Exhaust Systems: How to kill noise?

[claybodie]

I finally realized that the incredible amount of noise my truck puts out is produced by the exhaust system, specifically the section behind the firewall and under the driver's feet.

I tried to find a leak and am so far unsuccessful. What should I do? Could the pipe be so old and rusted that it just doesn't contain the noise anymore? Should I replace that section?

Any suggestions?

It's a 1982 Datsun 720 King cab diesel

Any help would be appreciated,

Clayton

[asavage]

I find the stock exhaust, in good condition, to be very effective at squelching noise. Perhaps it's rubbing/hitting on part of the floor? That makes it very loud inside the cab.

[claybodie]

Yeah, it's definitely not hitting anything. I'm totally stumped on how the noise is getting out. I was also thinking of scrapping it and starting over.

[asavage]

I tried to find a leak and am so far unsuccessful. What should I do?

Get it idling, put your foot (or your hand with a rag) over the tailpipe. The exhaust system should pressurize. If it doesn't, you have a leak. In the above test, a small leak will exhibit as a hissing sound. A large leak may not sound any different when plugging the tailpipe.

Could the pipe be so old and rusted that it just doesn't contain the noise anymore?

Yes, but the noise will not come from that section, it will come from the back, where the exhaust exits. A quiet or noisy (bad) muffler doesn't make appreciable noise at the muffler, unless it has a hole.

[claybodie]

Thanks for the help.
I'm going to try plugging the end and see what happens.

The noise isn't coming from the back, where the noise exists - so based on what you said it has to have a leak.

If it's a leak, it's got to be a small one based on what I've looked at so far. Basically all the engine noise is coming from it though.

[asavage]

Basically all the engine noise is coming from it though.

Great! When you fix that problem, it'll be a silent diesel

One other thing to check: does the noise change in amplitude (loudness) when you press in the clutch? I mention that because several SD folks have noisy transmissions in neutral.

[ecomike]

Mine had a tiny leak at the donute in the exhaust manifold to tail pipe flange. Found it one cold day when it started and I was actually under the hood (someone else started it) and I saw some smoke leak out of the flange area for a few seconds until she stopped smoking. (A bad glow plug can be a useful diagnostic tool!, I since replaced all the glow plugs).

It had been like that for 4 years, ever since I bought it, so for four years I just thought the SD22 was that noisy! Sealed that leak and a pinhole on the top side of the pipe just past the fire wall and it almost sounds like a gas engine now! It also vibrates less, for some reason.

[claybodie]

This is pretty depressing considering how much money I've dropped into this thing already...

Ok, so I have been to one mechanic and a muffler shop. The muffler shop said there's no leak they can find. This is pretty obnoxious to hear, because they do all the diagnostics with a stationary vehicle, but the real noise comes out at higher RPM under during actual driving conditions.

It sure as hell sounds like an exhaust leak when I'm driving it, and it comes right through the hood and firewall. So far, the hair-brained ideas I've come up with to fix it include:

a) replacing the whole exhaust system to increase exhaust gas flow and see if that sends all the noise out the tailpipe

b) putting a resonator as close to the exhaust manifold as possible to see if I could kill the noise

c) insulating the bejesus out of the exhaust shielding and the firewall.

None of these options seem likely to help, and so I'm stuck here trying to decide if I'm going to sell this thing to save my ears.

Any thoughts? I can't imagine a diesel datsun sounding like a gas truck, but if they can that gives me a little hope. I don't even know where to start at this point, but I seriously need some direction.

I would be extremely grateful for any help.

Clayton

[asavage]

Clay:

I'm in McMinnville (Carlton, actually) all this weekend, and I have to travel to just south of Dallas sometime to pick up some Mazda parts for an acquaintance. If your travels bring you up anywhere near this way, I'd be happy to give you a third opinion. PM me for my cell.

[zen]

take that offer!!

(sd engines are not that noisey..cracked manifold opening up on load??)