Odd ignition/misfire problem

[feef]

Car: 2002 MX5 1800, supercharged

The car has been running fine until earlier this week where it developed a misfire

I checked the plugs and found that the plug in cylinder 3 had had the electrode bent to the point that it was touching the center electrode, but there wasn't any obvious signs of contact

I re-gapped the plug and refitted it (as it was sunday evening and everywhere was shut) and went for a run, and all seemed fine

Today, I swapped out the plugs with some new ones and did an oil and filter change while I was at it

Went out for a short blast and noticed some hesitancy at the top end under load, and on returning home, the misfire had returned, however, it was the plug in cylinder 1 that had suffered the bent electrode now, number 3 looks tickety boo

Cambelt was changed less than 10k miles ago
both coil packs were replaced last year (2 packs for 4 plugs, wasted spark)
Both misfires developed after a blast with high rpms

Any ideas?

[Steve_D]

Are you sure the plugs are the right type.
If they are then logic says there is too much clearance somewhere allowing the piston to contact the plug. Big or little end bearings.

Steve

[feef]

Steve_D said:

Are you sure the plugs are the right type.
If they are then logic says there is too much clearance somewhere allowing the piston to contact the plug. Big or little end bearings.

Steve

They are the listed part number, the only difference is that I've gone one step cooler to allow for the additional power and supercharger

Odd, however, that it's affected two different cylinders tho, I'd have thought if it was bearings then it would be more consistent

[Borla]

Read up on indexing spark plugs, I'm sure others could explain it far better than me.

For sure sounds like plug/piston contact, doubt if it's bearing trouble though, you'd have heard that.

[feef]

The plugs are identical other than the temp rating. The same plugs were also used for the last couple of years until this weekend and the new plugs suffered the same issue in only a day which suggests the plugs are showing symptoms but not the cause.

[FordPrefect56]

Sounds like you've got "long reach" plugs by mistake. Anyway whatever you've got it's certainly the pistons hitting them.

[feef]

FordPrefect56 said:

Sounds like you've got "long reach" plugs by mistake. Anyway whatever you've got it's certainly the pistons hitting them.

The plugs are physically identical to the old ones, I've compared them and measured them and double checked the part numbers. The old ones worked fine for 2 years until this issue arose this weekend when no3 bent the electrode on the old plugs, and number 1 bent the new plug.

[stevieturbo]

Has it ever been tuned correctly ?

bent electrodes where you're sure there is no physical reason is probably detonation. Get things checked before you do some major damage.

If it has run fine for some time prior to this...then there may well be other problems that have lead to incorrect tuning now.

[FordPrefect56]

stevieturbo said:

Has it ever been tuned correctly ?

bent electrodes where you're sure there is no physical reason is probably detonation. Get things checked before you do some major damage.

I've seen plenty of det over 40 years but never a bent plug electrode from it. I don't even see a mechanism for this. The pressure would be evenly distributed around the electrode. No reason for it to move one way or the other.

Take a plug out, bring the piston to tdc. Measure how far the piston crown is away from the plug seat. Compare to the plug. Less than 1mm to the elctrode and contact could occur at high rpm especially if the bearings are shot or the piston crown is getting very hot and expanding a lot. It's happening at high rpm, that's the time for max rod stretch, supercharged maybe indicates non std pistons too.

[stevieturbo]

FordPrefect56 said:

I've seen plenty of det over 40 years but never a bent plug electrode from it. I don't even see a mechanism for this. The pressure would be evenly distributed around the electrode. No reason for it to move one way or the other.

Take a plug out, bring the piston to tdc. Measure how far the piston crown is away from the plug seat. Compare to the plug. Less than 1mm to the elctrode and contact could occur at high rpm especially if the bearings are shot or the piston crown is getting very hot and expanding a lot. It's happening at high rpm, that's the time for max rod stretch, supercharged maybe indicates non std pistons too.

I hear ya, but I've seen it quite a few times and also seen it happen to others ( including it getting totally broken off or melted )

[feef]

stevieturbo said:

Has it ever been tuned correctly ?

bent electrodes where you're sure there is no physical reason is probably detonation. Get things checked before you do some major damage.

If it has run fine for some time prior to this...then there may well be other problems that have lead to incorrect tuning now.

Had the ignition/injection map on the Emerald ECU set up by Emerald on their own rolling road last year, where it was putting out 225bhp

It was remapped at that time for a change in the supercharger pulley, when it was fitted with a 17% reduction pulley, prior to that it had run for a year at 180bhp, again, having been mapped by Emerald

I've been monitoring the wideband o2 gauge and haven't seen anything untoward on that while driving

Stock pistons and rods

I'm tending towards rod-stretch as a possible culprit, and may look at replacing them with some stronger items

I'll be taking a look down into the combustion chamber with a borescope tomorrow, just to see what's what before maybe taking the head off at the weekend

Edited by feef on Tuesday 17th May 14:13

[FordPrefect56]

feef said:

I'm tending towards rod-stretch as a possible culprit, and may look at replacing them with some stronger items

Well you'd be totally wrong and wasting your money. In a typical engine at 7000 rpm the piston comes about 0.5mm (0.020") closer to the head than on a static measurement. About 1/3 of this is down to thermal expansion of the top part of the piston. The rest is down to rod stretch, flex in the crankshaft and the take up of play in the crank bearings. Perhaps 3 thou total in the main and big end bearings if they are in perfect condition and maybe 5 thou each for the rod and crank. You could put rods made from unobtanium in the engine but it ain't gonna make diddly squat difference if the plug gaps are closing up by the best part of 1mm. Easy option is to grind a little bowl in the piston crown where the plug electrodes sit. Commonly done on things like Ford Pintos where the pistons hit the plugs if you try to get the CR very high. However if the engine has been fine for years then something has changed and it's either the plugs themselves or the bearings. However I find it hard to believe that an engine with stock internals could get the piston even close to the plug if the plug was stock no matter how worn the bearings were without the engine going pop first. 99.999% sure it's the spark plugs sticking out too far into the chamber.

[feef]

Yup. Some simple measurements and a chat with another mx5 type confirmed that the clearance is so big that it can't possibly be rod stretch.

I'll be sticking a borescope down tomorrow to see if there's a foreign body in there.

[Allan L]

feef said:

I'll be sticking a borescope down tomorrow to see if there's a foreign body in there.

That'd be my guess as many years ago I had the closed-up plug event. Took the plug out at the roadside and found the electrodes closed, so spread 'em, replaced the plug and started up, only to have them closed again p.d.q.
The carbs had pancake air-cleaners held together by a central bolt which was tightened into a captive nut on a bridge across the carb intake. The villain of the piece was that someone had fitted that with a locknut, which had come loose and been ingested to give the piston crown a fine pewtered finish and close the plug electrodes.

[stevieturbo]

feef said:

Had the ignition/injection map on the Emerald ECU set up by Emerald on their own rolling road last year, where it was putting out 225bhp

It was remapped at that time for a change in the supercharger pulley, when it was fitted with a 17% reduction pulley, prior to that it had run for a year at 180bhp, again, having been mapped by Emerald

I've been monitoring the wideband o2 gauge and haven't seen anything untoward on that while driving

Stock pistons and rods

I'm tending towards rod-stretch as a possible culprit, and may look at replacing them with some stronger items

I'll be taking a look down into the combustion chamber with a borescope tomorrow, just to see what's what before maybe taking the head off at the weekend

Edited by feef on Tuesday 17th May 14:13

Wideband isnt going to tell you if it's been detonating. You may see indicators with a borescope so that would be worth doing for sure.

And rod stretch is really stretching the realms of possibility here.

[feef]

Let's play Spot the Difference

[stevieturbo]

yup, detonation and busted pistons. Surprised it wasnt breathing heavy and using oil too.

Time for a rebuild.

[Munter]

stevieturbo said:

yup, detonation and busted pistons. Surprised it wasnt breathing heavy and using oil too.

Time for a rebuild.

Rebuild or replace. MX5 engines are relatively cheap...
Hello is that autolink. I'd like to swap 500 earth pounds for a replacement engine. http://www.autolinkmx5.com/engine-mx-5-mk25-18-bp6...

Unless the OP want's to rebuild with low compression pistons or something.

[stevieturbo]

Munter said:

Rebuild or replace. MX5 engines are relatively cheap...
Hello is that autolink. I'd like to swap 500 earth pounds for a replacement engine. http://www.autolinkmx5.com/engine-mx-5-mk25-18-bp6...

Unless the OP want's to rebuild with low compression pistons or something.

It may not need low compression pistons, just stronger pistons and then ensure everything is running as it should be afterwards in case there are other problems that led to it going bang.

But yep, if replacement engines are only £500....fire one in and then maybe look at rebuilding the other or at least seeing how bad it is.