Classic ignition problems

Does it have individual coil packs, or a classic coil and distributor?

If it's misfiring on one cylinder that suggests the plug/coil on that cylinder. If it has a classic distributor then suggests that or the HT leads.

Or anything perhaps related to moisture in the air intake?

Theo

A few points.

Older plug leads (carbon string) can fail. Examine them carefully for signs of cracking. Measure their resistance (they are so much per foot, so will vary by length) But a faulty one will be very much higher per foot. Don't forget to check the king lead too.

While the leads are off, look for signs of corrosion where they plug in.

If replacing, get a decent quality conductive plastic type. Bosch, for example. No need to buy expensive snake oil ones.

Examine the cap and rotor arm for damage and tracking. And note many replacements are pretty poor quality. Same applied to the ignition amp, if it has one.

In the 'good ole days' I would typically pull leads off spark plugs to find the offending cylinder.

If random misfiring I would suspect mixture rather than ignition.

The latter.

I recall from somewhere it's possible to check the HT leads by watching the running engine in the dark with the hood up and check for any visible sparks around the leads. Someone else suggested spraying salt water over the leads while it's running AFTER having run long enough to idle smoothly. Not sure what you lot think about those ideas?

I gave you the method of checking them. If using a DVM is outside your pay grade just by new ones. ;-)

yes, but that's not going to show up any insulation breakdown, though. And you didn't say what values I should expect when I measure for resistance. I guess the reverse of the spraying salt water idea would be spraying with WD40. Use salt water if the air is dry and warm when you test; use WD40 when it's cold and damp. In the first instance, listen for misfiring when you spray and in the second, listen for any misfiring clearing up when you spray.

You do that by a careful visual examination. Any tracking will show. And you also need to check the connections between the leads and cap etc for corrosion.

Because not all makes are the same. Somewhere about 5000 ohms per foot. Total, including king lead and carbon brush of the order of 30k. But the idea is to find the faulty one or ones. Which will read very much higher per foot than the rest.

If you want to invent and test a new method, carry on. Let us know how it succeeds.

I think you get the impression DR is not impressed with your idea. The idea of spraying salt water over high voltage electrics sound perverse.

The stock way to check for a faulty plug or HT lead is to manually remove them one at a time. Ideally using decent dry gloves. If you hear the 'tick' from the spark jumping the gap you know it's the plug or something more serious.

If you can't decipher which cylinder it is then I would pursue the mixture/carburation route.

Salt water is not a good idea. Nor is WD40 or any other can filled with propane if there is any sign of external sparks.

My car was left at Heysham docks for a long weekend when I went to the Manx GP with a friend. Got back to it and it wouldn't start, everything under the bonnet was damp. Towed it out off the docks using the van. Eventually it started firing on one or 2. Popped the bonnet and my mate sprayed the sparks with WD40. Though the gap at the back of the bonnet I saw the look of surprise his face as the can of WD40 turned into a veritable flame thrower. The flame was about 6ft long and lit up the night really well.

A good lesson for all.

I recall when cans were filled with CFCs, you didn't have the problem of flammability.

Friend was carrying out a compression check on an early injection engine. Which runs the fuel pump with the starter motor. Dizzy ignition. With all the plugs out, a spark occurred somewhere and ignited the mixture being pumped out of the spark plug holes. He's in the US, and it was 'his' mechanic carrying out the test.

That's the bit I needed to know; many thanks.

The salt water spray just seems kind of wrong for obvious reasons. However, it seems to be regularly promoted on US websites. My variation was intended to be a kinder version of it, but unfortunately I can't use it on my left 'ooker cos the HT leads run through grounded metal sheaths for most of their routing and access to get a can of WD40 in there is impossible.

I've certainly done the "lightning" check successfully in the dim and distant past on British 4 cylinder cars. I think salt water would be asking for trouble, but WD40 makes more sense. You could also often detect leakage by touching a lead or plug cap.

The classic way to find which cylinder was not firing (e.g. because of a fouled plug) was to remove the plug caps one at a time and see which one did *not* cause the tickover to collapse. Usually best to do this by unclipping the plug caps and letting them rest on top of the plugs.

Of course if you go right back to the days *before* plug caps, where each plug had an exposed screw terminal, you could short these out one at a time with a screwdriver resting on the block, or with an "earthed" screwdriver.

I'd be looking for problems where the leads enter and exit the trunking.

I know it was once common to run the HT leads in earthed metal trunking, but to me, this is simply asking for trouble. The normal way is to keep them a reasonable distance clear of any grounds, by the use of plastic spacers, etc. Even more so if the ignition has been upgraded to produce a better spark.

I fully agree. It does seem a daft idea just to keep things neat under the bonnet and like you say, there will be stresses at the entry and exit points which over time could lead to insulation breakdown I would imagine - and right up against grounded metal to boot.

That was NOT my idea, though! I came up with the WD40 variant (which I"ve since binned as a result of the flamethrower post).

Indeed, yes. Another brainwave I've come up with is to record the sound from the exhaust. The problem with V8 engines is that it's possible to have them running on only 7 cylinders without knowing it. So I thought about setting up a laptop running Audacity (or whatever other you please sound recording app) next to each of the exhausts in turn (2 in total in my case) and then view the graphical results expanded in the timeline so all the pulses are visible. In this way any regular or irregular loss of a cylinder would be easy to spot and hopefully trace to the cylinder responsible. The addition of a piece of tracing paper taped as a flap over the exhaust outlet would perhaps improve the sound level and crispen-up the pulses. Having said that, I do have a habit of coming up with overly complicated solutions with a less than stellar success rate. But hell, I'm not short of imagination!

The modern way seem to be to move to coil packs and do away with the distributor and HT leads. If it was my car I?d be researching the cost of converting to this.

Tim

On a V8 that can get expensive! There might be limited room above the plug too.