Misfire - high resistance in HT leads EVEN AFTER REPLACING

[thegman]

Hi

Had intermittent misfire issues about 6 months ago. Replaced a lot of electric bits including HT leads and extenders - all from a TVR specialist supplier recommended on here (whose service I have found very good). The HT leads were not Mags or any brand I recognised.

Over the last couple of months its started happening again. Took the car to garage and they detected very high resistance on a couple of the leads. They said the dizzy readings were fine.

Is it normal that leadres would deteriorate so fast - would think not. Is it likely to be the leads themselves or could it be something else. Am I really going to need to replace all the leads again?

Any ideas?

[K4TRV]

There isn't an easy answer here.

All leads have resistance for supression requirements. You need to measure each lead and see or ask what the Dealer means by High resistance?? Too high and you will get misfires. Also, if the coil is getting a bit the worst-for-age, its performance with high resistance can give misfires on some plugs.

Your measurements need to be similar with all leads - the longer ones with slightly higher resistance - can't give you a figure, but its K Ohmns.

If the Dealer is reputable as you say, then ask some questions - you may have a coil on the way out though.

Not a lot of help? Bye-the-bye I use Bosch leads from Halfrauds, a fraction the cost of Magnecore and change them after 2-years - it will take 6-years of this to match magnecore prices......!!

Trev McM

[thegman]

Coil is new.

[piston-broke]

What plugs have you got fitted?

[thegman]

NGKs.

Basically lowest lead is reading about 11 ohms, highest one is reading nearly 20.

I also suspect the ign. amp, but will try some new leads first.

Edited by thegman on Tuesday 9th September 10:58

[piston-broke]

It can only be the leads from what I read into it. The coil doesn't stipulate which point gets which power, and you've confirmed that the resistance is equal at the distributor. Stick some decent leads on it I reckon and all will be fine.

[BBR]

I had exactly the same symptoms. Magnecor leads cured it.

[FlipFlopGriff]

Genuine Lucas rotor arm fitted - made a huge difference for us as it stopped the shunting too. AFM? - is it kicking out more crap up the back end (ie black sooty stuff?)
No easy answer I suspect but these are cheap easy things to change, borrow another AFM, £20 max for a rotor arm
FFG

[griffter]

thegman said:

NGKs.

Basically lowest lead is reading about 11 ohms, highest one is reading nearly 20.

I also suspect the ign. amp, but will try some new leads first.

Edited by thegman on Tuesday 9th September 10:58

11-20 ohms must be metal (ie copper) leads - not high resistance carbon leads like Magnecors. Surely you should be running carbon leads? Are your plug extensions resistive (ACT ones are not, oe are).

[thegman]

Both leads and extenders were sold as OE by a well regards TVR parts -supplier I'm sure they are - but are suggesting that ACT stuff is better?

[GreenV8S]

thegman said:

NGKs.

Basically lowest lead is reading about 11 ohms, highest one is reading nearly 20.

I also suspect the ign. amp, but will try some new leads first.

Do you really mean Ohms, or could it be KOhms? It would seem strange for a TVR specialist to supply non-resistive leads. I don't know what sort of leads you are using, but some (many) are very fragile and can easily be damaged by rough handling (especially when pulling a sticking plug cap off). So it wouldn't be too surprising to yank a lead off and then find that it's faulty.

[thegman]

they are the bugiecord leads (or whatever they are called). might be kohms.

[piston-broke]

bungie cords?!?!, yeah, they won't work too well as ignition leads

[thegman]

they are apparently the OEM ones.

i have heard some bad stuff about magnecors....

[Johno]

I wouldn't fit Magnecor leads, they only ever broke down quickly in both the TVR's I used them in.

I have been converted and am much happier running Moroso leads. As is my Dad in on his Chimaera and a couple of other friends runnings RV8 TVR's. Much better longevity, runnnig and 75% the price plus . . .

You can cut them to the length you require, they come with a choice of fittings for both plug and dizzy ends (alhtough only one size fits the RV8 dizzy) and if you're bothered on colour they come in 3 different versions. Also available in 8 & 10mm.

Available from Real Steel. You will need a clamping tool they can supply for about £6 to finish each one, then you cut them and finish them to the lengths you want.

IMO a much better lead than the Magnecors at 75% of the price with a the flexibility to allow you to shorten the routing or extend as you see fit.

I use 135 degree ends on my set up as the plug end's with shrouds on the sit parallel with the rocker covers keeping them as far away as possible from the manifolds. You can get them at 90 degrees or straight like the Magnecors which makes the leads sit closer to the manifolds than they need to.

[griffter]

thegman said:

Both leads and extenders were sold as OE by a well regards TVR parts -supplier I'm sure they are - but are suggesting that ACT stuff is better?

The original ones came from a Vauxhall and were fitted because they fitted. The ACT ones are non-resistive and allow a higher discharge current because they are non-resistive. However you must have at least one resistive component in the HT system and given that the standard plugs (B7ECS) are non-resistive that means the leads should be.

As an aside I thought you couldn't measure HT lead impedance with a voltmeter as they only effectively conduct with a very high voltage applied? (I may be wrong)

[taylormj4]

You can test with a voltmeter but the resistance you get with a low test voltage will be different (higher if anything) than when using a higher test voltage, which breaks down resistances in the leads. If you can get your hands on a 500V or 1kV Megger tester, that may provide a more reliable reading.
Matt

[blitzracing]

taylormj4 said:

You can test with a voltmeter but the resistance you get with a low test voltage will be different (higher if anything) than when using a higher test voltage, which breaks down resistances in the leads. If you can get your hands on a 500V or 1kV Megger tester, that may provide a more reliable reading.
Matt

You are talking about two different readings here: one is the end to end, and the other is conductor to ground. A megger tester is for testing insulation, ie conductor to ground, but at only 1kv, is far less that the the HT its self so a waste of time. The end to end can be done with an ordinary test meter for a basic resistance check. The reason the resistance changes with the high voltage is due to the very rapid rise times of the spark voltage, that looks like an AC waveform. Magacors use a wound inner core that looks like a coil winding to AC, and due to an inductive effect resists the rapidiply rising voltage, so it looks like a resistance, and controls the spark discharge better than straight copper. You wont see this with your meter however, as this is straight DC. Normal high resistance leads will appear the same resistance whatever voltage or waveform (withing limits). As a matter of interest a neon screwdriver was used to trace an HT fault recently, as there is enough energy to light the neon if the tip is held against the HT lead. Simple comparison between the leads will show up any differences in the HT firing voltage as different intensities of the neon.

mark

[911motorsport]

blitzracing said:

taylormj4 said:

You can test with a voltmeter but the resistance you get with a low test voltage will be different (higher if anything) than when using a higher test voltage, which breaks down resistances in the leads. If you can get your hands on a 500V or 1kV Megger tester, that may provide a more reliable reading.
Matt

You are talking about two different readings here: one is the end to end, and the other is conductor to ground. A megger tester is for testing insulation, ie conductor to ground, but at only 1kv, is far less that the the HT its self so a waste of time. The end to end can be done with an ordinary test meter for a basic resistance check. The reason the resistance changes with the high voltage is due to the very rapid rise times of the spark voltage, that looks like an AC waveform. Magacors use a wound inner core that looks like a coil winding to AC, and due to an inductive effect resists the rapidiply rising voltage, so it looks like a resistance, and controls the spark discharge better than straight copper. You wont see this with your meter however, as this is straight DC. Normal high resistance leads will appear the same resistance whatever voltage or waveform (withing limits). As a matter of interest a neon screwdriver was used to trace an HT fault recently, as there is enough energy to light the neon if the tip is held against the HT lead. Simple comparison between the leads will show up any differences in the HT firing voltage as different intensities of the neon.

mark

Bloody hell

[rev-erend]

Personally - I would check the plug and it's gap and possible make it a bit smaller as see if the problem goes away.