sincitybimmer

Girlfriends '99 ML430 failed smog. Smog tech gave her two codes PO130, PO150 for failed O2 sensors. I bought the two front O2 sensors but did not install them. When I hooked up my OBII scanner it also showed PO301 (cyl 1 misfire) PO304 (cyl 4 misfire) PO303 (cyl 3 misfire) PO300 (Random Mult misfire) PO135. Since the truck has 155k and I don't know the history, it is conceivable the O2 sensors are bad, but I figured with the other codes the problem was more likely electrical. My scanner also takes "live" readings so I could see the O2 sensors receiving info -- i just didnt know if the flows were where they were supposed to be. Anyway, I replaced all 16 plugs with Bosch IR-Fusion 4s, Replaced the plug wires, air cleaner, and cleaned the MAF. I cleared the codes, but a few days later they were back showing at first the misfires, then a week later the O2 sensor faults. Any ideas what else could be the problem? Coilpacks? Something else? The temp smog passes are costing money and she's freaking out.

Any help is appreciated

sosh

The other faults you are showing are most likely not an electrical malfunction, they do most likely exist. Were the 02 sensors OEM?? If not that could be your issue. Do not think that the plugs you used are the correct ones called for by MB and that could be your issue. Another possibility is that the wires are bad or you could have damaged them during the plug change which is easy with 155K on the ML. Do your girlfriend a favor and take it to an MB shop, either indy or dealer and let them resolve the issue. Sounds like you do not have the knowledge or correct equipment to go any further than you have. Pick up the bill and she will be pleased.

sincitybimmer

Thanks but no thanks Sosh. I think I'll wait for someone to reply with useful information. The plugs are in fact the high end of MB calls for. If you'd read my initial post you would also see that I replaced the wires with genuine, factory plug wires when I replaced the plugs. Since the codes are EXACTLY the same as before the work, logical deduction would show that there is something else going on. O2 sensor malfunction would not cause engine misfires. But spark/ignition problems WOULD cause O2 sensor faults. And since those faults showed up again a week following a code reset, it stands to reason the engine misfires are to blame.

As for "Sounds like you do not have the knowledge or correct equipment to go any further than you have"... is a waste of my time and yours. I've had over 25 years of rebuilding cars ( mechanical, electrical) foreign and domestic. I'm putting aside my BMW for a time being to fix my lady's MB. The great thing about about forums such as these, it's a fantastic resource for people that have experienced similar issues to share those experiences for the sake of others.

Now, anyone else out there that has had a similar issue or relevant useful ideas on what might be going on -- please feel free to drop a line

sosh

Well if you have done all the stuff you said and you are certain the the 02 sensors are OK the next thing to look at is the coil packs. Try swapping with a good cylinder and see if the problem follows the pack.

lkchris

It's ususally pretty foolish to take the messages at face value.

Most of the time the problem is the MAF sensor, which then results in improper exhaust mixture downstream that creates the O2 sensor message.

While you're at it, replace the plugs (again) with the correct ones--KMart Bosch don't work.

sincitybimmer

Thanks for the info, but why all the flamming on the message board?? Bosch IR-4's are most definitely NOT Kmart Bosch, if there is such a thing. Should probably look something up before putting down. IR-4's are the top-of the-line Iridium plug for the ML430 -- they retail over $20 per. I got for about $7 each at the same place I get all my German parts, Autohaus

sosh

No flaming, just never saw those plugs used before and my suggestion was to get the car fixed and back into your girlfriends hands ASAP. Don't be so defensive and sensitive!

paulv

Can your scanner read the LTFT percentages (long term fuel trim)? These are recorded when the CEL goes on in the Freeze Frame data (Mode $02 on your scanner) -- that would be a good place to start as all those misfire codes suggest air/fuel issues (as lkchris was eluding to). If the LTFT %'s look OK (approx +/- 10% with the same range between cylinder banks), then that tends to rule out ignition problems. Also, from your response that you have many yrs experience, I will assume that you used something like CRC MAF cleaner and not some other "incorrect" cleaner.

With what you described, I tend to agree with lkchris' assessment. Try and back-probe the MAF and see how the voltage swings are. Also, don't manually reset the codes and let the PCM does its testing to clear the CEL. Good luck.

Regards,
paul...

sincitybimmer

Good info there. I checked the LTFT percentages in freeze-frame and the were within 10% - one was 6.10 and the other 10 i believe. So looking like MAF? I did use the CRC brand of MAF cleaner because everyone seemed to recommend. I had planned on swapping coil-packs between cylinders but thinking now should just replace MAF

sosh

MAF's are really not designed to be cleaned. Its only a band aid. They are throw aways...and cheap.

paulv

Since the LTFT numbers are looking good, this would suggest that the fuel adaptation functions are not correcting for any type of an overly lean/rich condition. Typically, good LTFT numbers with misfire codes indicate ignition problems as high LTFT numbers (either pos or neg values) tends to indicate fuel delivery, mechanical (burnt exhaust valve), or other sensors (i.e., MAF). If your scanner can graph live data (Mode $06 and $07) you may want to see how the O2 sensors are performing (although not realtime but possible useful info) and the MAF sensor (either by voltages or grams/sec). Check the live data readings at idle as well as approx 2000 RPMs. You may want to try an isolate the problem some more before you start throwing parts ($$) at it.

A big problem is that if you don't have any OBD2 diags data for the car, you won't know what all the conditions are required before you test a component, and many of these components depend on another one (i.e., crankshaft position with camshaft position sensors). Good luck.

Regards,
paul...

sincitybimmer

Replaced MAF sensor last night. Don't remember the live data readings on the old one but new flow readings seem increased. Hopeful this does the trick. Know in a few days