Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

2006 Toyota Prius, Misfire at idle

Dear Tom and Ray:
I have about 140,000 miles on my car, a 2006 Toyota Prius that misfires at idle when cold, and for the last 10,000 miles the engine has been using between 1 and 2 quarts of castrol synthetic 5w30 oil between the 5,000 mile oil changes. My check engine light naturally stays on constantly now and the engine also appears to misfire on acceleration also.

For the first 60,000 miles I had the oil changed at various places every 3,000 miles using nonsynthetic oil that was recommended in the car manual. Since then I have been getting the oil changed at the dealer where I bought it new and they switched to the synthetic oil, so I asked the service department, why the misfire?
They said the new type spark plugs are very sensitive to oil burned in the firing chamber, causing the misfire. They told me to check the oil often which I do and the engine has never been more than a quart low.

Is there a spark plug I can use that is not as sensitive to the oil, or should I be looking for a junk yard engine or is it cheaper to have my engine taken apart and repaired. Will it damage the engine further to keep running it, there does not appear to be any smoke coming from the tailpipe.

From new I got mpg in the 40’s now I am getting 33 to 34 mpg.

This problem doesn’t sound like an oil burning problem. Get the codes read that are triggering the the check engine light and tell us what they are. Didn’t the dealer do this? Their explanation sounds bogus.

The dealer did 2 diagnostics on the engine and told me - They said the new type spark plugs are very sensitive to oil burned in the firing chamber, causing the misfire.
By the way the engine seems to be burning more oil.

Without you telling us the trouble codes we can’t help you.

The trouble codes were (P0300), (P0301), (P0302). These codes were read at the last oil change at the Toyota dealer where I bought the car and the car is due for another oil change by the end of the month. Should I get another diagnostic test done on the engine at that time. The dealer charges $100.00 for a diagnostic test.
I had one done about two years ago, for free at an auto parts dealer. Are the diagnostic results reliable at the auto parts dealer.

Here are possibilities for P0300/01/02:

“A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn’t tell you specifically which cylinder(s) is/are mis-firing, nor why.”

•Faulty spark plugs or wires
•Faulty coil (pack)
•Faulty oxygen sensor(s)
•Faulty fuel injector(s)
•Burned exhaust valve
•Faulty catalytic converter(s)
•Stuck/blocked EGR valve / passages
•Faulty camshaft position sensor
•Defective computer

I strongly suspect your problem is being caused by a component that needs to be replaced, not by oil burning.

Have you posted this question on a Prius forum like Priuschat?

You are pushing your Prius way to hard. Buy a car with more power or replace the engine with a new one. If you get a replacement from the bone yard get a good one.

If the plugs are oil fouled, that could possibly cause a mis-fire. A visual inspection of the plug electrodes would confirm or disprove this theory straight-away. If there was any doubt, simply replacing the plugs should make the misfire go away. So that’s another easy to do test at the mechanic’s disposal.

Puzzling. I don’t see how the oil being low would cause oil-sensitive plugs to misfire. If anything, with less oil in the oil pan, it seems like that would cause them to misfire less (although not good for the engine in other ways of course.) 2 quarts in 5000 miles? What’s that? 2500 miles per quart? That is maybe more than normal for a new car, but isn’t considered excessive oil consumption on most engines. Especially with the thinner oils new cars spec these days. Generally excessive oil consumption starts around 500-1000 miles per quart.

I think the others here a right, this likely has little or nothing to do w/the oil. Or if so, it is an indirect effect. For example, if the oil gets one quart or more low, that could cause the engine to overheat, and overheating could well cause a misfire.

Problem solved: I went to another toyota dealer and they swapped some parts and found a bad injector. They had none in stock so I had to wait a few days to get new one installed. Total cost for all diagnostics, parts swapping, new parts etc. about $1000.00.



I’m surprised the Toyota dealer had to resort to swapping parts to find the bad injector

Many skilled professional mechanics with decent tools at their disposal are able to zero in on that bad injector without resorting to parts swapping

Anyways, I suspect your fuel economy will be back to normal, now that the other 3 cylinders aren’t working so hard to make up for the bad one.