Originally posted in Engine, drivetrain.

Coupes misfiring - it's a common fault and I've come across a few people mention it on here, as well as sorting it out weekly in the course of my job.

There are three things that can cause this to happen:
1. Lack of compression - Easily proven with a compression tester. (if you don't keep an eye on your timing chain or service your car regularly it could happen to you).
2. Fuel delivery system problems - Injector failure or fuel pressure.
3. By far the most common - Ignition system problems.

1.6/2.0 Beta, 1.6 Alpha and 2.7 Delta engines have whats known as a wasted spark ignition system. This means there is one ignition coil (known as an inductive double ended coil) serving two cylinders. On the firnig stroke the ECM will trigger the coil by earthing it and both sides will operate their respective spark plugs, even though only one is required, thus wasting the other spark.

So if your car is misfiring on one cylinder it usually means a HT Lead or spark plug failure. If it is misfiring on two cylinders (1 and 4 or 2 and 3 on a four cylinder or 1 and 4, 2 and 5 or 3 and 6 on a six cylinder) it usually indicates a coil or ECM trigger fault. The coil packs only have one live feed only and one trigger wire per coil (2 triggers on a 4 cyl and 3 triggers on a 6 cyl).

If this post is of any interest to anyone I can continue it, including test procedures and expected results.

Sorry about the delay, I've been busy keeping the Coupes in and around Birmingham on the road.
In answer to Uncle Buck:
Gen 1 - both 1.6 and 2.0 are Beta engines.
Gen 2 - some very early 1.6 had Beta, but all later ones were Alpha engines. 2.0 Beta.
Gen 3 - all 1.6 Alpha, all 2.0 Beta 2 and 2.7 V6 are Delta engines.

Right... to business, as has been alluded to before, by far the most common cause of misfire is the HT Leads. The Korean OE leads themselves aren't very good, but I have found a lot of vehicles with failed leads have plugs that are burnt around the lower part of the ceramic. Now the plugs should be changed every 20,000 miles, or 2 years on all 4 cylinder coupes with standard plugs in, (preferably NGK). V6 engines are at 50,000 or 5 years. Test the leads with an Ohm meter - looking for about 1000 - 1500 ohms per foot, e.g number 1 lead gives about 3000 ohms. As mentioned in the first post, lead or plug failure usually will present as a single cylinder.

When the vehicle presents with multiple cylinders misfiring the coil pack is the most likely cause and you will find the cylinders misfiring will be:
4 cylinder - 1 and 4 or 2 and 3.
6 cylinder - 1 and 4, 2 and 5 or 3 and 6.
But it could be the ECM power transistors, so to be sure, carry out the following:

There is only one live supply to the coil pack, so logically if this has failed the car wont start. But test for battery voltage at pin 1 (all Gens, 4 or 6 cyliders) anyway.

Coil primary resistance (Low Tension) - 1 ohm, approx, between pins 1 and 2 and 1 and 3 on the coil itself.

Coil secondary resistance (High Tension) - 10,000 - 14,000 ohms between HT lead connectors on each individual coil.

Do NOT start sticking things up the wiring harness connector or you will blow the ECM transistors, this is an earth circuit and carries almost no voltage when operational.
If the resistance values check out then you may have an ECM issue. The only realistic route in this instance is a new ECM, or repair of your exsisting unit. Sourcing second hand ones can be problematic due to the immobiliser coding that is unique to your ECM and the requirement to neutralise the ECM before removal from the vehicle.

If you want any more info on this, or any other Coupe related technical issue I will be more than happy to do more posts.

hi there mr sharky

i have a missfire when i first tap the accselerator now im suspecting its ht leads and either way they need changing as i have no idea when the last time they where changed.

in your last post you said the genuine hyundai ht leads arnt very good?

was speaking to my mechanic and he said its best to get the genuines as they fit properly. and by geting pattened ht leads they dont always fit correctly

my question is this, do i buy genuine HT leads ive been quoted £41.00 inc vat from my local hyundai dealer. and if not which leads do you sujest for me to buy. im looking for mid range leads nothing to fancy.

thanks. hodsy

Hi,

If it's a hesitation/misfire on light acceleration, more likely than not it's going to be your leads. When I said the OE leads, I meant Original Equipment, as fitted in production. The dealer replacement leads are much better quality and should do the job nicely. If you want to go mad, however, you could invest in a set of good high performance leads like Magnecor, but they'll set you back over £100.

Hope I helped, all the best.

Moved back to Engine, drivetrain etc. and made sticky

Nice one, thanks.

I can get the genuine leads for £37.95 inc vat for members but I have no idea how much p&p would be. I suspect it would bring the price to around what you would pay locally. My non-member customers pay £40.88 inc vat.

Hi,

I've got a 1.6 51 reg coupe, i havent had it long and its developed a problem. Firstly, it seems to be idling a little bit rough. Secondly, when its running and you put you foot down more than just a touch, it lags big time and feels like its misfiring. The engine management light comes on as well. Then it seems to catch up with itself and the revs pick up and the light goes off.

Any ideas? Leads and plugs?

Cheers