Skip the auto shop - Our mechanics make house calls

Q: Has a rough idle with no CEL on

asked by on

Felt like a misfire, so I replaced plugs and wires. The issue was still there with idle dropping a little low after giving it gas and just a rough in general. Took it to a friend with a scanner because it had a check engine light on and the code was for cylinder #8 misfire. I swapped the coil to the #2 cylinder and then took it back to read but no CEL on. I did a vacuum test and it was at 18in hg and went to 21in hg at 2000 rpm. I took PCM fuse out turned key to start until I heard two chimes and then reinstalled. No CEL on and it drives great but when it's stopped, the idle is slightly rough. It's pretty minor, but it's not smooth like it was before.

My car has 111000 miles.
My car has an automatic transmission.

I would try and have the fuel injectors and throttle body cleaned out. This should clean out injectors and intake valve carbon deposits that can cause the rough idle. If after driving the vehicle, and the CEL comes on, then have a mechanic diagnose the rough idle and do not disconnect or reset PCM unless directed to by a pinpoint test. This just puts the PCM in factory setting and may take a few weeks for it to see a problem again.

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・Fair and transparent pricing

Get a quote

What others are asking

Q: Charging system light.

The charging system light will illuminate when the vehicle control system detects a problem with either the battery or the alternator. Typically, the alternator is the cause for the light. If the alternator is not able to maintain the battery...

Q: Gear shift very hard to move

Hello, thank you for writing in. The vehicle's shifter uses cables and linkage to communicate what gear it is in. If it is difficult to move, there is likely a mechanical error with these cables. This would also explain why...

Q: Rough idle start stalls

Hello. The symptoms you are experiencing can be caused by a wide variety of issues. Engines require 3 things to operate: fuel, spark, and air. If there is any issue with the supply or system of any of these three,...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
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...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.