My car as of today has adopted a misfire very slight under load it will then clear and be fine, no starting problems and idels fine with revs maintaining ok when this misfire occurs so I dont think it is the MAF I have all ready got it booked into my garage for Monday whom I bought the car from, I am not overly concerned as it has a FSH and actually due its next service in February and has been throughly reliable I was just wondering if any of you had experienced this before? I have had similar issues on previous cars I have owned but they all differ. any input would be great. Thanks
On assorted other cars of mine over the decades it has usually been either a spark-plug going high resistance or its lead going open-circuit. The real problem with two of those cars came when the coil tracked across because the spark could not get to the spark gap, and this then damaged the coil such that the problem did not go away completely after the lead/plug had been fixed. Run the engine in the dark and look for a spark happening where it should not!
Also with older cars, where the ECU mapping cannot cope with modern cheapest lo-octane bio-ethanol fuels, they can be difficult to start, run rough at low revs, and miss and be hesitant when accelerating. My old Vectra B shopping trolley was cured of this ECU problem by simply feeding her V-Power, and fuel economy immediately improved from ~32mpg to ~38mpg, actually saving money on petrol.
Most likely plug leads and if its not improved then coil.MX5 ,s give a lot of coil troubles but we have several different types of coils for sale depending on model/year /eng size etc.New plug lead set may only cost approx 20 pounds so worth changing unless you know they have been changed before.
This time of year condensation builds up inside the spark plug holes. Last year I found a good quantity of water at the bottom of the plug holes which was causing the missfire. Just blew the water out the plug holes with compressed air and dried the plug leads and all was well.
As Mozza mentioned try basic first, water in the spark plug chamber, then if that fails test for a strong spark to each plug. If that fails test the leads, then last but not least the good old coil pack. even check the plug to the coil pack is on correctly, good luck and happy motoring.
Just to say thanks for all of your input, car was ok driving to the garage that morning but for peace of mind the owner ran a diag test on it just in case any codes had been logged by the ecu he explaned that any problem that is logged would not always bring up any engine warning light for sometime until the fault had been logged for a while. All of that came back ok with no faults, they checked the spark plugs, and spark plug chambers for moisture nothing found and of course the HT leads and then as I mentioned the coil pack however nothing found so they re greased with points etc with silicone grease car seems to fine now It is due for a service in February with them so will monitor it from now.
Misfire under load is often weak spark as it takes more voltage to jump the gap at higher combustion pressures (why turbo cars use a smaller plug gap). If the misfire is bad, 2 or more cylinders, suspect a coil pack or consider another problem such as fuelling. If it seems like just one cyl, suspect a plug or lead. For what it costs I’d change plugs then HT leads then coil pack. They are very difficult to test other than by substitution. We had a similar misfire recently and it turned out to be a brand new NGK which had a split lengthways along the ceramic, fine till it was under load.
Thanks, sparks were checked the grease was applied to stop any rouge sparks from coming out of any gaps as I was told the coil pack can be a problem so this was done to contain any sparks where they should be. They roadtested the car and it didnt miss a beat all day I will leave it for now as I have confidence in the dealrship two mazda trained mechanics one of whom ive known since primary school and the owner who happens to be a mechnic and mot tester very rare for a salesman/owner.