What could be wrong with my car?

Factual Questions
1

I have a 2004 Toyota Corolla, manual transmission. I know already that my clutch is going, and needs to be replaced (and will be in the next month or so). In addition, it has an odd habit of sort of “shuddering” wherein the RPMs drop while idling, the car shakes like it’s about to die, then it comes back to normal idle. It’ll do this a couple times consecutively while I’m waiting at a stop light, but doesn’t occur normally right after I start the car up in the morning. It never stalls completely, just goes up and down a few times.

Any ideas, car-dopers?

2

Does it do it if the AC (including defroster) is off? It sound to me like it could be the AC clutch engaging/disengagin, something else to check would be if it’s the radiator fan turning off and on. You could try opening the hood with the engine running and seeing if anything starts/stops when you hear it happen.

3

It happens whether the AC is on or off.

4

I’ve had this exact thing happen with my previous car. It was the timing belt/chain about to break (which it did).

5

It could also be the idle air control valve (IAC), which could simply be dirty. If it’s the timing belt, though, you want to get that taken care of ASAP (a timing belt snapping while the engine is running is doubleplusungood). Generally, timing belts should be replaced every 60,000 miles, but your car’s maintenance interval may vary.

6

I will get the timing belt looked at. Thanks guys, if there are any more suggestions I would still appreciate them. I have my car serviced by a friend, so I’m not charged crazily if I ask them to look at different things.

7

I think your engine actually has a timing chain, not a belt.

8

Luxury Yacht: That’s true, the Corolla does have a chain. As far as I know, timing chains rarely need to be changed before 100k miles, if at all. But the Toyota-recommended maintenance intervals trump my WAGs.

I still say it’s most likely the IAC.

9

It is a timing chain engine.

The first thing I’d do is inspect for vacuum leaks. Some are simple, like a faulty vacuum hose, others are more involved, like a leaking intake manifold gasket or faulty EGR valve. Next, I’d check for carbon build-up in the throttle body. This is a fairly common problem, and typically results in unstable idle. If there is significant carbon there, that should be cleaned before investigating anything else.

If the problem persists with no vacuum leakage and a clean throttle body, it’s time to do some testing of the engine management system (computer and related components).

10

I had exactly the same problem with a 2002 Nissan Altima.

The dealer tried the following fixes, in this order:

  1. Replaced EGR valve
  2. Inspected throttle body
  3. Replaced mass airflow sensor
  4. Inspected timing belt
  5. Inspected head gaskets
  6. Threw their hands up and refunded half of my money

I wound up trading it in. Had to make sure the engine was cold when they test ran it, though.