23 replies to this topic

[Jordan]

Hey guys I’m after a bit of help on a problem i’ve had in the last day.

 

2001 Forester GT

I’ve had a 3” x-force turbo back exhaust installed, and now i’ve developed a pretty nasty hesitation/miss at low revs or when i jab the accelerator. No CEL showing.  

 

Basically, If i give the accelerator any good press from idle, it will stutter for a good half a second before the revs pick up, or if i try and rev match downshift, it will hesitate/miss, sudden rev spike and backfire as i go down a gear. If i press very lightly, it will rev out no problem.

 

Also, I've noticed i'm now idling at around -25in vac whereas i used to be around 20?

 

Other than that, it still revs out fine and drives well. I’ve reset the ECU (By disconnecting battery, pumping breaks and waiting), a few times, but it hasn’t really fixed the issue. 

 

Going through all my hoses today searching for a vacuum leak i found that the BOV gasket wasn’t on properly (I took one bolt off and the gasket fell out), but after fixing that it still isn’t entirely fixed. 

 

Can anyone give any advice on what else to check for before i get it to a mechanic that isn't useless? I’ll clean the MAF sensor tomorrow but i don’t think that is the problem seeing as it came up in one day. 

 

Thanks.

[duncanm]

ECU reset?

[Jordan]

ECU reset?

Yeah mate done it a few times, still happening

[duncanm]

Sorry - didn't read your first post properly.


Less vac at idle certainly suggests you have a new intake leak, as you've obviously suspected already.

wouldn't be surprised if your exhaust people disconnected something.

Is the O2 sensor and wiring all ok? Presumably it had to move with the new exhaust.

[Jordan]

Sorry - didn't read your first post properly.


Less vac at idle certainly suggests you have a new intake leak, as you've obviously suspected already.

wouldn't be surprised if your exhaust people disconnected something.

Is the O2 sensor and wiring all ok? Presumably it had to move with the new exhaust.

 

 

O2 sensor is connected fine, and was only moved around 1inch from where it was originally. 

 

Going from -20 to -25in Vac wouldnt that be more vac at idle?

 

Im also thinking possible a dirty throttle body that is sticking perhaps

[duncanm]

sorry - brain isnt functioning tonight. You're right More vac means less air getting in (and probably lower idle speed).

IACV ? - but why would any of this stuff have changed with an exhaust install?

[Jordan]

but why would any of this stuff have changed with an exhaust install?

Thats my thoughts, Only other thing i can think is that they had the intercooler off to get the heatshield out, and they have missed something or split a hose, but i havent found anything.

[Reevesy]

Could also be dirty plugs.
With the exhaust change it had possibility of running too rich before the ecu was reset and fouled the plugs?
I doubt there would be any split hoses as your vac reading is good

[Jordan]

It didnt seem to be running rich before but i guess it could have been, Exhaust was clean and didn't really have any black soot. 

 

It was running the same way before the ECU was reset too but it may be something to check out.

[LPM_B4]

I haven't had any trouble on the standard map with a turbo back. Same car so my thinking is the problem may be elsewhere. only an idea though.  

[Jordan]

I haven't had any trouble on the standard map with a turbo back. Same car so my thinking is the problem may be elsewhere. only an idea though.  

Yeah a few of us have done the same exhaust and havent had issues, so i'll agree its a problem elsewhere. 

[Jordan]

Alright so cleaning the MAF hasn't helped, and I haven't found any other leaks or issues

I doubt this is something the ECU is learning, going to go grab my obd2 port and start scanning

[Jordan]

Ecu scan has showed no codes, and the miss is getting worse , probably twice as bad now and lasts longer

If it was an oxygen sensor or MAF sensor it would either throw a code or a CEL already?

I'm starting to think maybe leads/plugs/coilpacks coincidentally went at the same time as exhaust install?

Any last suggestions to check before it goes down to Ichiban next week?

[Reevesy]

Was the exhaust new or second hand. If second hand it may be a blocked cat. But I would be checking plugs/leads first.

[Jordan]

Brand new xforce system

Yeah I'll get on it

[Robert]

Foz uses coil on plug yeah ?

[Jordan]

Foz uses coil on plug yeah ?

Yeah man, bit out of my depth now 

[Robbks]

MY01+ requires a re-tune when you mess with the airflow characteristics.
 

[Jordan]

MY01+ requires a re-tune when you mess with the airflow characteristics.
 

I've had a couple mates + Lachy as above who haven't have any problems with turbo back exhaust only, but it will be getting a tune when new turbo+supporting bits go in. 

 

The fact that the problem is getting worse makes me think some part is failing rather than just ECU problem, and then again it would have thrown a code by now. 

[Robbks]

more vacuum at idle is also indicative of a less restrictive exhaust (which you now have)
So the engine expels more gas easier, and can then inhale more intake charge easier.
And this happens for just about all engine load scenarios too.

problem with this is the AF learning values go out the window,
So each time you touch the throttle the ECU pumps in a little bit more fuel (called acceleration enrichment,) to account for the in-rush of air.
Problem here is these tables are dependent on Throttle Position. But you are actually allowing comparatively more air in that the fuel that has been injected.
SO you end up with the car running lean for a split second, so it hesitates and mis-fires.

Conversely, the o2 sensor can notice the lean condition and richen up the fuelling,
this can then cause the car to run rich in other areas of the mapping, again not ideal, it may begin to foul the plugs

All this can be seen if you interrogate the ECU (a $20 VAG-Com cable and Rom-raider on a cheap laptop is all you need to read/ Log ECU data)