What can cause a rough idle?

[TheV6Bird]

My bud has a Mercury Topaz, and he says his car idles very rough. What are some causes for a car to have a rough idle?

[TheV6Bird]

C'mon guys, it can't be that hard of a problem to diagnose, right? TTT

[TheV6Bird]

TTT [img]graemlins/crybaby.gif[/img]

[uno]

random guess, i would say a timing or spark problem

[Fin]

Bad IAC (Idle Air Control) valve, TB pintle could be at the incorrect position at idle, bad plugs or wires or other ignition components. I'm sure I missed a few, but that's a start.

[xFactoRx]

worn main bearings.

[TheFireChicken]

Actually, the number one cause of rough idle is needing throttle valve servicing. Here is how you do that.

1. Remove airfilter and induction system up to the throttle valve.

2. Manually open throttle valve, using assembly attached to cable that goes to pedal.

3. Spray throttle body valve/carb cleaner on the plate of the butterfly valve, and wipe it off. also make sure to wipe off the seats (places that plate touches against when closed).

4. The valve is a fail safe closed valve, which means when nothing forces it to open (pedal cable), it will close swiftly, and crush your fingers. So be careful.

5. A second useful step is to buy a bottle of concentrated fuel injector cleaner made by STP that goes in the gas tank. use that at the same time.

6. By the time you have gone through a half a tank of gas, you will notice a powerful difference in the responsiveness and acceleration curves of your car, as well as a smoother idle.


The valve gets dirty because of your exhaust gas return system, which pumps exhaust gas, containing burnt byproducts, mostly carbon compounds, into your intake manifold. as your engine shuts down, it no longer pulls in as much air, which allows some exhaust gas to be trapped in your manifold. It expands, and pushes against the walls of your manifold, and the throttle valve, depositing carbon and gunk on the sealing surfaces of the throttle valve. Over time, those seats build up with gunk, and in idle, when the valve is only open a tiny bit, the amount of air getting into your intake system decreases, screwing up your fuel air mixture ratios, and causing rough idling, even stalling. This procedure above should be done approximately every 3000-6000 miles to keep your car at peak efficiency. Incidently, a lube shop or dealership calls this a "Fuel System Service" or a "Throttle Service". It usually goes for a pricetag of up to $50 if you have them do it, if you do it yourself, its about $6. There is a seperate service, where they flush fuel lines with pressurized cleaner to wash your fuel injectors, but this usually runs around $80-$125.

[strange_trp]

<blockquote>quote:</font><hr>Originally posted by Fin:
Bad IAC (Idle Air Control) valve, TB pintle could be at the incorrect position at idle, bad plugs or wires or other ignition components. I'm sure I missed a few, but that's a start.<hr></blockquote>

also emissions components like egr and evap. you don't want either egr or canister purge at idle.
also fuel problems like insufficient fuel pressure or leaking injectors.
then there are base engine things....
i'd start the diagnosis by checking engine vacuum and compression just to rule out base engine problems. vacuum test will also point you in the direction for diagnosis.
is the check engine light on??