Misfiring under load - any ideas?

[marshman]

Not sure what those sparkplug testers actually achieve. Yes they will allow you to see them sparking in the open air, but they are not testing under actual operating conditions and more importantly when the engine is under load and voltages required to get a spark are much, much higher and breakdown of insulation is much more likely to occur. We already know the spark plugs "spark" ok under less stressful conditions as the engine revs freely without misfire when not underload.

[richardthestag]

Not sure what those sparkplug testers actually achieve. Yes they will allow you to see them sparking in the open air, but they are not testing under actual operating conditions and more importantly when the engine is under load and voltages required to get a spark are much, much higher and breakdown of insulation is much more likely to occur. We already know the spark plugs "spark" ok under less stressful conditions as the engine revs freely without misfire when not underload.

it is a good point, sometimes the same "underload" misfire will exist when under no load, however way less noticeable. I have found the tester to be very useful as it provides a continuous feed, admittedly not under load and especially without the silly plastic cover (remember not to drink battery acid folks) you can see and more importantly hear a miss beat from the plug.

And yes it has a switch to turn off for fitting and removing the plug.

Recently used to find a misbehaving plug that was causing a barely perceptible flutter at 1300 rpm. able to replicate the issue on the bench and actually see what was going on. Yes I could have done that on the engine but time = money and customers want me to be as efficient as I can be

On an entirely related note, Ian (Milo, formerly of this parish) shared with us a vintage spark tester that would simulate load conditions also. wish I have seen it before the hammer dropped at auction. Nice bit of kit that bucks the trend of the 21c throw away society to save the planet etc

[marshman]

Fair enough in a business sense, but in my opinion an extravagant purchase for your average car owner. To the unknowing could give the impression the plugs are OK when they are failing underload conditions.

Those vintage testers used compressed air to pressurise the the air around the electrode to about 150 psi to simulate the combustion chamber which is a better BUT it still will not simulate trying to ignite a rich mixture which requires a higher voltage to make the spark jump than a weak mixture.

My preferred approach when you have issues like that is to swap for a "known good part" that works fine on another vehicle. Not always possible and again not infallible. The danger there is sods law creeps in and you end up with two cars with "issues"

[MandM]

Mabel, my poor old abused Stag seems to be getting fed up. In the past couple of weeks she's started mis-firing under load - in other words, if I'm accelerating away from a junction, or up a hill and the selected gear is too high, the engine mis-fires, or hesitates. If I select a lower gear and the revs come up she pulls away beautifully. Other driving is normal - I've just returned along the M4 at 70mph, temperature steady at 92º (soak temperature no higher than 98º), I'm getting a reasonable 300 miles to the tankful, oil consumption slightly worse than I'd like but about a pint per 1,000 miles. She's due a 6,000 mile service next week so I'll no doubt find something slightly awry but I thought I'd ask....

I was going to say "answers on a postcard...." but they never arrive these days, so email or on here will be fine!

Hi Richard did you find the problem with a miss fire under load?

[mole42]

No so far Martin, I found a split in the rubber t-piece in the carburetter vent pipe and I’m awaiting a delivery of parts from Peter before I tackle it. I did 250 miles over the weekend without any trouble so it’s not too serious :-) I’ve just realised that I’m not working this weekend so there’s some time available. I’m trying to decide whether to tackle the slight coolant leak or just keep topping up for now, it’s either the pump cover or the inlet manifold, the coolant appears in the V…..