low speed shunting

[dings]

I have a 4ltr 1995 and it shunts at low speed or when you ease off going round an island and apply a little throttle, while I can live with it i wondered if there is any thing that can be done any suggestion are welcome

Cheers

[selnic]

Usual answer is to clean the stepper motor as it becomes coked up during normal use, next step from experience is to change the plugs, had exactly the same issue with mine, also found that it certainly make a huge differance which petrol you use.

[That Daddy]

This may also be of use www.pistonheads.com/gassing/topic.asp?h=0&t=47...

Edited by That Daddy on Thursday 17th January 22:36

[FlipFlopGriff]

Change the rotor arm to a genuine Lucas one also - I don't know whey but they may look the same but cause no end of problems - if not you got a spare.
FFG

[blitzracing]

Also see:

http://www.pistonheads.co.uk/gassing/topic.asp?h=0...

If you dont mind chopping into your loom, I dont mind sending a couple of trial resistors, but you do need to be able to measure the lambda probe outputs as you go, so you dont overfuel at tickover. (its in the range of .3-.7 volts)

Also see

http://www.g33.co.uk/fuel_injection.htm

[dings]

Cheers blittz, but my knowledge on mechanics is zero i need to get someone sort it out, went out tonight just cut out, i disconnect the stepper motor and it started, got back home reconnected and it was ticking over on 4000 rpm, stopped engine disconnected then reconnected and seems ok now, I am just starting to get rattled with it now, I will see what tomorrow holds out.

[Ordinary Bloke]

Isn't "Low Speed Shunting" what Thomas the Tank Engine has to do when he's naughty?

[Chimjunkie]

This might help!

http://uk.youtube.com/watch?v=G-OHUZ_4Qwg 

edit to say...defy anyone not to watch the whole collection

Edited by Chimjunkie on Saturday 19th January 01:35

[dings]

The way its driving Thomas the Tank would probably leave standing still,

[Sam Gamgee]

dings said:

Cheers blittz, but my knowledge on mechanics is zero i need to get someone sort it out, went out tonight just cut out, i disconnect the stepper motor and it started, got back home reconnected and it was ticking over on 4000 rpm, stopped engine disconnected then reconnected and seems ok now, I am just starting to get rattled with it now, I will see what tomorrow holds out.

Sounds like could deffo be the stepper, then?

Dave.

[blitzracing]

The stepper is not a reason for "shunting". It only makes the engine over rev or die when it gets faulty. Basically the shunting appears to be due a combination of things but its basically poor combustion burn at around 1500 rpm and low throttle. (It can be made worse by transmission backlash.)This could be simply an over lean mixture as I found, or possibly a lean mixture combined with poor HT. It all seems very marginal. Something like failing ignition amp, worn rotor cap or distributor cap, HT leads, or failing coil, could fail to ignite the lean mixture fully, (its much hard to keep it burning when its lean, and it needs a longer spark duration) so replacing one or more of these components may be enough to overcome the shunting as some people have found. The car I looked at closely had just had a £600 bill from a main dealer and it was no better. (Latterly reduced to £300 when this was pointed out- personally I still don’t know what they had done for this money...) My philosphophy would be to replace the HT leads, rotor cap, plugs, rotor arm (Lucas original only) and stock HT leads (don’t use magnacors) as these wont cost a fortune as standard service items and it MIGHT just do the job. What I can tell you is tweaking any ECU controls like airflow meter, throttle pot, or stepper motor etc or even a rechip will not cure it. Having said all that, an re-read your post about it dieing on you, Id definatley start with HT first, the two problems may well be the same cause.

Mark

[dings]

Hi Blitz, I had new leads from Castle racing, the ht lead does interfere with coil as the tacho needle jumps about but Jamie is on the case for that, if I replace cap rotor arm etc where is the best place to get them from, as you mentioned it is only when going along between 1500 - 2000 rpm or taking your foot of then reapplying something so minor but very annoying.

[That Daddy]

dings said:

Hi Blitz, I had new leads from Castle racing, the ht lead does interfere with coil as the tacho needle jumps about but Jamie is on the case for that, if I replace cap rotor arm etc where is the best place to get them from, as you mentioned it is only when going along between 1500 - 2000 rpm or taking your foot of then reapplying something so minor but very annoying.

Strange,both my brother and i have these leads (12 months+)and have had no problems(the pale blue 8mm ones),are you sure you used castles plug extenders too?just curious,because they are for these leads(Unresisted)

[Sam Gamgee]

Blitz, why avoid the magnecors....? And, doh!, youre right, stepper little to do with shunting.

Dave.

[Chimjunkie]

Magnecors do highlight and cause problems down the line of weak ignition systems. However, I have these with new coil, plugs etc and they are great, look great, and are much more temperature durable than normal leads. Mark Adam recommends them so cant be bad.

[blitzracing]

Oh boy ...how many times have I gone over this one. Low resistance leads like magnacores cause the coil to discharge much faster. You do get a hotter spark, but as the magnetic fields collapse in the coils secondary, it also glitches the primary windings. This produces much higher voltage spikes in the primary side and increases the coils over shoot voltages (called ringing). This could go from say 250 volts to around 500 volts. The rev counter triggers off these pulses and the switching transistors in the ignition amp have to deal with the higher voltages. This is why the rev counter goes nuts, as it see's multiple voltage spikes as extra firing strokes. So you get a hotter spark, but ask the question:

I have a bucket full of petrol vapour, and I can light it with a small match, or a flame thrower....Do you think there would be any difference in the following explosion?

Just the same with the spark. All you need is enough to reliably start the flame front in the combustion chamber and no more. Why don’t you see rolling road results for Magnacors? That’s because there NO power increase over a decent set of standard leads. Its all hype, and with big profit margins. The only environment to benefit from low resistance leads is very high compression ratios or specialist fuels that have enough resistance to prevent the spark from starting in the first place. Not your TVR RV8 !!

To cure the jumping rev counter, replace the king lead with a standard one, it should be enough to calm the back emf's in the coil. Im surprised that Mark Adams recommends them (unless he’s selling them),or maybe he simply likes the robust construction ??

I spent some time messing around with plain copper leads (virtually no resistance), and could not understand why I kept blowing up rev limiters, until I studied the various waveforms and realised I had doubled the voltages in the coils primary and hence rev limiter control circuit. Magnacores are about a tenth the resistance of standard leads, so not as bad as plain copper, but its still basically a bad idea. The secondary reason for using high resistance lead is it reduces radio interference, but the magnacores are shield to prevent this being a problem. Of course you can go to far, high resistance leads, suppressed plug caps, and resistive plugs, all add up together to the point the spark becomes just too weedy. Also as the leads resistance increases with its length, the plugs further from the coil and distributor will have the highest resistance path, so its not an exact value at best of times.

Mark.

[Chimjunkie]

Thanks for the informative explanation. I dont notice any strange goings on with the rev counter. Also didnt buy them for supposed power gains. They look great, cost twice as much but probably last many times longer than std leads. The latest Sprint mag has a great article on V8 tuning by Mark Adams, quote "the leads that we have found to meet every requirement are those by Magnecor, and we have used thousands of them" The average person does not have the technical appreciation to understand how different lead resistances can affect the ignition system performance. We are therefore left to take the opinion that we read or hear from the professionals that do. Problem is when two professionals disagree!! Where does that leave us??

I am a scientist by qualification and go by scientific experimentation which brings me towards your camp (Blitzracing). However Mark is regarded as one of the foremost experts in the V8 tuning world! Dont want to start a fight but would be nice to get a reasoned argument from Mr Adams.

[Aubrey]

blitzracing said:

This is why the rev counter goes nuts, as it see's multiple voltage spikes as extra firing strokes.

Ah thats interesting, I've noticed occasionally under hard acceleration the rev counter needle will 'bounce' up and down briefly and then settle and rise smoothly. Might this be a similar problem?

[blitzracing]

Ok, if you dont mind paying the price for Magnacors, like the look and construction, and can live with a possible jumpy rev counter so be it, and although the primary voltages will be higher, the Ignition amp transistors obviously cope OK. The rev counter problem may show up as intermittent, as the exact voltage the plugs fire at will change dependent on the compression and fuel amounts, so the wave forms will vary a bit. I can only think that loss of spark intensity could be an issue at peak RPM when the dwell time on a V8 is so short that the coil cannot fully charge, so low resistance leads could help. I will confess that my HT work was not done on an 8 cylinder engine and not having held the v8 at 6500 rpm, with a 'scope looking at the coils "charge" waveform I could not verfy this one way or the other! I just dont like the "you must have magnacors", with the silly price attached, when I think ordinary decent leads should do the job (bearing in mind the total resistive path with caps and plugs already mensioned). Ill drop a line to Mark adams for his input, as I stand only for the prosecution so far.

Mark

[dings]

Blitz, Looks like Ive started something here, would a ht lead from a Range Rover spare shop do the job, and is it ok to leave the Castle leads on, where can you get rotor arm and diz cap from, my diz cap is blue but not sure what make.