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Cold starting issue - please help!


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HI all


Quick introduction as my first post is a mechanical issue lol


I have  V3 STI with SC42 twin scroll conversion, Deatchworks 740 injectors, HDI FMIC, Walboro fuel pump, ESL ECU, forged short block with cosworth pistons. Its running mafless too


OK here's the issue:



Since having the twin scroll conversion done the car is really difficult to start sometimes when cold or left overnight. I literally have to have the accelerator flat to the floor to get it to fire up, and when it does it fires up lumpy at about 100, then climbs to 1500 after a while and is fine.

However its intermittent - wont do it all the time, sometimes she fires up perfectly?


But another issue which im sure is related is -  if I drive the car up to temp then park up and let it cool for say an hour, when I start her up again the engine fires up literally as soon as it starts to crank over - almost as tho its starting too easily? If that makes sense? It will start when it does this but again limping on at 100, then if I blip the throttle it seems to sort itself out.


Both of these issues are intermittent and seem totally random, but its happening so often now that I just want it sorted.


Racedynamix has looked at the map twice now so pretty sure the map side if things is fine.


So far I have replaced - Brand new Subaru Oxygen sensor (genuine part), Brand new Walboro fuel pump,


I have ordered a new Crank sensor from ICP to see if that helps, and I also have brand new coil plugs to fit


Anyone have any ideas at all?




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Have you got a ait sensor fitted ?

As Mafless can cause a bit of rough idle as the air density sensor and mass air flow meters are taken out of the ecus parameters and it has to function purely from the map sensor .

So if it was mapped for warmer air in the summer the colder denser air in the winter can make a difference , so most mappers recommend fitting a air intake temp sensor to the tmic or fmic pipework to help with fuel trim

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Hi mate welcome, sounds like your going down all the avenues I'd be looking at to rectify the problem, i'd guess at the coils maybe with them being stone cold there's a break or back connection and once up to temp somethings expanded or warmed enough for it not to be an issue ? Just clutching at straws really. Have you checked the idle control valve ?

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Hi all thanks for the help -  it has a air sensor fitted - but this is the one that runs of the same bit of loom as the maf connector - the one that usually sits in the plastic air feed near the head light? At the moment its tucked in the wing, but im not sure if its still operational? Thing is the outside temp doesn't seem to make much difference, but to be honest its hard to tell. I just went out to it now where its been sitting for over 4 hours and she fired up perfectly...


Thing is its always had a weird starting issue, but before the twin scroll was done it fire up fine from cold, but now it seems to have manifested itself to the cold starting too.


Could a failing crank sensor cause this intermittent issue?


Not checked the IACV yet as I don't have a gasket to replace the old one - will order one now

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What power rating is the new walbro good for as I'm sure you know the turbo's good for 420+.

Mappers normally run the ait info to the ecu via the old maf wires but it's better situated just before the throttle body like this post-1399-142576236351_thumb.jpg

Few things that might be worth a look

If it's got a aftermarket fpr with a gauge does it read a steady pressure ?

Have you checked the vaclines to the fpr ,purge valve and charcoal canister ?

How old are the spark plugs and what grade are they ?

Are the engine earths and grey loom plugs clean ?

if it runs separate coil packs have you thought about a newage coilpack conversion .

Cleaning or replacing the icv is a good shout as well .

Intermittent faults are a pita best of luck bud

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API fitted some ngk7b spark plugs few months back.

The walboro is a 255lph one so should be ok and the map is relatively mild as I still have the standard 752 on there lol bhp I would estimate at about 360 as was mapped on road.

Its got the standard fuel pressure reg so I might swap for a fuelab one.

I've not checked the vacuum lines yet as I don't really understand what each one does but they all look ok? Will give them a check over tomorrow.

Got new coil pack to fit tomorrow so see if that helps.

Also its been suggested that leaving a stick on the accelerator over night so its open allows any dribbling fuel from a leaky injector to evaporate so will also try that.

One last question - where abouts is the coolant temp sensor located? And how easy is it to change?

Many thanks again

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Well I changed the coil pack over today and also cleaned the Crank sensor - but its made no difference :(


Coolant Temp Sensor next on the list - but it looks a PITA to do? Is it necessary to drain all the coolant before doing? Or can I remove old one, soak up any dripping liquid then quickly get the new one back in?

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Hi mate once i had my car rebuilt with different compression and forged it hated starting, this was caused by it running lean, have you had it on a dyno to make sure its getting the right fuel for the cold starts

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Not had it on the dyno yet no, I was saving that for when I get a 6 speed :)


I'm trying the 'stick on the accelerator' over night method to see if that helps, or at least guide me in the right direction lol Apparently it can help vent any leaking/dribbling injectors that maybe causing flooding - Certainly feels like its flooded first thing after being sat all night - i.e cranks but wont fire - then when it does eventually fire up there is a really strong smell of fuel


Thing is it did this with the old injectors pre twin scroll but never had this cold starting issue. But I'm not convinced the stick method will work, as its unlikely to have 2 sets of leaky injectors - but not impossible I guess. Anything is worth a shot I guess. But even if this does work its not really sorting the issue, kind of like taking strong painkillers for a toothache instead of going to the dentist lol

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  • 3 weeks later...

Hi everyone - just to update


I have replaced the following brand new genuine bits - (except coil packs as couldn't find genuine Subaru ones from ICP)


Coil Packs

Coolant Temp Sensor

Crank Sensor

Throttle Body + Throttle Position sensor

PCV Valve

Cleaned IACV and replaced with genuine gasket


And it appears to have sorted it out :) I'm guessing that it was the IACV all along. 


Its early doors so will need to see how it behaves in future, but so far so good

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