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Is there any way you can check for degrading HT leads. Car has had new plugs and disi car and rotor arm over last year but it still seems offbeat slightly. Its not affecting driving but do feel it could be a little better. The engine uses little oil for a 15 year old engine although a litle every 3 or 4k maybe higher than a new car. I'm happy to spend the 45 or 50 pound for new leads but just wanted to check first?
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Use a multimeter to measure the resistance of each lead.
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and for leaks to ground, open the bonnet at night, and look for jumping sparks - take great care you won't be able to see the fan belt and pulleys as easily - so, be careful if you go poking around with the engine running.
It might be better if you describe the problem more clearly rather than continuing the scatter gun approach. What exactly is the engie doing? Does it do it from cold?, or does the engine have to be running a few minutes first? What happens under load?
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>>Use a multimeter to measure the resistance of each lead.
Most modern leads are resistive, it's true but the bit that usually lets the side down is their abilities with high voltage electricity.
I have met failed leads that would have showed up on a multimeter, but as the core had broken, a gloved hand removing and replacing leads while the engine ran quickly pin pointed the culprit. Quite surprising how badly a V8 runs on 7, I had to do 20 miles like it before I got home and could investigate.
Vehicle was a company one, and the previous driver insisted all was ok for him. Perhaps, but I'm not convinced!
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>> I'm happy to spend the 45 or 50 pound for new leads
How much?
It's been decades since I've bought any, but that seems to in the realms of rattle's scart leads from Curry's.
What's the car?
Have you had a look under the bonnet in the dark with the engine running? If you can see sparks, then that's the problem.
Edit: Great minds etc NC..........
Last edited by: bathtub tom on Mon 13 Aug 12 at 14:05
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>> How much?
>>
>> It's been decades since I've bought any, but that seems to in the realms of
>> rattle's scart leads from Curry's.
£45 to £50 is cheap for a set of HT leads. Most now have moulded spark plug caps, etc which push the price up considerably.
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Sorry for the Rattle style post. I'll try and describe the situation.
From cold overnight, it sometimes fires up quickly and cleanly but not always. Most of the time though it either takes 2-3 seconds of cranking and then it seems to run rough for a second or so before idle settling (imagine an old diesel being started without glow plug light going out first). This happens also after an overnight rest but also after being stood for a few hours. Warm engine starts are fine.
Once started, the first few miles or until the temp needs starts to rise it just seems to hold back and is a bit jerky at low revs (mainly under 2000 revs and town stop/start driving)
Otherwise when warm and over 3000 revs, smooth and pulls very well and doesn't give any bother.
I agree there are other factors in play, it has 102,000 miles and that cant be discounted.
Re the leads, it's a six cylinder car (Mazda MX-3 1.8) so will be more expensive as there are more of them but the leads aren't anything special.
MODS please move this to technical
Last edited by: nice but dim on Mon 13 Aug 12 at 20:39
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"Can suffer from ignition problems. Repair kit from Ford dealers is £164 (inc VAT) for the V6. No longer necessary to buy a new/reconditioned distributor"
~www.honestjohn.co.uk/carbycar/mazda/mx6-1992/?section=bad
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>>From cold overnight, it sometimes fires up quickly and cleanly but not always.
One possibility here is low fuel pressure. To check for it, cycle the ignition a couple of times to run the fuel pump to build fuel pressure before cranking.
For the general malaise until the engine is hot, a sluggish lambda sensor is a possibility. To check, connect a multimeter to the sensing wire and you should see regular pulsing between about 0.3 and 0.8 volts. This should be about once per second at idle, getting faster if you rev up. Another check for a sluggish lambda sensor is to temporarily use your left foot under the accelerator pedal to hold the pedal absolutely still, and see if you can feel the car pulsing at ~30 mph (obviously, find an empty road for this test!)
If the HT leads were the problem, I would expect there to be a full misfire, and more serious bad running than you're describing.
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A mild HT fault, lead, plugs or coil is most noticeable when you want some power, ie overtaking or going up a hill. Cruising or idle can seem near normal.
Reason is, a richer mixture when you want power is harder for the spark to travel in, so any weak point in the HT circuit will show itself.
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>>Reason is, a richer mixture
It's actually because the pressure in the cylinder is higher when you've got the throttle open - but, I agree, the way to bring out an HT fault is to put the engine under load. As nice-but-dim doesn't mention misfires and problems under high load, I woudn't initially suspect an HT problem.
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From the way NBD describes the problem, it sounds to me like a dicky MAF sensor. In my experience they can cause all kinds of weird behaviour, including the above.
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Rough old motor that's been run gently and neglected... The first thing you do is install new plugs, leads, distributor cap and (in the old days) contact breaker, get them adjusted and then try to make the carburettor work properly.
It's more complicated now I know. Or perhaps simpler. But plugs and leads are still a good idea. Perhaps a coil pack. Obviously you start with the cheap stuff.
Plug leads of all types do deteriorate and do fail.
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>>Plug leads of all types do deteriorate and do fail.
Yes, but, plug leads for modern motors aren't cheap anymore.
It's better to carry out some kind of diagnosis before throwing parts at the car.
As an example, the MAF sensor (if this car has one?) has been mentioned. Often, these can be "tested" by temporarily disconnecting the sensor, and seeing if the engine runs better without it.
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>> Yes, but, plug leads for modern motors aren't cheap anymore.
>> It's better to carry out some kind of diagnosis before throwing parts at the car.
I don't doubt that for a moment NC. It's true that in the traditional days those parts only cost ten or twenty quid and it wasn't a big loss even if the old ones were 'good enough'.
These days I have to take advice on quite simple-seeming things. One is being gradually swallowed by the industry and its connected avatar the aftermarket, like an Amazonian child by an anaconda. That's what it feels like, a bit.
:o}
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I had a whole reel of HT lead and a bunch of distributor and plug connectors which screwed into it.
Doubt I ever paid as much as a tenner for a set even on the rare occasions I did actually buy them ready made.
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A brief update as I'm at work. "Tested" the MAF this morning by unplugging the connector and it barely ran. Replugged a few miles down the road, felt like it had a massive air leak, severly lumpy idle and reluctance to rev. Replugged and back to as it was before (better!), so it isnt the MAF. I should add I have a spare MAF because prior to this I had a split in the rubber flexi hose. A replacement from a spare or repair car I got had the MAF attached but I used my existing one as the other one looked a bit manky. I might try that at the weekend.
I cant rememebr it getting worse, the car gets used quite a bit. I'll report back how I get putting the spare MAF on.
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The MAF may work but may be badly contaminated. Remove and carb cleaner and cotton wool swab does the job but gently...
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HT leads - VxFan
