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Old 09-15-2018, 05:27 PM   #1
Itsme2970
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Default Sohc to Dohc swap done with issues

Swapped out a motor for single mom because thatís what she had and had no funds to purchase a new motor. The swap actually went well with few issues, however I have a po171 code that I have yet to chase down.. as far as I can tell there are no intake leaks to cause the lean mix. I am curious to know if the injectors being smaller due to bein sohc is the problem. I did not change out the pcm because the car has antilock and some other things the donor motor didnít have. The swap is 96 sohc to 96 dohc.

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Old 09-15-2018, 06:20 PM   #2
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Default Re: Sohc to Dohc swap done with issues

The SOHC PCM will run the DOHC as a grocery getter with no issues. The code is likely real. Check your fuel pressure as that is the most likely cause. You are looking for 38-42/44 KEY ON Engine Off and 33-36 at idle. Regulator on the rail can be bad and the one from the old engine should fit if you are lucky.

Exhaust air leaks can also cause this code.

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Old 09-15-2018, 07:41 PM   #3
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Default Re: Sohc to Dohc swap done with issues

Thanks. I will check that Monday first thing

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Old 09-15-2018, 10:37 PM   #4
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Default Re: Sohc to Dohc swap done with issues

Did you swap in the SOHC injectors or are you using the DOHC injectors. You do want to keep the SOHC PCM or you might end up buying a world of expenses/trouble swapping in a DOHC PCM. It should run fine on the SOHC PCM.

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Old 09-16-2018, 08:15 AM   #5
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Default Re: Sohc to Dohc swap done with issues

P0171 exhaust lean, maybe check/replace the O2 sensor. It may be on its way out, and just hasn't set an O2 sensor code yet.

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Old 09-16-2018, 10:43 AM   #6
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Default Re: Sohc to Dohc swap done with issues

That is a hillbilly fix that almost never works as the O2 sensor is closely monitored and they either work or not, no half way. If there are no O2 sensor codes then it is very likely not broke. Lean codes are very difficult to fix.

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Old 09-16-2018, 12:20 PM   #7
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Default Re: Sohc to Dohc swap done with issues

Quote:
Originally Posted by OldNuc View Post
That is a hillbilly fix that almost never works as the O2 sensor is closely monitored and they either work or not, no half way. If there are no O2 sensor codes then it is very likely not broke. Lean codes are very difficult to fix.
I agree with OldNuc don't replace an O2 sensor because you get a lean code error.

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Old 09-19-2018, 02:05 PM   #8
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Default Re: Sohc to Dohc swap done with issues

https://www.motor.com/magazine-summa...-january-2018/

Here is an excellent tech article, that is very detailed, and covers the multiple causes of a lean condition.

And, yes, the O2 sensor CAN be a culprit for setting the P0171 code in OBD-II systems.

as Quoted: "In general, a rapidly and regularly switching front O2 sensor is the best predictor of a fuel mixture thatís in control. However, a damaged, contaminated or degraded sensor may develop a bias that skews its performance away from stoichiometry, causing it to switch at a different air/fuel ratio. This, unfortunately, will not in and of itself necessarily result in a DTC if the total fuel trim corrections (STFT + LTFT) remain within an acceptable mathematical range. From time to time, a faulty O2 sensor that cannot identify or report a rich mixture may cause a false DTC, so you should always verify O2 sensor function and response early in the diagnostic process (see ďTrust, But VerifyĒ on page 30)." Page 30 is directly under this quoted area, in the article.

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Old 09-19-2018, 02:23 PM   #9
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Default Re: Sohc to Dohc swap done with issues

I think the writer should take a close look at the O2 sensor DTCs and figure out how they function. If there is no DTC and the 171 is a continuous event then the sensor is functional and if there is a O2 sensor DTC active the 171/172 codes are locked out. By definition and programming if you set a rich/lean DTC then the fuel trim is out of limits.

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Old 09-24-2018, 08:23 PM   #10
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Default Re: Sohc to Dohc swap done with issues

We can verify the condition of the actual fuel mixture by pulling the spark plugs and looking for white, blistery grounds.

Part of isolation troubleshooting is to eliminate all possible causes. It is better to at least verify it, by taking a spark plug out of the cylinder, than to do everything else and not eliminate the code.

I may not be correct in stating to replace the O2 sensor, which I will very easily say I am sorry for if I am wrong.

I am not incorrect by stating that verification of O2 sensor functionality as part of the OPs troubleshooting steps.

I went through a rigor Moro on troubleshooting a misfire and excessive power loss on my Grand Am, because I overlooked something that seemed as though it was NOT the cause and was NOT throwing any DTCs.

Replaced the fuel pump, the fuel pressure regulator, the coils, the spark plugs, the wires, the MAP sensor, the O2 sensor, fuel filter, EGR valve(I was getting a Code 32), and even put a whole throttle body on the replacement engine(when all the previous repairs did not correct the issue, eventually the misfires burned a valve and I had to replace the entire engine. The replacement engine had the same symptoms as the old engine did, and was about the same amount of mileage).

None of that corrected the issue on two engines. You know what the problem was?

The design of the crankshaft position sensor, and its location inside the engine block causes it to develop a hairline crack in it at about 90-100k miles. It NEVER throws a code, because it does still send a signal to the ECM. It just sends a skewed signal that causes incorrect ignition timing and fuel delivery.

So, a $22 part and 1 hour of labor would have saved me over $1,400 on a replacement engine, not counting all the other costs in repairs that didn't correct the problem.

So, IF is CAN be a PROBLEM, it SHOULD be VERIFIED as functioning CORRECTLY.

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