RT

MY2000, 35000kms.

I have read several past posts on this topic. Here's my 2cents worth.

The reported symptom is that the car pulses/surges on light throttle between 2500-3500rpm when cold. If near-full throttle is given, or if the throttle position is constantly varying, then this phenomenon does not occur. I have experienced this from taking delivery of the car.

Speculated fixes posted on this bulletin board in the past include a replacement ECU, faulty/dirty MAF sensors and sticky solenoids, to name a few.

On the contrary, I feel that this is related to the lamda sensor (mounted immediately post-turbo, in the downpipe). As anyone who has used an aftermarket wide-band exhaust gas analyser will know, the lamda sensor only provides accurate values when hot. The heating of the sensor to its proper operating temperature may take as long as 5 minutes with an idling engine.

Now, the factory ECU operates in closed loop mode (to target lamda=1) when the car is driven on:
1. steady cruising throttle,
2. or during idle.

In all other instances, the ECU ignores inputs from the landa sensor and operates in open loop mode. Thus, when driving on steady throttle on a cold morning, the lamda sensor is providing "fishy" readings to the ECU. What the ECU does with these readings that eventually leads to the surging, I have no idea.

This theory is further reinforced by the fact that an aftermarket ECU that operates only in open-loop mode (in my case, an Autronic replacement board) totally solves this problem. The Autronic does not use the lamda sensor at all, relying only on its stored calibration for fuel mapping and ignition timing etc.

I hope this helps and makes some sense.
Cheers!

BHORT

RT,

Excuse me for being a bit thick but what does the lambda sensor do?

Brendan

Bob Rawle

Good theory but unfortunatly not it either, the lambda sensor is heated and comes up to temp withing a couple of minutes of start up, the ecu runs open loop off defaults until the engine warms to about 60 deg C then goes closed loop, its closed loop all the time until you get on boost, then it goes open loop but still uses the mass air flow meter to vary the map within limits.

An aftermarket ecu overcomes the problem because it uses different software routines, also, yes, they can be set up not to use the lambda sensor and also most do not use the mass air flow meter either but rely on the map sensor for engine load info. I think that the non use of the maf is probably more significant here.

RT

BHORT, the lamda sensor measures the lamda ratio in the exhaust. On steady throttle or idle, the target lamda ratio is 1 for max effiency and clean emissions.

BOB, why I don't feel its MAF related is because after 3-4 mins of driving, it all goes away. The MAF does not get significantly heated during normal driving when compared to the lamda sensor (unless the 30-40deg or so of underbonnet heat makes the difference). Also, changing the MAF sensor for lots of guys doesn't seem to help.

But I'd agree with you. Perhaps the engine warm-up Open Loop defaults in the factory ECU are screwy, which causes the rpm to hunt about a datum for a given load and throttle, causing the surging.