the Machinations of a Mysterious Misfire

[Brucey]

some you will have noted that I have been muttering about an intermittent misfire recently. This post pretty much summarises what has happened to date.

Background; my 4/87 built UK spec RHD 635CSi has about 165K under its belt and is (mechanically) not in bad shape overall. However several years ago I converted the old girl to run on LPG, simply because fuel costs meant that I wasn't using the car as much as I'd have liked. The car mostly does occasional long runs rather than lots of short ones. Obviously it starts and runs for the first few minutes on petrol, but then runs the rest of the time on LPG unless that runs out or the system is manually switched over.

Overall the LPG system has worked pretty well but several times I've had episodes of lean running; this can happen if there is an air leak (surprise surprise) and also if the vaporiser unit isn't up to full temperature. The second of these happens quite often if the water filter that protects the vaporiser unit gets blocked. The first you know of it is that the engine starts to miss after about tens seconds at full chat (eg pulling up a long uphill in second gear at 5000rpm+ WOT). Typically the engine is less likely to miss when running on petrol than on LPG; LPG is more difficult to ignite at the best of times and lean LPG mixtures are more difficult again.

When I've pulled the plugs there have been clear signs that the LPG running has been eroding the electrodes slightly faster than normal (i.e. with the same plugs on lead-free petrol), and also that the deposits on the insulators were slightly different in character from what I've been used to. I suspected that the plugs had seen higher temperatures too, probably because of the lean running.

When I last examined the plugs (in 2015) they looked OK to me, but the deposits on the insulators looked a bit weird; I'm used to brown or tan deposits but these were distinctly red in colour, something I've never seen before. In terms of mileage, with these plugs in this engine, I'd estimate that the plugs were less than 25% of the way through a 'normal' (on petrol) lifetime.

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More recently: I didn't use the car for two or three months over the winter. When time came to start the old girl up again (spring 2016), she started up first turn and ran sweet as a nut. I had to clean the water filter on the LPG vaporiser unit, and fiddled with a few other things but mostly things were pretty hunky-dory.

Then when I was driving the car hard for the first time, a misfire appeared rather suddenly. This appeared and disappeared over the next few hundred miles, during which time I discovered that;

- the hose clip on the AFM boot was not quite tight and this (combined with some damage to the hose spigot on my spare AFM ) may have lead to an air leak that varied with engine tilt (i.e. engine load)

- my AFM appeared to be doing something wrong so I fitted my spare one. It turned out that both AFMs had a little corrosion on the terminal pins, and that is why they were giving intermittent results

- I checked and cleaned every connection I could find in the fuel injection system in a bid to find the culprit.

- most of the time when the misfire was there, it was worse on LPG and also worse at mid throttle. Often WOT performance was unaffected, as was performance when running on petrol. Other times the misfire was there on petrol and LPG

- I examined everything I could in the HT system and I couldn't find anything obviously wrong anywhere.

- several times I did or changed something and thought I'd fixed the problem, only to have it recur later on.

Sometimes the car would do a long trip (100 miles plus) with no trouble, other times it would play up within a few miles of leaving home.

Last sunday the misfire appeared and this time it was there to stay, and affected running on petrol as well as LPG. Once or twice the engine pulled hard on all six (on petrol) but mostly it didn't. After driving for an hour so I stopped and checked things over. To my surprise I found that the distributor cap was still cold to the touch; the weather was cool (about 5C) and rather humid. I began to worry about damp in the distributor cap.

Anyway I found myself stuck in a car park an hour's drive from home with the airbox and AFM off, the radiator off its mountings, fan off, fan shroud off, distributor cap off, rotor arm out. I started removing spark plugs but having removed the #1 plug concluded that it was pointless; the plugs looked just the same as they had done the previous year.

I drove home slowly with the misfire there most of the time. My only consolation was that on LPG, at least the bore wasn't so likely to get washed in the event that there was fuel but no spark.

Once I got home I resisted the temptation to walk away from it and started to do a little more diagnosis. I realised that there was an unburnt fuel smell in the tailpipe (on both LPG and petrol) and I started pulling the connectors off the LPG injectors one at a time. I found that the #2 injector didn't affect idle but there was clearly less unburnt fuel smell in the exhaust when it wasn't connected.

The following day I pulled the #2 spark plug and it looked just like the #1 plug had done; light brown/red deposits, not bad. I fitted a new plug in #2 anyway and started up (from cold) on petrol. Same misfire. I started having visions of burned valves, sticking valves, failed head gaskets....

Then I started to think a bit more, and I remembered that I'd not plumbed the LPG injectors in sequence; the pipe runs worked out better if some crossed over, so what I'd actually done was to unplug the injector for #1... d'oh!

So I whipped out #1 plug and hey presto, I was looking at a plug that hadn't been firing correctly; clearly wet-fouled from the cold start on petrol. I put the plug I'd taken from #2 in there (which didn't look much different apart from the wetness) and then the old girl started and ran normally.

On test (in air) the #2 plug worked OK but even once it had dried out the #1 plug was still a duffer. I'm used to looking at fouled plugs in engines that have run on petrol (gasoline) with and without 2s oil in it, and I've seen plenty of fouled plugs. However this plug wouldn't have gone into the 'definitely won't work' category.

Today I went for a test drive with six new plugs fitted; everything was just fine. I've also cleaned the old plugs. The old plugs cleaned up very easily when grit blasted, so I'm supposing that one possibility (that I'd actually overheated/glazed the insulator, perhaps with some contaminant, during lean/hot running) is thus eliminated. I've yet to reinstall the old plugs but I suspect that the result will be the same, i.e. that everything is back to normal.

I've taken some photos of the spark plugs (before and after) which I have posted below;

dirty plugs, #1 to #6 (R to L)

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All plugs bar #2 were in for a cold start.
All plugs bar #1 were in for a run of ten minutes or so; you can see that the #2 plug looks less sooty than #3-#6 which have all had one cold start more on them.

clean plugs, #1 to #6 (R to L)

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Now it may be that there is some lurking underlying condition that will yet bite me in the behind but for now I'm supposing that the problem is (for now) resolved. Compression figures on #1 and #2 are very similar and running is now quite 'normal' again.

My takeaways from this are that

1) it is still worth carrying spare spark plugs. I'd previously driven several hundred thousand miles in a variety of vehicles with no problems but that doesn't mean that these simple parts can't cause trouble.

2) LPG running is not only different ( some folk suggest closing the spark gap to allow the plugs to fire more easily, which they mightn't otherwise because firing voltages are a lot higher with LPG) but reading LPG spark plugs is different to reading Petrol (gasoline) spark plugs.

3) I don't discount the possibility that some contaminant (in either the LPG of the petrol) isn't causing weird deposits on the spark plugs that greatly reduce the service interval, (much as lead fouling used to in times past).

4) don't take anything for granted; just because a spark plug looks OK it doesn't mean that it is; some kind of test is a good idea. Sparking in air isn't proof that you will get sparking in the combustion chamber but it is at least a good start; in the end my 'duff plug' didn't even spark in air.

so there you have it; a sad tale of several frustrating weeks, apparently all due to slightly fouled spark plugs! Has anyone else seen these weird red deposits?

cheers

[hornhospital]

Excellently documented. Thank you for writing it up. Yes, plugs can fool you, but as simple as it is to change them, they should be the first thing changed when a miss presents itself.

[slofut]

Is that a commonly used plug in an m30? I know that on the jag xk engines, you can run any brand you want but regular old champions are considered the best performers and last longer than any new tech plug. Don't know if these motors are particular to plugs but I've always been told to run old school NGK's.

Hey Brucey, you know I just did a similar brainfart with a bad dist cap, so now I don't feel quite so foolish!

[Brucey]

Is that a commonly used plug in an m30? I know that on the jag xk engines, you can run any brand you want but regular old champions are considered the best performers and last longer than any new tech plug. Don't know if these motors are particular to plugs but I've always been told to run old school NGK's.

these are old-school NGK's.... but better. Commonly used? Maybe not, even in standard form. But with six ground electrodes... no one but me has been daft enough to bother I think.

But my logic is thus;

1) NGK '6' is about the right heat rating for engines of this sort.
2) There is already plenty of resistance in the plug leads and rotor arm, no point in having yet more in the plug itself.
3) Nickel electrodes with a copper core is a known evil.

-so if you were to choose an old school NGK it would be BP6ES (non resistor) or perhaps BPR6ES (resistor).

4) Triple ground electrodes allow a longer wear life, so BP6ET is like BP6ES, but better.

5) Triple ground electrode design also allows the spark gap to be further into the combustion chamber (which mirrors the Bosch plug specified for the M30B35 engine which is a 'LR' designation plug). Until there is a clash of some kind a long reach plug is nearly always a better choice.

6) Bisecting each ground electrode reduces the wear life of the plug a little but allows even better access of the mixture to the flame kernel. It also increases the voltage stress in the spark gap which allows either easier firing (at the same gap) or a wider gap (for the same firing voltage).

Hey Brucey, you know I just did a similar brainfart with a bad dist cap, so now I don't feel quite so foolish!

I dunno if I'd call it a brain fart per se, the behaviour of this misfire was quite unlike any other I have had, as indeed the nature of the contamination may have been. I think that if you produced a spark plug reading chart for plugs that have come out of LPG fuelled engines, it would look a lot different to the ones that exist for petrol fuelled engines.

Take a look at the 'dirty' plugs; none of them look that bad to me and of course if they come out of an engine running LPG (and in contrast to petrol spark plugs) they look dry even if they are not firing correctly. All very misleading, certainly to me.

cheers

[Brucey]

.... Yes, plugs can fool you, but as simple as it is to change them, they should be the first thing changed when a miss presents itself.

I have mixed views on this. On the one hand swapping the spark plugs out nearly always makes things better, not worse, but on the other hand...

1) if a spark plug goes bad this is nearly always a symptom of 'something else'. Treating symptoms whilst leaving root causes unidentified doesn't sit well with me.

2) changing the plugs in an M30 ought only be moderately awkward (in the grand scheme of things) but in a post 9/86 RHD E24 the brake MC/servo obstructs access to the rear three plugs; doing those is a PITA. In addition to that, the last few times I've removed plugs from this particular engine several have threatened to seize up and strip the threads in the head; I've had to use penetrating oil on them and it has all been rather fraught to say the least.

So given this and that plug fouling has been (for me, with these plugs/this engine)

a) incredibly rare and
b) normally of the sort that 'burns off' in hard use

and the plugs didn't look that bad to me, you can understand that it wasn't top of my 'usual suspects' list. The first time it misfired on a cold start (running on petrol) I was immediately able to diagnose a wet-fouled spark plug; (until then it had started and run OK from cold on petrol, and had only started to misfire at all once it was warmed up).

So prior to that I'd only had the chance to observe a dry plug with light deposits on it; not an obvious candidate for a misfire IME. In fact when I tested for spark on the duff plug (in air) I had to swap between a good plug and the duff one several times before I believed what I was seeing, i.e. no spark.

cheers

[Brucey]

well I don't like unsolved mysteries; they are the sorts of things that build in unreliability IMHO. So I carried out a few more tests;

1) I pulled the new spark plugs after a few tens of miles to look at them;

shiny happy spark plugs...

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1-6, R to L. I'm happy with that look; the plating on the ground electrodes is burning off nicely, and the slight discolouration on the #5 plug is (I think) due to penetrating oil residue running down the cylinder head threads.

2) having cleaned and regapped the old plugs I put them back in (into the same cylinders as before). I got a misfire on #1 as before. I pulled the #1 plug, looked at it, couldn't see anything wrong with it. I inspected it under a microscope, still couldn't see anything wrong with it. I tested it in air, and got what looked like normal sparks. I tried it again in the engine, still a misfire, where with any other plug, everything was fine.

I did various more tests including swapping the duff #1 plug into cylinder #2 when the engine was hot. I still got a misfire, this time on #2 of course.

I've never had a plug before where it misfired and I couldn't see a problem with it, so the fact that it wasn't obviously bad was troubling to me; despite everything I still had a lurking doubt that it was the spark plug; maybe I disturbed the plug leads each time and they were bad or something...?. Thus I inspected the plug (yet again) under a microscope, without any great hope of seeing anything. However this time, I did see something, which I think is my 'smoking gun';

a tell-tale plookie....

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If you look carefully, you can see a wee bleb on the side of the insulator nose. I think this is where the HT has punched a hole through the side of the insulator, melting it slightly. This definitely wasn't there looking like that before BTW; the final test must have made it more melty and obvious. Maybe there were conducting deposits on the insulator before, and it punched through and tracked then. On the clean plug it may have been doing something different to that, but still no good.

So at least I've found a real fault, which is something. However I don't fully understand why the plug failed like this. I can think of a few possible reasons;

1) deposits on the insulator degraded the insulation value (if so, what caused those deposits...?)
2) the plugs were worn (to about 1.2mm plug gap in some cases) thus stressing the plugs more than normal;
3) LPG requires a higher firing voltage, again requiring more voltage stress;
4) Maybe cylinder #1 is different to the others somehow.

I've run plug gaps that size before on this engine and it was fine, so it can't have been that alone. However several things can each make a contribution to a failure perhaps.

On point 4) I think this may be true; the cylinders don't work equally all the time on my engine, because the Fritz's manifold has an unusually long header pipe on the #1 cylinder. As it happens I think the manifold would work better if they were all that length.... but when I put the duff plug into cylinder #2 the misfire both sounded and felt less bad than when the misfire was in #1. Quite possibly at various throttle openings the cylinder pressures are not equal and the #1 cylinder -if it (say) fills better through improved scavenging- has higher pressure, and needs a higher voltage to fire the plug, more of the time.

Anyway, finding the fault in the spark plug makes me happier because at least it means I am less concerned about the rest of the HT system.

Has anyone else ever had a plug fail in this way?

cheers

[hornhospital]

Yes, almost identically, although with a single-ground-leg plug. I very seldom disagree with you, but I still say plugs should be #1 on the list of things to change when you get an unexplained miss. I've seen too many of them, even new ones, go bad for no apparent reason, and replacing them cured the miss permanently. If it's "covering up" some other fault, that will make itself evident soon enough. By replacing them, you eliminate that item from the "what's causing this" list, even temporarily.

[Brucey]

I've seen no end of bad spark plugs but I've never seen one that failed quite like this before.

Some engines I've had seem to consume spark plugs for a hobby, (and on those engines the plugs would be the first port of call) but IME previously, not this engine, and not these plugs; I've previously done far higher/harder mileage on these plugs with no problems of any kind.
I guess the LPG-ness fooled me appearance-wise but there is probably something else going on too, I'd have said.

In the bad old days (leaded petrol, carburettors etc) you'd be lucky to get 6000 miles out of a set of spark plugs; the lead would erode the electrodes at high speed and if the heat rating of the plug or the carburation wasn't quite right the nose could glaze up and soon cause a misfire, even if the plug didn't foul up with some other kind of crud. But since decent fuel injection, unleaded petrol, and low ash oils have been the norm, IME spark plug problems have virtually disappeared on most road car engines; they have pretty much gone from being the least reliable part of the ignition system to the most reliable part of the ignition system, with an expected service life of many tens of thousands of miles in an otherwise healthy engine.

I guess there must be the occasional spark plug that is no good from new, but these weren't like that; they were only part way through their expected service life; apart from slightly higher wear rate and the weird deposit colour, (which I'd -rightly or wrongly- attributed to the LPG fuel) they had been behaving 'normally' over the previous few thousand miles.

I guess only time will tell if there is something else going on here or not...

If anyone has any pictures of their failed spark plugs I'd be interested to see them.

cheers

[Brucey]

last year I had no misfire issues and when the old girl fired up promptly this spring I was looking forward to plenty of happy miles.

For various reasons I didn't drive the car far until about two weeks ago. Guess what.... another ruddy misfire.....

At first a misfire running on LPG only and then a misfire (when hot) on either petrol or LPG.

After a short period of messing about it turned out to be more failing spark plugs. Weirdly I'd reinstalled the residue of the set that had one fail in 2016 and they have been fine all last year..... but one or more of these plus has failed in 2018. The two 'new' plugs that have been in the last two years still look like (and work like) new. There was little or no sign of the weird red deposits as seen upthread.

If anyone has any idea why these plugs can be OK for ages and then fail shortly after a winter lay-up, I'm all ears....

BTW I stocked up years ago and the 'new' plugs are the same type and from the same batch as the old (failing) plugs. In other engines these plugs (same type and from the same batch) have managed 80000 miles without trouble.

Q is it possible for petrol to go bad over the winter in such a way as it causes spark plugs to fail?

cheers

[wattsmonkey]

Spare set of plugs now in boot of M!