ChrisC

I noticed a slight hesitation yesterday on the motorway at about 5,000 rpm.
So this morning found my favourite quiet straight stretch and opened it up. Again the hesitation at about 5000 rpm (in 3rd) and then when I changed up the revs just died to 0 very briefly (although the car was in gear) and then the engine picked up - along with my heart rate.
So any ideas ?
I posted elsewhere and was told it might be overboost - a subject covered here. I can find some references to same but no description and nothing to explain exactly what this is. (Forgive my ignorance)
So was the diagnosis right ? Whatcauses the problem and how do I fix or get it fixed?

GrahamF

No helpful sugeestions as to what it is, but unlikely to be overboost, especially in 3rd. Normally felt pulling hard in 5th and feels like someone has switched the lights off - once felt never forgotten.

graham

Bitten Hero

Overboost is where the ECU detects the boost going out of range for it's fuelling and timing maps etc, so it temporarily releases all of it to bring it back down to an acceptable level. Net effect is all boost vanishes (and possibly there's a fuel cut in there as well?) as it's similar to driving into a brick wall. Apparently! I've never had it happen - for me it always seems to back off in a sensible, smooth way.

Normally happens in very cold weather and high gears (4th, 5th) as the boost builds better in these gears.

As for your problem - not sure I'm afraid! Sounds like it might be worth taking it to a dealer, or a rolling road tune as that will tell you. Sounds weird... probably a sensor problem (MAF, timing or similar).

Hope you can sort it soon...

Cheers
Richard

NM

Hi Chris

I had Overboost on my MY98, It started after about a month after purchase (me 2nd owner). I had purchased the car from a none subaru car dealer who had done a oil and filter service b4 I bought the car. I began feeling sudden missing around 4000/5000 rpm in 5th. Then later more urgent (hitting a brick wall feelings) a few days later in more gears. I read up on overboost and found the dealer had put too much oil in the engine (over max level). The oil contaminates the boost sensor and causes false reasings which the ecu thinks is excessive boost thus breifly shutting down fuel and cutting the boost. To remove the oil from the sensor I used brake cleaner fluid in all the boost pipes and removed excessive oil from the engine. After 3 sensor and pipe cleaning sessions all was fine.

Any questions mail me.

Cheers

Neal

MAFFA

Chris,

A few helpful hints were given in my thread "bloody car!!!!" under drivetrain. Take a look.

MAFFA

MAFFA

I'm as stumped as you are about these sort of things but that Bob Rawle seems to know what he's talking about. Try putting a thread for his attention coz he'll probably be able to help.

MAFFA

Bob Rawle

Hi Chris, your car is a MY93 WRX ? This will use coil packs, i take it you don't have a boost guage installed to see what boost level you are at when this happens ?

Overboost cut is like hitting a brick wall, as has been said you cannot mistake it, face into the screen etc, this sounds very much like coil pack issues, the coil packs gradually age and then won't generate enough spark to fire the mixture properly, the dying would be cos the plug(s) have got wet from unburnt fuel, I've known cars refuse to start for a couple of days from being flooded, its common enough. As a check remove all pugs and regap to 0.6mm, check again, if the hesitation reduces/dissapears then its coil packs, they should all be replaced if it is this along with plugs if more than 15,000 miles on them.

Hope this helps

Cosie Convert

Hi

I read somewhere that it is recommended that plug gaps are reduced from the manufacturers gap setting - if you are increasing the boost ? I think 0.6mm was the setting suggested.
I have a 95 WRX with coil packs running 1.2 bar and do get the occasional stutter at approx 5k revs.
Would you recommend the reduced gap for this car ?

cc

ChrisC

As I've explained I have now fitted a boost gauge but I am not sure I understand what it tells me. Being a Blitz gauge from Japan - see the earlier message - it shows bar above 0 (up to 1.5 although I've not pushed it beyond about 0.8/0.9) but something different below that !
Only under clear acceleration does it go above 0. I've cleaned out the solenoid with carburettor cleaner and it seems a lot better but there is still a minor hesitation. So what should I look for ?

Dyney

ChrisC,
0.9 bar is about std. If the boost hits 1.2 bar(18psi) (I think??)
that is when you get overboost. You will know it as you'll probably $hit yourself thinking the engine has died The car will suddenly stop accelerating and your face will be fired towards the windscreen, then it will pick up again and off to the horison as usual.

It sounds like coil packs as said above.
I'll have a look with Maffa at his car as he's local and we'll let you know if we find anything.

Iain.
Fingers crossed.

Bob Rawle

Boost behaviour is perfectly normal, push the engine and the boost builds, ortherwise it will drive like an aspirated car ... sounds like you are not overboosting if thats all you are getting which is about right for your MY car. Not overboost.
Correct plug gap is 0.7/0.8mm, 0.6 is just to check the coil packs out and isn't a solution just a way of checking things out.

ChrisC

Maffa
Taken a look and they all seemed helpful but now is a MY 93 WRX different - I am confused but I might try cleaning the solenoid once I am sure what I am dealing with. I think I've found it but it looks more like gun metal than aluminium on my car. Stll it has three pipes and seems to go to the right places.
But following the thread I dug the Blitz Racing boost gauge out from the boot where it had been put(disconnected for some reason by the importer ?)and connected it up with a 60mm A pillar holder that was there. It works but what should I be expecting ?
I get 0 with engine off, almost down to -.6 bar when idling and then rising over 0 as I go above about 3000 rpm. What's the maximum at what RPM etc. Presumably it should top out at +.9 bar or thereabouts if the car is set to standard and problem free ?Forgive the ignorance but I'm a turbo "virgin" and nowhere helps like this place.

[Edited by ChrisC - 1/9/2002 9:36:23 AM]