I smoked the evap and the intake yesterday. I found a small leak around the vacuum tree. Replaced the O-ring and the vacuum plugs which were also hard and loose but the P0171 returned even after plugging the purge valve. I'm at a loss as to what to do next. This car has 192k miles. I have one more thing I need to check and that's the throttle body. I wonder if it's worn out and air is entering around the shaft. I get a large negative fuel trim in both the short and long term of about -10 to -20% on deceleration. It continues at idle but the fuel trim start climbing back to normal. As I said before, the most normal fuel trim readings is light cruising around 1500 rpm. But I see large swings that are troubling. The car drives fine.
Another week or so of this, and I'll have to have my mechanic look at it.
P0171, Getting Closer to the Problem
I finally found the culprit. It was the new Bosch MAF sensor I installed last September. This P0171 has been going on for 4 months but its been more persistent recently with the CEL coming on every other drive cycle. I replaced the new MAF with an old Bosch unit and my fuel trims are no longer swinging back and forth wildy and there is no engine stumbling after a start. I became hopeful. With the P0171 issue, my STFT would go +25% on acceleration and then go negative during deceleration. Occasionally I see normal fuel trims during cruise but eventually the LTFT would start marching toward the negative until the damn CEL. Also, with the bad MAF, there would be a bad stumble or stall after a start. The RPM would momentarily go low then high. After replacing it, it roars after starting (I like that). I had no real reason to suspect the new MAF sensor other than I've tried almost everything except a new catalytic converter and believe me that was on my mind. Now with the old MAF sensor, the fuel trims looks unusually normal never exceeding +/- 10% and best of all, no P0171!
You may ask, why was the old MAF sensor replaced last September? Well, Mortitia was stalling out on a warm autumn day at a stop. She became hard to start and would not stay running and fearing being stranded and having to call AAA, I recalled a trick from this or some other Volvo site that the MAF sensor can be disconnected and if the engine started and run, then this may be a bad MAF sensor. The new one fixed this problem at the time but I soon had a CEL, P0172, a very rich condition. That was quickly remedied by replacing the swollen elbow that slipped off the flame trap. Some months later, the P0171 appeared. It became more persistent even after I replaced the PCV system, vacuum elbows, the IAC, camshaft sensor, fuel injectors, o-rings, air and snorkel hoses, cleaned the air box, etc. I had my mechanic take a whack at it and he found a vacuum leak I missed. He even replaced the computer for no charge since he replaced an earlier one that I wrote about. He did warn me that he didn't think it was the computer causing the P0171 and he was right.
This P0171, lean condition, has been a real pain-in-the-ass in terms of time, money, and worries. The most deceiving part about it is that the car ran fine even with the error. I've tried bypassing every vacuum related subsystem except the power brakes and I still couldn't find the problem. I would never suspect that a new MAF sensor would screw with me like that. Mortitia even past the California bi-annual smog test last month with the cleanest emissions ever! I have mis-diagnosed this problem so badly that I could kick myself but now this weight has been lifted off of me. Well, on the positive side, I got a refund on the MAF sensor and the car has some new parts. All the vacuum leaks are fixed. There's no smoke in the crankcase thanks to the new PCV system I installed. The fuel injectors and seals are like new now. Anyway, until the next problem.....
Lessons learned:
1. Disconnecting the MAF sensor is a work around and not a clear diagnosis that the sensor is bad.
2. Don't rule out newly installed parts. They need to be tested too. They could be bad even a Bosch.
3. Do a more thorough job of testing and stick to the results. E.g. if fuel pressure is fine then the pump is OK, move on.
4. Resist the temptation to just replace parts no matter how strong the rationalization is.
5. Run a test twice if it will remove all doubt.
6. Make sure pre-test conditions are adequate e.g. adequate smoke and pressure for the smoke test, double check wiring diagrams for proper ECU connector numbers, etc.
7. There maybe more than one cause for an ailment e.g. I thought the stumbling start was a fuel injector issue and so I replaced all them with the rationale that they have 192k on the clock.
8. Sometimes going by instincts is wrong.
9. If there's a checklist or procedure, follow it. If not, find one.
You may ask, why was the old MAF sensor replaced last September? Well, Mortitia was stalling out on a warm autumn day at a stop. She became hard to start and would not stay running and fearing being stranded and having to call AAA, I recalled a trick from this or some other Volvo site that the MAF sensor can be disconnected and if the engine started and run, then this may be a bad MAF sensor. The new one fixed this problem at the time but I soon had a CEL, P0172, a very rich condition. That was quickly remedied by replacing the swollen elbow that slipped off the flame trap. Some months later, the P0171 appeared. It became more persistent even after I replaced the PCV system, vacuum elbows, the IAC, camshaft sensor, fuel injectors, o-rings, air and snorkel hoses, cleaned the air box, etc. I had my mechanic take a whack at it and he found a vacuum leak I missed. He even replaced the computer for no charge since he replaced an earlier one that I wrote about. He did warn me that he didn't think it was the computer causing the P0171 and he was right.
This P0171, lean condition, has been a real pain-in-the-ass in terms of time, money, and worries. The most deceiving part about it is that the car ran fine even with the error. I've tried bypassing every vacuum related subsystem except the power brakes and I still couldn't find the problem. I would never suspect that a new MAF sensor would screw with me like that. Mortitia even past the California bi-annual smog test last month with the cleanest emissions ever! I have mis-diagnosed this problem so badly that I could kick myself but now this weight has been lifted off of me. Well, on the positive side, I got a refund on the MAF sensor and the car has some new parts. All the vacuum leaks are fixed. There's no smoke in the crankcase thanks to the new PCV system I installed. The fuel injectors and seals are like new now. Anyway, until the next problem.....
Lessons learned:
1. Disconnecting the MAF sensor is a work around and not a clear diagnosis that the sensor is bad.
2. Don't rule out newly installed parts. They need to be tested too. They could be bad even a Bosch.
3. Do a more thorough job of testing and stick to the results. E.g. if fuel pressure is fine then the pump is OK, move on.
4. Resist the temptation to just replace parts no matter how strong the rationalization is.
5. Run a test twice if it will remove all doubt.
6. Make sure pre-test conditions are adequate e.g. adequate smoke and pressure for the smoke test, double check wiring diagrams for proper ECU connector numbers, etc.
7. There maybe more than one cause for an ailment e.g. I thought the stumbling start was a fuel injector issue and so I replaced all them with the rationale that they have 192k on the clock.
8. Sometimes going by instincts is wrong.
9. If there's a checklist or procedure, follow it. If not, find one.
-
- Similar Topics
- Replies
- Views
- Last post
-
- 1 Replies
- 2625 Views
-
Last post by frogprince
-
- 13 Replies
- 4378 Views
-
Last post by Sandawgk5