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Pontiac vibe intermittent rattling in rear when idling

Hi everyone,
I’ve got a 2009 Pontiac Vibe GT. A few days ago, my battery went dead when parked and listening to the radio. The terminals looked corroded but we jumped it and it was fine. Monday, my friend removed the connections and cleaned the terminals. The next day, I noticed that upon starting it, there was a noticeable rattle coming from the rear area of the car. Once I hit the gas, it’s fine, and after a 50 min interstate drive it seemed fine. However, leaving work today it happened again-- a rattling when stopped at red lights or stop signs. Once I hit the gas it goes away, but starts on again as I slow to a crawl.

Is the battery issue a red herring? What could be the cause of this?

Thanks in advance for your help!

Scott

Pop the hood, stick yuour head underneath, have a friend rev the engine (stand t the side rather than in front of the Vibe), and see if you can see what’s rattling. If you’re not sure, touching possible places of origin with a metal rod (or better, a mechanic’s stethescope) should help.

Post back with the results.

Well, I had the battery checked again and a cell was bad, so I bought a new battery. This didn’t fix the problem. I looked under the hood with the engine running and it seemed as if the whole engine was shaking! When we hit the gas it stopped.

It’s sort of hard to see how a bad battery could cause a shaking noise in the rear of the car, but since your battery was bad anyway, it was good you replaced it. That part is done.

So what is causing the rattling in the back of the car? I expect it is some exhaust part. You’ll need to get your mechanic to check it out on the lift. Probably an exhaust hanger has fallen off, or the cat coverter shield has come loose from its moorings

Engines normally shake a bit at idle. But if they are shaking excessively, that could be caused by a number of problems. Usually it is something to do with the idle air/fuel mixture. Vacuum leaks are a common cause.

Thanks George.
Actually, while I originally thought the rattle was in the back, I think that it was truly coming from the engine shaking. It’s definitely rattling/shaking more than it normally would. Would bad gas cause this? That was a thought. I’m going to drive it again today and see if by emptying the tank and refilling the problem improves. If not, I’ll bring it to the dealer Thursday.

Just some more information on this problem…it seems the engine only starts to shake when the RPMs go around or below 0.5 (500 RPMs). Anything above that, and it’s fine. Sometimes at a stop sign or red light it’ll go that low and the engine rattles. Sometimes, the RPMs won’t go that low and there’s no problem. Any clues? Thanks!

How low is the idle normally on that car? 500 rpm seems very low to me.

That’s a good question. I’m not sure, but it is probably around 1,000 RPM or even more. Why would the car suddenly dip below that and cause the engine to rattle? It also doesn’t do it all the time. Odd.

A car will rattle when it idles too low. Stuff starts to resonate.

Perhaps have another look at those battery terminals. Clean the terminals and make sure that the wires are properly crimped, clean and tight. Also check the grounds.
Most cars have a way to measure the electric load imposed onto the car and adjust the idle accordingly.
The measuring for this is usually done through a wire that shouldn’t have have any real current going through it so it gets a precise reading.
If you have a bad connection, that reading will be way off and could mess up your idle.

Well, the battery is brand new - just had it installed last night. I presume that all the connections are ok. I don’t really know how to check the grounds…

The battery may not be a red herring. The battery may have been dead because the alternator was not putting out sufficient voltage to keep it recharged… and that might also be the cause of the low idle and shaking. Your engine may just barely have enough voltage to be barely firing. Have the battery and charging system tested at the local parts store. Most of them wlll do it for free.

Post back with the results.

There’s been some reports of bad gasoline, now being recalled by the oil company involved. You might google “gasoline recall” and see if it affects your area. This bad gasoline apparently is clogging fuel injectors in some cars.

Yikes…I will have to get an alternator check today at Advance.

I have noticed that it seems to be getting better. It isn’t idling below 500 rpm and when it approaches that number, it only does so briefly then goes back to 1,000. I plan to put some fuel system cleaner in on my next fill up as a precaution.

I’ll let you know what I find out. Thanks for all the advice!

Just a quick update…the problem seems to be ceasing. The RPMs haven’t approached 500 or lower, and I’m not getting the familiar engine rattle. I am hoping the problem was some bad gas. I don’t have any other common symptoms of an alternator problem (dimming lights, etc.) so I sure hope that’s not the issue!

Well, let’s hope that’s all it was. “Bad gas” is usually said by a mechanic that is in over his head on an intermittent problem but BP actually released to bad gas, as barkydog mentioned.

Yes, they did, but I didn’t get it from a BP. But I did get it from a rickety station way out in the country (I had no other option), so it wouldn’t surprise me. I will continue to monitor it. I appreciate everyone’s opinions and advice! Thanks again!