Dear Tom and Ray:
My husband and I changed the oil, spark plugs, wires, cap and rotor on our '92 Chevy Blazer yesterday -- something we've done many times with no problem. When trying to restart the car, the engine literally exploded, flames came up out of the throttle body, smoke came out of the valve covers, and seals were broken. What could possibly have gone wrong -- and should we donate or scrap our car? Many sincere thanks! -- Jackie
TOM: This is why we always keep marshmallows on hand at the garage.
RAY: You made a very simple mistake, Jackie, but as you now know, it led to some very flammable repercussions. You reattached the spark-plug wires incorrectly.
TOM: The spark plugs fire one at a time. They're all timed precisely, so they fire at exactly the right moment the spark is needed in that cylinder. If you put the wire for, say, spark plug 1 onto the top of spark plug 4, spark plug 4 is now going to fire at the wrong time. And if it fires when the intake valve is open, it could set fire to the fuel-and-air mixture that's waiting in the intake manifold and the throttle body. This will lead to a distinctive sound, best described as a "phooomph," followed by a nice, Christmassy yule scene under the hood.
RAY: The reason the explosion continued into the valve covers is because you have a faulty PCV valve. That's a check valve that's supposed to prevent flames from a backfire from traveling into the valve-cover area.
TOM: So, next time you work on the car, in addition to making sure you label the spark-plug wires very carefully, add a PCV valve to the things you need to replace.
RAY: The good news is that you probably didn't do any serious damage to the engine. You might need a valve cover gasket or two if you blew out yours. And in the worst-case scenario, if you put all the wires on wrong, and then cranked the engine for a long time, you could have created an explosion in the crankcase, which could have blown out a crankshaft seal. But even that's easy to replace -- at least the front one is. The rear one's also easy to replace, once you have the engine out!
TOM: So, start by hooking up the wires correctly. You'll have to find the proper firing order for your particular engine so you can get the engine running again. Then, after that, you can assess the long-term damage to your car -- and your self-confidence.