Would misfire fail an MOT test?

Answer 1

If it's a constant misfire, it will. The vehicle would not be declared roadworthy.

Here's a few suggestions you may like to try.

Check the distributor cap for cracks. Moisture can get in and interfere with the spark path.

While the distributor cap is off, check the rotor arm. It may be sooty or oily and need cleaning with a dry kitchen tissue and perhaps some very, very fine emery paper (1000 to 1200 wet & dry) - don't use coarse sandpaper or a file.

Check the HT leads. you may find one of them is damaged, maybe trapped between the bonnet and a high point on the engine or worn - crackling appearance on really old leads.

Check all leads are pushed home fully as there may not be a proper circuit.

Check the plugs are set correctly and they are not worn. you can usually find a link to a website where the plugs are described. The plugs should usually be a light to medium sandy appearance at the tip. Any oil or soot indicates a fault which should be dealt with. Oil may mean that a piston oil ring is faulty or broken - usually three but they all need to work (major job - trained mechanic). Soot may indicate that the fuel mixture is too rich (carburettor needs adjusted - mechanic or good DIY motorist medium work). Worn gap or crack in the plug means it must be replaced - best to replace the whole set at once (minor job - regular motorist).

You need a plug spanner and you need to be careful that you do not cross the threads.

If you are in any doubt, consult a qualified mechanic and allow them to do the work for you. Mechanics can charge between £20 (local) and £100 (dealer) per hour (or part) for their time. Will take less than one hour to change plugs and leads and rotor arm and distributor cap.

Depending on what parts you need, a local Motor Factors (yellow pages - Motor Factors) will have the parts you need at a fraction of dealership prices. Also visit Halfords who will fit most parts for a small fee - typically a few pounds.

Set of HT leads, £20+ [includes one for each cylinder and one for HT coil]
Single HT lead, £5+
Distributor cap, £15+
Rotor Arm, £5+

I am not a mechanic but I have fixed many misfires by following these guidelines. One time, a car was with the dealership for an hour and the mechanics could not find the fault. When I lifted the bonnet, I saw the damaged leads, replaced them and there was no more problem. (Dealership leads, £45, not in stock. Motor factors, £25, in stock) 10 minutes to replace. Do them one at a time. They are usually numbered or they are different lengths that only fit one way. Still, do them one at a time.