CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Just changed the coil and it's still running rough

Symptoms: If I’m going 40 and try to increase to 70 MPH the car sputters, Lousy gas milage and just started idling rough. Took it to my mechanic who said the coil was bad. Changed that and it was better for about a day. Advice … anyone … plugs and wires tonight just to make sure. I think it’s the oxygen sensor. Would that make it not take gas correctly?

Fuel and air filters?

Test the fuel pressure.

Any check engine light?

No check engine lights but I’m not good enough to check the other by myself. I guess I assumed the mechanic checked those. I also assumed that the fact that it’s eating gas tells me there is no blockage.
Sorry I work on computers not cars.
TjD

Don’t consider the oxygen sensor, for the moment. A rough running engine can be the result of multiple causes. Fuel filter, air filter, spark plugs and wires, and distributor cap and rotor (if it has them) are much cheaper to change than to perform diagnostics on.

EGR system…? How does it perform at full throttle? If you have a misfire, and you hold it down to the floor for a few seconds, that should force the CEL to come on and then you will have some codes to work with.

Mass Air Flow sensor and coolant temp sensor are two other possibilities, once normal tuneup parts can be ruled out.

Thanks, Yes I would hate to buy another $60.00 part that doesn’t fix the problem.
I changed two of the six )plugs & wires) last night before I ran out of daylight and feelings in my fingers. More tonight.
Thanks for the help.
TjD

I like that suggestion because YES when I floor it … vroom, pause, bang!
Your saying if its hooked to “da machine” they’ll be able to tell why as it happens?
Thanks for the help.
TjD

With 247k on the clock, most of the stuff that people are mentioning is liable to be suspect and should probably be examined anyway. I’m putting my money on a simple vacuum leak though.

This thing has a dist. cap and if it hasn’t been changed in a year - do it (rotor too). It needs changed regularly anyway as normal maintenance. The contacts in any cap will corrode. You would too if you had 80,000 volts zapping you hundreds of times a second (ha ha). It’s a cheap (around $25) performance booster. My Dodge and Ford trucks get a new one once or twice a year and it always runs much better. I buy the ones with brass contacts as they seem to last longer. Also with the price of sensors do the cheap stuff first. If you do need sensors look for them on Ebay -much cheaper.

P.S. If you want to really be cheap - remove the cap and rotor and scrape and lightly sand the crud off of the contacts. Be sure to blow the dust out of the cap before reinstalling. This should give you an idea if this is the problem. If its really bad this solution will not last long as the crud will come back quickly.

A clogged catalytic converter could cause a problem like this but you might define “better”. Better as in very good or better than it was but still not perfect?

If the mechanic changed the coil then he should have changed the spark plugs and wires because, often as not, a faulty plug will kill the wire which will in turn kill the coil if the problem goes on long enough.

And just my opinion but an engine with 247k miles should have a compression test performed. This is true of any engine with any mileage but especially so on one with a quarter of a million miles.
One cylinder down on compression can kill a spark plug. (see above paragraph) The domino effect often comes into play.

At 247K I would replace every original ignition part, including the distributor itself.

Has anybody checked the timing belt(s)/chain?.
The check engine should have come on but who knows. Over the years I have found that this sems to be the last thing thought of and for some reason difficult to diagnose.
Jamesrsavage

An evaporation controls system problem would make it not take gas correctly. You did not state a model or year, is the problem with filling and running bad related?