This post discusses fundamental procedures as regards engine health checks.
First up you’ll want to use BTssm, Romraider or Cobb Accessport to datalog your state of tune, here’s how: viewtopic.php?f=6&t=36932
Engine health checks revolve around the Air, Fuel, Fire triangle and this FAQ covers many of the factors relating to AIR.
The faq is mostly specific to stock cars that are mostly emissions compliant, and use a MAF (mass airflow sensor) and a return-to-inlet PCV system (positve crank case ventilation)
Before proceding, check your MAF and engine air filter is clean and that it, and the air filter, are properly seated in the airbox.
Other basic health checks are exhaust pressure test and crankcase pressure tests.
Vacuum and boost checks, inlet pressure testing part 1.
Firstly check your maf is clean and that it and your air filter are properly seated, airtight: viewtopic.php?f=6&t=36513&p=451943#p451943
The most basic version of an inlet pressure test.
This is an engine off test.
I want to say performing this check is reasonably simple to undertake, you can definitely do this at home with minimal tools.
You'll need:
A screwdriver or basic 1/4' drive socket set.
This is used to undo and tighten the clamps on your cars "S" pipe, aka SFB (short fat bit)...
Or whatever pipe joins your your airbox or maf housing outlet to the inlet pipe (from the front of the intake manifold to the turbo compressor inlet)
A bung, aka blanking plug.
This must be the same, or very slightly smaller external diameter when compared to your airbox, pod filter or maf housing *outlet* diameter.
For most stock Subarus the out side diameter of the plug will be around 80mm or 3", but check yours by diameter or circumference.
You can use various measuring methods, eg a piece of string, a ruler, calipers and then look in your recyling bin, jar stash, hardware shop or supermarket for suitable jar lids or pipe fittings, aka bungs.
The important point being able to tighten the usual inlet clamp with the bung *inside* of the SFB, the S pipe or the main inlet pipe *after* the filter/airbox/pod (without crimping the that pipe which would cause an air leak).
Your inlet tract is now sealed and you can pressure test it at 1-2psi max *regulated* pressurised air.
Your inlet is temporarily sealed for pressure testing from the bung in the inlet all the way to the exhaust valves, which will leak a little requiring you to maintain regulated pressure at 1-2psi maximum. Go over this pressure at your own risk of popping cam and crank seals.
You can add pressure using your breath (circular breathing required and yes I can) a bicycle floor pump, a 12v tyre inflator or a shop compressor. The more powerful your pressure source the more likely you’ll need a regulator, but an emergency 12v tyre compressor will struggle to keep up with the loss: keep an eye on its gauge and or your boost gauge.
You can add pressure:
at the bung itself,
the bpv pressure reference hose between the bpv and the intake manifold adding pressure to the intake manifold.
the bpv return hose, using another smaller bung in the return hose leading to the intake,
the brake booster hose: if the brake booster one way valve is *not* in the brake booster hose
the oil cap, if on a car with pcv valve *and* by closing off any vent to air catch can, *if any*.
Bear in mind a closed throttle can be a restriction between the inlet and the intake, so either open the throttle plate, (key on not running for dbw cars) or add air to both sides of your inlet tract.
Why though?
The air filter is (probably incorrect terminology but anyway) a restriction at the start of the inlet tract and creates a pressure reference against which the MAF (mass airflow sensor) is calibrated, and against which engine vacuum pulls.
The inlet pipe, PCV (positive crankcase ventilation) system and crankcase operate under engine vacuum.
The intercooler and intake manifold under engine vacuum and turbo pressure.
The fuel purge system operates under engine vacuum, and has returns to the inlet and intake manifold which are solenoid controlled.
The inlet tract essentially ends at the exhaust valves in the cylinders.
Over time with heat cycles and miles, the hoses, plenums, gaskets, seals and orings will harden, compress, loosen or crack and cause inlet tract leaks.
Any leaks in the entire inlet tract can cause drivability and reliablility issues because the calibration and operation of the engine management system is thrown out of whack.
You can find evidence of the fault:
In the drivability experience, bad mileage, rough idle, stalling.
ECU (engine contol unit) Learning Values eg Fuel Correction, Fuel Learning and knock related parameters FLKC (fine learning knock count), FBKC (feedback knock count) and Knock Sum.
ECU DTC's (diagnostic trouble codes, aka CEL codes aka check engine lights), typically P0171, system too lean or P0172, system too rich. Not suprisingly the diagnosis for the former is the same as the latter.
Misfire codes or cylinder roughness counts in logging. The FSM DTC diagnostics for P0304 include P0171.
To test for leaks, its possible to replace the engine air filter with a plug or bung that should seal the inlet tract all the way to the exhaust valves in the engine.
Generally the stock airbox is not a convenient place to seal off the inlet tract completely.
If you can, go for it as you'll also be testing if the airbox and maf sensor are sealed.
So disconnecting the next pipe in-line, the SFB, is the perfect approach to a quick inlet pressure test.
Back to the test!
Ok your SFB pipe now has a bung in it, temporarily for the test.
To make putting the bung in easier, undo the clamp closest to the inlet pipe and rotate the SFB upwards by 90' for easier access.
Find the smaller pipe (of two pipes) leading from the intake manifold to the BOV (blow off valve, back pressure valve etc), remove the pipe and blow into it, using your mouth and breath.
Thats going to be an issue for some home mechanics, but you wanted the very simplest approach.
Its also going to be not very ergonomic, bent over on the LHS engine bay, but it will make for a quick indication of the presence of a leak or leaks in the inlet tract, plus you'll more easily hear hissing and wheezing from any partial leaks.
There's four possible outcomes at this point:
You'll "pump up" the inlet tract with a few to half dozen or so breaths, until you'll need to take a short break before adding more air. The inlet tract will take a similar amount of time to decompress, eg it will return to atmosperic pressure slowly. This is good, but you may still need to escalate the test further.
The inlet tract will "pump up" but you hear wheezing, sighing and the inlet tract loses pressure quickly. You'll be escalating the test further for sure.
You're trying to inflate the inlet tract but there's simply no resistance to the air you're trying to introduce into the inlet tract at all. You have a huge leak and the inlet tract is not airtight. You'll need to locate the huge leak and blowing cigar or vape smoke into your test hose will help hugely for this fault, when performed in a draft free area. If you find and fix the huge leak, you'll still need to repeat the test to look for more inlet tract leaks.
You cannot blow any air into the test hose at all. This is an unlikely outcome, but is possible with non standard fuels like e85 or perhaps cars that have never had UEC treatment (upper engine cleaner). In this instance you'll need to clean the affected hose or replace it, and repeat the test.
Just bear in mind, finding one leak in the inlet tract may reveal another leak if you continue testing and thats a good thing. Depending on the age of your car some leaks may be deep down and really hard to find, we'll get to that in "escalating the test procedure further".
So depending on the outcome of the test, you'll have no apparent leaks in your inlet tract, as such your drivablilty issue may not be leak related, eg the
AIR leg of the combustion triangle: it might be related to:
FUEL: fuel pump, jet pump, fuel filter, fuel pump relay, regulator, control module dampers, lines, looms and plugs; or
FIRE: spark plags, coilpacks, battery, grounds, alternator, looms and plugs or:
a control system eg AF (air/fuel) and O2 (oxygen) sensors, the exhaust or catalyctic converters.
Escalate the test and add some extra things to the test kit:
You can go to the car parts shop and buy a hose butt connector (or find one at the self serve wreckers) and a piece of hose the same diameter.
Put that on the end of the bov reference hose so you can stand back and blow, or use an air mattress or bicycle pump, or your breath still.
Or a step up is to buy some rubber tyre valves, sealant and a step drill (or scissors) and inset it into your lid or can inlet plug.
Remove the tyre valve.
Apply a pressure source, eg breath, bike pump, smoker, tyre compressor, but keep it under 1psi. Eg regulated air.
Use soapy water, eg hair shampoo and dish detergent/water mix in a trigger spray bottle (or a smoke source) liberally.
Big leaks/no pressure in inlet: check your oil cap is tight, or use smoke in the pressurised air.
From there, if you have tmic, use a plug or blank plate at the turbo & throttlebody hose to inspect all the pipes under the rear of the inlet manifold.
One leak found will reveal the next, so don’t give up looking till you’re sure and your learning view is correct.
If you’re still experiencing hesitation, rough idle, you can blind test the front o2 sensor by unplugging it temporarily, endure the resultant CEL, and see if the hesitation/rough idle improves.
If it does, replace the sensor with the oem or denso equivalent part number.
Now imagine finding a mechanic to do all of the above...
And being able to afford it
The thing is turbo cars over 250k km are absolutely cooked in the cyl3 area.
I just did lower inlet manifold gasket, upper orings, breather pipe, injector seat & oring, Pcv and breather hoses, check pcv (ok), bov ref, fpr ref/mod, and stock tmic... all leaking
My inlet and Tb hose all good since redone 50k ago.
Blank plugs sizes.
Stock airbox outlet OD: 80.5mm
Kobe SFB big end OD: 88.5mm (for upgrade clamp)
Kobe SFB small end 82.5mm (for upgrade clamp)
Stock postfl tmic outlet: 79.5mm
Inlet pressure test part 2.
It will help to be able to id all the hoses.
Ziptie everything without a clamp.
Going clockwise around the engine bay:
Inlet to turbo (rots underneath), replace with silicone inlet.
Wastegate to compressor outlet, tee’d to boost control solenoid.
Turbo compressor outlet to intercooler gasket/s
Blow off valve gasket
Blow off valve vacuum reference hose
Blow off valve return hose to inlet
Intercooler to throttlebody hose (stock hose has a gasket that causes leaks that may vary according to boost/vacuum)
Fpr vacuum reference hose to lhs rear inlet manifold runner (backfire under boost)
Front of inlet manifold: left to right
Fuel purge solenoid/s: check the hose is not cracked before the solenoid and after it at the return to front of inlet manifold
MAP sensor
Boost control solenoid, from wastegate tee, return to inlet manifold.
Around the top of engine: head breather balance tube and head blow by and crank case breather returns to inlet pipe
Injector seats. Really difficult to spot these leaks on cyl3 under the turbo coolant tank.9
Cam position sensor o-ring: viewtopic.php?f=10&t=36503
Don’t forget that for this test supposed to block the inlet tract post maf.
Note that for this test the engine should not be running.
The idea is to pressurise the entire inlet tract at no more than 1psi, after the maf.
Usually if doing a high pressure test, you’d isolate eg inlet tract from the throttlebody to prevent blowing out your cam seals.
Eg isolate the inlet to compressor outlet and throttle-body entry to low pressure check the inlet tract and crank case using a low pressure test to 1psi.
Then the intercooler and piping can be tested separately using high or max boost pressure.
Stock intercoolers with plastic end tanks will seperate. Use parallel pliers or clamps to re-crimp the tabs.
Prefl GT’s are notorious for rough idle issues.
There’s sharpies uec fuel rail soak for that, linked here: viewtopic.php?f=17&t=36178
Or get your injectors replaced or serviced, eg cleaned & tested, new orings and seals.
However that’s really a great excuse to refresh every gasket, hose and oring in the general area, as IME, based on my 280k postfl GT, they’ll mostly all be compressed, hardened, cracked and leaking.
Another consideration I spotted over at LGT is that side feed generation engines (usdm at least) have an (adjustment?) screw in the middle of the fuel pulse damper which, again on side feed usdm LGT at least, is located on the fuel rail on each engine bank.
Apparently if this screw backs out it can also cause issues.
I’d imagine some Locktite or a dab of paint might trap the screw at the desired setting, probably all in, or a few turns out (conjecture see FSM or forums for more info.
Now my issues were only bad enough to cause negative fuel trims and the occasional iam drop during cruise, plus extra fuel use, black smoke on boost etc.
What was leaking?
Stock tmic, bov ref, head pcv & balance breather pipes, upper and lower inlet manifold gaskets, injector seat oring, and right now, cam sensor oring.
A bunch of basic stuff wasn’t leaking due to being replaced a year or two ago but also check or replace the inlet & throttlebody hose and all related returns, and check, clean or replace pcv
Now paying someone to do all this would have been completely prohibitive.
I have a bunch of mods for the car but there’s no point putting them on a half rooted car.
Whilst I could have just replaced a bunch of stuff during the modding procedure, I have to say I didn’t entirely understand the scope of what’s required on such a high k car.
Well I sure do now.
I’ll fix the leaking cam sensor oring, see how it goes, then write a big list for all the extras I’ll need to take it up a notch with the mods.