Rough idle/misfire and dealer blamed on cheap gas...?

I've found my baby runs much better on real dino-fossil fuel than on ethanol, and that the cost is comparable. It actually gives me better mileage, too!

Bad gas could make for a rough idle...or at least it did in every other car I've had. Their suggestion has more to do with the consistent quality of the big name brand gas stations, than anything else.

Having said that, the Prius is so sophisticated that it can adjust to running on any crap they decide to push on us.

*Really* rough could imply a problem. A little rough could imply bad gas.

Chuck

 

The Prius engine is designed to work on 87 octane unleaded gasoline. In fact the Toyota tech training docs indicate that the engine may have starting issues when higher octane fuel is used. See p. 3-4 of the Hybrid13 Fuel and EVAP System.pdf file that you can find in the Technical Articles section of this website:
Automotive Training and Resource Site

It could be that the advice was given in hopes that one tank of premium gasoline may help to clean engine deposits. An alternative would be for you to buy some Chevron Techron and add that to the tank when you next refuel.

There's something to be said for buying gasoline from a name-brand station, though, in order to obtain the additives that may be missing from gasoline sold by a no-name place.

 

I second the Chevron Techron. I use it on a semi regular basis in our chevy Venture since it helps keep the different things in the fuel sender clean (a GM issue), but recently put in a bottle in my Prius since I thought it idled very slightly rough. It seems to have done the trick since my Prius purrs like at kitten at idle now.

 

The Prius ignition timing is tuned for 87. Running higher octane will result in decreased performance and a rougher idle.

Your dealer is an IDIOT for suggesting that you use premium given that:

1) It costs more.
2) Will decrease performance.
3) Will decrease gas mileage.
4) Will add to carbon build up.

Always run the octane your vehicle is tuned for to get optimal performance.

 

Had a similar problem at the end of a cross Canada trip a year ago. However they were able to pull a code which indicated "misfiring".
The mechanic suggested I picked up some water so I added 1/2 container of STP water removal and have not had another problem. My local dealer said he had a customer with a similar problem which continued for some time until he changed where he bought his gas.
Gary

 

That's really interesting because my Prius seems (I emphasize seems) to run better on 93 octane (I run a tank through every once in a while). Specifically, the engine appears to run somewhat more quietly with a smoother rev response. Again, this is a very superficial seat-of-the-pants impression (added to calm the apparently now-rabid Prius fanboys).

 

You should try a blind test. Have your wife or coworker fill your Prius the next four or five tanks. Have them switch between 93 and 87, but don't tell you which is in the tank. Write down your results. At the end of the trial, you can compare your perceptions with reality and see if there is any correlation. No cheating!

Tom

 

Interested to hear more feedback, as my car just started doing this at idle too.

 

try the Techron first.
It is a very strong (good) fuel system cleaner. Much stronger than whatever little bit might be added to premium fuel.

 

Have the plugs checked. While doing that they can inspect to see if the plug inserts are leaking allowing water in there. This was a known problem and I'm surprised the techs. didn't check that (well not really after what the dealer told the OP).

I find that E10 cleans up engines quite well and also ensures there is no water pooled in the fuel tank (it absorbs it and allows the engine to "burn" it). You do get a mileage hit. I've switched now to only using a tank of E10 once every two months. So that's one every four or so tankfulls.

 

Thats what I was thinking too. To the op:
Has the car been in very heavy rain and or a carwash where water may have gotten into the engine compartment?

There is an issue where water can get past the water dam tray below the windshield and leak over onto the top of the engine and get into the spark plug wells. There's 4 rubber boots on the top of the engine over the spark plugs that are the plug igniters. If water gets in a well it will fill up the well and short out the igniter possibly intermittently. Since it will cause a certain misfire it usually throws a check engine light. If you do get a check engine light in the future make sure you get that code it throws from the dealer when they pull it and let us know what the code was.

Also be careful when washing the car at home. A strong stream of water at the wrong angle can allow alot of water to enter the engine compartment. So after a good carwash always open the hood and check the engine.

When I first bought my Prius the wife put in a few tanks of premium fuel by mistake. The car ran really good on it just got bad fuel mileage. But no perfromance issue at all.

 

Whenever I wash my car at home (spring to fall) I also wash the engine. Have never had water get into the spark plug wells, but I do believe those who say it can. I don't use a pressure washer at home. I use a garden hose.

My understanding was that the leaking spark plug wells were "fixed" after some model year, possibly 2005. Don't know that for sure so don't quote me.
Even so, stumbling idle would make me very suspicious of them.

Check the plugs or have them checked.

 

Higher octane gas won't result in decreased performance or rough idling. But it does cost more and shouldn't be necessary in a Prius which is designed to run on 87 octane.

 

I can confirm this also. In fact my 07 ran very good on it. Very peppy pickup. Just did not get very good mileage.

Now I'm not advocating running higher octane as many times mentioned its tuned for a lower octane which is exactly what I run.

Do not run Premium!

But by mistake it did see a few tanks of Premium when I bought it
years ago with no performance issue at all.

 

I would approach this as a step by step process. I would agree with those suggesting the easiest first step would be to try a tank of Top Tier Station Premium Gas.

I would also then fill with a Top Tier station regular and add a bottle of Techron..run it through.

See if any of this helps.

I can understand not wanting to go to Premium permanently, but if you have build up or deposits running some techron through might clean it up. Then you could go back to regular.

 

Your car doesn't need higher octane, however, there are few incidentals about higher octane gas choices: they tend to have less (or none, with Chevron's top octane choice, for one) ethanol blended in, and more cleaners (Chevron again).

 

Check for a dirty MAF sensor. This shouldn't be the case at only 22k miles but the symptoms seem consistent with it.