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P0131 & P0171
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Author:  Woosch [ Thu Jun 19, 2008 11:09 pm ]
Post subject:  P0131 & P0171
Guys,

First, this is on the "new" L36. It has 42k miles.

The code scanner arrived today and I downloaded two codes. The first was P0131 "Oxygen sensor signal too low" and the second one was P0171 "fuel trim sensor signal lean." My CEL came on the other week, and the car has been really low on power since I bought it. Changing the plugs and wires helped a bit, but the car has never been as strong as it should be.

I've been checking the coolant level religiously since I bought the car because I knew I was running on borrowed time. Up until yesterday, it hadn't used any coolant. But when I got home yesterday I smelled coolant when I parked the car. I had to add a gallon to bring it back to "full." It's definately leaking externally. The oil level wasn't high and it didn't look milky, so I'm hoping I caught it before the car started digesting coolant internally. I won't know until I get it torn apart. I changed the oil as a precaution... When I squeeze the upper radiator hose, coolant squirts out a plastic cooling elbow on the right (passenger) side of the engine.

Fortunately, I ordered a new APN intake last weekend. If it arrives tomorrow I'm going to do the UIM and LIM gaskets this weekend.

I though I'd better check and see what these codes are and if I need to deal with anything else while I have everything else torn apart.

Any thoughts? Advice? I've read and re-read the UIM section in tech info.

Thanks in advance!
Author:  bonnevillain [ Thu Jun 19, 2008 11:39 pm ]
Post subject:  Re: P0131 & P0171
i tried tracking down those exact 2 codes on my 95 L36. i think it may be a PCM problem, they are finicky onthe 95a. i replaced my 02 sensor and nothing changed. phantom codes. i found several other 95s with this problem that was never resolved. my car runs fine so i dont worry about them. they show up once in a blue moon, and then go away

as fat as coolant... you say it squirts out a plastic cooling albow on the right side... i would assume that's where you lost your coolant, and your intake gaskets are fine. Also, right hand side coolant leak makes me think water pump. but if you can see it coming from a coolant hose, then hopefully that's the only place youre losing it from.
Author:  lash [ Fri Jun 20, 2008 9:27 am ]
Post subject:  Re: P0131 & P0171
It sounds like you'll need to replace that coolant elbow going into the LIM while you're in there. It should almost be expected that rubber and plastic parts will begin to fail on a 13 year-old car. Even if there's low mileage, you get age related failures at this point.

As to the two codes you got, I don't believe that there will be much internally you can do to chase those down right now, since those can be tricky ones to diagnose. While you have the top end apart, you should go ahead and pull that O2 sensor to see it's condition. You'll have good access.

When you are buttonning things back up, take your time to make sure that all electrical connections are clean, tight, and corrosion free. Many times that P0171 can be caused by corrosion at the grounds, though you should make sure your O2 sensor wires are as far from your plug wires as is possible.
Author:  Woosch [ Wed Jun 25, 2008 1:59 pm ]
Post subject:  Update
I figured I'd post an update. This is on my '95 L36 I just bought with 42k miles. The UIM replacement went off without any problems. The LIM gaskets were about gone, and the coolant elbow had a big crack in it that was causing my leak. The UIM hadn't failed yet, but I'm glad I have the updated APN one installed. Hopefully it'll last a lot longer than the GM parts.

I wasn't able to track down the problem with my car not running right. The two codes haven't appeared again, but it still stumbles and bucks at low speeds when cold. When warm, it runs better, but still feels like it's running at about 60% power. Here is what I have done so far:
New fuel filter
Fuel injection cleaning (3-part, one in fuel, one in oil, one through vacuum line)
new plugs (autolite)
new plug wires (Napa premium)
I checked the wiring to the O2 sensor. It appeared to be in good condition, and doesn't contact the plug wires.

I finally took it to a mechanic friend's shop to see if he could figure anything out. He suspects it is a fuel pump issue. He put a sensor on the fuel pump wiring and said the fuel pump's wave form is almost flat. Apparently it's the flattest he has seen. He thinks the pump is still building pressure, but doesn't have enough volume. Any thoughts on this? I'll probably pay him to do the repair rather than fight with the gas tank myself, and I'd hate to spend the money if this won't solve the problem.
Author:  lash [ Wed Jun 25, 2008 2:21 pm ]
Post subject:  Re: P0131 & P0171
It sounds like your mechanic may have hit it dead-on. It also sounds like you have one of the smarter techs around.

Here's a great tech/experience article that talks about waveform testing and the three most typical hard-to-diagnose fuel pump failure modes:

http://www.asashop.org/autoinc/jan98/techtotech.htm
Author:  Woosch [ Tue Jul 22, 2008 12:50 pm ]
Post subject:  Re: P0131 & P0171
bonnevillain wrote:
i tried tracking down those exact 2 codes on my 95 L36. i think it may be a PCM problem, they are finicky onthe 95a. i replaced my 02 sensor and nothing changed. phantom codes. i found several other 95s with this problem that was never resolved. my car runs fine so i dont worry about them. they show up once in a blue moon, and then go away



I wanted to give everyone an update. After replacing the fuel pump, the car still wasn't running right. The mechanic said that the oxygen sensor looked to have been recently replaced, and after doing a few more tests, he recommended replacing the MAF sensor. This took care of the problem. I've driven about 1800 miles since the repairs. The car is now powerful, doesn't stumble, and is running like an L36 should. I also haven't gotten any more codes.

You might want to have someone with an oscilloscope check your fuel pump. My pump tested fine for pressure and flow. The problem didn't show up until the mechanic tested its waveform. I'm glad he thought to test it that way. Even though the "usual" tests showed the fuel pump was okay, it was definately on its way out. We're taking the car on a road trip to northern MN in a couple weeks. Losing a fuel pump during that trip would've sucked a$$. Also, you might try to find someone nearby with a '95 L36 that you could swap MAFs with to see if anything changes.
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