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Boost issue!


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ive been round the houses with this one!

would anyone be kind enough to offer some advice / opinions / general knowledge?


1995 jdm wrx saloon with standard internals fmic and td05

im experiencing fuel cut and boost issues -  turbo spools up kicks then boost goes crazy sometimes up to 1.8bar!! then fuel cut

Things I have checked so far:

bcs (3 port)  cleaned with carb cleaner

maf cleaned

actuator arm adjusted to 3mm of preload

direct vac line on turbo (boosts to 0.5 bar and holds)

check fittings on intercooler and vac lines

read fault codes under the dash ( did return maf sensor code 23 but put a power probe on it and it raised with revs also dies when unplugged)

I semi mechanically minded but begrudge going to a fitter for diagnosis that's even if you can find someone who would want to touch an impreza!!

im really losing my patience a jap specialists did tell me that the intercooler is very poorly fitted prob a cheap £150 eBay job


Next job is a smoke test may help to add that today it developed a schreech at mid boost which reading up may be the up pipe shagged? is only done it since I re fitted the air intake silicone pipe on the turbo.


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Is the screech a bit like when you blow through a blade of grass like a harmonica ?

As it does sound like a boost leak probably from the intake pipe or fmic pipework.

I assume you have a boost gauge ,where is the vac line it's getting a reading from plumbed into ?

Ideally these need to be fitted directly from the inlet manifold . Or tee'd into a pipe directly from the inlet in order to get a correct reading.

The pre v3 (97) map sensor will only work up to 1.2 bar and cause fuel cut after that ,even when mapped and the boost map threshold is increased (so most mappers will suggest that you upgrade the early map sensors)

Has it been mapped and if so who and what method ?

As classic ecu's do self learn to a certain degree but if you've got a decat turbo back ,fmic and intake mods ,it would probably breathing out of the standard ecus map parameters

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it does sound somewhat like blowing through grass yeah but not throught the whole rev range but that has only developed today? I still had fuel cut and hesitation / mostly through 3/4/5th before it.

yes it has turbo back de cat and z4 ecu which I think is standard ecu? I haven't took it apart to see if there is a daughter board inside.

My boost gauge is apexi and directly on the top nipple of the intake fed through the bulkhead.

I have no idea if its been mapped I bought it with this issue but really am getting to end of my teather mate.

would a smoke test be the best option for finding a leak or should I drive it in test mode and see what gets stored?


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A smoke test should show up any boost leaks but I'd check the intake pipe you fitted as you said it started screeching after you fitted it .

You'd get more pressure in the vac line system when the engines under greater load in the higher gears .Which in turn would change the boost limits achieved.

If it holds 0.5 bar when running "actuated" boost without fuel cut kicking in . I'd guess that it's due to the lack of back pressure (decat) and increased air flow (fmic /air intake). Are causing the boost to peak above the maps standard duty cycles which activates fuel cut .

I wouldn't keep driving it on boost if it's cutting ,as it's probably trying to keep the boost in control or maxing out your injectors (If they're grey 380's ) so it doesn't eat itself by running lean .

Is there a 4wd dyno near you as a dyno run (£60 ish) would probably be the best option for identifying leaks and over boost .

If you can't afford the £550 to get it mapped you could try to get the intake /tmic back to standard and see if the problem persists

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  • 2 weeks later...

well the good news I found out the issue with over boost fuel cut!

running suberp! bad new was it decided it didn't like being enclosed in the bay and wanted to come out, diagnosed as piston ring failure plooms of smoke

and no boost!

Have now took it off the road and will start from scratch! engine suspension gearbox diff will be rebuilt, I like this car more than my misses.

I believe it is worth it on a near mint v2.

ive started a new thread ' pop goes the scooby' and will document everything I do as I enjoy reading build threads.

time of completion I would want this side of xmas.


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