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Matt
Matt, Engineer
Category: Subaru
Satisfied Customers: 20588
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Subaru engine surging while driving up ll on hot day, within

Customer Question

Subaru engine surging while driving up hill on hot day, within 5 miles after fillup. I geared down from drive into 3 and turned off a/c as I crested hill and it stopped. Not sure what the issue is, if contaminated gas and need to change fuel filter, or
weakening fuel pump that might need replacement, or issue with head gasket straining in 93 degree heat, though it was replaced 4,000 miles ago. I need ideas on what to look for to insure smooth reliable running when travelling up hill.
Submitted: 1 year ago.
Category: Subaru
Expert:  Matt replied 1 year ago.

Hello

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile

Customer: replied 1 year ago.
I suppose I could take an old Berzomatic propane torch and hold the nozzle near the vacuum hose, and hope for engine surge on problem area. Will look at Haynes manual, look at Youtube Subaru and Japanese car videos for air leak vacuum hoses and see if I can then narrow it down to specific hoses. Not sure where MAF wire and sensor/air meter is on Suba. Will poke around today and tomorrow and get back to you.
Expert:  Matt replied 1 year ago.

Understood

the MAF meter is just after the air filter box as part of the intake tube