Tracking down P0171

[jtp]

Long-Term Fuel Trim Lean @ 25%.

I've had this code for about a month and am really getting tired of it. Really don't want engine damage on this car. I'm putting together a flowchart to tackle this thing once and for all and would appreciate your input. Please help me form a logical to-do list and I'll update as I go through each item. Here's all the information:

The Car:
95 850 Turbo Wagon 100k miles
Aftermarket intercooler and rad hoses, IPD intercooler
Recent tune-up replaced cap, rotors, plugs wires, fuel filter, PCV system (No code after this)
Large diameter turbo intake pipe
Kand N oiled filter. Let it dry 48 hours after oiling before putting it in the car. (Code 2 weeks after this)
NA Intake Mani, Throttle Body, and Manifold Spacer

Symptoms:
-Car is in some kind of limp mode. Boost limited to 5psi.
-Stumbles somewhat initially @ idle. Pump the gas once and it smooths out. Drives around without a hitch.
-Code P0171

What I've done to track down the problem so far:
1) Smoke tested for vacuum leaks. None present in the intake air system.
2) Cleaned the MAF with MAF cleaner
3) Hooked the car up to a Pegasus Scanner (professional grade/fast updates). Showed pretty good responsiveness in both the front O2 sensor and MAF.

My personal suspicions
1) Last time I put the fuel rail back on the car I tried really hard not to drop any o-ring spacers that go between injector and rail. If I did lose one though, I'm thinking that could be the vacuum leak. And the smoke test wouldn't have caught it.
2) Bad fuel pressure from a done fuel pump.
3) Bad/dirty IAC valve
4) Bad o2 sensors or MAF. Even though it wasn't readily apparent on the scanner.

My plan right now, based on what I have on hand and is the most affordable:

1) Clean IAC
2) Smoke test for vac leaks again
3) Swap-out MAF with an old one I have (not sure if that one is good or not)
4) More O-ring/spacers are on order, put them in when they get here next week if the first three items don't take care of it.
5) Check fuel pressure (will have to buy a fuel pressure gauge that fits the Volvo)
6) O2 sensors, I know I could use them anyway but the data doesn't suggest they are the issue. That's why I have this further down the list.
7) Get a new MAF if #3 didn't work.
8 ) Blame it on the ECU. I think all flow-charts end this way.

[regent]

Have you tried to make the comp "forget and re-learn" its LTFT charts by disconnecting the battery?

[rmmagow]

I have the same problem on a 98 V70 LPT unmodified. Doesn't obd reset essentially issue a "forget and re-learn"?
I probably have a vacuum leak someplace but can't figure out how to smoke test the thing. Light will actually stay off for days at a time. I've got a persistent P0455 that may relate here too. I get P0243 on hard acceleration and my hoses to the turbo from the controller are in not so great shape and will be replaced as soon as we see warm weather.

[jtp]

I've cleared the code after each attempt to fix it. It's come back like 3 times now. P0171 relearn is two drive cycles I believe. Mine is coming back at about that rate.

If you don't have access to a smoke machine look with throttle body cleaner. Spray the vacuum lines and listen for the engine to rev. Pretty effective. Automotive smoke machines are nice but costly.

[jtp]

@regent I cleared the code with a codereader. I will disconnect the battery today and reconnect it after 15-20 minuted and see what happens. I didn't know the LTFT memory remained after the code is cleared.

That might make sense though because my short term fuel trim readings are never up where the long term ones are when I read them live as I drive around. And I thought the long term was just an average of short term.

[JRL]

Of course it will come back, it's an 02 sensor code

[regent]

3) Hooked the car up to a Pegasus Scanner (professional grade/fast updates). Showed pretty good responsiveness in both the front O2 sensor and MAF.

If according to the scanner, the O2S and MAF perform, then the code may not necessarily come back (if it does, there should be a different reason). I say the O2S is usually the last on the offender list, and would put my money on a vacuum leak.

Clearing codes with a scanner will NOT reset or initiate the complete LTFT relearn routine. The LTFT matrix resides in volatile memory (RAM), and therefore power needs to be disconnected and filter caps discharged completely to make sure the comp 'forgets'. With the battery off, I turn the key to pos. II (Starter), and the solenoid coil is a good drain path for the Main Filter Cap. The wait time is also required (for the filter caps inside the comp to drain).

One more thing, bear in mind that the comp may have no clue if the O2S is reading properly in certain cases. Say, an O2S were sticking rich, the comp would assume it was reading correctly and begin taking away fuel to compensate. (This would be a "false rich" condition.) The comp would be leaning the engine and setting a possible P0172 or P0175. The codes would indicate rich, however the AFR is really lean.

If you use only the false rich codes to diagnose and don't observe all the fuel trim and O2S data, you may make a false diagnosis. Hope this helps.

[jtp]

Thanks regent and JRL. If this gets much more complex I'll just grab two o2 sensors. Main reason for holding off on them is that I'll be doing an exhaust at some point here and have read new o2 sensors are a necessity then.

Turned off the battery today twice but don't think I ever got a full drain. Accidentally left a scantool attached the first time, which really made things weird. The service light was on before and after battery disconnect both times.

I'll give it a real good drain tonight and then have a good bit of driving tomorrow to and from work. The way things have been going, P0172 should pop up halfway through my drive home tomorrow night. I'll post back then...

[jtp]

P0172 popped up halfway through my drive home tonight, as scheduled.

Tomorrow I start my list. Clean IAC and smoke test are on the schedule.

[echase76]

I'm having similar issues but with more idle/stumble issues and not the boost problem. Car finally set a P0171 tonight.

You can do a quick test of fuel pressure with a tire gage to check your idle fuel pressure.

What is your idle vacuum?

I'm down to MAF, Fuel Pump Relay, or ???? Yeah, ECU is always there too.

Here is my thread https://www.matthewsvolvosite.com/forums ... 62#p271962

Hope you get it resolved.