"Popcorn" rattle in exhaust

[StephenSanchez]

Hi All,

Looking for any feedback on this. Here's the story: Have a 2000 XLT 4.0 SOHC with about 40,000K original. Picked it up from an old lady in PA, her son maintained the truck and despite running an oil heating business ironically he only changed the oil every year and half simply going on the number of miles the oil had. The oil was old when I got it despite the truck being in great shape I expected some sludge buildup in the motor. After a few oil changes I got a tick/knock on cold starts. (The truck already had a distinct vibration but only while idling in drive, otherwise ran perfectly.) Figured the knock was due to the sludge and the vibration due to carbon buildup. Instead of going for a motor flush thought I'd give it Seafoam through the brake booster. Got a lot of smoke and it reduced the knock and the vibration in drive lessened, but afterwards it developed an intermittent popcorn rattling sound on deceleration and in idle which seems to be coming from the catalytic converter. The truck also feels sluggish. I've read elsewhere that the honeycomb structure inside the cat can break up and that's what causes the rattling sound. My questions are 1) did the seafoam do this? 2) is it dangerous to drive around with the cat in this condition? 3) Is the Magnaflow cat/exhaust set up the best choice for this truck or Flowmaster? 4) Would I be crazy to run another seafoam treatment? I'm assuming I'm going to have to change the cat anyway.

I'm an amateur car guy, thanks for any help!

 

[Flash]

Lie under it and hit the cat with your fist to see if it rattles.

It might be possible that the seafoam dislodged a big chunk of carbon that's sitting in the cat banging on the monolith.

The only way to check the cat is to disconnect it and look at both ends.

 

[2000StreetRod]

Sea Foam is flammable

According to the Sea Foam Material Safety Data Sheet it is a flammable liquid and vapor. Adding it to your intake system richened the air/fuel mixture which probably increased the temperature of your catalytic converters. Depending on how rich and for how long the Sea Foam may have damaged the converter. The only time I have used Sea Form was to loosen stuck rings in my Volvo engine that was parked in a field for more than a year by the previous owner. I removed the spark plugs, cranked the starter and dropped a small amount of Sea Foam into the cylinders every day for 5 days. When the compression increased to normal I drained and replaced the engine oil. I never started and ran the engine with Sea Foam in it, nor will I do that to any engine. I only use Techron fuel injector cleaner in my fuel systems and have for more than 30 years. The exception is my current Explorer Sport when I added fuel stabilizer to the fuel tank because I knew it might take 6 months or more to install my supercharger.

 

[StephenSanchez]

Thanks Flash and StreetRod! I'll try knocking the cat to see if there's something in there. It was my first Seafoam and I did accidentally pour most of the can into the brake booster - it never stalled but I got distracted once the smoke began.

 

[StephenSanchez]

Update 7/18/14

Thought I'd post an update: I used Seafoam for the first time about a week ago (through the brake vacuum line), lots of smoke, the next couple of days the car was significantly sluggish (felt like I was driving through water) and there was something rattling around in the exhaust sounded like intermittent popcorn or little stones. Thought I had clogged the cat. Regular city driving since the seafoam treatment with a little time on the highway. Since yesterday the truck has been responding very well. Feels much lighter, running very smoothly, and I just touch the accelerator and it takes off like a bandit. Still some popcorn rattle but I'm happy with this.

 

[rb142]

Given the responsiveness of the vehicle now, it is unlikely that the cat is plugged. There could be rust or other debris rattling around in there. (You won't know unless you open it up and look.) Another thing is to check that it isn't a heat shield that is loose and rattling.