Big misfire when accelerating

[artistjohn]

Original poster, are you saying the car misses only at 70mph/highway speed, while it never does so at lower/town speeds?

Yes. It takes awhile - maybe 30 minutes or more of highway cruising before it does this. Only notice it when on a trip. Short bursts of highway speed don't cause any misfires. Something's getting hot?

[June]

On my 2004 S80 T6 I had the same symptom and swore the engine was misfiring only at interstate speed after a good thirty minutes and it turned out to be a bad axle.

No engine light, but it seemed like the engine was bucking with the lightest acceleration varying severity the more I pushed the pedal down. No problem in town or clicking/popping when turning.

I was so frustrated I told the service manager to have my mechanic drive it home for however long it takes to figure it out. The axles were about 10,000 miles old and it didn't seem likely to me, but I figured they know best about these things. Volvo replaced both axles under the warranty and it fixed the problem. It never was a engine misfire. Definitely a learning experience for me. June

[artistjohn]

Thanks. Never would have thought this was related to an axle. Have replaced both in last year, but could have gotten a bad one I suppose. Frustrating.

[oragex]

As mentioned above, I would first have a look at the spark plugs. Even just by the looks, may tell there is something not right in one/more cylinders. I would also have a close look at the wiring going to the ignition coils, these may get crushed under the plastic cover. It seems the issue happens when the engine is fully warmed up, perhaps a check at the small fan hidden inside the tube behind the ECM box (the black box just in front of the power steering reservoir), if this fan doesn't turn easily, the ECM modules (engine control modules) may overheat and cause engine management issues. Another possibility that happens once in a while is contacts corrosion at the bottom of the two modules of the ECM.

[June]

Wouldn't a serious misfire due to plugs etc create a rich code turning on the engine light? I didn't see where a code was present or the engine light was on. June

[abscate]

Misfires need to be regular to store a code, so one that only happens under certain conditions will slip through.

Likewise, if it runs really poorly, it can throw phantom codes or no codes. A slight leak in my induction on my turbo gave me 3 cylinder misfires, general misfire, knock sensor error, fuel trim error, cat error.

All from a loose clamp

[June]

Thanks. Never would have thought this was related to an axle. Have replaced both in last year, but could have gotten a bad one I suppose. Frustrating.

I took some photos of the original axle replacement at 109k bill and notice I thought the engine misfired but axles bad then at 124k reported misfiring no codes found, than 125k it is written on the next two tickets as torque vibration, but I still was complaining the engine was misfiring but advisor wrote it as torque vibration, also notice I specifically said only after a hour drive on highway misfiring starts. It looks like they felt it was from the inner joints and on one of the tickets a engine mount was replaced which I wondered how that could cause a misfire, but I figured Volvo knew best. Then the ticket the axles were warranty replaced. I was convinced something was getting hot like in the old days when cars had sparkplug wires and one went bad it would miss when hot under load but be fine cold. It really shocked me that axles could cause no vibration then after extended highway driving suddenly start causing the car to buck like a misfire. I am quite sure my mechanic hates my car and probably me for bringing it in. I left the dates and mileage uncovered but did cover my personal information I hope that does not offended anyone, these are the tickets fore my 2004 S80 T6. June

[abscate]

Withholding your personal information from unknown multitudes of psychotic Volvo owners on the internet? How could that possible go wrong??

Thanks for sharing that info.

[June]

Artistjohn have you found the problem? June

[artistjohn]

I think we found the misfire problem - oil collecting under plastic valve cover rotted wires to coils. Oil cap gasket leaked. Now replaced everything and running well.
Thanks for all the advice!