Wierd muffled exhaust noise

[Ricardo Miramontes]

I need your help! It's the second time this has happened in a span of about four months. I have a 1998 4x4 XLT SOHC with 85,000 miles on it and regularly scheduled fluid changes.

The first case:
It started developing on start up only a muffled exhaust noise, as if it was clogged or there was a re-direction of the exhaust (not too sure how to explain the noise). After about two months of this noise only on start up it finally gave up, on my way home from school, one or more of the pistons was mis-firing. The next day I took it to the shop and they found that one of the spark plugs was dirty with oil and another was kind of dirty but not too bad. They replaced all plugs and wires and sent me on my way, I was happy, the engine sounded great once more.

The second case:
About four days ago I started noticing the same muffled exhaust. This time around I didn't let it give up on me completely. it's in the shop at the moment and I haven't heard from them yet.

I'm wondering what may be getting my plugs dirty if that's what's wrong this time around. I hope this isn't an expensive fix.

Thanks in advance.

 

[Alec]

Sounds to me like they didn't replace the cause, they replaced the symptom. If your plugs are bad again, you might be getting blow-by in your pistons or have some loose gaskets, seals, or bolts. But oil on the plugs is never good

 

[Ricardo Miramontes]

Well, I picked it up after work yesterday and the mechanic (a family friend) said he could not detect anything. He said he even took out every plug and tested it for spark and they were all clean and in working order. He tried getting any possible codes but none were present. After I turned on the car for him and pointed out the noise he crouched under the car and inspected the muffler and the pipes. Apparently there is a hole in the pipe exiting the muffler. Could a hole the size of a quarter make such a noise?

 

[Ricardo Miramontes]

Alec, what is blow by in the pistons?

 

[Alec]

A hole in the exhaust can definately cause weird noises and performance problems. I didn't suggest it cause that answer seemed too easy after being in the shop twice. Guess they missed it.

Piston blowby is when oil in the bottom of the engine gets past the piston rings and into the combustion chamber. Basically passing the piston heads.

 

[Ricardo Miramontes]

Do you think it would be a good idea to take the truck into Midas and have them help me figure out whether it's the exhaust making that noise or it's something else?
One thing I have discovered is that the noise only occurs on start up and after the idle come back down to normal, the noise seems to go away until I rev it up and let go of the gas and as the rpm's start to decrease the noise appears again. But the noise is not there while the rpm's are high.

Thanks for your help Alec!