Check engine light, codes P0303 and P0170, cylinder three misfire

CDBELL98

MEMBER
2001 MAZDA MILLENIA
2.5L
V6
2WD
AUTOMATIC
83,843 MILES

Initially steady check engine light. Code P0303

1) Had valve cover gasket, spark plugs, and spark plug wires replaced. Oil change.

Light continued to come back on every time reset. Did nothing for third cylinder issue.

2) Added detergent to oil and ran through engine for half an hour. Swapped out recent oil change with clean synthetic oil. Fixed compression problem on third cylinder temporarily. Steady light came back on.
Told to run two tanks of gas through and see what happens.

After running only half a tank of gas. Currently steady check engine light when started, switched to flashing, runs hard. Code P0303 and P0170.

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi and thanks for using 2CarPros.

If the light is flashing, you could damage the catalytic converter because the fuel in that cylinder is most likely not being burned. Now, what do you mean you repaired the compression temporarily? I never heard of that. Is the compression low on cylinder three?

As far as the P0170, that is related to fuel trim. The computer recognizes that the fuel trims are beyond the preprogrammable criteria.

If you look at the first picture I attached, it indicates possible causes. One of which is low compression and another a misfire. With that in mind, I need to know what is happening with cylinder three. What is the compression? How does it compare to the other cylinders?

I am going to attach a flow chart designed to identify the cause of a P0170.

Also, here is a link that shows how a compression test is done along with causes and symptoms:

https://www.2carpros.com/articles/the-reasons-for-low-compression

https://www.2carpros.com/articles/symptoms-of-low-compression

https://www.2carpros.com/articles/how-to-test-engine-compression

Here are the specific directions for testing compression on your vehicle. It includes the minimum compression required and the maximum tolerance between cylinders.
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COMPRESSION INSPECTION [KJ]
1. Verify that the battery is fully charged.
- Recharge it if necessary.
2. Warm up the engine to the normal operating temperature.
3. Stop the engine and allow it to cool for about 10 minutes.
4. Perform "Fuel Line Safety Procedures". Leave the fuel pump relay removed. Refer to Powertrain Management; Fuel Delivery and Air Induction.
5. Remove the spark plugs.

Picture 6

6. Install the SSTs into the No.1 spark plug hole.
7. Fully depress the accelerator pedal and crank the engine.
8. Record the maximum gauge reading.
9. Inspect each cylinder as above.
- If the compression in one or more cylinders is low, pour a small amount of clean engine oil into the cylinder and recheck the compression.
- If the compression increases, the piston, piston rings, or cylinder wall may be worn.
- If the compression stays low, a valve may be stuck or improperly seated.
- If the compression in adjacent cylinders stays low, the cylinder head gasket may be damaged or the cylinder head distorted.

Picture 7

Compression

10. Remove the SSTs.
11. Install the fuel pump relay.
12. Install the spark plugs.

___________________________________________

Let me know if any of this helps or if you have other questions. Note: If the check engine light is flashing, you really shouldn't drive it. It will end up causing other problems and damage.

Take care,
Joe

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