Skip the auto shop - Our mechanics make house calls

Q: My Hummer shows code P0300 after getting gas

asked by on

Put 40 bucks of gas in tank from same place as always super America. Got on freeway about 10 min on freeway check engine light came on. Stopped at orileys to check code and got p0300. Random misfire drive home. When on freeway could feel the loss of power. I have good oil pressure not overheating truck starts no problem idles slighty ruff.

My car has 173000 miles.
My car has an automatic transmission.

A: You may have got contaminated fuel from the...

You may have got contaminated fuel from the gas station. The code P0300 indicates you are getting a random engine misfire. This random misfire does not point to any one cylinder misfire and indicates that the fuel may be contaminated with something like water. All gas stations have some water that is in their tanks in the ground. The water normally sits in the bottom of the tank and if it becomes excessive then the gas station will need to have it removed. Now if there is about 2 inches of water in the tank settled on the bottom of the tank and the refueling truck dumped new fuel in the ground tank then water will temporarily mix with the new fuel for about an hour. If you happen to get fuel at the time the water is mixed up in the tank then you would get some in your tank. This water will cause erratic running and misfires in the engine. If you go to the parts store and get some dry gas additive it will disperse the water so you can run it through the engine and as soon as you get to half tank then refill tank again to disperse the water more until the engine starts running better. Then you can get the Check Engine Light cleared.

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: Truck won't start from store

Hi there, thanks for writing in. From the symptoms you described, it sounds like your battery or battery cables are bad. You can try jump starting, but a bad battery may not allow a jump start if it is...

Q: Don't understand CEL Codes.

The codes P0300 through P0306 are for misfires of random cylinders or different cylinders. The second code P1456 is for the evaporative fuel tank system having a leak somewhere in the system. Since you are getting random misfires, then I...

Q: overheating problem

Thanks for writing in about your Hummer H2. Let's cover the basics of diagnosing a overheating issue. First, verify the actual temperature at the thermostat housing. Cooling system (and cap) has probably been pressure tested by now. If it tends...

Related articles

How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
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...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...