BigDude

I keep on hearing of people blowing their engines after long high speed driving when they finaly lift of or slow down. Can anyone please explain why and the mechanics of it?

BigDude

I guess it just seems to happen but no one knows why...

scoobyslut

They all seem to have the sad same ending, as in major engine damage, but so far as what causes it on all of then does vary from engine to engine. One of the main causes is loss of oil pressure, sometimes due to sticking releif valve,too thin oil used, or type of oil as turbo's dont like fully synthetic at all,other than that, possible MAf sensor causing engine air/fuel ratio mix to run over lean, burning out pistons.
But, who these days has a magic wand and can fortell anything?

BigDude

Come on! Every one's talking about it but no one knows why?

LG John

I had a theory about this. I stress I'm not an mechanic, engineer or anything like that - just my thoughts.

When your tanking it you've usually got a lot of boost for a sustained period of time so all the temps start to increase quickly - I think! This will (I think) be paricularly the case at high load (wind resistance) in 5th gear (130mph+).

When you lift off at these speeds (well I personally found myself doing this) I didn't just come straight up off the gas. I found I acutally eased back and slowly came off the gas (don't want jerky movements at such speeds) I then slowly feathered the brakes. Here's my totally useless theory. If you lift off slowly when at full boost when all the temps (charge, oil, water, etc) are already very high then in the first half second of your lift off you are cutting the fuel the car is getting (by not having the throttle fully open) but the boost doesn't drop back much until the throttle in 50% closed - this would be worse with a dawes. Basically you have a brief moment where already hot pistons suddenly find they have less fuel but the same boost - BANG!

I'm 90% sure that this will be total **** but it's something I was wondering about once and thought I'd share it.

Denmark

iTrader: (2)

I had this to problem on mine!!.
I am sure it is becourse of not enought fuel when you lift off coursing it to knock, mayby just ones , but if it´s really hot it´s enougt to burn the piston!!..


Skassa

Dave T-S

Saxo Boy
EVERYONE knows the engine blowups are due to fuel surge, or bald tyres

BigDude

Thanks every one. So what's the solution? I have an STi7 tuned to 305 PS/427 Nm (315lb) and do a lot of driving between Zurich and Stuttgart at fairly high speeds. Over here you often get trucks or slow cars that just pull out in front of you as you get to their level forcing you to sudenly lift off and break. Should you use the clutch more?

john banks

Don't lift off then

Amanda-Jane

Dave T-S, you ain't funny anymore

Floyd

iTrader: (9)

Dip the clutch, ease of the throttle (hold it at 2k rpm) and possibly break at the same time.

I suppose that could get quite 'exciting' doing this on an Autobahn at 150mph, especially in traffic.

IMHO of course.

F

LG John

AJ, and you wonder why I can't post under the name 'Saxo Boy' anymore

LB4

Hey Big Dude,

There is a reason that an engine will blow on lift off after high revs, especially in top gear .... its to do with the fact that when running WOT you have lots of air being forced/drawn (this applies to NA engines too) along with fuel going into the cylinders and this actually provides a kind of cushioning effect on compression ..... imagine the speed the pistons are stopping and accelerating when the crank is doing 6000rpm say!! ....now suddenly lifting off the throttle, all but shuts off this air and fuel supply!! .... so now the pistons are being thrown about at the same kinds of speeds but doing no work, and in top gear obviously its going to last a lot longer before the crank speed drops.
This is the actual physics of why, of course oil temps etc, will also play apart and the condition of the internals will decide if the engine is going to throw its hand in.
I'm not an engineer, i'm an electrical and instrument mech, so maybe someone more knowledgable than me could explain it better, we'll see...

What Floyd says could help .... not sure i'd want to be dipping the clutch and loose engine braking at the speeds i guess you are doing though!!!..... if you got to brake, what can you do?.... if you can slow down over a longer time then as Saxo said try easing off the throttle in keeping with the car slowing as you brake, its not that hard to learn left foot braking.

[Edited by LB4 - 11/14/2002 6:29:21 PM]

Adam M

i dont believe any of this.

I am still convnced it is due to the oil pressure relief valve getting comfy in an open position for a period of time, and picks up on the edge of the bore it normally slides in. Then when you slow down afterwards, the oil stuck oil pressure relief valve remains open, so that when you slow down there is no oil pressure at all, hence big end bearing failure.

many people recognise this and modify the valve itself when rebuilding.

apparently there are very few if any reported failures of engines having had the mod done.

jazzer

Some driver Mr Floyd, dip the clutch, feather the gas and brake at the same time? Are you Jake the Scooby pilot?

J

mutant_matt

jazzer,

it's easy, left foot on the clutch, ball of right foot on brake, heel of right foot on the throttle....heel and toe'ing

No idea if it would help though. Anybody know if the MY01> have had a change of oil pump/relief valve design?

Matt

jazzer

Let me get this right. You are blasting along, full bore and then you need to slow down due to Gatso or Nissan Micra pulling out into your lane so you do the 3 leg thing to stop the engine bursting?

J

MGJohn

Never experienced engine blow up on lift off but, by trying to visualise things in my mind, need to know if the "blow up" is IMMEDIATE on lift off or some considerable time after? What is the usual extent of damage to the engine; Mains, big/little ends and/or holes in pistons etc, rods poking out of crankcases or what?

Cant see a possible sticking Oil Pressure Relief Valve causing so much damage on IMMEDIATE lift off .. maybe some considerable miles later or if it's one lift off too many in a long series on a journey.

I did experience holed pistons in a Ford some years ago which did show up on lift off, but I suspect that was merely a coincidence. There again ... ?

MGJohn

Never experienced engine blow up on lift off but, by trying to visualise things in my mind, need to know if the "blow up" is IMMEDIATE on lift off or some considerable time after? What is the usual extent of damage to the engine; Mains, big/little ends and/or holes in pistons etc, rods poking out of crankcases or what?

Cant see a possible sticking Oil Pressure Relief Valve causing so much damage on IMMEDIATE lift off .. maybe some considerable miles later or if it's one lift off too many in a long series on a journey.

I did experience holed pistons in a Ford some years ago which did show up on lift off, but I suspect that was merely a coincidence. There again ... ?

owbow

yeah, are we talking holed pistons or big ends?

i've got a theory if it's big ends.

Owain

exesspeed

CHOCOLATE ENGINES BUD SEEMS ONLY ON SUBARU,S had 3 cosworths never any trouble last one was e reg , just got a subaru , allready makin funny noises after 4 weeks ,very poor engine it seems talkin to loads of owners , shouldnt be though, jap stuff is usually good , well their bikes are

Floyd

iTrader: (9)

Jazzer, I didn't say it was easy You'll have to weigh up whether you can do the three leg thing safely or whether you need to just brake as normal. You could always drive slower

MGJohn, it's after a few miles later so Adam M could be right, which means that it's pot luck who'll be next and dependant on how fast you drive. It'll also mean that oil, fuel pressure and uprating stuff etc won't matter so much unless the relief valve has been modded.

F

Gridlock Mikey

I've done the German Autobahns at "daft" speeds for 30 mins + and my car is fine! I reckon it's the old scoobynet effect. 1 person says my engine has blown up, one other person describes the same symptoms, another agrees and says the same happened to a mate and suddenly scoobs are crap and should only be driven at sub 70 mph for no longer than 10 secs

Everytime someone offers a viable explination, it is rubbished and then an element of "grey" gets thrown into the mix ".....it happens a few miles after the lifting off...". Could be bloody anything and you know it

Drivel....it's a car and as such will suffer many different kinds of failiure across the range (as does any model) For every one person who has had a problem, 000's will be fine.

Just an observation and obviously all IMHO

Mikey

nom

Well, there's 2 things going on here - pistons & big-ends. Pistons are pretty simple - big hole in the middle points to pre-ignition. Pre-ignition can come from a number of places, most likely sustained det which is easy to do on an unmodded JDM running on the 'good' Euro (especially UK) fuel, & remember that although the ECU can retard, the engine temperatures will still rise while this det/retard is going on - especially over a long run - so pre-ignition can still happen. And the iffy MAF which will almost always (IMHO) lead to det/pre-ignition if it's misreading and the car is driven hard.
The 'hole in piston' can happen on any car at all, it's not a Subaru thing, just indicative of the way that the cars are driven (ie considerably harder than average!). Oh, but the dodgy MAF issuse doesn't help here...
Big-ends are something else altogether.

[Edited by nom - 11/15/2002 9:46:49 AM]

BigDude

Thanks every one! It's just a new concept for me (these engines blowing up) and had me a little concerned after reading several threads on this site. My prvious impreza MY98 station wagon was tuned to 265 PS/374 Nm (276 lb) and was driven quite hard in Germany for sometimes hours at a time. I traded the car in at 187,000 kms (116,000 miles). All I ever did was keep up the maintenance and change the oil every 4,000 miles. Was I just lucky? And has anyone had that problem with the STi7?

ragnarock

I wasn't aware turbos did not like fully synth??? Does this mean I will be safer to run my MY98 on semi??

nom

Nope. Turbos don't like semi-synth; it can't cope with the heat.
One of the reasons that turbos used to go all the time & were known as very unreliable is because the oil wasn't up to the job (it is now - what's known as full synth!). Nor was it changed enough.
I haven't put it into the scoob, but I had a different turbo'd car a couple of years back which went in for a service (dealers), came out with semi-synth (and a 'no, it's fine sir') - 500 miles later it was a thick black tar. And the car was noticeably running worse after 100-odd miles. A non-dealer (ie real mechanic) looked at it, gawped somewhat, swore quite a bit & flushed it, used some weird light oil, filled with synth, emptied & filled again & advised to not go back to the dealers again. Never used the s**t since.

scobymars

I ve got a my 99 and here in france I very often do travel of 3 to 4 hour at minimum speed of 12O MPH and I have allready got a piston hole after only 45000 km.
Now on my new engine with tek I have a knock link and you can see that after more than 45 minutes cruising at 130 MPH on light throtle with 0 to 5 PSI boost you get a lot of orange flashing.
If i do small curvy road at wot on evry gear for the same time I don't even get a green light.
So I think subaru are not made for motorway cruising.
A friend of mine mesure is egt
AT wot on evry gear 30 minute he got 800° C
30 minute at 110 MPH 890° C

scobymars

May be it's because in japan they got this speed limit at about 115 mph so they don't do so much work on what is hapening after.

Sure it's a stupide idea.

nom

I think you're spot on, scoobymars! And that's a useful bit of info you've provided there as well! Any idea what the oil temperature was doing after the 45-minute run with the orange flashing light?