Skip the auto shop - Our mechanics make house calls

Q: Check engine light is on. Misfiring on Cylinders 3 and 4.

asked by on

I don't think the plugs have ever been changed, but am not sure. I wanted to have the plugs changed and all of the coils. I can get Bosch coils on-line rather inexpensively. The rear plugs and coils are hard to reach. The car has 202,000 miles but is a one owner car. It has a new timing belt and front crankshaft seal. What do you recommend? I don't want to spend twice as much as the car is worth. It also needs front end work.

My car has 202400 miles.
My car has an automatic transmission.

A: Before replacing all the coils and plugs yo...

Before replacing all the coils and plugs you may need to have the engine decarbonized. The engine can have excessive carbon build up on the exhaust valves, cylinder head, and exhaust manifold. You should use engine combustion chamber cleaner on a cold running engine.

Let the engine suck in the cleaner or simply spray it into the engine while it is running. Once the whole can is run through the engine, shut off the engine and let it cool for at least 30 minutes. This will allow the cleaner to loosen up the carbon deposits.

Afterwards, start the car and run a second can through the engine. Once this is done, run the engine for about 5 minutes at 2000 RPMs to clear out the cleaner and get rid of the carbon.

Now you should replace the coils and spark plugs as needed. If any oil is on the spark plug wire boots then replace the valve cover gasket.

Once you've done all of this I would recommend running fuel injector combustion cleaner in the tank to keep the carbon from returning. However, carbon build up can be extreme and can require additional cleaning or replacement of the parts that are affected.

If you'd like some assistance with this, consider YourMechanic, as a certified professional can come perform this service on your vehicle and properly diagnose your Check Engine Light. Good luck.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Car has trouble starting and misfires.

A new ignition coil is a very likely suspect for this issue. I would recommend enlisting the assistance of a YourMechanic technician to diagnose your issue. Have the technician switch the suspect ignition coil with a known good one and...

Q: Repeatedly blowing the same fuse

Hi. I can't help you much without knowing which fuse is blowing. Fuses are designed to blow to protect the circuit in the case off too much current draw. You're correct that a short can cause a fuse to blow....

Q: Check engine light

You would need to list all the codes you received so we could help you isolate the problem. You may have a more major issue that needs to be diagnosed. I recommend having a mechanic come and diagnose the Check...

Related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.