I've just picked my RB5 up from a dealership, it has some faults with it since i bought it from the 5 weeks ago, the main one being a not so bad misfire on no4 cylinder BUT a serious misfire on no2 cylinder which does'nt fire at all after a certain rev, 3500 i think.
They've come back to me after having the car since Tuesday and said the reason why its misfiring is because it has a de-cat downpipe and theres not enough backpressure(downpipe was on the car when i bought it) so they have a brand new downpipe with cat on order for me as well as a new clutch and diff seal.

Are they talking bollox, is the lack of backpressure causing the misfire??
Getting really pi$$ed off now please help me
Many thanks in advance
Matt
RB5 - PPP and de-cat downpipe, running on optimax

 

Well never known a downpipe for cause a miss fire on any other car..

Mine runs fine with a full decat.. and so do a lot of others including RB5's..

JGM

 

Yup!!

 

Thinking about it.. for a miss fire I'd be replacing ht leads, spark plugs and if still not solved then coils..

I think possibly that cylinders 2 and 4 share a coil on the RB5? but not sure..

JGM

 

Yup i told them they were talking sh!te when they said that.
My RB5 has 1 coil pack for all four cylinders, i'll wait until they put the cat and new clutch in then i'll go elsewhere and put the de-cat d/p on, woohoo free £500's worth of downpipe

 

I thought the RB5 had the wasted spark twin coils system on?

Two coils on top of the inlet manifold with ht leads from there to the spark plugs?!

JGM

 

Its more likely that the coil pack is down, hence the misfire, though also worth checking the connections & spark plugs.

 

I've checked the spark plugs they're fine (NGK PFR6's at 0.7mm) HT leads are ok as we swopped them round and its still doing it.
I did take my coilpack off and noticed some brown deposits on the connectors and what looked like very very small scorch marks on the connectors, but not on the ones thats misfiring ?!?
Could it be the MAF doing this??
I disconnected the MAF whilst idling and there was no difference, revs did'nt drop or stall for the first and second time anyway, the 3rd and 4th time it stalled but ran normal when engine was re-started.
Thanks for your help guys, but if anyone can help me with this i'm really getting fed up now.

 

If you've fitted a decat downpipe, there is a good change you're getting a touch more overboost as the turbo comes on. You will also get better breathing over all, meaning more (mass of) air being breathed in by the engine overall. If your ignition system is marginal, this may be enough extra cylinder pressure to cause the misfire.

The likely causes are going to be HT leads, coil block or ignitor module.

If you can drive with a touch less throttle and keep the boost down, I would imagine you're not going to get the misfire.

Paul

 

Thanks for that Paul.
I understand what your saying just done a search on overboost but the misifre starts from around 2000 rpm.
My boost has never peaked beyond 15psi and holds anything from 12-15.
This is starting to puzzle me now...