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When the engine is completely cold or at normal operating temperature it consistently cranks quickly and starts great within two seconds.

However if the dash temperature gauge shows between 1/4 to 1/3 I often get longer cranking (5 to 10 seconds) after pressing the engine start button. This happens say maybe a couple hours after parking up. So far starting didn't fail and once started she runs OK.

Some recent work may have introduced a small air pocket or bubble into the coolant. I'm wondering if the longer cranking at warm start is caused by this. Perhaps the warm engine partially exposes the coolant temperature sensor to air at at different temperature to the coolant which confuses the EFI system ?

EDIT: Updated title as I was likely over-complicating this for myself and others

The issue has only started happening in the last few weeks following recent activities. The climate here is around 20C/68F.

Recent activities which could have triggered the problem include checking the EGR value (clean), checking engine coolant sensor for cracks (none) and having the fuel injectors pro-cleaned.

There are no coolant leaks or fuel leaks around the injectors (new o-rings were used). All fluid levels seem OK and are not changing. She drives OK and mileage consumption is normal. Long term fuel trims are within 5% on both banks and there are no current or stored CEL/OBD codes.

Could an air bubble in the coolant cause such symptoms or another issue ? Appreciate your help here as I am learning to troubleshoot these things.

This is a six cylinder 2007 Subaru Legacy 3.0R at 110k km with push button start, serviced regularly with recommended parts and fluids.

  • What is the outside temperature? Is this hard to start problem the same summer and winter? – zipzit May 12 '18 at 11:59
  • When it does eventually start, does it immediately run smooth, or is there any kind of miss or stumble that clears up? – Milison May 12 '18 at 12:36
  • What is it when normal? 1 sec? 5 sec? What does "much longer cranking" mean? 20 sec? 30 sec? Does it sometimes just not start? – Pᴀᴜʟsᴛᴇʀ2 May 12 '18 at 12:41
  • Are there any OBD II codes? Have you checked the fuel temperature sensor? – GdD May 12 '18 at 18:14
  • hi @zipzit this only started recently following the changes listed – ajayel May 12 '18 at 20:33
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To close this out the problem was with the fuel injectors after having them pro-cleaned. Replaced the injectors and the the warm start long cranking issue has gone.

The troubleshooting was to connect a fuel pressure gauge to each fuel rail and clamp upstream after turning off the engine. With the engine cold the pressure maintained but with the engine hot the pressure all dropped off over an hour each side.

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