Random misfire when AC is on

shawny · Post by **shawny** » 20 Jul 2021, 01:06

2004 V70 NA 5spd at 286k mileage.

Wagon runs well if air conditioning is off. When AC is on, the car will throw P0300 and P0420 code. Check engine light will flash and show "Try to slow down or shift up" message. If I keep driving, the car will run rough and hard to keep speed over 50 mph.

The small scanner gave some reading of fuelsys is CL under idle.

When I raise the RPM to 3500, the fuelsys is OL-Fault. The CEL starts to flash and misfire.

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I have replaced spark plugs and wonder what could be the problem only AC is ON.

Post by **jonesg** » 20 Jul 2021, 15:18

gather a bit more information, does the radiator cooling fan run when AC is on?
does the misfire go away when the engine is restarted ?
is the compressor draggin the engine down?

shawny · Post by **shawny** » 20 Jul 2021, 20:06

Thanks.

Radiator cooling fan runs when AC is on.

Misfire goes away if we restart the engine. Misfire will not be present if the AC is off.

Is the compressor dragging the engine down? I am not sure how to tell that based on what I see.

Post by **jonesg** » 21 Jul 2021, 01:28

shawny wrote: ↑20 Jul 2021, 20:06 Thanks.

Radiator cooling fan runs when AC is on.

Misfire goes away if we restart the engine. Misfire will not be present if the AC is off.

Is the compressor dragging the engine down? I am not sure how to tell that based on what I see.

AC always loads the engine, if something else is marginal it will often show up when fully loaded by the AC.
Marginal battery can cause it, low fuel pressure too.
If fuel pressure is good then load test the battery. AZ does battery tests for free.
You can release the acc belt and check the clutch by hand....feel for bad bearings.