Ross82nd

Hello everyone, I have a 68 Oldsmobile 442 with a 79 403 sitting in it. I purchased the car as is and it developed a bad exhaust manifold leak and a valve cover leak so I replaced both gaskets of each and I decided to replace the plugs while I was at it. Before I did this work the engine had a decent tick on the driver side but that is the side of the exhaust leak. Now that I have done this work the engine seems to be misfiring. it idles ok but every few seconds the engine does a big shake and it does not like to go when put into drive it runs rough and shuts off. I figured I may have to replace the plug wires because I did not know how old they were so I did that. The engine is running smoothe except for every cycle it sounds like its misfiring or just not running right. The tick appears to no longer be an issue . Anyone have any suggestions on what I'm looking at?

oldcutlass

Make sure all of your spark plug boots are seated right.

Sugar Bear

Check the firing order.

joe_padavano

I moved the thread because the 403 is a small block.

If the problem started when you replaced the plugs, swapped plug wires would definitely be the first place to look. Also check that you didn't accidentally crack an insulator installing one of the new plugs. Ask my how I know this...

oldcutlass

And sometimes a new plug can be bad right out of the box. I always keep 1 set of old plugs, wires, (points, condenser if applicable), cap, and rotor around for testing or temporary fixes.

joe_padavano

Also a good point.

The best way to approach troubleshooting is to first verify the plug wires are correct. Once you do that, connect a tach and with the engine at idle, pull one plug wire at a time. Obviously be careful of the spark - despite the low current, it still stings. I the RPM drops, replace that wire and move on to the next one. If the RPM stays the same, you've found the problem child. If you do find one bad cylinder, the first thing to do is to swap that plug with one in a good cylinder. If the miss moves to the new cylinder, it's a bad plug. If it stays on the original cylinder, it's a wire or internal engine problem.

Ross82nd

Thank you for this info. I put back in the origional plugs before I replaced all the wires. It runs smoother but there is still what I think is a misfire that continually happens. I've checked the firing order online and when I installed the new wires double checked. Tom I will try this test approach. Thank you

Ross82nd

Thank you all for the responses. I'll see what tomorro brings and be back for more! I also have a 69 455 sitting in a donor next to it that has supposively been rebuilt that I'm seriously considering making the switch. But I do not have the space, tools or time to do this so I'd have to pay someone. Anyone know someone in northern Illnois who is good at this type of work?