Misfire Diagnosis

[Alex1997]

Hi all,

I’m really hoping someone can shed some light on this. I’ve got an issue with my mk3 rs200 where the car keeps misfiring on idle and gets worse when hot and under load. There’s absolutely no stored codes and no check engine light (or any other warning light for that matter) but I can hear it physically misfiring when it’s idle if I stand and listen at the exhaust I can hear popping noises every so often. When driving it’s absolutely flat as anything, no power at all really and as I say once it’s hot the misfire gets worse and far more noticeable, but again, no codes. So far in the last week and half that I’ve been trying to pin this down I’ve eliminated (or at least think I’ve eliminated):

Injectors removed and professionally tested, all fine
New spark plugs
Coils checked
Manually checked the dephaser solenoid, actuates as it should
Checked all wiring
Checked earth points
Read live data on my scan tool, dephaser appears to be working as it should as cam angle is changing with revs, lambda sensor voltage appears to be where it should. Pre cat lambda fluctuating from around 0.09v to around 0.8-0.9v and post cat lambda holding at around 0.45v
Manifold pressure sitting at around 500 mbar when on idle
No vacuum leaks
Engine compression tested (all 4 cylinders between 150-155psi)
Resistance tested all 4 coils as well as the lambdas and all appear normal

Absolutely at a loss here. Would be much appreciated if anyone has any ideas of what else to check or point me in the direction of where I may have gone wrong.

Thanks in advance

 

[JSD]

Has the timing been checked? (I'm no expert by the way)

 

[Frank Perry]

You checked coils but didn't replace them... they're not that expensive to replace, but if you have somebody nearby who can lend you a set (or borrow known working ones from a friend) then I'd look at this as an option.

What plugs did you replace with? Did you have the misfire before the plugs were changed?

Check crankshaft sensor - could be gummed up and cause timing issues which could be related. Take off - clean up, replace and test. May be worth replacing for new if it's bad. Some tell-tale signs are usually RPM jittering at idle; worth a shot. Check the wiring on this too - heat can make cabling brittle and give unstable signals due to deteriorated wiring.

Dephaser pulley could stick, even though you have inspected and tested... Sometimes these can silently fail but still "appear" to work when tested manually.

Any exhaust leaks? Specifically near the manifold?

Check intake manifold gasket(s) - could help explain when cold it's less prominent but warm (metal expanding etc) - it's worse.

Fuel pressure - kind-of hard to diagnose this one but the fuel pumps can go.. albeit usually they just outright die. If you have the ability to test the pressure, that could rule out fuel pump beginning to fail... but wouldn't really affect cold or warm so probably a long-shot.

Lambda sensors might appear to be "working" but can still be an unhealthy sensor - worth noting.

Failing the above, I'd run it down to a specialist in your area to do some testing...