Strangest Missfire

Post by **MoggyTech** » Thu Jan 17, 2008 3:37 pm

Got a call from a friend last night, who's Moggy had developed a horrid missfire after he replaced a heater hose! First thoughts, were coolant had maybe dripped onto the dizzy when he did the work. Nope, he had replaced the dizzy cap and leads during a service, at the same time he did the heater hose.

So he limped the car round to my garage, and boy did it sound rough. Poor idle and no power worth mentioning, sounded like a blown head gasket.

Lifted the bonnet, and noticed he had made up very long HT leads for some reason, leads 3 and 4 neatly cable tied to the new heater hose.

So we start it up, turn out the garage lights, and notice HT tracking along the new heater hose, to the brass heater pipe that goes to the bottom rad hose!

Turns out, the new hose, is one of the Kevlar reinforced ones, and if you stick a digital multi meter between earth and the hose, set to Meg Ohms, you get a resistance reading of between 3 Meg Ohms and 1 Meg Ohms, depending how far you measure up the hose.

I cut away the HT lead cable ties, and the missfire stopped! So the moral of the story, is that these hoses have a resistance, and will conduct HT voltage to earth.

Post by **Rob_Jennings** » Thu Jan 17, 2008 3:49 pm

I think you should replace those leads.

A good HT lead should not 'leak' even if placed close to the engine block which has almost no resistance to ground, let alone to a component that has a few Meg ohms to ground

Post by **Axolotl** » Thu Jan 17, 2008 4:10 pm

And cut 'em shorter!

Post by **MoggyTech** » Thu Jan 17, 2008 8:21 pm

After further investigation, I discovered cheap and nasty silicon leads, that have now been replaced with good old fashioned copper core leads of the correct length. The car was also running a 6 volt coil, that was getting 'toasty', and the engine diag scope couldn't even display peak HT value as it was off the scale!

Interesting to discover rubber hoses that have a measureable resistance though, I guess Kevlar must have a carbon component in there somewhere.

Post by **Kevin** » Thu Jan 17, 2008 10:58 pm

I discovered cheap and nasty silicon leads,

I didnt know there was such a thing as cheap silicon leads, something else learnt.

Post by **Axolotl** » Thu Jan 17, 2008 11:03 pm

Kevlar itself is an insulator related to nylon, with very low electrical conductivity.

However, it is mixed with graphite in some materials to increase abrasion resistance and reduce the risk of sudden failure (snapping) of the material.

Could be the graphite element in those hoses doing the conducting.

Post by **bmcecosse** » Thu Jan 17, 2008 11:12 pm

All black rubber contains carbon!

Post by **Axolotl** » Thu Jan 17, 2008 11:23 pm

As do most life-forms known on earth.

Post by **badfelafel** » Fri Jan 18, 2008 12:30 am

Are you saying the hoses are alive? Scary!

Post by **Orkney** » Fri Jan 18, 2008 7:40 am

So we start it up, turn out the garage lights, and notice HT tracking along the new heater hose, to the brass heater pipe that goes to the bottom rad hose!

Like the idea od looking for tracking in the dark - never occured to me once that !

How often the HT leads get overlooked for sevicing & tuning is quite amazing.