Technical My cats misfiring

[YSporting2003]

Looking for some help, I’ve taken a mk2b 1.2 16v engine and put it in my mk2a, it was running good before swapped but now isn’t misfiring, I’ve changed spark plugs, coil leads, coil pack, new spark plugs. Just had a thought that maybe the spark plug gaping has a play in this, google recommends 0.8mm has anyone done any different sizes?

 

[bugsymike]

Looking for some help, I’ve taken a mk2b 1.2 16v engine and put it in my mk2a, it was running good before swapped but now isn’t misfiring, I’ve changed spark plugs, coil leads, coil pack, new spark plugs. Just had a thought that maybe the spark plug gaping has a play in this, google recommends 0.8mm has anyone done any different sizes?

That may be clutching at straws, if plugs are in good order .8mm should be fine, if you have a wider gap generally the coil need to generate a higher voltage to fire them so more likely to misfire at higher revs and loads when pulling hard up hill or on damp days more likely to short out/misfire.
Have you checked the continuity from one end of HT leads to other using an Ohm meter, the main thing is that there is a reading, as what often happens with suppressed/carbon leads is they break internally when pulled off the plugs, engine still runs except under load the lead can cause a misfire.
I see you have changed the leads , but still worth checking.

 

[GettinBetter]

As per above.. and suppressed plug caps can go open circuit (or very high resistance) which usually leads to tracking

Never assume that new parts are faultless.

I experimented with plug gaps when I was younger, and thought bigger was better. But nope, manufacturers setting always proved to be best. I also learnt that a good benchmark, was that the plug electrode erode at roughly 0.001" per thousand miles, so at 5K miles plugs should be cleaned and regapped. The iridium plugs may differ? don't know.

I personally never replace plugs, only clean & regap. I do carry a couple of spares though, as you never know.

 

[bugsymike]

As per above.. and suppressed plug caps can go open circuit (or very high resistance) which usually leads to tracking

Never assume that new parts are faultless.

I experimented with plug gaps when I was younger, and thought bigger was better. But nope, manufacturers setting always proved to be best. I also learnt that a good benchmark, was that the plug electrode erode at roughly 0.001" per thousand miles, so at 5K miles plugs should be cleaned and regapped. The iridium plugs may differ? don't know.

I personally never replace plugs, only clean & regap. I do carry a couple of spares though, as you never know.

Back in the old days we had a Champion Spark plug cleaning machine that sand blasted and tested the plugs, interesting enough if you tested a brand new plug then sand blasted it with the machine it broke down earlier in the test.
If spark plugs running clean, then regapping was well worth doing.
The other thing was we had a Krypton Tuner/diagnostic machine with Oscilloscope etc. What I found was that the old cars with basic copper HT leads and non suppressed ignition systems only required around 5K volts to happily run the ignition system, but once we were forced to go the suppressed plugs,HT leads, rotor arms etc.etc. then the voltage needed rapidly exceeded three times that and much more.
Probably why back then you could wash an engine down and it would keep running, but now a heavy dew will cause misfires as the electricity takes the easiest route and shorts out.
When servicing my customers vehicles, if there was an option I used to go for NGK spark plugs but the non resister version and generally replaced them at each service as a minimal cost in those days and my customers appreciated the reliability of their vehicles.
One of my sisters who lived many miles away had a Honda Jazz bought new and serviced by the main dealer, she mentioned to me that she was repeatedly going back to the dealer for a misfire and they were fitting expensive coil packs at her expense. I asked her to insist they change the spark plugs which they said were were not due at that mileage, strangely enough misfire problem solved!!!

 

[judderbar]

Looking for some help, I’ve taken a mk2b 1.2 16v engine and put it in my mk2a, it was running good before swapped but now isn’t misfiring, I’ve changed spark plugs, coil leads, coil pack, new spark plugs. Just had a thought that maybe the spark plug gaping has a play in this, google recommends 0.8mm has anyone done any different sizes?

Did the cars have the same engine ecu?

It is fairly easy to get the plugs leads installed incorrectly and never get it right until you have a picture

 

[GettinBetter]

Back in the old days we had a Champion Spark plug cleaning machine that sand blasted and tested the plugs, interesting enough if you tested a brand new plug then sand blasted it with the machine it broke down earlier in the test.

I quite believe it, under close inspection (using a magnifying glass) it was surprising how much of the particles used for blasting them, got stuck down the side of the centre electrode, and the dust also formed a fine layer down the centre electrode. Not exactly sure what they used back then as a blasting medium.

One of my sisters who lived many miles away had a Honda Jazz bought new and serviced by the main dealer, she mentioned to me that she was repeatedly going back to the dealer for a misfire and they were fitting expensive coil packs at her expense. I asked her to insist they change the spark plugs which they said were were not due at that mileage, strangely enough misfire problem solved!!!

When I left school I worked for a garage that had a mechanic that worked like that, when in my innocence, I mentioned the likely fault, it was met with a sharp elbow to the ribs. As a kid I vacated the area, one of lifes lessons learnt.