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Initial hesitation on HOT acceleration - 1998 Subaru Legacy Outback

posted Apr 13, 2016 15:20:43 by RobBrown
Hey all,
Gearing up to do some diagnostic work this weekend. I have a 1998 Subaru Legacy Outback with the 2.5 engine. Acceleration from stop is smooth under most operating conditions. But after an extended time of cruising, when the engine is really warm and has been running at high RPM's for a long time, initial acceleration has a half second RPM drop/hesitation that is immediately overcome and normal acceleration returns. So I cruise on the highway, take an off ramp, stop at the stop sign, hit the throttle and there's a brief RPM nose dive/hesitation that goes away until the next time I come to a stop and try to get going again. (I hope that's clear). ONLY happens after extended cruising.

I'm hoping to get some ideas about where to start with my diagnostics given these symptoms. Things that could contribute to this issue, as I understand it, might be: fuel pump/pressure/volume concern that's exacerbated by heat, some sort of vacuum leak that's made bigger as the engine warms, EGR that's sticking or slightly opened after extended operation, a MAF sensor that is glitching when really warm, a TPS that's glitching when really warm.

Any other ideas what might cause a symptom like this? Something I should check out? Again, this brief hesitation ONLY occurs after extended crusing.

Thanks for your input!
Rob
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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Noah said Apr 15, 2016 11:50:47
Sounds like you got the right ideas, just gotta find the right lead to follow.
Start with fuel trim numbers and find the next most logical, easiest step first.
Massachusetts, USA
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