Boost leak or maf?
 

So apologies to the regulars who probably see 'boost leak' threads come up all the time but, iv been shirking around in this forum for ages and searching for all sorts of things but can't quite get to the bottom of my problem.

Now what happens is when I'm driving along all is fine but if I give a quick blip on the throttle or even give it the full beans if I depress the clutch in and wait for the revs to go down they drop way past the normal idle of around 850. Iv replaced a couple of iffy lookin hoses with new Silicon hoses. Including the intercooler one. (Top mount) and also replaced the iacv as that used to play up. Seems happy now. Now I'm not sure if it's coincidence or not but it only seems to have started doing this since I had my up pipe replaced with the Harvey trick jobbie. Could be in my head but wanted to run it past you all.
It does have a cheap dv on it. I didn't put it on there. Came with. Will ideally replace that but wanted to see if the symptoms sound like the dv, boost leak or the dreaded maf. It's a 94 classic sti. I took it out earlier as replaced those hoses and it didn't seem to do much. Still revs dropping and occasional stalling. It's going my head it :mad: any pointers would be greatly appreciated. When you do get the toe down it boosts to 1 bar happily. Not that I'm making a habit of it ATM due to its condition. Would rather find out first before I go thrashing it around.

Cheers

Dave

Also to note. It has a hks filter and full 3" exhaust. And ofc the dv.

the idle is lower than that , it may look like it's going too low but maybe it's not , the up pipe did you replace a cat or was it a non catted sti , if you have lost a cat and not remapped that maybe the problem , could be a boost leak , i some times have the revs drop low on the fsti , plus worth cleaning the maf any way , do a reset and let the car re learn it's plugs under the dash for a reset or disconnect battery pump brakes about 50 times leave for 30 mins and put battery back on , worth cleaning boost solenoid too

I think you've answered your own question "cheap dump valve" bin it!

The idle should be about 850 at warm idle. Which it does sit at but when I blip the throttle and get any sort of boost it sometimes drops down to 200 when I let off the throttle. As iv never been out in anyone else's scoob or owned one before it's very hard to know a bench mark isn't it. It didn't have a cat on the up pipe. Have ordered a new forge vta so that's coming this week, I'm currently spending more on this than my girlfriend lol, it doesn't hurt if she doesn't know :D If the new dv doesn't make a difference I'll clean the maf (again) and then continue in my search for a boost leak. The only thing that concerns me is it's the maf as iv heard these symptoms also point to that. That is correct isn't it?

I had a simalar problem mines used to bounce at idle only a bit but car used to rattle as ot dropped like it was going to die read a post on youtube vid other day sayin its the dump valve so i bought a standard one and tried it now the one i have is a genuine forge motorsport one with the blanking bung fitted so i changed it to a standard one as i bought car with the forge onit and gess what no more roughy idle or judder at idle i used to leave my blowers on low and it wouldnt judder but if i had it off it would so just to get rid and make it run smoother i have left standard recirc on only cost me £20 too :) now selling the forge on the net so its a win win bud

maf can cause issues but does not show a cel , worth a clean and see what happens

Fitted my new forge bov today. No difference. Replaced a couple of boost hoses, no good, have taken the maf off and cleaned it out. Will see what happens. Does anyone actually know how to ACTUALLY test the maf to see if it works?

Although the forge vta bov is a quality dv it's still a vta which to run correctly needs mapping in,a vta bov has no benefits to performance apart from going pshhhhtt.
Best trying a recirc valve,this way the ecu is getting the air expected rather than it being vented and causing a brief rich moment.

Have you tried a stamdard recirc valve mate

I havnt no. As the car came with a crappy copy one. I did see your post about using a standard one. I suppose my issue is that previously the car has been fine with this other dv on so thought it couldn't be that it needed a standard recirc back on as thought if it's fine with one why would it need a standard one back on? If you see what I mean. It's been fine before but has now decided to play up. Maybe I should give that a go. Loathed to as I'll lose that lovely whoosh noise :D it idles fine. Drives fine. Boosts fine. When it's cold it starts up no problem and when warm it's no issue. I don't want to buy a new maf for the sake of it and it doesn't change as that's a lot of dollar for nothing. Saying that it's cheaper than buying another engine I suppose. Thanks though guesty. Maybe I should give that a go though see what that does

Well the mafs can be cleaned with electric contact cleaner ive been told and the standard recircs do make a noise just not as loud mate and there only about £20 off a breaker mate so cheap enough to rule out have you had battery disconnected to reset the ecu ?? Also worth a try it costs nothing to try ?

only way of knowing is borrow a working one or buy a new one , import car parts prices are good , at least yours in cheaper than a new shape one , have you reset the ecu yet , i take it you always use sul

Yeah that's true. Worth a punt isn't it. Have cleaned the maf with contact cleaner, have taken it off the car and done it properly, and yes have disconnected the battery. Probably won't get a chance to have another look till Saturday now. What's 'sul'? Although some say you have to properly reset it by the method on scoobypedia I think it is. Connect the connectors under the dash.

hi super unleaded , if a maf is going symptoms can be stalling, inconsistent boost sometimes more sometimes less , rough idle , but other things can cause same issues so no easy answers , mafs are known to fail so if it's not a lot it's worth a punt , i take it services are up to date plugs leads , coil pack ok , fuel pressure ok ? an ecu reset is important as you may cure the issue then wth out the reset not realise you have , i take it you don't use a bit of the accelerator to start the car as that confuses the ecu too

Ooh sorry yeah I do use v-power all the time. It's difficult isn't it as iv heard the same. Could be maf but then could be lots of other things. Thing is it idles lovely. No jumping around. Sits good at 850. Plugs have been done maybe 3 years ago. Not sure on the leads tbh. I have been thinking about doing that for a while along with the fuel filter aswell. I'll do what I said above, cleaning etc. Put it all back on after the reset and see if anything changes, if not then maf and get another dv, we'll do that first otherwise it's maf I suppose.

dirty throttle body or throttle position sensor , other things can sometimes cause issues are crank and cam sensor but from what you are saying not likely as they tend to cause starting issues and running issues , leads may be breaking down pedending or mileage , it's not easy to diagnose , a maf borrowed would be worth a try

No test afaik, apart from replace it with a known working one.

I assume that ALL changes to the engine have had map tweaks, especially things like the up-pipe and the air filter?

Ok yeah. thanks for your help. Process and elimination seems to be the key.

Alxazar - well when I purchased it from the previous owner (my sister) she had it rolling roaded at zen probably around 5years ago and they said it all ran fine just slightly rich at top end, it came from Japan with the exhaust, air filter and dv already installed so iv tried to stray too far from what it already has. As regards the up pipe when I purchased it from alyn I asked whether it'd need a map tweak and he said not. I'm not sure what ecu it even has on it tbh. Still learning what's what with it. How can I tell? As it's a v1 sti I have read that the original ecus aren't mappable so have been looking at race dynamics ecus.

breaking down depending on mileage .. i love auto correct especially when it makes no sense

Since the plugs have been in there for 3 years I would be taking them out and at least checking gaps and for fouling,even putting new ones in,far cheaper than buying new maffs and dump valves.

Plugs are £18 each and theres four second hand maf £70 standard valve £20

Cleaning and checking gaps are Free
And I'm not sure where you're buying your plugs but my last set of 4 PFR7B's were £31.21 :thumb:

the up pipe may be the issue the only way you find out if it needs a map is to check it

Yeah cleaning and checking pipes are free and only plug recommended by supplier
https://cimg5.ibsrv.net/gimg/www.sco...583da12ed5.png

For a start off the op has a GC8 so your recommendation is wrong.
Ngk pfr6 are standard and 7's uprated

Ahhh my bad i didnt know

https://cimg9.ibsrv.net/gimg/www.sco...7b0cce69ef.jpg

Al of the above advice is greatly appreciated cheers lads. I will check them. Today had bit of a breakthrough I think. So I wanted to clean everything as one last ditch attempt to get it all ticked off the list. So as already had the maf off for cleaning decided to remove the throttle body and gave that a clean. Was fairly grubby inside but also whilst it was there I cleaned and checked the top mount and this is what I found. The pipe that goes to the turbo had splits on it. By the turbo itself. And there was a bolt missing out of the top mount and the gasket has popped out and was leaking. Can see the black make around the fitting. So I'm hoping that was/is my problem. Will buy a new pipe and gasket and refit all that and then go from there.