K series misfire (light throttle only)

[fergus]

I've got a light throttle misfire on my 1.8 VHPD K series. When under full load/high revs, there are no problems. The dyno trace does not show any loss of power from previous historic runs when over 3500 rpm.

I've checked/changed the following

- TPS (via the data logging trace at Emerald whilst on the RR)
- new set of plugs
- TBs within balance
- no air leaks on the TBs that I can determine

Haven't yet changed/checked the following

- rotor arm
- HT leads
- dizzy cap
- coil (although this was working fine prior to the rebuild over the winter and was replaced last summer)

Any other thoughts/things to check other than the list below?

- HT feed to the dizzy isn't either touching anything or close to the crank sensor

Thks

[Shaun_E]

Hi Fergus - my money would be on the coil but the rest of your "not yet checked" list are all prime candidates. I went through 4 or 5 coils in about 3 years and have now changed to wasted spark ignition using a coilpack mounted on the bulkhead. In the past I've also had a rotor arm fail and had to replace HT leads. I used to replace the dizzy cap every year as a precaution.

[fergus]

Shaun_E said:

Hi Fergus - my money would be on the coil but the rest of your "not yet checked" list are all prime candidates. I went through 4 or 5 coils in about 3 years and have now changed to wasted spark ignition using a coilpack mounted on the bulkhead. In the past I've also had a rotor arm fail and had to replace HT leads. I used to replace the dizzy cap every year as a precaution.

A new coil may also be on the shopping list then! Cheers Shaun. Is the coil not more likely to break down under load, i.e. when the spark needs to be stronger?

PS - when you moved to the wasted spark ign using coilpacks, is this simply a case of doing a straight swap, or will I need an ignition remap? Apologies for my ignition ignorance!

Edited by fergus on Wednesday 7th April 11:00

[Shaun_E]

Each coil failed in a different way - I had one that initially just misfired at around 2000-3000 rpm but was fine at higher revs! It did eventually get worse. There doesn't seem to be any rhyme or reason to the failures but as the coil is bolted to the engine block it gets a pretty hard life. I am sure vibration has a lot to do with it hence the symptoms are often related to a particular rev range.
Switching to wasted spark doesn't require a re-map - you just need to change a setting on the Emerald and wire in the second ignition driver (wiring pin out is in the Emerald manual on their website). I got the coil pack and the plug to fit it from SBD. You will then need some custom HT leads - I splashed out on a set of Magnecor leads. Let me know if you want a picture of my install.

[Shaun_E]

Here's the coil pack I used - http://www.sbdev.co.uk/Electrical/HT%20Leads/2L_HT...

[Incorrigible]

Is it running a MAP or TMAP sensor ? What does the ECU think is happening to that ? And the engine coolant come to think if it

It sounds like the part throttle map isn't quite right, you could play with the TMAP compensation (when it was originally mapped it won't be 100% accurate for these cold air temps (guessing))

Also you could retard the ignition by a couple of degrees at the effected revs

As you say it could be any of those other things, however, as it idles fine and runs fine at high revs I would guess (and it is a guess) not

[fergus]

Incorrigible said:

Is it running a MAP or TMAP sensor ? What does the ECU think is happening to that ? And the engine coolant come to think if it

It sounds like the part throttle map isn't quite right, you could play with the TMAP compensation (when it was originally mapped it won't be 100% accurate for these cold air temps (guessing))

Also you could retard the ignition by a couple of degrees at the effected revs

As you say it could be any of those other things, however, as it idles fine and runs fine at high revs I would guess (and it is a guess) not

Ben

It was missing when on Dave Walker's RR @ Emerald yesterday, so we tried everything we could with what we had available. Unforutnately, I only had a spare set of new plugs with me (didn't help). The session was to revise a previous map since I've subsequently had some work done on the throttle bodies to increase the airflow. Dave's wideband lambda didn't show any goofy readings. The ign map shouldn't need to have changed since it was last mapped.

I'll give you a call re setup once I got the thing running smoothly again.

PS Howcome Eliseparts only want circa £16 for a cap and rotor arm and caterham want £47 for the same parts? I've got a spare coil at home I've just realised....

[Spingo]

Are you a member of the AA/RAC etc? Might not be as daft a suggestion as it sounds as they should be able to narrow down the list of potential problem areas - and it won't cost you a bean (if you're a member...)

[fergus]

Spingo said:

Are you a member of the AA/RAC etc? Might not be as daft a suggestion as it sounds as they should be able to narrow down the list of potential problem areas - and it won't cost you a bean (if you're a member...)

Good plan. I use the German company ADAC, who use the AA in the UK, so may try a crafy call out after I've explored all other obvious routes.

[Incorrigible]

Ah, didn't realise you were at Emerald. What have you had done to the TBs ?

[fergus]

Incorrigible said:

Ah, didn't realise you were at Emerald. What have you had done to the TBs ?

I'll drop you a PM!