Car not running right. Broken spark plug?

[James k]

Hi,
yesterday I was driving back from a fairly long drive (20 miles) and my car started occasionally jolting and feeling like it was misfiring. It had done it occasionally before but it's got slowly worse and now suddenly much worse. It feels like when I had a faulty condenser but it can't be that as I now have electronic ignition.

Anyway, I got back from a drive this morning where it had been jolting again and had a look at the engine. I took out the plugs to do a compression test and found that one had a very worn down positive electrode and had a gap of about a quarter of an inch, the rest were fine. The compression test was fine (a bit low as before but consistent across cylinders). I bent the top electrode of the spark plug to try and make the gap a bit smaller but it hasn't really worked.

So, could my problem just be due to that broken spark plug? Would that give similar symptoms to a faulty condenser? Or have I probably missed something more sinister? As I said, the compression was fine.

Thanks

[James k]

A couple of other things I should mention. My ignition has been a bit too advanced for a while, I retarded it ever so slightly the other day but before then, the engine was fairly frequently pinking under load.

The other thing is that when I go for a long drive, my radiator always blows some rusty water out of the overflow in the cap. IS this normal or does it mean that it's overheating?

[panky]

Just try another plug, with one in such bad condition it was bound to affect the running. Try a new radiator cap if it continues to spit water out once the plug has been sorted. Also don't over-fill the radiator, an inch from the bottom of the filler neck is enough otherwise it will chuck some out as it warms up and expands

[James k]

Can I replace just one or should I do the whole set? They're only £2.50 each.

[panky]

No harm in replacing all of them, and keep the three good old ones for spares

[James k]

I wonder what caused it to do that though. I only replaced the plugs in November :/

[panky]

It's a strange one, hopefully just a dodgy plug. Use Champions if you don't already.

[James k]

I wondered if the frequent engine knocking had damaged it. Or if the overflowing radiator meant that it was overheating. I've got champion in there at the moment. I might have to get NGK today though. I need the car and that's all the parts place have in stock.

[philthehill]

You do not say what type of plug you have been using.
Champion N9Y is a good all round plug for use on a standard Minor.
Champion plugs (those made in England) used to last at least 12k miles plus.
Using the wrong plug especially the wrong heat range can cause all sorts of problems.

[James k]

I've now put a new set of plugs in and it's running fine. The previous plugs were champion n9yc. Three were fine but one had clearly overheated. The centre electrode and insulator are basically non existent and the body has gone a dull grey instead of its normal shiny colour. The question now is why it overheated and if it's going to do it again.

[bmcecosse]

The pinking may have damaged the plug....but why just one? Possibly the gasket is leaking water into that cylinder, and that would do it.... I never use NGK plugs.

[James k]

The gasket leaking water into the cylinder would make it overheat? How can I test if that's happening?

[panky]

It could pressurise the cooling system and push water out of the radiator cap Can you see any bubbles in the radiator water when the engine is running? Any steam coming out of the exhaust?

[James k]

I've just checked and there are no bubbles or fumes from the radiator cap when the engine is revved. Perhaps I've just overfilled it, there's no overflow pipe so the overflow does look a lot more spectacular. My worry is that that spark plug overheated, my radiator feels VERY hot after a long drive (I don't know what's normal) and the heater is very warm so there must be some underlying problem. Would water leaking into the cylinder not show up on a compression test?

[panky]

It will get hot after a run, it would be difficult to put your hand on anything over 50 deg C so it might not be an issue. I bought an infra red thermometer of ebay for a tenner, point it at anything and it will tell you the temperature - if you're unsure it would be a good investment and show you what's going on. Oh and nothing wrong with a good heater, it shows the coolant is circulating properly

[Trickydicky]

As Roy says, The plug most likely failed due to the ignition being advanced too far, but why it should only affect one plug is strange. With the ignition too far advanced it could also make the engine overheat which would explain the water is overflowing from the radiator.
Is the ignition set correctly now?

[James k]

The timing seems to be set correctly now, it's not pinking. It was set correctly before but then the mechanic who did the MOT leaned out the mixture and it started knocking.

[Trickydicky]

Which cylinder was the failed plug in? It could be that the water jacket is silted up/blocked and the area around that cylinder is overheating causing the damage to the plug.
It may be a good idea to flush the cooling system as the rust in the water may be the clue.

[James k]

It was cylinder two that overheated. There's rust in the water and a fair amount of brown sludge on and around the radiator cap. The oil hasn't got water in it.

[grumpydad]

Hi
put some dish washer tabs in the rad and run for 3 or 4 days
then drain and refill