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Old 08-29-2019, 09:28 PM   #1
bheston
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Help diagnosing P0172, P1174 codes

I have a 2008 Uplander with 3.9L engine. I have a couple of codes coming on, they come back after a short drive after resetting them, usually P0172 and P1174 together, sometimes one or the other.

I started by cleaning MAF sensor with no change. I then tested fuel pressure at the rail, I got 60 psi with key on but engine off, and 67 psi with engine running at an idle. This seems a little high, the correct operating range I found was 59-62 psi. I was going to test the fuel pressure regulator but this engine doesn't have a vacuum controlled regulator, it has an electronic unit on the return fuel line. I don't know how these work or how to test them.

Is 67psi at an idle too high? How can I test the electronic regulator on this engine?

Thanks for the help!

Last edited by bheston; 06-04-2020 at 05:56 PM. Reason: New information
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Old 06-04-2020, 06:13 PM   #2
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Re: Testing fuel pressure - P0172, P1174

I've struggled to figure this one out, and let it go with the check engine line on for a while because it is running well and mileage hasn't seemed to change either. But I'm thinking about selling this van soon and would like to get this problem fixed so the light isn't on.

I'm now getting a P1174 code to set after a few minutes of driving after resetting it. I've replaced the pre-cat O2 sensor, replaced the MAF sensor, and checked the purge valve above the fuel tank. As stated above checked the fuel pressure which may be a little high, I ohm'd out the injectors and get similar readings on all 6 injectors within acceptable range. I have also reset the alcohol % calculation a couple of times because this is a flex fuel engine. I can't figure out why I keep getting this fuel trim code. What else can I test???

Does anyone see any readings out of normal range? Here is the freeze frame data that my scanner gave me the last time it threw the P1174 code (pictures attached, but I'll type text below also):
Fuel Sys 1: CL
Calc LOAD(%): 27.4
ECT(deg F): 168
STFT B1(%): -1.5
LTFT B1(%): -4.6
MAP(inHg): 12.4
Eng RPM: 1087
Veh Speed(mph): 44
Spark Adv(deg): 8
IAT(deg F): 109
MAF(lb/min): 1.091
TPS(%): 25.4
Run Time (sec): 169
command EVAP(%): 52.5
Fuel Level(%): 52.5
Warm-up DTC Clr: 2
Clr Dist(mile): 1
EVAP P(inH2O): -0.584
BARO(inHg): 28.6
CAT Te 11(deg F): 1344.2
ECU Voltas (V): 14.094
LOAD Value(%): 19.6
EQ ratio: 0.999
Rel TPS(%): 11.7
Ambient(Deg F): 109
TPS B(%): 25.4
ACC Pedal D(%) 19.2
ACC Pedal E(%): 9.4
Command TAC(%): 16.4

Thanks for the help!!
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Old 06-04-2020, 06:38 PM   #3
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Re: Testing fuel pressure - P0172, P1174

There is a GM service bulletin for this. 08 06 04 028.
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Old 06-05-2020, 07:08 AM   #4
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Re: Testing fuel pressure - P0172, P1174

Quote:
Originally Posted by maxwedge View Post
There is a GM service bulletin for this. 08 06 04 028.
Thanks for that info, I hadn't seen that bulletin. While it may be related, that bulletin covers 4.3L engine in Silverado and Sierra trucks. As it was dated 4-28-2008, I assume this was an issue on brand new vehicles, and required new ECM software to fix the problem.

This van has run well for me for a couple years before this started, so it seems like I should be able to get it back to the state that it doesn't throw this code without changing ECM software. Any other ideas?
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Old 06-05-2020, 10:18 AM   #5
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Re: Testing fuel pressure - P0172, P1174

Not common but look at your baro readings in your area and altitude, then key on engine off get a baro reading on your scanner and compare them.
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Old 06-05-2020, 03:21 PM   #6
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Re: Testing fuel pressure - P0172, P1174

Quote:
Originally Posted by maxwedge View Post
Not common but look at your baro readings in your area and altitude, then key on engine off get a baro reading on your scanner and compare them.
Right now barometric pressure for my area is 29.9, my computer on the van reads 28.6 with key on but engine off. This looks like about the same variance as the data that was stored when the code was stored.

Is this close enough or do you think that variance could throw things off? What sensors does the van use to calculate this?

Thanks for the suggestion.
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