A Previa OEM Muffler Autopsy

[man_btc]

I have an 1992 All-Trac whose non-whistling muffler was replaced with a Bosal in 2013 after 19 New England winters (2 winters not driven in a garage while head gasket, etc. got (eventually) fixed - another story). When that Bosal rotted out a year ago, I had just acquired a '91 RWD from Phoenix not deemed reliable enough for a return trip from Massachusetts and left with relatives to sell on Craigslist not far from me. Mechanically on its last legs, but a miraculous find of rust-free metal around here.

I was excited to see it had an OEM muffler which I installed on my '92. It whistled on acceleration 2 minutes into my test drive. Resigned to another Bosal, I decided to do an "autopsy" once it was replaced. The results were posted on the Yahoo! Previa site, but I though they be of interest so I'm reposting here.

It had some rust along the pipe leading from the cat to the muffler, but otherwise looked perfectly fine to me inside and out. No sign of a high-flow-activated "door" anywhere - all the pipes were clear inside. There was a small pipe (internal drain?) on the side of the pipe just before the tailpipe exit, which was partially blocked. Just for grins, I blew air through the pipes with a leaf blower before and after clearing it - actually got a whistle whether it was open or blocked. In fact, blowing air through any of the internal baffled sections created a whistle, so that experiment didn't seem to obviously prove anything.

Given the lack of an obvious defect that might be the source of the whistle, maybe it's reasonable to assume that the design itself depends too much on a narrow range of construction, and that variations in assembly or metal deformation over time are the cause? I certainly don't have the answer.

As to age, most likely from 1991 with 436K miles (on the odo) if it was never replaced, but I have no maintenance history.

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[kamesama980]

OE muffler design can be fascinating. Interesting how much damping can be done in just a few chambers. I have a 95 SC RWD I may do similar with on a lazy afternoon. I have some motorcycle muffler cutaways that are similar: 3-4 chambers with pipes between.

[pdgizwiz]

Wow, I've thought of doing something like this, but never got around to it. Since you've gone to the trouble and still couldn't find the source of the whistle, I'm glad I never tried. Thanks for posting your findings!
I put a Bosal on a '94 and got rid of the whistle, but didn't like the extra noise. Later, my "keeper" '97 started whistling, and I splurged for an OEM muffler. This solved both problems but left a gaping wound in my bank account. The same van has an after-market front exhaust section that started rattling two years in.

[man_btc]

The only thing I could think of as a cause might be that small "drain" or pressure-equalization whozit or whatever it is just shy of the tailpipe exit. Perhaps as it becomes occluded, it allows some high frequencies to resonate there? I suppose an experiment anyone with a "whistler" could do would be to somehow thread a bendable wire into the tailpipe and down the 90-degree angle into that thing to clear it, and see if the whistle disappeared. Nothing I could try with it torn apart, and as I mentioned, just forcing air in with a leaf blower (and getting a whistle) wasn't a controlled experiment. It's not like the airflow is normally anything approaching that.

[tfreed1]

My original muffler lasted 18 years and I'm on after market muffler number 3 since then. I did not have the whistle problem but the drain pipe looks like a good place to investigate. I wish the original mufflers were still available. Toyota still has the mufflers for my 87 Cressida and I am thinking of getting one before they are all gone.