[jonte]

2,3 V5 (AGZ engine), 1999, 100000 km/62500 miles.
Motronic M3.8.2 injection.

I don't think these engines are sold in the US Passats, they are quite rare here in Sweden too, more common in Golfs.

Have had intermittent problems during last year with very bad performance during acceleration first 2-3 mins when cold started, but not always.

Replaced O2 sensor 6 months ago, slightly better but problem still there.
Mechanic suggested trying replacing MAF sensor too, but hadn't anyone avaiblable.

Now: Needs 4-5 tries when starting cold, then starts reluctantly.
Really bad performance first 2-3 mins, then ok but still not really good.

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EDIT: "Bad performance" means accelerator works like inverted. That is, when I press the accelerator on 2nd gear, the power decreases. When I release the accelerator, (some) power gets back.
Feels like the engine doesn't get enough fuel.
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Gets these two faults now and then:

00561 - Mixture Adaptation
12-00 - Adaptation Limit (Mul) Exceeded

00525 - Oxygen Sensor (G39)
27-10 - Implausible Signal - Intermittent

00561 has not returned for a couple of days, 00525 returns more often.

In the "readiness" window everything usually looks ok except the last one, catalytic something...

Have cleaned MAF sensor with electronics cleaner.

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EDIT: Tried disconnecting the MAF, two starting attempts failed.
Then I reconnected the MAF, and it started rightaway!

Repeated next day (cold engine again) this time one failed attempt with MAF disconnected, 2nd attempt with MAF connected, started!

But if I try with the MAF connected at 1st attempt, I need 4-5 attempts.

Does this make sense to anyone!?
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Looking at the VAGCOM displays when running most things looks sane, including the two temperatures (coolant and, I suppose, intake air) and air flow ranges between 3 and 30 g/s.

The O2 value is +/- 25%, but when idling it's +/- maybe 2% only.

The ignition timing is about -30 degrees, seems like some misinterpretation by the program?

So, any suggestions are welcome.
I have some VAG logs too.

Should I be looking for a new MAFS?
Could a bad MAFS really make the engine that hard to start only when cold? To me it seems like something else, maybe something with the fuel pressure?

Could the new O2 sensor have gone bad in just 6 months?
Or is this an indication of something else?

Thanks for any input,
Jonatan Lindell

 

[jonte]

Anyone?

Added some more info (see EDIT)

 

[Boris]

Is it running OK when the engine warms up? If yes then you may have:

- Thermostat stuck in the open position.
- Coolant temp sensor mulfunctioning

Which would also explain the out of range O2 sensor readings.

 

[trlyka]

You don't have to replace the MAF, but you can clean it.

http://www.ajdist.com/crc.htm

 

[jonte]

Boris:
About the coolant temp sensor, I have been watching the two values in VAGCOM and they seem to match reality. I have VAGCOM logs showing temp around 25 C when cold, and slowly rising up to 90 C.
I will check this again for sure.
I suppose I can trust what I see via VAGCOM? I mean the values I see are the same that the ECU "sees", right?

About the thermostat, I suppose a stuck open one just would make the temp rise very slow?

And yes, the engine runs "OK" when it's warm, but I still can feel some hesitation, it's not perfect.
But it always starts nicely when warm, and there are always problems when cold.

trlyka:
I have tried cleaning the MAF with electronics cleaner, contaning Isopropanol and CFC22. I have been reading about people using carb cleaner and other stuff, I'm not sure what chemicals I "dare" to try.
A problem with my type of MAF sensor is that I can't see the platinum wire, so I don't see the effect of the cleaner, I'm just spraying the cleaner through the "intake".

 

[Boris]

Also check the vacuum hoses. If there is leak, you may be getting lean mixture which is not good when the engine is cold and need the opposite rich mixture to run smooth.

If the VAG is showing coolant temp OK then the sensor is OK.