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996 misfire / rough idle

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Partyvan

Spa-Francorchamps
Joined
21 Aug 2011
Messages
324
Hello everyone

I developed a rough idle on my 996, feels like it's running on one or two cylinders down.

So far I've done the following, partly in seeking the fault and partly as general maintenance

- Compression test (all good)
- Oil and filter change (no metal particles in old filter)
- New set of plugs
- New set of coilpacks
- New lambda sensor (this came up on the fault code reader)
- Brief check for air leaks
- Inspection of earth straps

The rough idle remains.

I'm thinking of cleaning the MAF next. Any other ideas, or common faults I'm unaware of?

Thanks in advance
 
Your Missus hasn't caught a switch on the dash with her high heels as she? :hand:
 
kas750 said:
Your Missus hasn't caught a switch on the dash with her high heels as she? :hand:
Click to expand...

:floor:
 
If by '98 you're talking non e-throttle, these cars seem to be ridiculously sensitive to air/vacuum leaks...!
- I suspect the e-throttle cars compensate more than their owners realise...

I had a similar issue re. hanging/climbing idle which I tracked down to being due to the poor design/seal/fit of the large air/oil breather pipe that connects to the lefthand side of the throttle body, after the butterfly plate
- I ended up cutting off the plastic ring that is supposed to be squeezed to remove the pipe from the throttle body and wrapping the end of the fitting with plenty of PTA tape before wedging it back in snugly to seal properly (then cable tied various pipes/hoses to each other to ensure it also had pressure to hold it in...)

This then cleared the related fault codes which I was getting i.e. "idle exceeding maximum stop"

Need to get a fault code reader on it, but you might want to look at this in the meantime...
- there's also a fuel/air vapour vent valve you might want to look at (#996.110.129.52) which can cause problems if old and fouled up - about £30 from memory to replace.....
 
Thanks guys, no it wasn't the Mrs!

Thanks for the tips I'll get it back on the fault code reader, and try leak finding!

I've removed the oil filler cap when running and the was no suction or change in idle, so I assume the AOS is okay?

Cheers
 
Partyvan said:
I've removed the oil filler cap when running and the was no suction
Click to expand...


There should be some suction! These engines run -ve crankcase pressure.

I had a bad idle, no fault lights. A scan showed enrichment limit reached, so pointed at an air leak. My fault was down to the rubber hose connecting the AOS to the crankcase had split.
 
almost certainly a split vacuum pipe which should be a mix of good and bad news for you.

good because it can be fixed, bad because you have shelled out plenty of £££ on things that weren't broken..... :frustrated:
 
I had a bad idle, no fault lights. A scan showed enrichment limit reached, so pointed at an air leak. My fault was down to the rubber hose connecting the AOS to the crankcase had split.
Click to expand...

this is my own next area of investigation as I think I've ruled out all breather pipes, vacuum lines and seals in the engine bay, but I bet this could be commonly overlooked culprit judging by the basic design of the connection at the TB end.... :mechanic:

there's been at least one revised design issued of the AOS, which suggests that there have been a few issues with it... doubt many people look at it until it fails completely and plumes of white smoke billow out the exhausts...
 
Okay air leak sounds plausible then!! Good news!

I don't mind spending on other items as it's always good to have new parts.

Is there an aerosol I can use to help detect a leak? Like the "boost start" type sprays, where if I caught a leak it would change the revs
 
I removed my NSR wheel and spied this AOS -> crankcase boot split and caked in oil and muck:

78iRTWy.jpg


So the intake was drawing fresh air through the AOS rather than sucking oily mist from the crankcase.

Follow the guide by steveoz to replace: http://911uk.com/viewtopic.php?t=61942

Just a note - there should be some vacuum at the oil filler cap, but not so much its tough to remove the filler cap.
 
you could unplug the maf and see if its ok,
it will run without the maf, just not as economical.
 
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