95 1MZ MAF Sensor

[ErikS]

Newbie/lurker posting.....
After failing emissions test (only high HC), and reading up on what might cause high HC (tho car is running good) - decided to change plugs and check out a few her things along the way.

At the upstream end, the MAF sensor is showing high resistance values at 60 F, tho cold values are withing specs.
-2 F = 12.2-12.8 KOhms (spec 10-20 KOhms @ -4 F)
58 F = 2.7-3.3 KOhms (spec 0.4-0.7 KOhms @ 60 F)

Bought some CRC MAF Sensor cleaner, but after recommended cleaning (and then some), resistance values haven't changed. No MIL shown, and car runs great - can a malfunctioning MAF cause high HC readings?

This seems to be an expensive part to replace - list ~ $330, online prices ~ $250. Is it worth going to a pick-n-pull boneyard to save some $$$$ ? Original part # = 22020-0A030 Replacement part # = 2202020020

Thanks in advance for any insight -

[Mister_Perkins]

Usually having a high Resistance on the MAF will cause a High CO and not a high HC.

You've read up on the High HC causes already, so you know that the following can cause it to fail:

1. Engine was not at normal operating temperature.
2. Ignition system malfunction. Spark failing to occur for any reason will send unburned hydrocarbons (HC) down the exhaust pipe. Example: bad plugs, plug wires, distributor cap, rotor, coil wire, coils, etc.
3. An extremely lean fuel mixture that causes misfiring. Examples include disconnected, leaking or misrouted vacuum hoses, intake gasket leaks, EGR stuck open, low fuel pressure.
4. Over advanced timing. Insufficient spark duration.
5. Low compression or mechanical problems. Worn rings, burned valves, bad or misaligned timing gears.
6. Over rich fuel condition causing both HC and CO failures.
7. Contaminated, bad, or restricted catalytic converter.

If the MAF is reading out of spec though, I would suggest replacing it anyways. You can probably find a good one at the salvage yard or in the classifieds here.

Good luck!

[ErikS]

Thanks - I had no idea what effect the MAF has on emissions. The car passed for CO, so something else must have more influence than the MAF. Changed plugs Saturday (color good, but worn), as it's been 60K miles, and along the way cleaned throttle body, Idle Air Control valve, and the Intake Air Control valve (all new gaskets).

Guess I gotta get a hand-held vacuum pump and diagnose the EGR now.

I'm wondering whether the CAT may be at fault - the car tested OK in '07, and then an aftermarket CAT was installed in '08 (exhaust pipe rusted through), followed by fails in '09 and now.

[Pvt-Public]

I know this will sound weird but, change your oil. We used to do this at a shop I worked at years ago if the HC was a little high.

[ErikS]

Thanks, but I DID change the oil the day before the emissions test, and also took it out for a 40 minute expwy ride just before the test.