328 intermittent engine sound - any thoughts?

Any thoughts as to what this might be? Should be able to hear the abnormal sound here, which is most prominent at startup. At times it sounds like the butterflies on a motorcycle carb fluttering open at WOT with the airbox removed. It seems to subside as warms, but won't necessarily stop (though it's hard to hear on the road so not sure if it abates, though I think it does).

Can't really tell from which side it's originating, but can tell it's towards the firewall. To me, sounds from the driver side, to others, the right.

Doesn't seem to be AC-system related (no change with AC on/off), belt related (hit belts w.dressing). Doesn't sound as 'clicky' as symptoms of water pump going, but maybe that's it? Has no apparent affect on any operation - all gauges remain normal and constant.

[ame]http://www.youtube.com/watch?v=GJH83a8BS1E[/ame]

 

Try to listen with a mechanic stetescope at various places (camshafts,waterpump, etc.)

 

Rough counting puts the rate at ~80 counts in 10 seconds = 8 events/sec (and you say it lessens with increased temp), so I'm going with "exhaust leak":

1000 rev/min * 1 firing event per cylinder/2 revs * 1 min/60 sec = 8.3 events/sec

just a guess...

 

[QUOTE I'm going with "exhaust leak"QUOTE]

I agree with Steve on this one. I had a nearly identical sound on my 328 a while back and tracked it down to a corroded exhaust gas analyzer port. After examining all of the tubes, I found several corroded and ended up removing them and inserting stainlss plugs. During my major this last winter I also removed the AIR injection tubing and plugged those ports as well using Verell's plugs purchased from Ricambi America. While I didn't find any obvious leaks in the AIR injection tubing, there were several areas on the rear bank that looked suspect. Hope this helps.
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I agree with exhaust.

Ettore's post shows some significant culprits, and he did well fixing.

The exhaust flanges are areas too...rusted out donuts and bolts.

I just removed my cat on my 328 and installed a test pipe (didn't want fire during a loss of ignition event). Afterwards, I heard more leaks and afterfires (backfires in the exhaust...these were minor). Also, during the replacement, I had to shear off some of the bolts because of corrosion; this was not a difficult effort...they gave way (too) easily. Later, I will consider a full exhaust replacement. The only part that seems to have any real life left is the muffler.

Caveat: the cat can cover-up a lot of exhaust issues.

 

I'll second this - had a similar noise and it turned out that an exhaust sample tube had cracked. On looking closely, it turns out that the same thing had happened to the adjacent tube. The PO apparently tried to remove it to install a plug, but it looks like the connector crumbled so they just pinched the tube closed. That's what I've done as well until I can try and get the plugs installed.

A mechanics stethoscope was great to narrowing right down to where the sound was coming from, highly recommend getting one if you don't have one.

Good luck!

 

Is the AC belt plastic?
When mine has been sitting a while, it gets a horrible clack-clack sound, intermittent, which always goes when warm.
The mech (admittedly without actually hearing it) suggested the plastic belt, which "sets" in shape after a while, and slaps around until it heats up and softens.
That's my 2 cents

Patrick

 

I get some belt slap, that sound in the you tube is not belt slap, almost certainly an exhaust leak, makes sense it might diminish when things get hot and things expand and tightened up. Maybe a loose o2 sensor or cat overheat probe as two easy things to check? My bet is a failed check valve, one on each bank that is part of the egr iirc, mine rusted at the hose connection but was hard to spot visually, my stethoscope pinpointed it, and the metal of the valve had very bad corrosion and was held together by rust and leaking obviously once i new what was going on. These cross ref to a gm part, see the parts thread for my details i posted acouple years ago.

 

Re previous post, its the air injection check valve. They should be replaced as a service item, the exhaust is corrosive, but they are usually replaced when they fail, probably have a 40,000km life or so.

 

How do I get at the front bank to check for leak there? The leak (assuming it is that, which seems most likely) doesn't sound to be coming from the rear which seems pretty easy to access.

 

Removing the rear wheels and the rear wheel well liners should give some (limited) access from the sides.

 

Thanks Steve. Was afraid that would be the case. Was hoping to get at it before going to Lime Rock tomorrow...going to be a lot of furrowed brows as I roll through the grounds.

 

depending on where exactly you find a leak - and listening to the sound, I think you will -
it`ll be more or less impossible to seal/weld in situ...

in case you cannot see/reach it...to remove the whole front header, the a/c compressor, alternator and passenger side tank have to come out -
not really something you can do in an hour on a rainy sunday morning...see post 4

 

You could always remove the back deck to get to the front side of the engine if you need to do so. It is light in weight but the hardest part is getting it on properly aligned when reinstalling. If you choose to remove it, drill two small holes, say 1/8 inch a couple of inches apart near the bolts that secure the hinge mechanism to the hatch. Do this on both sides and obviously do it before you loosen bolts. This way a return to the exact same position is guaranteed when you reinstall the deck lid if you place 1/8 inch bits in the holes passing through both the hatch and swing arms.

I do it in my basement where I have a hoist that holds the deck hatch in position while I properly position the lid while inserting drill bits.

BTW, my 328 is making the same sound. I need to do the belts and a valve adjustment this fall and will tend to that matter while I'm at it.

 

I get that on mine but its a much lower repeat rate than this sound. This is not belts, agree exhaust leak.

 

That was a great post!

I could have written that myself (and supplied nearly identical photos) because that exact thing happened to my 308 last July. You beat me to it.

From what I could hear from the video, I think yours makes three. . . that we know of.

 

****** FOLLOW UP & RESOLUTION *******

Exhaust leak it was. A gasket on the 5-8 header was pretty much blown out (vs the analyzer port which my Euro doesn't have). Car sounds great and runs incredibly now.

 

Thanks for reporting back on that. I was curious what the source was, as I'm starting to develop a similar sound on my 328. Good to know where to look.