Noise from exhaust in 78 Chinook

[george101]

Hello,

I was wondering if anyone with a 78 Chinook has ever had a funny noise coming from their exhaust.

It only happens when I take my foot off of the accelerator going at high speeds (upwards of 40 mph). It sounds like something strange is going on inside my exhaust. It makes this "wuuuuZUP thud" noise. It's pretty weird.

I wasn't sure if it might have something to do with either the muffler or the catalytic converter. Does anyone even know if 78 Chinooks had cats?

Thanks,

George

[Excalibur]

I have a '78 chinook newport, and it does not have cats. Under my hood is a sticker that says "Non-Catalyst" indicating it did not use a cat for emissions.

It does have an elaborate air pump setup for emissions.

If your rig was originally sold in California, that could be different, however. It might indeed have cats. The easy way is to look at your exhaust pipe. It will have a muffler inline, anything else bulging (usually smaller than a muffler) and inline with the pipe is a cat.

Without hearing it in person, it sounds like you might have a slight backfire. How long since its last tuneup? Running a bit rich with the timing off a bit can cause that.

One other thought is a rusting out muffler, with a baffle inside coming loose that moves when the pressure from the engine changing. It would do this also in the driveway - run it up to 2500-3000 rpm, hold it for a 5 or so seconds, and then remove your foot from the gas. It might still do it.

Edited July 22, 2010 by Excalibur

[george101]

hmm... Interesting. I'll have to check to see if my Chinook is a non-cat. Thanks for the tip about the sticker.

I'm not really sure if it was ever in Cali or not. It has a sticker from Oregon on the bumper, so it was apparently around the West Coast at some point. I picked it up from Minnesota from a Turkey hunter. He said a lady owned it before him from Minnesota.

Ya, I think I'll climb underneath and check to see if it needs a new muffler and if it might have had a cat installed. This whole air pump system thing is new to me but it sounds kind of interesting. It would probably be a pretty big pain to need to replace that thing I'd imagine. Not sure if you could just get it at an auto store, or need to get at auto parts store.

I wonder if my rig is backfiring. Maybe I should get it looked at. Probably wouldn't hurt. Not sure if that could mess up the engine or something.

Cool - well thanks for the tips man! I'll check it out and let you know what I find.

[Maineah]

If it has an air pump and the diverter valve is not working it will backfire like crazy! Generally by this time the pumps have locked up and been taken off but if it's still there the diverter valve shuts off the air flow to the exhaust when you let off the gas it is vacuum controlled so it may just be nothing more then a broken vacuum line. The valves do make a noise when they bypass the exhaust because it just blows air out of the valve instead of into the manifold.

[george101]

Thanks Maineah. It sounds like the vacuum line could be the likely culprit then. I'll have to look into that. So is the vacuum line found within the air pump or does it just connect to the air pump somehow?

[Maineah]

Thanks Maineah. It sounds like the vacuum line could be the likely culprit then. I'll have to look into that. So is the vacuum line found within the air pump or does it just connect to the air pump somehow?

There should be a diagram under the hood of the vacuum lines if not the valve will have a small hose it should have vacuum with the engine running at idle you should be able to feel it with your finger.