R129 SL500 misfire

[E55BOF]

I have a regular misfire on my '94 SL 500. I've changed the plugs,distributor caps and rotor arms, but it has made no difference. Seven plugs are the right light brown colour; the front left plug is just oily - not carbon fouled - with no sign of combustion in that pot. I'm thinking a new ignition lead is needed. Does anybody disagree?

Can I get a single lead from the stealer, or will it have to be a complete new set, does anybody know?

 

[Flyinspanner]

Does anyone live near you with the same model, maybe you could borrow a lead for half an hour just to rule that out/confirm diagnosis?

 

[Doodle]

You can build individual leads via the dealer, but they're furiously expensive.

 

[E55BOF]

They certainly are; £48 EACH.

I think I'll get the multimeter out and measure the resistance; that should give me a clue. More anon.

 

[Doodle]

For consideration, Fastlane do a replacement lead set at a much more reasonable price.

 

[E55BOF]

I'm pretty certain I'm on the right track; it's no. 5 cylinder, and the lead has higher resistance than the much longer lead that goes to no. 8 at the back of the cylinder bank. It will spark a plug in air, but it seems not under compression; the misfire is unchanged when the lead is not connected to that cylinder's plug.

Fastlane certainly do do a set at a much better price, but they don't have all the components in stock at the moment. Eurocarparts calling...

 

[markmifsud]

Have you tried to swap out know good leads with your known bad ones? if the problem moves with the leads then your one step nearer to solving the issue?

 

[E55BOF]

It's a regular single-cylinder misfire, no. 5 is not firing, and the HT lead for that cylinder has higher resistance than another one about twice the length from the same distributor cap to another pot on the same cylinder bank. That's good enough for me.

A set of new leads is only £80, anyway, delivered to my local UPS pick-up point tomorrow morning, and I'll be quite surprised if it doesn't solve the problem. I'll be swapping out known good for known bad leads then...