P0305 code

LG LOGAN

MEMBER
2004 MAZDA MPV
3.0L
6 CYL
2WD
AUTOMATIC
220,000 MILES

Hi, really appredicate your time and advice in helping me.

I bought this car three months ago and drove it for a long trip (around 5,000 miles) without any issues. But these days, I found P0305 (cylinder five misfired). After I changed the ignition coil and spark plugs, the code P0305 is still showing and the worse thing is that there are white smoke and bad smell under the hook after running. In addition, there is oil leaking at the two valve cover bolts beside the cylinder five.

The mechanic checked the spark plugs and found that both of them are in very good shape without oil leaking at them. He then checked the engine compression for both cylinder four, five, and six and found that only cylinder five has a very low compression (only below 90 while other two are around 150). He told me that there are some serious internal problems in the cylinder five and it needs to be rebuilt which will cost tons of money.

I can tell that the previous owner is very honest and this car had no big trouble before except a little bit oil burning. I have checked the engine following your article (it is really professional and thanks for sharing so much), I think that it may not be caused by the blown head gasket since cylinder four and six are good and there are no signals that the coolant system has problems.

I could not afford $3,000.00 to rebuild the engine. Could you help to give me some suggestion? Will oil leak at valve cover bolts beside the cylinder five cause P0305 for my car? Must I open the valve cover to stop the oil leaking at valve cover bolts? Once I open the valve cover, must I change the valve gasket? Any other ideas?

Thanks a lot.

5 Replies

STEVE W.

MECHANIC
13,463 POSTS

From the description I would say you have a couple issues. The cylinder five misfire could be caused by oil if it pools around the plug and allows it to fire through the oil.
Because it is misfiring it would wash the cylinder with fuel, that will cause the piston rings to wear very fast. That can be the cause of the low compression.

Or it could simply be that something in cylinder five has failed and the lowered compression is causing the misfire because cylinder five is no longer contributing enough to the power of the engine.

I would do the compression test again and do both a dry and wet test. The dry is done with the ignition disabled and throttle wide open. The wet test is the same but you add a tablespoon or so of oil. It will get around the piston rings and stop compression loss past them for a few piston strokes. If you get 90 psi on the dry test and 120-140 on the wet test the rings are shot.

If there is no change then the compression is going someplace else. To find out where you can do a leak down test. It applies air pressure into the cylinder and you listen at the intake and exhaust for the air leaking out.

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Monday, October 9th, 2017 AT 11:17 AM

LG LOGAN

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3 POSTS

Thank you very much, Steve. I will follow your instruction to do the compression test again and do both a dry and wet test.

If I get 90 psi on the dry test and 120-140 on the wet test, do you mean that the rings in cylinder five need to be fixed?

Will AT-205 re-seal and Engine Restorer help to fix the problem? If they could help, which product should I add first?

There is no oil leaking at cylinder five's hole and the spark plug, but there are oil leaking at two valve cover bolts' holes beside cylinder five's hole. What does that mean and how could I stop the oil leaking?

Thanks again for your professional help, Steve.

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Monday, October 9th, 2017 AT 12:08 PM

STEVE W.

MECHANIC
13,463 POSTS

If the compression comes back on the wet test, it means the rings are worn. The thing is that you need to determine if the misfire is the cause for that or is the misfire the result. That is why I suggest the compression and leak down tests, they will show you what is leaking and where. It could be the rings or it could be a lot of carbon build up under a valve that is letting it leak. Once that is determined by the tests you can determine which things to repair.
I would not try anything until the tests are done. If it is a physical problem with a valve or bad rings no chemical will help.
I might go for a used engine if the body is in good shape. Rebuilds are good but you can spend a lot of money and have the transmission fail a week later. Maybe look at the you-pull-it type yards and get an engine yourself. Then you know how they come out and how to put it back in.

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Monday, October 9th, 2017 AT 3:28 PM

LG LOGAN

MEMBER
3 POSTS

Thanks a lot for your expert advice, Steve.

I have done the dry test and wet test following your suggestion.

For the dry test, I recorded twice for each cylinders, and the dry test result is:
Cylinder 4: 170, 175
Cylinder 5: 80, 78
Cylinder 6: 180, 180

It's obviously that Cylinder 5 has a low compression, so I done the wet test for Cylinder 5, and the wet result is:
Cylinder 5: 110, 100

From your analysis, this result means the rings of Cylinder 5 are worn. I can't afford to rebuild the engine, so I would give "Engine Restorer" a try since this product says that "The unique CSL formula fills worn areas of the cylinder walls to reduce blow-by and restore compression to near original levels."

I will keep updating this post if I see any changes of the compression after I use "Engine Restorer".

Appreciate your help so much, Steve.

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Thursday, October 12th, 2017 AT 12:31 AM

STEVE W.