Exhaust has a "missing" like sound

[Cool67]

I have a stock 67' mustang, 289, 4V, C4 with about 10K miles on the rebuilt engine. When the engine is warmed up, running and in park, every so often, the exhaust has a poompf sound, kind of like a miss. It sounds like it's running pretty smoothly and then I hear a poompf sound once. It runs smoothly again and another poompf sound. The car runs good otherwise. Does anyone know what may be causing this sound, and how to verify what the cause is?

 

[D.Hearne]

Might have a plug that's fouled more than the rest and it skips a beat now and then. Or a bad plug wire. Hard to say.

 

[Cool67]

Might have a plug that's fouled more than the rest and it skips a beat now and then. Or a bad plug wire. Hard to say.

I recently put in new plugs and new wires and and still has the same sound. Thanks for the reply.

 

[NasaGT]

I had a very small exhaust leak on my 02 GT that gave the exhaust the same sound. I tightened up the catback a little bit and the 'poompf' sound went away.

 

[Cool67]

I had a very small exhaust leak on my 02 GT that gave the exhaust the same sound. I tightened up the catback a little bit and the 'poompf' sound went away.

Pardon the ignorance but what's a "catback"?

 

[NasaGT]

lol - that's the part of the exhaust that is aft of the catalytic converters. Hence the name cat-back. Since these classics didn't have catalytic converters, thats not a term used much in this section.

 

[badazz68stang]

you might want to check the firing order and make sure u put the new wires back on the cap correctly because it sounds like you have a dead cylinder. im pretty sure the 289/302 order is 15426378.

 

[stangonline]

You probably have a stock cam? What you are hearing is the normal sound of exhaust scavenging. If the car runs fine otherwise and is not down on power, I wouldnt worry too much about it.

Jeff

 

[mustangdave]

Do a search on "poomph" J/K. Are you sure the plugs are gapped correctly; I never trust them out of the box. Also check for loose plug wires, worn dizzy cap or rotor, and as mentioned, an exhaust leak. There is also a small chance that there is some trash in the fuel system that is clogging the filter momentarily and then letting the flow resume.

 

[Cool67]

lol - that's the part of the exhaust that is aft of the catalytic converters. Hence the name cat-back. Since these classics didn't have catalytic converters, thats not a term used much in this section.

OK. Thanks for the explanation.

 

[Cool67]

You probably have a stock cam? What you are hearing is the normal sound of exhaust scavenging. If the car runs fine otherwise and is not down on power, I wouldnt worry too much about it.

Jeff

Jeff,

Yes, I do have a stock cam. The car runs fine and the power is OK. I'll research "exhaust scavenging" a little more to find out exactly what it is. Thanks for your input.

 

[Cool67]

you might want to check the firing order and make sure u put the new wires back on the cap correctly because it sounds like you have a dead cylinder. im pretty sure the 289/302 order is 15426378.

I think the wires are on OK. I changed them one at a time so as not to mix them up. Thanks for your input.

 

[Cool67]

Do a search on "poomph" J/K. Are you sure the plugs are gapped correctly; I never trust them out of the box. Also check for loose plug wires, worn dizzy cap or rotor, and as mentioned, an exhaust leak. There is also a small chance that there is some trash in the fuel system that is clogging the filter momentarily and then letting the flow resume.

I gapped the new plugs to 0.34 and I also have a Petronix Ignitor module in the stock distributor. I did tighten the bolts on the exhaust manifold but it still makes the sound once in a while. Thanks for your input.

 

[Jester67]

My car will do the same thing from time to time on the passenger side bank but to date I have not located the cause. If it is terminal I guess I will let you know what it is when she lets goes.

 

[thehueypilot]

You probably have a stock cam? What you are hearing is the normal sound of exhaust scavenging. If the car runs fine otherwise and is not down on power, I wouldnt worry too much about it.

Jeff

I agree with Jeff. There is no comparision between todays smooth running, fuel injected cars and our 40 year old cars that miss, spit and rock. A carbed car will never idle as smooth as fuel injection cars do.

 

[Cool67]

My car will do the same thing from time to time on the passenger side bank but to date I have not located the cause. If it is terminal I guess I will let you know what it is when she lets goes.

OK. Thanks for the input.

 

[Sodaholic]

well i have the same problem with my 67 cougar.. look closely along your exaust pipes and everything.. mine is caused by a crack where the pipe meets the driver's side muffler.. so everyonce in a while it will be in a possition where the exaust escapes and i get the noise. make sure to tighten your header to exaust thingies(still dumb dont know what they are) cause those could be moving around and letting exaust out everyonce in a while aswell..