What the clunk is wrong with my corolla?

October 16, 2007 12:07 PM

[my old jalopy filter] What is my Corolla's clunking problem?

So my dearly beloved car, a 1996 Toyota Corolla sedan, has been making some funny noises lately. Since I don’t know an engine manifold from a monkey’s uncle’s sister Aster, I thought I’d ask for some off-the-cuff diagnostics before bringing it into the shop. Let's play Car Talk.

Here are the symptoms:

1) When I start the car I get a few clunky-rattly sounds, like a shock cord repeatedly whapping against a metal bin.

2) When I try to accelerate from – say – 10 – 25 miles per hour I get the same clunky-rattly shock-cord whappy sound, and really poor performance. The whole car seems to shudder and lurch, and takes twice as long to reach the desired speed as it should.

3) Not sure if it’s related, but the whole car vibrates rather intensely when I’m idling at a light, even if I’m in park.

Thoughts? Ideas? How do I go about repairing this [X] without getting ripped off? Any good mechanics in Culver City?

posted by mr. remy to travel & transportation (9 comments total)

Is your check engine light on?

Have you looked under the hood at all yet? This is a totally wild guess, but it almost sounds like the belt that drives your AC is broken, and your engine is trying to run it without it being attached to anything. I'm not 100% sure how the on/off belt linkage for the AC works, but I feel like if your engine were trying to drive the AC, and the belt was broken, it would slap around inside your engine compartment like crazy, and sap your power like hell.
posted by autojack at 12:27 PM on October 16, 2007

A broken AC belt would not make your car accelerate slower - just the opposite, as it takes no energy to drive a belt that isn't attached. and the clutch mechanism for engaging the belt is on the AC compressor, so again, a failed belt won't cause this problem.

The actual cause of the problem could be a variety of things. Have you tried opening the hood and seeing if anything is obviously wrong? Anything that makes a big "whapping" sound as you describe is going to leave a mark.(And you can always take a picture of the engine compartment and post it on Flickr if you need additional eyes).

Oh, and get thee to a mechanic.
posted by mosk at 12:37 PM on October 16, 2007

The check engine light has not come on.

The AC works fine -- I use it quite a bit.

I might have been a bit too figurative describing the sound as 'like a shock cord repeatedly whapping against a metal bin' because I don't really think anything has come loose. I was just trying to explain the quality of the sound that way.

I went out to take a look under the hood; there's nothing obviously wrong.

Another important clue I forgot to mention:

4) The car performs much better when it's got a full tank of gas.
posted by mr. remy at 12:45 PM on October 16, 2007

The noises on start could be related to the performance problem or not -- for example, my car has a loose exhaust heat shield that makes an awful rattle but causes no harm. So the rattle may be a clue, or a red herring.

How loud is the exhaust? Is it louder than normal?

Does the tone or speed of the rattle change with engine speed?

Can you expand upon "performs much better on a full tank of gas"?
posted by mosk at 12:57 PM on October 16, 2007

I reckon that sounds like a broken engine mount. Get someone to start the car while you look under the bonnet. If the engine moves a lot as it cranks over, then one of the rubbers on the engine mount (the one at the middle of the car, counteracting torque affects is my guess).

I don't know as 2 is related, and some of that may be psychological. The times that the noise happens suggests torsional engine movement to me - the rubber part of the mount that isolates vibration is toast, and so you get metal to metal contact. Hence the row.
posted by Brockles at 1:27 PM on October 16, 2007

Hmmm Unless the torque whip of the engine (in relation to 2) is totally confusing the torque convertor, of course. Then they might be related.

Changing an engine mount on a Corolla should be a relatively straightforward job. It shouldn't take any mechanic worth their salt much more than half an hour to an hour (depending on how well hidden the mounts are on that car).
posted by Brockles at 1:29 PM on October 16, 2007

If I didn't know better, I'd say it was a loose timing chain. These actually have a belt, however, and wouldn't be very noisy though performance would certainly be lacking. It's probably due for one of these by now, fwiw.

Are you sure this noise isn't spark knock? Try a tank of high octane and see if it helps. I say this because you hear the noise at startup at which time the knock sensor would react and attempt to richen the mixture, calming it down. On acceleration the engine is then starving for fuel once again and knocking/pinging. Something like this would be caused by a weak fuel pump, clogged injectors or an obstruction in the fuel inlet line. Again sounds more likely when you combine the fact that it does better with a full tank (more pressure helping out the pump). Easy enough to test the pump with a fuel pressure gauge (HF ~$30, very handy).

Also, when it's cold, reach under the car and try to move the exhaust around sharply by hand to make sure it's not rattling. The more noises you get out of the way the better the remaining clues as to it's real problem. I say this because it's very likely that this is a combination of problems and not one individual part.
posted by IronLizard at 5:21 PM on October 16, 2007

Seconding engine mount. I will even go so far as to say it sounds like a front engine mount, which is a cinch to replace if memory serves. Another giveaway of a bad front engine mount is a very rough "kick-down". If the Corolla has an automatic transmissions, it will be very rough when when the transmission switches from 3rd to 2nd, when it kicks down. This type of thing happens when you're gunning it to pass for example, not when normally decelerating to a light or stop sign.

I hope that helps.
posted by KevCed at 5:33 PM on October 16, 2007

Thirding engine mount - the noise is probably the exhaust banging against the floor or another part of the underbody as the engine moves under torque. I suspect that you have an ignition problem of some sort, which explains the rough idling and the shaking under acceleration - when was the last time the car was serviced? As for the "performs better with a full tank" thing, I have no idea - area you absolutely sure this is true? Does it still make the noise with a full tank, or does it just seem to run better?
posted by dg at 9:00 PM on October 16, 2007