Mystery Misfire
Discussion
What appears to have started as an intermittant fault on my (1999) Superlight R seems to have become a permanent one. 
For quick throttle inputs the rev counter goes mad (flicking rapidly to zero and back) and there is a misfire (across all cylinders it seems, but I haven't confirmed this). The engine will then run rough and often stall. It can be restarted (usually after a short wait) and will idle normally. For constant and smooth throttle inputs it seems to be OK. This can be reproduced with the car stationary, and seems to be worse when warmed up.
My best guess is an electrical fault given the flickering rev counter, but where to start in the diagnosis?
Any ideas from the PH massive, especially from those who have experienced something similar with their Caterham?
If it helps, car is a standard Superlight R with Rover MEMS ECU (an ECU2 version, complete with single coil and distributor).
For quick throttle inputs the rev counter goes mad (flicking rapidly to zero and back) and there is a misfire (across all cylinders it seems, but I haven't confirmed this). The engine will then run rough and often stall. It can be restarted (usually after a short wait) and will idle normally. For constant and smooth throttle inputs it seems to be OK. This can be reproduced with the car stationary, and seems to be worse when warmed up.
My best guess is an electrical fault given the flickering rev counter, but where to start in the diagnosis?
Any ideas from the PH massive, especially from those who have experienced something similar with their Caterham?
If it helps, car is a standard Superlight R with Rover MEMS ECU (an ECU2 version, complete with single coil and distributor).
Edited by jeremyc on Wednesday 2nd January 16:59
That sounds like an ignition fault, probably either the coil or ignition amp breaking down. If it's worse when it's good and hot, you may be able to help narrow it down by cooling components down - a bag of frozen peas does the job nicely! Otherwise you will have to start monitoring the LT side and see where the change occurs when the problem happens.
I had a problem with miss fires, it seemed when the coil HT lead was cable tied lightly in place it was shorting out. Only discovered at night when in the dark, basically the HT lead was knackered, but they were new magnacors. Try inducing the fault in the dark!! it worked for this one if not a pure electrical testing procedure.
I read somewhere, probably google search, about misfiring Elises. They put a rubber barrier between the ECU and the bodywork.
I read somewhere, probably google search, about misfiring Elises. They put a rubber barrier between the ECU and the bodywork.
Edited by sam919 on Monday 31st December 18:35
But Jeremy's saying it occurs on snap throttle openning not steady throttle or slow smooth opening?
If that's the case surely it implies something like the throttle pot (but then why would that bother the rev counter?)
You could also try checking that the alternator hasn't come lose, it's taken me 4 months and £100s of pounds of experts time, dyno sessions etc to resolve my miss fire on my new 2.3l Duratec and new Emerald ECU setup having tried everything we could think of - last week i discovered the problem totally by chance, turns out the alternator mount was lose (and guess who fitted that
) which was causing a voltage spike at certain revs.
If that's the case surely it implies something like the throttle pot (but then why would that bother the rev counter?)
You could also try checking that the alternator hasn't come lose, it's taken me 4 months and £100s of pounds of experts time, dyno sessions etc to resolve my miss fire on my new 2.3l Duratec and new Emerald ECU setup having tried everything we could think of - last week i discovered the problem totally by chance, turns out the alternator mount was lose (and guess who fitted that
) which was causing a voltage spike at certain revs.MikeE said:
it's taken me 4 months and £100s of pounds of experts time, dyno sessions etc to resolve my miss fire on my new 2.3l Duratec and new Emerald ECU setup having tried everything we could think of
Mike, look on the bright side, you must have a superbly mapped car now though?!
Well it's progressed to a permanent fault now: I can't get the car to start at all.
It certainly isn't the coil (new one fitted today) and I seem to have sparks at all four plugs with plenty of fuel being delivered.
I just know it's going to be a wierd one.
Tomorrow's investigations will centre on the battery cutoff switch (make sure it isn't intermittantly connecting the alternator output to ground) and the crank position sensor ... 
It certainly isn't the coil (new one fitted today) and I seem to have sparks at all four plugs with plenty of fuel being delivered.
I just know it's going to be a wierd one.
Tomorrow's investigations will centre on the battery cutoff switch (make sure it isn't intermittantly connecting the alternator output to ground) and the crank position sensor ... jeremyc said:
Well it's progressed to a permanent fault now: I can't get the car to start at all.
That's a relief. It's usually much easier to fix a permanent problem than an intermittent one.From your original symptoms I would be looking for a fault on the ignition side. The discharge characteristics of the plugs vary as you open the throttle and also depending how hot / clean they are etc and this can feed back into the behaviour of the LT side when the coil fires. If the tacho is doing strange things that suggests there's something funny happening on the LT side so I would check that the ignition switched feed is OK, correct dwell, good rpm shown at the ECU, then start looking for HT problems.
JC
As posted already, it could be a duff TPS. If the signal is weak or non existant, you will not get the car to run at all. Can you swap this out with another car to check? Also, check the wierd multiplug that sits in the middle of the std VPHD intake manifold, as this may have corrosion in.
As posted already, it could be a duff TPS. If the signal is weak or non existant, you will not get the car to run at all. Can you swap this out with another car to check? Also, check the wierd multiplug that sits in the middle of the std VPHD intake manifold, as this may have corrosion in.
Thanks for your help and suggestions chaps - I've now managed to find the fault. 
The battery cutoff switch seems to be faulty - bypass it and all is well. I guess this means that either of the pair of normally open contacts (which are closed when the switch is 'on') were opening intermittantly (so interrupting the main battery or the ignition supply) or the normally closed switch was closing intermittantly and earthing the alternator output via a protection resistor.
New switch ordered, so hopefully back to full blatting force soon.
The battery cutoff switch seems to be faulty - bypass it and all is well. I guess this means that either of the pair of normally open contacts (which are closed when the switch is 'on') were opening intermittantly (so interrupting the main battery or the ignition supply) or the normally closed switch was closing intermittantly and earthing the alternator output via a protection resistor.
New switch ordered, so hopefully back to full blatting force soon.
jeremyc said:
Thanks for your help and suggestions chaps - I've now managed to find the fault.
The battery cutoff switch seems to be faulty - bypass it and all is well. I guess this means that either of the pair of normally open contacts (which are closed when the switch is 'on') were opening intermittantly (so interrupting the main battery or the ignition supply) or the normally closed switch was closing intermittantly and earthing the alternator output via a protection resistor.
New switch ordered, so hopefully back to full blatting force soon.
JC,The battery cutoff switch seems to be faulty - bypass it and all is well. I guess this means that either of the pair of normally open contacts (which are closed when the switch is 'on') were opening intermittantly (so interrupting the main battery or the ignition supply) or the normally closed switch was closing intermittantly and earthing the alternator output via a protection resistor.
New switch ordered, so hopefully back to full blatting force soon.
should have added - I had a master switch probelm out at the ring this year: due to a loose connection the resistance was such that the fuel pump (very sensitive to current) would prime OK when the engine was idling, but under load, would stop delivering fuel. To 'breakdown' before T13 is a tad embarassing to say the least. Moral of story - ensure all electrical connections are sound and also tight (where connectors are involved)
Gassing Station | Caterham | Top of Page | What's New | My Stuff