evap or EGR throught the PCM? Sombody knows this. I'll try to find out.

 

[bigfootchiro]

Yea, your post cats are not an issue. But I don't know if I would bother switching pre cat o2's either, s1b2 seems to be working because it is changing values when you flip the a/c. Are the converters gutted/gone? That shouldn't matter either. It has to be like your first instinct...somehow the a/c wiring is jacking with bank 2 injectors making 1 or more quit or run lean, or activating a vacuum solenoid causing a lean on bank 2 only. Which vacuum lines run to bank 2? Fuel tank evap? Is EGR tied into a/c? Does a/c somehow interact with evap or EGR throught the PCM? Sombody knows this. I'll try to find out.

It has practically no exhaust right now. They are dumped right behind the transmission for right now and no crossover. Wouldn’t think that would affect just the one side, though. I’m wondering the same thing- how the AC could be tied into anything vacuum and/or electrical that would only affect Bank 2. Great minds think alike!