My grandparents have a 91 explorer. they let some old man change the spark plugs and now its putt putting. they are on a fixed income and i know a little about cars and want to know what you guys think it might be.
I agree - pull the plugs, and make sure they are gapped correctly - depending on the engine size the gap is different.
Also check the spark plug wire routing.
All the wires go to certain plugs, if one is messed up then 2 are wrong, and this will cause a bad spark, and very poor sound, as well as bad mileage.
In other words, check the firing order of the spark plug wires. It's not real hard to do. You should be able to find some info at autozone.com about the firing order. If this is correct then I would be looking at the spark plug gap.
chech the gaps on the spark plugs,check that the HT are in the right order on the plugs too,he might have switched no2 lead with no3 lead and your miss firing coz of it,check fuel filter and air filter too