Q: PO300 and PO420 and rough idle and RPM fluctuation

asked by on March 05, 2018

For the sake of diagnosing, I have installed new spark plugs, spark plug wires and boots, new engine coil packs, EGR valve, EGR modulator, new tubes in the EVAP system, vacuum switch valve, vacuum control valve, Purge solenoid and a new Idle Air Control valve.

My car is having a rough idle every once in a while (and seemingly getting worse) and would hesitate accelerating because of whatever issue is going on. There are no audible sounds but it feels like the car is kicking while trying to accelerate and there is a hesitation to the acceleration. It also has a rough idle just sitting in drive or reverse with the brake on. This only happens in drive and reverse.

It would help with diagnosis if you included the vehicle year, make and model with engine size. Since the engine is running good out of gear and only gets rough when in gear you either have a bad motor or a transmission mount that is not supporting the engine and allows excessive movement when accelerating.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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


Experienced Mechanic?

Earn up to $70/hr

What others are asking

What is the brake-shift interlock?
All cars today have an interlock system that’s designed to prevent the car from being put into gear if your foot isn’t on the brake. This does two things. First and foremost, it ensures that you can’t accidentally knock the...
Getting error code P0304
P0304 means the engine computer has detected a severe misfire on cylinder 4. This could be due to a bad ignition coil, spark plug, fuel injector, or valve train issue. If the issue is not resolved, further damage to the...
Leak from under dash
Hello. It is likely the heater core causing the leak because it is on the passenger side and when they leak the passenger floor becomes wet. It may also be a heater core gasket that is leaking but the heater...
It dies at random times also the check engine light blinks at certain times and it stays on all the time
When the vehicle dies, you will need to see if you are losing spark or fuel, and scan the computer for possible codes relating to a sensor problem. The Check Engine Light blinks when an active fault is happening, and...
Blower motor only works on high and wont shut off
Hello, thanks for writing in about your Jeep Grand Cherokee. The blower control module and connector got hot and burned from the blower motor drawing too much power. It should be replaced to fix this issue. Your A/C blower switch...
Previous owner had a fender bender in rear driver's side. The Service Parking Assist light is coming on. What do I need to do?
The Parking Assist warning message may simply mean that the sensors in the rear bumper are dirty, covered by a bumper sticker, or they are damaged. Wipe the sensors with a wet cloth or paper towel. Drive forward at more...
i need a new rear bumper on a 2016 chevy cruze
Hi...the parts and labor will depend on exactly which components are missing and/or damaged. Therefore, the only way to obtain an ironclad estimate is to have a certified mechanic come to your location, inspect the damage, determine why the bumper...
Significant gas mileage drop after tire change
If the vehicle fuel efficiency has dropped 15%, and there is no obvious explanation, something is wrong. The timing of the tire replacement to the drop-off in mileage is undoubtedly just a coincidence unless the tires are wildly defective thus...
I have a 96 Mercedes recently it started rattling in park but as soon as i put it in gear it stops eat cud be wrng
Under hood rattling can be caused by loose exhaust components, faulty pulleys and accessories, motor mount faults, loose heat shields, or any loose component. The reason you are noticing the noise only under certain conditions ("park" versus "drive") is because...

Related articles

A Buyer’s Guide to the 2012 BMW X5 xDrive35d
The The 2012 BMW X5 xDrive35d offer contour seats and smooth acceleration. The suspension offers a smooth ride to the driver and passengers in the vehicle, even for long road trips. It can handle tight curves without any problem while...
How to Buy Good Quality Struts
Vehicle Vehicle struts serve a number of different purposes, including adding control to extreme handling conditions such as sudden turning and braking. They also serve to provide a level of comfort to passengers in the vehicle as they provide a...
Rules of the Road For Maryland Drivers
Driving Driving requires knowledge of the laws so that you can be safe when traveling to your destination. While you likely know the driving laws in your own state, that doesn’t mean they are the same when you visit or...