I have a RB5 with the "official" Prodrive WR Sport kit (ECU and exhaust).
Over the past few months it has issued a series of "check engine" red lights:
1) MAF sensor - fault went away by itself (???)
2) Lambda sensor - replaced with Halfords jobbie for 25 quid
3) Knock sensor - replaced with pukka Scooby item.
I now have no red light, but the engine is running like a dog. Large throttle openings give full boost (have a gauge) but the power delivery feels like it is stuttering and well below par. One cold evening it actually decided to let loose and went like a train, so its not a mechanical fault. It just feels like the ECU is strangling the power.
I have tried cleaning all the hoses, the boost solenoid and the MAF sensor, but no great improvement. Now the bloody thing has decided to put me in "limp home" mode (0.5 bar max boost). Bah humbug!

Any ideas on how to make a logical diagnosis of the fault?

The car has just had its 75K service and the garage completely failed to improve things. "If the computer says its OK, then it must be OK Sir..."

 

Find another dealer. Consider replacing the MAF sensor anyway - they are failing on lots of 99/00 cars and despite what you have been told they often do not produce an active fault code when they are failing. In addition, driving your car with a failing MAF can make the engine fail.

 

The MAF sensor went on my RB5 without giving a fault code, luckilt it was over fueling so much it was easy to spot. For the sake of 70quid i'd change it just incase.... in fact i did!

 

I have ordered an new MAF, but I hate trying to solve a problem by randomly replacing parts. Sure, it will work in the end, but how many perfectly good components will I have binned by then?

My understanding from searching previous threads was that the MAF sensor caused poor idle stability if it was damaged. My idle is fine. How else can I test the sensor? If necessary I have access to bench power supply, oscilloscope, climatic test chamber, etc.

I did note when cleaning all the hoses that the material has hardened which could possibly cause leaks. What can I replace all the pressure sense pipes with (to the t-piece, boost solenoid, etc.)? Can you get suitable replacement pipe by the foot? Gets a lot of heat off the turbo so probably needs to be a bit special.

 

You can have a perfect idle with a failing MAF sensor.

You could measure the MAF voltage at wide open throttle at 6000 RPM. It should be about 4.3 V from memory.

 

MY99 STi-5 went into linp home mode the other night, I've put it down to rubish fuel, filled up with super plus unleaded at a better station and added octane booster, then once home I removed the earth lead from the battery for an hour to reset the ecu...

Hasn't gone into limp home mode since...

Just an idea for you to try, worked for me...

 

UK cars should not go into limp home mode because of dubious fuel. If they go into limp home mode the ECU is seeing serious amounts of knock on a UK car.

Resetting to get rid of a limp home problem is usually just putting off the inevitable and is a cheap dealer's trick to get rid of you