W163 270CDI Dodgy MAF readings?

[manga-blue]

I have a 2005 ML270CDI with 170,000kms on the clock. Never had a problem until recently when it started to toggle in and out of Limp Mode.

We use an ELM327 OBDII scanner with EasyOBD USB software.

When I start the engine, all OBD values (e.g. RPM, coolant temp, intake temp, manifold pressure, etc) make sense, except for the MAF reading which is locked on 8.88 g/sec. After a few minutes of normal driving the MAF starts giving sensible values ranging from 6 to 150 g/sec. If I back off for a few seconds, the MAF will hover normally around the 6-10 g/sec marks for a little while and then lock on 8.88 again.

Whenever the MAF is reading 8.88 the car is in limp mode, with 1/2 power, 3000 RPM rev limit and gearbox extremely reluctant to kick down.

Whenever the MAF reading is constantly varying with values consistent with load at the time then the car has full power, full RPM range and normal gearbox behaviour.

There are absolutely no stored fault codes recorded - which I find a bit strange for a 10 year old car assembled in Alabama by workers trained by Chrysler.

In the olden days before OBDII we used to tap into the signal wire from a sensor to the ECU and measure its voltage or frequency directly. So if a MAF was outputting silly values then we could plainly see it. These days we depend on the ECU to tell us through OBDII what the ECU is interpreting from a sensor output. So it's possible for a sensor to be OK, but for the ECU, for its own fell purposes, to decide it's something else and give us that value instead.

I have two questions:

1. Does the fact that the MAF intermittently locks on 8.88 indicate an intermittent fault in the MAF or is the ECU assigning 8.88 to it because of some other condition?

2. Is it normal for there to be no fault codes when a car is going in and out of limp mode?

 

[manga-blue]

It's very quiet out there ...

For what it's worth, here's an update:

I finally managed to unstick some very stuck MAF connector lock pins and got the original MAF off. It had a light covering of black gunk which cleaned off easily with a spray can of electronic components cleaner. I think there was roughly about a teaspoon of gunk and a few unsavoury looking lumps of harder stuff in it.

I refitted the MAF and started the engine before I reconnected it to the loom. OBD MAF reading was a constant 8.88 g/sec. So that tells me that 8.88 is the default for a totally failed MAF (or at least that's one condition that does it).

Reconnecting the MAF immediately gave me sensible readings, albeit slightly higher than before the clean-up, which would indicate to me that a layer of grease and varnish on the wire was causing it to read low previously.

On the highway the car had full power and continuously varying MAF readings for 10 minutes. Then the first time I let it coast for an extended period the readings hovered around 6-10 before locking on to 8.88 again. When I put my foot down it stayed at 8.88 and in limp mode for a second or two and then the MAF lit up and the power came back on. Repeated this a few times: so it seems if you let it coast down to low RPM it will go into limp mode but will recover after you put your foot down. It's a bit like having a 2 second turbo lag.

 

[Simzy]

It's very quiet out there ... For what it's worth, here's an update: I finally managed to unstick some very stuck MAF connector lock pins and got the original MAF off. It had a light covering of black gunk which cleaned off easily with a spray can of electronic components cleaner. I think there was roughly about a teaspoon of gunk and a few unsavoury looking lumps of harder stuff in it. I refitted the MAF and started the engine before I reconnected it to the loom. OBD MAF reading was a constant 8.88 g/sec. So that tells me that 8.88 is the default for a totally failed MAF (or at least that's one condition that does it). Reconnecting the MAF immediately gave me sensible readings, albeit slightly higher than before the clean-up, which would indicate to me that a layer of grease and varnish on the wire was causing it to read low previously. On the highway the car had full power and continuously varying MAF readings for 10 minutes. Then the first time I let it coast for an extended period the readings hovered around 6-10 before locking on to 8.88 again. When I put my foot down it stayed at 8.88 and in limp mode for a second or two and then the MAF lit up and the power came back on. Repeated this a few times: so it seems if you let it coast down to low RPM it will go into limp mode but will recover after you put your foot down. It's a bit like having a 2 second turbo lag.

This is a lot of effort by yourself for a part that is easy and relatively cheap to replace but better than guessing I suppose. Are you sure your obd2 scanner is picking up codes as I have two scanners for my car, one picks up codes and one does not. If you are sure there are no codes then just replace the maf if it's no good and be done with all the diagnostics.

 

[E270 Owner]

EasyOBD even in the full version format is quite limited compared to MB Star.

Star has a MAF test procedure to follow so you know if its at fault.

All stored codes will be available under MB Star but not found on EasyOBD

 

[manga-blue]

Thanks, I was wondering about that. EasyOBD is fully functional on our W168 A-class and our W203 C-class petrol engines; there are no problems reading fault codes on them. However it registers the M-class diesel as "non-OBD compliant" but does state there are no fault codes present. My problem is that the nearest service shop with a STAR system is over 200kms away with a 31 km steep mountain ascent between us. I'm not really confident of getting there ...

New MAF is ordered but it will take 2 weeks to get here. That's what makes it worthwhile trying to get it working in the interim.

 

[E270 Owner]

Sometimes going with your instinct will beat any diagnostics machine due to some fault codes becoming almost impossible to trace (Toyota's P0420 or GM's P0170) without doing a parts swapping exercise £$£$£$£$£

If it looks like a MAF Smells like a MAF and is producing a MAF fault then it probably is the MAF

 

[kie010]

have you sorted this issue, I have a similar issue, I can start my car and rev it, will be fine until I let it coast or get stuck in traffic, anyways long and short, give it the beans and it will go! Funny thing is, it has a brand new MAF !?

 

[Simzy]

have you sorted this issue, I have a similar issue, I can start my car and rev it, will be fine until I let it coast or get stuck in traffic, anyways long and short, give it the beans and it will go! Funny thing is, it has a brand new MAF !?

Did you diagnose the fault via a diagnostic system ? It amazes me the amount of people on this forum get a fault on their car and then want to guess what is wrong with it. Put it on a diagnostic then come back with the codes then we can help you .

 

[E270 Owner]

Did you diagnose the fault via a diagnostic system ? It amazes me the amount of people on this forum get a fault on their car and then want to guess what is wrong with it. Put it on a diagnostic then come back with the codes then we can help you .

It amazes me how people put so much faith in a fault code, diagnostic codes are at best a guide not a definitive answer.

 

[Simzy]

It amazes me how people put so much faith in a fault code, diagnostic codes are at best a guide not a definitive answer.

So guess work is better ?

 

[manga-blue]

have you sorted this issue, I have a similar issue, I can start my car and rev it, will be fine until I let it coast or get stuck in traffic, anyways long and short, give it the beans and it will go! Funny thing is, it has a brand new MAF !?

Not yet. I'm still waiting for the new MAF to arrive but I'm not really hopeful a new MAF will fix it. Further investigation reveals oil all through the intake system, starting right back at the PCV assembly. It's pretty thick and nasty. I guess that somewhere this is causing another fault which is causing the ecu to decide either to ignore or apply default values to sensors.

I can see a major job ahead to dismantle the entire intake system to scoop, scrape, wipe and wash the gunk out, basically everything from the exit from the air filter to intake ports in the head is involved: all pipes, intercooler, manifold, hopefully not turbo, ....

 

[manga-blue]

If it looks like a MAF Smells like a MAF and is producing a MAF fault then it probably is the MAF

New Pierburg MAF finally arrived after the first one disappeared in the post. That fixed it, thanks E270.

So it turns out that an OBD reading of 8.88g/sec can be either the MAF is disconnected or not working and it doesn't throw an error code.

Here's a good link from Pierburg telling you everything you wished you didn't need to know about testing, cleaning and replacing MAFs.

onlineshop.ms-motor-service.com/static_content/msi_pdf/tinfo/pg_si_0079_en_web.pdf

 

[E270 Owner]

Well done