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2005 Odyssey - rough start with overheating engine

My wife’s 2005 Honda Odyssey has this problem since June, 2009 and the dealers have not been able to figure it out. We have spent about $1500 to no effect.

This problem recurs about 3 weeks after a trip to the dealer to fix it.

The problem:

1. startup idle is rough

2. Engine overheats at low speeds (per the gauge)

3. The check engine light comes on (code is P0304 - misfire at cyl. #4)

We have visited two Honda dealers who have tried the following:

1. moved #4 coil to #5 spot (still had the same problem at #4, later on) - dealer #1

2. Adjusted valves - dealer #1

3. Replaced the engine thermostat - dealer #2

4. Replaced the coil pack for cyl. #5 - dealer #2

5. Adjusted coolant level

6. pressure test - no issues - dealer #2

The problems go away after the dealer clears the code and tops the coolant or whatever - but always recurs.

We have a recurrence now - and are wondering what to do. Any suggestions?

I don’t see on your list where either dealer pulled and inspected the #4 spark plug? I’d do that first to see if there is any evidence of coolant leakage into the combustion chamber. A good pressure test is a hopeful sign. It might be time to go ahead and replace all the plugs anyway, you will have to check your maintenance schedule to see when that is due and when it was last done.

Next thing to check would be the coolant temperature fan switch. Overheating at low speeds is commonly caused by the fan not coming on. At higher speeds, there is enough airflow through the radiator so that the fan is not needed.

BTW, are you loosing coolant? Check the cold level daily for awhile. A loss of coolant would not be a hopeful sign, but if its holding steady, then that would be good.

In addition to a check of the plugs, I would be wanting to check the compression. Since this can be a little difficult in these transverse 6 cylinders, you can get a quick and cheap look at the compression and test for head gasket problems with a vacuum gauge. If there is a compression problem the gauge won’t tell you much in detail (e.g. cylinder by cylinder), but a good and steady vacuum reading does make compression problems unlikely in which case I’d be inclined to continue normal troubleshooting.

“The problems go away after the dealer clears the code and tops the coolant or whatever - but always recurs.”

If you are telling us that the coolant has needed to be “topped off” multiple times in the past 5 months, that is a VERY bad sign. Among other possibilities, this engine might have head gasket problems.
If that turns out to be the case, then both dealerships are “rearranging the deck chairs on the Titanic”. (In other words, they are doing things that will not protect the ship from imminent disaster).

Have you examined the motor oil for evidence of coolant contamination (a “milky” appearance to the oil) and for evidence of motor oil in the cooling system (a dark line of oily residue in the coolant overflow tank)?

I had the same exact issues with our 05 Odyssey. Blown head gasket was the culprit. In the process as I’m typing this, it’s getting a bottle of bluedevil head gasket sealer run through it.