Rattling/knocking on deceleratih

[Rovin]

After searching several sites and terms, I'm only a bit closer to having a table-top diagnosis for a new noise coming from under the hood of my J30.

Background: check-engine light is on and I probably need an oxygen sensor. Car frequently floods since I got it a few months ago, which means lots of gasoline in the oil. Curiously, the noise started after an oil change -- the shop put in plain 5/30 which I replaced with synthetic oil today. I also removed and cleaned a sensor located on the exhaust manifold, passenger side, near the rear of the engine. That seems to have reduced the tendency to flood, but I've not driven it more than a few miles since then.

The noise occurs on deceleration, and only when the engine is braking. More prominent in second gear, it "chirps" a bit if whenever I let off the gas quickly. More recently, it sort of grinds -- not unlike the sound of a jake-brake, but it's also like that ugly metal-knocking-on-metal pitch I recall from hearing rods go. Seems to rattle in synch with rpms -- could be a mechanical rattle against the engine, but that's a wild dream - it's almost certainly related to something turning.

Rod bearings? Timing belt or tensioners?

I'm not commuting with the car every day, and am not beyond attempting to replace the belt myself. Replacing bearings would be a bit of a stretch for my set-up and tool collection.

Whatever it is, it hasn't failed yet and I'm in a position to baby it until I can get a diagnosis and repair lined up. However, it seems to be gettng progressively worse. -- i.e. more consistent and easier to trigger with less deceleration. Ideas?

96 j30 - approx 150K miles
Modified by Rovin at 5:46 PM 7/5/2009

[driverdriver]

Have you checked your heat shields?

They start rattling with time. Always on acceleration and deceleration.

You may need to tighten them or need new bolts.

[yodawill2000]

Fuel in the crankcase is murder on the Main bearings till the temp gets up enough to evaporate the gas.

[Rovin]

This car has as many heat shields as the Space Shuttle! There is that sort of rattling present on idle - a somewhat higher pitch. Are you thinking exhaust heat shields? Or maybe just work the whole thing for loose bolts?

[Rovin]

Would that deceleration knocking be consistent with main-bearing wear?

[driverdriver]

This car has as many heat shields as the Space Shuttle! There is that sort of rattling present on idle - a somewhat higher pitch. Are you thinking exhaust heat shields? Or maybe just work the whole thing for loose bolts?

I'm thinking exhaust heat shields and the shield that's directly under the motor, the one that prevents grass fires from starting should you park on dry grass.

I seem to have to tighten mine/or replace bolts every couple of years to stop the rattling.

[Rovin]

Thnks. I'll check those.

[Infinitiguy19]

You can try some BG Quick clean if the heat shield is not the problem. Go to oil change place or if your doing it yourself run it for 15 min at IDLE. The IMMEDIATELY drain then fill with new oil then add BG MOA.

BG Quick clean an MOA Usually come as a kit and are avalible from the dealer or BG Suppliers ONLY.

[yodawill2000]

You can find it on Ebay as well.

[Victor]

Check your cup holder. A knocking rattling noise just started today in my J30 and I finally accidently found out it was a windshield washer pump from my van that I had put in the cup holder, planning to stop by an auto parts store and I was bringing it with me to compare to what they had. The sound had appeared to be coming from under the dash on the passenger side.

[alex61089]

excuse my intrusion but where the hell are the heat shields located at? i have the same problem and it's like a never ending maze underneather there

[yodawill2000]

I believe they cover the bottom of the Cats on both sides.

[WA261]

I removed mine. Those thing use to rattle like no tomorrow.

[yodawill2000]

Don't go parking on tall grass !!!

[alex61089]

well i went to the junk yard today and i saw there are heat shields above the cats too; my cats were replaced so i dont think they are the problem.... any other heatshields that could rattle i can definitely hear it on acceleration