Limp home mode from nowhere.........

[jameswkr]

My ABY S2 has been running really well since the switch to copper plugs ( see previous thread ). Used the car all over the week end. Started her up to come home on Monday night and the engine is hesitant at low revs and stumbles pulling away. I can see the MPG dropping and on overrun at high revs the gauge is showing more vacuum than usual ( it usually drops down at 2000 rpm ). When stopping, the engine nearly dies and the ignition light flickers before revs jump to 1500 rpm and then fluctuate wildly between 500 - 1200 rpms...
ISV off cleand and lubed. Resistance seems OK. Throttle pot checks OK. All other connectors cleaned and re connected ( including MAF ). I even tried a different set of plugs (!) Pulled ECU fuses and out for a 30 mile road test. By the end the ECU seems to have re adapted and MPG ( driven gently shows 28.5 ). I can only assume this is limp home mode but do not understand what triggered it. THe MAF contacts looked fine and nothing was touched between going well and going badly. Looking through the archive it seems Im not the only one with gremlins in the engine...

[quattronutter]

sounds like one of those pesky boost leaks,check all pipes/hoses for splits or pressure test the system..

[dan_b]

Could be a failed/ failing coilpack? There have been a series of these going of late.
Are your plugs all in tightly enough, again there have been a series of incidents of late where plugs have come loose... Boost leak does sound rather likely too though.

[jameswkr]

Thanks guys. I think if it was a boost leak I would not see high recorded vacuum on overrun. There were no misfires ( although the plugs were v sooty compared to normal ).

[dan_b]

Is the ECU connected ok?

[alg]

I would check it with a known working MAF.

[dan_b]

@James - if you want to swing by my place at the weekend we can swap in my MAF to double check that if you like?

[jameswkr]

@dan_b. Many thanks for your kind offer. I am going to run the MAF checks as per S2 Central at the week end. If they show any probs I may be in touch. The car has been running fine since Monday.......(crosses everything in sight)...