When i come to a set of lights and am about to stop the revs needle drops to near 800rpm and stays there for maybe near 10 seconds then raises to 900rpm (where it should be)

some times if i rev the car in netural, same thing happens it drops below the idle rev and raises up again...!?

Is this normal?


--Sam

 

Sounds pretty normal. . if it were dropping a lot lower then I would look into it but at 800rpm I wouldn't worry. imho

 

All three of my Scoobs classic and newage have done it so i presume its normal.

 

Hi sam

My 99MY rev counter drops to Zero rpm and either stalls or just bounces back up to 1350rpm, occasionally settling at 8-900rpm. A realpain in the butt.

I wish mine was as good as yours!!!

JGMonster if you check this out again can you advise?

cheers

Dave

 

on a 99 I would say that is a MAF failure..

change the MAF

Leaking dump valve if you have a VTA can also effect it..

Simon

 

Thanks Simon

I have a rich mixture also, would the MAF alone cause this??? Temp gauge reads normal, however I understand that there is a temp sensor for the ECU. If this is faullty (ie tells ECU engine is cold, this enriches mixture) causing this 'hunting'.

Sam, are you OK with yours now?

cheers

Dave

 

Coolant temp failure will be obvious if rad fans are on when engine is cold.

the ecu doesn't know if engine is hot or cold so puts rad fans on and richens mixture.. covering both extremes.

MAF failure usually (99 times out of 100) causes a lean mixture but how are you determining it is rich? if it is very lean a slight misfire will occur sending unburnt fuel down the exhaust causing it to APPEAR rich.. if you see what I mean?

Simon

 

Thanks Simon,

Emissions report (Mot Fail)
Fast idle CO vol% high by 0.57 on run 1
Fast idle Lambda passed 0.011 within spec
Fast idle CO vol% high by 0.26 on run 2
Fast idle Lambda failed 0.014 over spec
Natural Idle CO vol% high by 0.12

While hunting at idle black smoke is visble from exhaust as it blips up
to 1350rpm from zero on rev counter. It either does this or stalls
completely.

Normally driveability is absolutely fine.

I am changing MAF first today and will do aother gas run afterwards.

Dave

 

Also check for a vaccum pipe leak, also the lambda sensor.. assume this is with cats fitted etc..

those results looks less like MAF sensor tbh

Simon

 

No, its full decat. And passed last years MOT no problem like this.

 

then they couldn't have checked the emissions.. full decat will not pass the emissions.

There is clearly a problem due to the stalling but regardless it is very unlikely to pass with no cats.

Simon

 

Agreed, cheers Simon. Idle problem is now cured with new MAF sensor. In fact I'd go more than that. Drives much better. We carried out another gas run and the result is CO%vol in only just a fail still. Unfortunately I dont have the original downpipe.

Dave

 

Try and get a UK centre section.. you can be sure it is a catted centre if you can see the honeycomb grill when you look in the end.

www.hdcsubaru.com might have one you could buy.

Simon

 

I don't agree with that Simon. I know it's difficult to measure the AFR when the idle is way too rich or too lean and it is easy to get it wrong.
What happens when the 99/00 mafs fail is that the sensitivity reduces, this causes a very rich idle, almost normal midrange and a lean top end.

It is possible as a temporary measure to map a car to suit a reduced sensitivity (read failed) sensor by removing fuel from the low zones and adding to the power zones. I have had to do this on ocassion during a multiple mapping session away from home, when I have already used my spare maf (which I always carry )

Andy

 

Thanks lads,

HC results are fine (pass) therefore problem is not unburnt fuel! Rich is the problem. I will put a standard front pipe on (Cheers Bryan, RB5108) for that. Should then be withinin spec for MOT.

I'll keep you posted ok. My car is running sweet though.

Dave