M54 P0491 / P0174 tomfoolery
 

If I didn't have my Ultragauge hooked up to show me the codes every time they pop up this wouldn't be so annoying. But now I'm to the point of just being pissed off about this. Damn near every ignition cycle I get a P0491, and on about every 10th I'll also get a P0174. When the P0491 was popping up just by itself, I figured, OK, I'll just get a new O2 sensor and be done with it. But now the P0174 accompanies it and makes me wonder if I'm not gonna have to chase a vacuum leak. Chasing a vacuum leak means sending the boss away on a shopping trip so she won't just be sitting around waiting for me to finish my fun. :rolleyes:

Would anyone be so kind as to point me in the right direction here? I'd perfer to get this resolved quickly, especially since the boss has to drive my X every couple of weeks while her F25 is in for regular inspection at the plant.

Any and all guidance / suggestions / sympathy is greatly appreciated! :thumbup:

Grab yourself a cold beer or whatever is your favorite beverage. If you have little one around grab a pair of headphone or turn the volume down. You don't want them to listen to all the expletives on this how to video.

https://www.youtube.com/watch?v=eUhrh87A6tc

Yeah, that guy's a real hoot... not the kind of guy I'd trust any of my cars to, even though he apparently does even fix something every now and then. :rofl:

I've considered maybe it's an issue with the SAP, and I'm even thinking about installing a simulator and removing the SAP (since here in SC, we don't have emissions checks, seeing as NC and GA filter all our air of pollutants for us...). What's curious to me, though, is that I never get a P0492, only the P0491. If the SAP was going, wouldn't I get both? Also, I've read that if I get a Euro tune for the MS43, I can forget about the SAP altogether.

Hell, I'd almost rather deal with another oil leak... :confused:

DO NOT remove the SAP system and try to work around the problem. You will be sorry if you go this route.

Resolving the problem is not a difficult as you may think. You may have 2 problems. Could be a lazy pre-Cat O2 sensor, could be a vacuum leak. Could be both.

A lazy pre-cat O2 sensor will not trigger Lean codes, but can cause SAP codes. Other problems can also cause SAP codes.

So start reading and starting thinking before you jump on anything. DO NOT replace sensors as your first order of business. Pre-cat O2 sensor replacement may be due anyway. They are consumable and should be replaced every 100k or if there are codes for the sensors before then. Since O2 sensors have been used, I typically only see O2 sensor heater codes, I rarely see codes for lazy O2 sensors, might be able to count on one hand the times I have seen an O2 sensor code for a sensor that is not reporting correctly.

Here is some light reading for you:

Readiness Monitors not clearing? - E46Fanatics

Emissions Readiness Help - E46Fanatics

P0411 solution for 330 with SAP MAF - E46Fanatics

Secondary air pump system P0491 and P0492 - E46Fanatics

Secondary Air Pump - E46Fanatics

Intake Manifold and Vacuum Caps - E46Fanatics

http://forum.e46fanatics.com/showthread.php?t=106959

Secondary Air Pump - E46Fanatics

http://forum.e46fanatics.com/showthread.php?t=106959

SAP (Air Pump) Simulator - E46Fanatics

Need to pass inspection, P0455 no CEL - E46Fanatics

Failed CA Smog for Secondary Air Injection Not Ready - Here's what I did! - E46Fanatics

Read my comments in this thread as well - Help With Misfire & Engine Fault Codes - E46Fanatics

You sir, are a gentleman and a scholar. I'll definitely do the suggested reading and get back to everyone on my progress in solving this. Of course, since I first posted about the issue, both DTC's have disappeared.



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Do not worry, they will be back and you will start to have more problems.

Vacuum leaks and lazy pre-Cat O2 sensors are the most likely culprits. I know the M54 3.0l quite well, the problems are the same across the body styles, however, the X5 has a slightly larger engine compartment so the problems show up a bit later than on the smaller 3 series bodies. More free air space and slightly cooler under hood temps in the E53.

I'm certain that none of the O2 sensors has ever been replaced, so should I just go ahead and replace the pre-cat sensors?

O2 sensors are like spark plugs, they operate in a very harsh environment and they are recommended to be replaced at 100k miles, rarely are they.

If you choose to replace them, I would just replace the Pre-cat O2 sensors with Bosch direct fit units, usually Amazon or AutoZone/Advance online with coupons are a good price. If you can get them for under $45 each, this is a pretty good deal.

Now, the Pre-cat O2 sensors WILL not resolve the Lean codes, may even make them a bit worse. They may help with the SAP code(s), but the SAP system also has a few hoses that deteriorate and fail that will cause problems.

O2 sensors and the MAF are really the main controllers of the Air/Fuel mixture. The MAF is the single most sensitive and precise sensor on the vehicle, but ALL the fuel control is influenced by the Pre-cat O2 sensors. The measure or feed back what the fuel control system is doing so it can make corrections. So in the big scheme of things, healthy Pre-cat O2 sensors are important for a properly running engine.

If you wait to get OBDFusion and the interface, if the sensors are lazy, you will be able to see this and it may make you "feel" better when you replace them that they were lazy, but again, O2 sensors do not last forever and should be refreshed as recommended.

fwiw ... i have P0491/P0492 codes that I believe are due to a bad vacuum line (rotted then damaged during a Valve Cover Gasket replacement). However, I have the 3.0L so this may not apply. I have had the P0174 (too lean) and this was a whole in the large rubber air intake elbow between the MAF and engine. Good Luck.

I had the same problem with the SAP vacuum line after a valve cover gasket replacement a couple years ago. Replaced that vacuum line and had no problem since. I've also replaced the air intake boot recently. My fear at the moment is I may have a bad vacuum control valve on the back of the intake manifold. I thought if I replaced the SAP with a simulator then I could forget about that valve and vacuum line. The engine seems to be running fine and is very responsive. Haven't checked the fuel trims yet or made any other deep analysis. Mostly just hate having DTC's.



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