D2 4.6 V8 Vacuum Leak Or?

**Battler** · 19th August 2015, 08:08 AM

The last few mornings the engine starts perfectly but then after about 30 seconds of idling the revs drop and nearly stalls. It continues barely running for about 15-30 seconds and then it comes good for about a minute and then it starts to almost stall again. Comes good again and stays good and drives perfectly.

Except for this morning when it was far worse. When driving it was down on power and nearly stalling at lights. After a 20min drive and leaving it for 30mins it started perfectly and drove perfectly.

No codes showing.

I'm thinking vacuum leak somewhere or could it be a faulty sensor?

Cheers

**bee utey** · 19th August 2015, 08:46 AM

Grubby oxygen sensors maybe. When they're cold the heater voltage leaks into the sensor part and makes the ECU think its running too rich. First one side then the other. Once the engine is fully warm the heaters are turned off and no moisture condenses out in the sensors.

**Battler** · 19th August 2015, 08:59 AM

Originally Posted by bee utey

Grubby oxygen sensors maybe. When they're cold the heater voltage leaks into the sensor part and makes the ECU think its running too rich. First one side then the other. Once the engine is fully warm the heaters are turned off and no moisture condenses out in the sensors.

Thanks bee utey.

Maybe that's why it was worse this morning as the ambient was about 7c.

Although, the sensors are in the extractors near the collectors and so I would have thought they would heat up very quickly.

I'll give them a clean and see what happens. What would you recommend I clean them with?

**bee utey** · 19th August 2015, 12:56 PM

Oxygen sensors are a service item and ideally should be replaced at regular intervals, say every 80K, not that anyone does that in Oz. I can't imagine you'd have any success cleaning them, they're too delicate. New ones aren't cheap, either.

**Battler** · 20th August 2015, 07:22 AM

Originally Posted by bee utey

Oxygen sensors are a service item and ideally should be replaced at regular intervals, say every 80K, not that anyone does that in Oz. I can't imagine you'd have any success cleaning them, they're too delicate. New ones aren't cheap, either.

Thanks bee utey,

A P0130 code showed yesterday so I'll replace both sensors.

Cheers

**PhilipA** · 20th August 2015, 07:57 AM

AFAIK on Motronic the O2 sensors are not active until after warm up, so the engine is depending on the MAF in the warm up phase.

MAF errors will often not show as errors, so if the new O2 sensors do not fix it , it may be the MAF.
Regards Philip A

**Battler** · 20th August 2015, 03:30 PM

Originally Posted by PhilipA

AFAIK on Motronic the O2 sensors are not active until after warm up, so the engine is depending on the MAF in the warm up phase.

MAF errors will often not show as errors, so if the new O2 sensors do not fix it , it may be the MAF.
Regards Philip A

Thanks Philip,

I've done some Googling on O2 sensors (Motronic) and you may be right. Also learned O2 sensors are ignored by the ECU when idling. And that's when my issue occurs on a cold start.

The MAF is only about 20,000 Ks old and it's a genuine Bosch.

I'm back thinking either an air or exhaust leak. Most likely air as I think that would screw up the air/fuel mixture. After warm up things expand and the leak closes or reduces enough not to cause a problem.

To eliminate the O2 sensor I'll swap them over and see if the problem switches to bank 2.

Cheers

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