QuestionQUESTION: I have a 2000 Elantra (2.0 sedan ~100K miles) which has a rough idle and has a difficult time starting in the cold. New plugs and wires. Fuel filer is not that old, could it be the fuel pump?
ANSWER: Rough idles tend to occur in three basic ways:
1. low engine rpm
2. engine misfiring
3. abnormal transfer of engine vibration to the engine compartment.
Since I don't know what your rough idle feels like, it'll help me if I know which of the above characterizes your rough idle.
If the rpm is low, you'll want to check the things that affect idle speed. In many cases, a check engine lamp will set if your rpm is too low.
If the engine is misfiring, the secondary ignition system is the first place to start checking. Since you've already replaced the plugs and wires, the coil would be the next thing to check. But misfire issues can also be caused by low engine compression or a faulty injector. In nearly all cases, a misfire will cause the check engine lamp to come on or flash.
If the engine is simply transferring excessive vibration to the body, chances are good that either the upper engine mount or the upper transmission mount aren't insulating the vibration properly. This issue won't cause the check engine lamp to come on.
I'll also need to know what you mean by "difficult time starting." Does the engine crank for a long time before starting? Does it crank slowly? Does it intermittently not crank at all?
It's not very likely that the fuel pump is the issue. Low pressure from the pump typically manifests itself in a hesitation on acceleration, when it's necessary to have more fuel available. A reasonable likelihood, however, is the fuel pressure regulator. If it bleeds off the fuel pressure by leaking the fuel back into the fuel tank, you'll have a long crank time while the fuel pump rebuilds the necessary pressure.
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QUESTION: I would describe it as low engine engine RMP. For example, when stopped at a light the RPM's will drop a few hundred and at times it appears like it will stall out. It's very sparatic and sometimes (30% of the time) doesn't happen at all. Also, no check engine light has ever come on.
As far as the difficult time starting. The engine always cranks (normally) but at times (especially when it's cold) it will crank for a a good 5-10 sec. and I need to give it gas to get it to turn over.
AnswerFor the idle, you should check the throttle position sensor and the idle actuator. Improper throttle position sensor readings can cause the vehicle to not attempt to idle, and obviously, a sticking idle control valve can cause the vehicle to not idle properly. If you get the opportunity while the condition is occurring, whack the idle actuator with a wrench. If the problem goes away, replace the idle actuator.
For your starting issue, I'd suspect the fuel pressure regulator. To confirm, attach a fuel pressure gauge and check to see that fuel pressure remains while the vehicle is off.
