Engine misfire

had what seemed to be and exhaust leak and wasn’t so we decided that we should change the distributor cap and spark plugs and wires and now the engine is misfiring. We did everything correctly but don’t know the reason for the engine misfiring and don’t know why the car had symptoms of a exhaust leak and when taken to exhaust specialists they say there was no leak

What were the symptoms that you thought indicated an exhaust leak? Have you had the misfire codes read? (That may tell you which cylinder is misfiring or if it’s a multiple misfire).



Did you remove all of them then replace them one by one? 1929 Hupmobiles hate it when you do that. It’s way too easy to cross up the wires. It’s a very common mistake.

What? You don’t have a '29 Hupmobile? Well gee, what DO you have?

Misfiring is a symptom of an exhaust leak? An exhaust leak where, exactly?

Can you describe the symptoms of the misfiring? If it sounds like someone is rapping on the engine with a hammer, you’re screwed up the firing order. Refer to my first paragraph. If it’s just a random missing ignition, a random roughness, you probably didn’t connect one of the plug wires correctly, or maybe used an incompatible new distributor cap with your old rotor… they should always be changed as a set.

Oh, and did you adjust the spark timing after doing the work? If it’s a really old car, did you replace the points and condenser? How did you set the dwell?

To start, you should go over your work. Assume for starters that you crossed the wires. Work to get it all correct. If you confirm that you did NOT cross the wires, start by checking the ignition timing. Then the dwell (if the car has points). NOTE: if you have to set the dwell, you’ll have to set the timing again.

There’s also the possibility (even a highly LIKELY possibility) that the original problem has become more pronounced with the stronger spark from the new parts. But without even knowing what kind of car it is, it’s hard to guess what might be wrong.

I would also like to know why OP was so sure there was an exhaust leak . . .

Fumes entering the cabin?

Ticking noise from the exhaust manifold?

More importantly, I’d like to know what this mystery vehicle is

You really need to post the make, model, and engine of this vehicle. Seeing as how plug wires and a distributor are involved it could be assumed to be an older model.

If the vehicle had no miss before and does after the plugs and wires then you need to go back over the wire routing.

Maybe that tick you heard was not an exhaust leak at all but the sound of a spark jumping from a bad plug wire (possibly exacerbated by an old plug) to the cylinder head.

Did you replace the ignition rotor with the distributor cap? Those wear together so should be changed in pairs.

Did you use OEM parts or aftermarket?
Some aftermarket parts, especially cap and rotor, are of shoddy quality and cause more problems than they fix.

1979 corvette, 350 small block I48 chevy with edelbrock performer intake, Rochester carburetor, exhaust manifold to 2.5 straight pipes to super 44 muffler, msd hei distributor cap with 8.5mm msd wires @ok4450

I haven’t had the codes read yet, but I thought it was a leak because when it would be driven and would accelerate it make a cracking kinda of noise that would sound like it was coming from the exhaust but also would here some back pressure, we took it in and they fix the welds on the exhaust and said it wasn’t a leak said we needed a tune up so that’s when we started on the distributor cap and spark plugs and not its just misfiring

Check the spark plug wire location on the distributor cap to cylinders.

Firing order; 1-8-4-3-6-5-7-2 going clockwise.

You’re not likely to find any error codes on a 1979 as ECUs did not arrive until 1981. Does this thing still run the air injection pump?
What about the converter? I vaguely remember this era as having converters full of BBs instead of honeycomb substrate.

This is a bit of ancient history for me.

Can you hear the crackling noise with the hood up and is it louder with the top of the air cleaner off?
If so, the carburetor may need work as fuel overflow from the primary discharge tubes can dribble gas into the carburetor bores and it will sound like a string of firecrackers off in the distance.
With the engine idling and air cleaner off you can look down inside the carb bores and visually determine this. You should see NO gasoline dribbling.


We have a 1996 GMC Topkick in our fleet that uses a cat with those pellets

I imagine it was still acceptable, because it was big. Anything over 14000 lbs is not OBD2 compliant, so those vehicles have to meet very low standards. Maximum allowable emission values are VERY high. No downstream sensor required. Very basic evap system. Misfire NEVER turns on the mil. Less readiness monitors. And so on . . .