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CEL P0300, P0456 and difficult starting after fill-up


2006 Kia Rio LX with 78K miles,

I recently performed several repairs in the car including replacement of the timing belt, all other drive belts (power steering, air conditioning, alternator).

Recently, the car has developed a new problem where it intermittently has been difficult to start, but only after filling up the gas tank. The engine cranks over okay but it seems like it might have vapor lock. It seems to occur every time we fill it up. Rarely, the car has appeared to run rough for a few seconds, the electronic control module seems to make an adjustment and then it runs better. The gas mileage has decreased from typical 28-32 miles a gallon down to 23-25 miles per gallon, despite getting new tires for the car.

Additionally, the check engine light has come on intermittently…once reset it stays off for up to two weeks then will come back intermittently with the following codes:

P0456 which indicates a problem with the fuel evaporation system, which may be related to the gas cap (sometimes when this comes on, the gas cap appears to be loose, but other times it appears to be tight).

P0300 which indicates in engine misfire
P0301 which indicates in engine misfire in cylinder #1
P0302 which indicates in engine misfire in cylinder #2
The first time that I received this warning, I replaced the 4 spark plugs and the check engine light did not come back on for a couple weeks.

I have also previously received a code for faulty oxygen sensor temperature regulation for the forward most oxygen sensor. From what I have read, this may be related to oxygen sensors were related to an exhaust or intake leak. I have attempted to carefully inspect the intake and exhaust systems and I don’t find evidence of leak. (From what I have read, a lean fuel condition which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open are possible causes.

I have visually inspected the 4 coils/plug wire units and they all appear to be intact and since the engine does not run rough (it idles smoothly, seems to have normal power etc.), I believe that they’re all functioning.
Any thoughts about what should be my next step to diagnose and to fix this problem?
Replace gas cap?
Replace oxygen sensors?
EGR valve?
Could this be related to the timing belt replacement (CEL did come on prior to the repair but gas mileage and problem starting after fill-up has started after the change)?
Carbon canister or solenoid valve for evap emissions needs replacement?
(I have never been in the habit of “topping off the tank”)
Thank you very much!

I think vapor lock is rare in a FI car but perhaps the vent tube or the tube from your evap system back to your tank is clogged. That’s where I’d probably start, if that was my car.


My first thought was that you are probably overfilling your gas tank and not stopping at the first click. But since you just had the timing belt replaced, it could be a tooth off.

These recent repairs, were they motivated by an operating problem or routine maintenance?
Did you do them yourself?
Was the EVAP system code occurring before the repairs?

I’m inclined to agree with Keith, but if you changed the belts to try to repair a malfunction it would be a big help to know the whole story.

Thank you very much. The accessory drive belt had minor cracks and the timing belt was supposed to be replaced at 60,000, so it was the routine maintenance schedule not vehicle performance prompting the belt replacement.

I was supervised in the timing belt replacement by a mechanic who works at a local kia dealer, we checked several times and the belt appeared to be spot on. However, i guess we could have been wrong. I don’t have any prior experience with what performance is like if the timing belt is off one tooth, but from what I have read the engine should run rough and power would be significantly decreased, neither of which seems to be the case.

If it stops raining, I plan to get under the car and inspect the evaporation lines and air filter next to the filling tube today. Thanks for your help!

OK then, are you guilty of adding more gas to the tank after the handle clicks off the first time?

Evap systems need a more detailed than normal scan. I have this issue. I had a gas tank changed and since my evap monitor has been nuts. The evap issue is really separate as it is its own test independent of the engine. It checks the vacuum integrity of the tank. You can try service on the solenoid and check the fuel filler for air leaks/ corrosion. The misfires are usually just engine system issues and not related to evap at all.