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1997 GMC Sonoma code P0171 but only when using 87 octane

Tiny
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  • 1997 GMC SONOMA
  • 4 CYL
  • 2WD
  • MANUAL
  • 155,000 MILES
As the tittle says. I get a lean code P0171 but only if I use 87 octane.
I verified this by filling the tank 3/4 of the way with 89 and ran it completely (almost of course) out and the CEL never came on.
I then put 87 in it and in just 20 miles, the CEL came back on.
so I put 89 back in, CEL never comes back.


a little background first,
I recently replaced the engine with a rebuilt one.
the old engine spun a rod and aparently had a cracked head.

ok, the CEL was on with the old engine also. I just thought it had something to do with the old worn out engine that was low on compression too.

but it's back on even with a new engine.

I've checked the fuel pressure and it seems fine. It's in the specified PSI ratings.

I also replaced the TPS because it had a glitch when I tested it with a multimeter.

the truck runs fantastic other than a slight hesitation under light throttle opening.

I'm pretty confident I have no vacuum leaks. And the brake power booster holds vacuum. So does the heater control vacuum line.
I also blocked off the EVAP system by unhooking the vacuum line to it and using a new line I zip tied off so it wouldn't leak.
the CEL came on with the evap system problem(which I expected) but the Lean code was also present.

I've also replaced the Coolant Temperature Switch.
(this truck only has this sensor to read the coolant temp)

I'm stumped.

would a leaking or clogged injector cause this?

I don't have a problem replacing all of the sensors, they are relatively inexpensive. But I'd rather not.
especially since the O2 sensor is pretty stuck. I doubt I'll ever get it out.
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Wednesday, February 18th, 2009 AT 8:08 PM

15 Replies

Tiny
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The o2 sensor is most likely what is causing the problems. They have reactive material in them and they only last so long, especially if the last engine burned oil/coolant it will coat the sensor and make it read out of range. I would start there. If this vehicle has a Mass Airflow Meter, it could be the problem as well. Those are about the only 2 things I can think of that an octane rating would effect. The MAF measures the amount of air coming into the engine if it is out of range it can cause lean codes
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Thursday, February 19th, 2009 AT 7:52 AM
Tiny
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Thanks for the response.
I don't have a MAF. So that narrows it down.
I was thinking it was an 02 also.
I just have not had the time to test it to be 100% sure.

And I did try to break it loose before but it seems like it might not ever come out.

I hope I don't break it off instead.
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Thursday, February 19th, 2009 AT 5:18 PM
Tiny
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I wasnt sure about the MAF, that is right around the transition period for those engines. Try soaking it with PB Blaster or other penetrating oil to loosen it up. Also break it free, spray it and thread it back in. Do this while backing it out a little more every time. This will work the oil down into the threads and help free it up.
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Thursday, February 19th, 2009 AT 5:48 PM
Tiny
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Well I got it replaced. It came out much easier than I thought it would.
Big 24" ratchets are handy. Haha.
I thought for sure it was the O2 since it had white soot on the tip.

On to the bad news, the CEL came back in about 10 miles.

So I replaced the Crankshaft position sensor.
So far, the light has not come back on.

But, I still get a slight misfire.
I'll keep this thread updated if I get it to quit misfiring.
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Friday, February 20th, 2009 AT 3:21 PM
Tiny
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That may be the injector, those had big problems. Keep me posted
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Friday, February 20th, 2009 AT 4:33 PM
Tiny
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It seems when the engine is cold it runs the best.
I will get a lot of misfire/hesitation after it's warmed up.

Especially if I lug the engine too much. Say like 25mph in 3rd gear, it will miss terribly.

What's your take on "rebuilt" injectors?
The ones I seen have a 24k mile warranty and cost $125 for a set of 4 shipped.

The CEL has never come back. Which I'm very happy about. I would have bet the farm the two problems were related.
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Friday, February 20th, 2009 AT 8:34 PM
Tiny
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Ok I think I've got it 100%.

I checked the plugs and the #1 has white deposits.
I guess this is the cyl that is too lean.

But, I changed the gap to the spec the owners manual says to use..045. The sticker under the hood says use.060

the misfire/hesitation is 100% gone. CEL still off.

I already ordered the injectors so I'll go ahead and replace them.
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Saturday, February 21st, 2009 AT 8:54 PM
Tiny
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New injectors are in. Truck runs a little better than before. The idle is much smoother and so is the power delivery.

I'll try changing the plug gap back to the hood sticker spec of.060 and see if the hesitation is no longer.

I just don't have the time right now.

Ok, I have not change the plugs back to.060 but it seems to have a slight hesitation. But much much much less, but still noticeable. Almost like it runs out of gas for a sec.
I'm getting a little frustrated with this truck.
I can't believe so many parts failed at once on this thing.

The new injectors have seem to fixed one problem but created another. Argggg.
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Wednesday, February 25th, 2009 AT 3:41 PM
Tiny
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Plug gap back to.060. And it's a little better.
The slight misfire I feel is about 97% gone. I guess I will have to live with it. Still no CEL.

89 still makes it run 100% though. I just don't get it.
Maybe I will see if anyone in my area can dyno tune the computer.
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Friday, February 27th, 2009 AT 6:08 PM
Tiny
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I have never come across one that is so sensetive to the octane of the fuel
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Sunday, March 1st, 2009 AT 8:58 AM
Tiny
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I'm starting to think something is wrong with the computer and the way it's adjusting the timing.

I mean there isn't anything else left except the computer and the Ignition module.

It also quit doing the high idle when cold a few years ago.
I've checked the IAC valve etc and it's working.
I guess it's the computer not actually opening it?

I have no idea.
It doesn't stall out or anything. It just doesn't behave like it's supposed to.
I'm positive none of the wiring is bad. There isn't one part of it that is exposed from the wire looms and everything seems to be getting voltage to all the sensors etc.
I cleaned all of the grounds when I had the motor out too.
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Sunday, March 1st, 2009 AT 9:30 AM
Tiny
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Does it have a knock sensor, If so maybe it is out of range (too sensitive) try wrapping the threads in pipe tape to dull its ability to read, worth a try at this point
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Sunday, March 1st, 2009 AT 10:18 AM
Tiny
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How much of it should I use?
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Sunday, March 1st, 2009 AT 4:14 PM
Tiny
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Ok I got this POS figured out.
The EGR valve had a code come up.
Blessing in disguise you could say, I replace it with a used part from a newer vehicle and it runs 100% perfect now.

Thank god that ordeal is over.

Btw, this kind of egr is $200 new. Ouch.
I got a used one for $8 that looks almost new. It doesn't have much carbon in it either.
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Saturday, March 14th, 2009 AT 3:32 PM
Tiny
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I just wanted to update this post.

After many attempts to replace parts in hopes one would fix this problem nothing has ultimately worked in the long run.

I finally bought a new flex pipe and replace it since it was leaking and starting to fall off.
So far after almost 2 full months the check engine light has stayed off.
I guess the 02 sensors were able to detect the extra fresh air in the exhaust because of the leak at the flex pipe.
However, I still (but only once so far) get the check engine light for a random misfire. And the spark plugs look like they are running really lean.
The only thing left to replace is a fuel pump, but it shows it has sufficient pressure.
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Saturday, December 19th, 2009 AT 5:54 PM

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