97 GMC C1500 P0175 and P0137

Hello,

I bought me a code reader last night to see what was going on with my truck and found two codes. The truck is ideling rough and the exhaust smells rich.

P0175 - system to rick bank 2
P0137 - o2 circuit low voltage (bank 1 sensor 2)

That is according to the book that came with it.

I found that the low voltage means that the sensor is reading lean so is it possible that that sensor is reading lean then the computer is adding fuel which caused the other side then to read rich? Does that mean that the sensor is bad?

Where do I go from here? Do I just replace the sensor with low voltage and see what that gets me? I have read some responses on here about each of these codes independently, but I haven't really seen anything with both of them and that is what is confusing me. One side is saying rich and the other is saying lean.

Thanks,
Chad

it sounds like an 02 sensor issue, if the mileage is high enough to justify replace the sensor in each side!

but which sensor? The code is saying that it is the post cat sensor that is showing low voltage. Do I change both of those? Or do I change both of the pre-cat ones?

Thanks for the response. Hopefully I can get this thing running again.

Thanks
Chad

3 o2's total or 4?

4 total. two before and two after the cats. I have two cats too.

try the downstream one on bank #2 to start and then see what other codes are left or develop, assuming the circuit is good that is!

ok well, I changed the front two 02 sensors first to see if it would help and then I would move to the back two. So far the truck is running better overall, but I still have a couple of other issues that surfaced at the same time.

Problem 1. When I start the truck it idles rough for about 15 seconds then it starts to idle fine after that. During that 15 seconds it is hard to tell if the truck is even going to stay running.

Problem 2. Once it gets past that rough idle I can take it out and have no problems. That is until I stop it and try and restart it. If I park the truck and let it sit for a few minutes it is real hard to start up again. It will crank over for a little bit, but it won't start until I give it some gas. Then it will start up and do the rough idle thing again.

By the way the codes are clear for now so nothing to report on that front.

Thanks,
Chad

check to be sure the fuel pressure isn't bleeding off !

would a bleeding fuel pressure cause the rough idle? I have been reading on some chevy sites that the prolonged restarts are probably caused by the fuel pressure regulator. I am going to buy a fuel pressure tester today and check this out.

Thanks,
Chad

Well, it sounds like you have two problems .A post cat. O2 sensor isn't going to cause your engine to run rich. THis is why: All post cat. O2 sensors(with the exception of Chryslers) are there only for detecting catalyst efficiency. They have no bearing on fuel trim readings. If they fail, they'll set a code but the PCM uses only the front sensors for determining fuel injector on-time. So, like someone else mentioned, it's a good idea to replace all 4 at once(pricey, I know but worth it). Now your code for a rich running bank sounds like a different problem. Fuel press. reg. sounds like a good place to start. THose lousy GMs have a central injector unit on most of their Vortec engines that are a pain and often go bad. The press. reg. is located inside the upper intake. If you have a Vortec eng. replace the whole unit NOT JUST THE REG. They also sell a replacement unit that relocates the injectors to the cyl. heads. (On the stock engines they are located inside that Central inj. unit under the intake. If it's a standard 350 or 305 engine, then your press. reg. should be located in the traditional manner.

Ok, so I checked the fuel pressure this weekend. Here is what I found.

1. When I first plugged it in there was no pressure at all.

2. When I turned to key to the on spot so the pump would kick on it went up to a bit over 60 and started to bleed off slowly.

3. When I started the engine it jumped to just over 80 and while the engine was running rough it was jumping around like crazy.

4. Once the idle smoothed out it was pretty steady at 82 psi.

5. When I killed the engine it dropped down to 55 and started to bleed off right away. It was not bleeding real fast, but you could see the needle moving as it bled off.

6. Also, while it was bleeding I crimped off the return line at the tank and it still bled off just as fast as when it was not crimped off.

The problem I am having is that the idle is real rough right at startup for about 15 or 20 seconds then smooths out. Also after the truck is warmed up it has an extended crank cycle to get it started the second time.


Any ideas.

Thanks,
Chad

Fuel Pressure Regulator is shot.

The specifications call for:
KOEO = 60-62 PSI
Idle = 50-59 PSI

There should be no bleed off.

wow, it is really shot if it is letting it run at 80 psi. I guess that would also explain the rough idle, it is basically flooding the engine until the computer adjusts for it. Is that what is happening you think?

that's defiently what's happening, regualtors go all the time on those engines, so do injectors!

well, I put in a new fuel pressure regulator and that didn't help the problem at all. The pressure still bleeds off with the key on and engine off. Then when I start it, it is still running VERY rough and the pressure now goes up to about 95 psi. While I had the upper intake manifold off I cleaned it up and cleaned the idle air control valve. when I shut the engine off it drops down to where it is suppose to be but still bleeds off.

What else could it be? Fuel pump?

Thanks,
Chad