Still no justice with code P0171

Tiny
PUREEVILBEAUTY
  • MEMBER
  • 2006 CHEVROLET TRAILBLAZER
  • 4.2L
  • V6
  • 4WD
  • AUTOMATIC
  • 154,000 MILES
One day I was driving and then the check engine light came on I called On-star and they said it was code PO171?
Edit
So then after getting information and having is looked at Firestone they said it was the one ignition coil and the spark plugs that needed to be change. So being the fact those were easy fix I changed all six ignition coils and the spark plugs. But the check engine light still came back on after a little bit of driving and we are still having problems cranking it when it is cold. So now I am here scratching my head.
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Thursday, February 9th, 2017 AT 1:41 PM

4 Replies

Tiny
CARADIODOC
  • EXPERT
"Problems" does not tell me what is happening. Is the starter cranking the engine too slowly? Is it taking too long for the engine to start and run?

Did the mechanic actually diagnose anything or did he give you his best guess based on your description of the symptoms?
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Thursday, February 9th, 2017 AT 2:37 PM
Tiny
PUREEVILBEAUTY
  • MEMBER
Sorry about that. Well the problems are as followed hard to crank when cold runs rough with shaking like it is going cut off. Cuts off while cold when trying to warm up. Also, does that when at a stop light waiting . During time it does this no power.
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Thursday, February 9th, 2017 AT 2:50 PM
Tiny
PUREEVILBEAUTY
  • MEMBER
Yes at Firestone they said the use the tester on it.
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Thursday, February 9th, 2017 AT 2:52 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. As you can see from this fault code, they never say to replace parts or that one is bad. They only indicate the circuit or system that needs further diagnosis, or, as in this case, the unacceptable operating condition. Running too lean can have a whole bunch of different causes, and a failing ignition coil or worn spark plugs can be one of them. With anything that causes the spark to not occur, the unburned fuel and air will go into the exhaust system where it is the unburned oxygen that will be detected by the oxygen sensor as a "lean" condition. You will smell the unburned fuel at the tail pipe, but that does not get detected by the oxygen sensor.

There is another missing piece of the puzzle. If a misfire is occurring, regardless if it is caused by loss of spark or something else, the Engine Computers on 1996 and newer models will detect that and set a different diagnostic fault code stating which cylinder is misfiring. You should have a misfire code if the engine is running rough.

Unfortunately there is not much you can do other than replacing the spark plugs, wires, and ignition coil(s). A scanner is needed to view what the computer is seeing and reacting to. An experienced engine performance specialist will watch what happens to the oxygen sensor readings when he introduces an artificial lean and rich condition. If the misfire is constant and steady, he will disable one cylinder at a time to see which one(s) have no effect on the symptom. Fuel pressure should be checked too. A lot of GM engines will not run right if fuel pressure is just a few pounds too low. There also used to be a common problem with fuel pressure regulators leaking excess fuel into the vacuum hose attached to them, and that dumped way too much raw fuel into the engine. That only applies to the engines that have the regulator on the fuel rail on the engine. On many newer models, that regulator has been moved to inside the gas tank.

When there is no fault code set related to the rough running, the mechanic has to analyze the sensor readings to figure out where to start looking. The scanner gives him the ability to turn systems off and on to see if that has any effect on the symptoms. Examples include the fuel vapor recovery system which does add fuel to the engine, and the EGR, (exhaust gas re-circulation) system that adds inert air to the engine. Either of those can cause a variety of symptoms.
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Thursday, February 9th, 2017 AT 3:57 PM

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