P0303/P0304 Above 70MPH - Subaru Outback - Subaru Outback Forums
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post #1 of 5 (permalink) Old 10-20-2018, 12:46 PM Thread Starter
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P0303/P0304 Above 70MPH

Most highways here in Washington are 60mph and I typically do the speed limit or about 65. From time to time though, I've been on a few highways out here that are 70mph. My '98 outback 2.5 5spd will do exactly 70 all day with no issues. However, once I push it closer to 80 for passing etc. for more than 30 seconds approximately, the check engine light comes on with P0303/P0304 codes for misfires on cylinders 3 and 4. I've never felt the car shudder or lose power when this happens. It continues to feel just fine.

The first time this happened, I erased the codes and they didn't come back. Thursday I was on one of these highways and it happened again.

I've only owned the car since the summer. As far as I know the PO had the following work done: New front CV axles, headgaskets, timing belt assembly, water pump, radiator, rear brakes, backup light switch, starter, steering rack, oil pan gasket, valve cover gaskets, spark plugs, and all the exhaust forward of the cats all within the last year. Personally, I've replaced the clutch disc, pressure plate, flywheel, clutch fork, rear main seal, clutch master and slave cylinders, shifter stay bushing and shifter joint.

As you may have seen from my other thread, the car has been bucking under 2,500rpm in first and second gear when the throttle is barely being pressed. I don't know if that could be related at all, just throwing that out there.

Sorry if this is a re-post. I swear I saw another thread about this pertaining to the ~70mph issue in particular when I looked it up before, but I can't find it now. Just looking for some advice or suggestions. I don't really want to just throw money at it.

EDIT: Actually checked the codes just now and it's showing P0301, P0302, P0303, and P0304 this time. So all four of them. Coil pack then?

Thanks, everyone.

1998 Subaru Outback
2.5L 5SPD
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post #2 of 5 (permalink) Old 10-26-2018, 04:55 PM
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Not sure why no responses, but my .02.

- The coil pack (CP) is an easy thing to change out. I'd grab one from a local used yard. Just note that your year was about when SOA went from 'innies' to 'outies' where plug wires connect to the CP. You can even swap to a Neon (Mopar) CP that supposedly has more 'juice'. There is mod info online.

- NGK (only) plugs gapped correctly?

- Valves adjusted when head gasket was done? More than one of these EJ25D's has had a burnt valve (or two).

RE: TPS- The throttle body should be the same (except maybe for a vacuum line) and includes the TPS, from any 2.2 or 2.5 from '95-'98/'99. Try swapping that out, too. Biggest pain is getting the TB coolant lines disconnected...and yes, you'll have some coolant/AF come out.

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post #3 of 5 (permalink) Old 02-23-2019, 12:46 PM
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Not sure if you got this straighten out yet but mine was doing something similar where over 70 the check engine light would flash and give me the same codes you have. My issue turned out to be the alternator. Once i replaced it no more flashing check engine light and the car generally felt better.
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post #4 of 5 (permalink) Old 02-23-2019, 01:06 PM
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yeah, if it's always that pair of codes - points to coil pack due to 'waste spark' ignition scheme.

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post #5 of 5 (permalink) Old 03-28-2019, 02:35 PM
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Start with the basics.

Ignition coil, wires and spark plugs.

The photo isn't correct - the part number is.



I did this on my wife's 1999 Outback 2.5 5-speed. That resolved inconsistent acceleration, random mis-fires and fuel economy issues (went from 12mpg to 25mpg).

Be sure to apply some dielectric grease to any electrical connector and the spark plug wire ends (flat end where they contact the spark plugs). This helps to prevent moisture from seeping in and causing issues.


I also put a small 'dab' of this (anti-seize) on the threads of the spark plugs - it really helps if you need to replace them anytime in the near future.


One last detail - when you're tightening down the spark plugs - there is a crush washer and is designed to be compressed/crushed, it creates a seal between the combustion chamber and the outside world. You basically want to 'snug' the plugs until they stop. DO NOT crank them in like your life depends on it, but you also don't want them loose (dealt with that BS - the threads were RUSTED and exhaust gases were seeping out via the plugs).

Again, I did the above to my wife's 1999 Outback 2.5 and it runs like a champ now! No CELs of any kind!
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