nom

I was just wondering 'which way' the lambda sensor goes (misreads) if/when it overheats, for example on a sustained high speed run.
And at the sort of load that a high-speed run would induce (to create enough heat to make the lambda misread), would the probe be being used for anything? I have no idea where the map changes from closed to open loop, and presumably this varies anyway...
Just a thought. And probably not a very good one


Just thought I'd add - as I'm on a drivel role here - This came to mind 'cos I've been going through the 'bigend' stuff a bit recently (cheery reading ) & noticed that engines that have go often seem to again, despite replacing many parts. So it seemed likely that there's something causing/contributing to the problem that is not changed... ECU has been brought up as a possible fault, but I haven't noticed anyone mentioning the lambda sensor being changed. It's proximity to the turbo exhaust means it could easily overheat, and therefore misread. If the engine is using it for chasing stoich, and it is not reading correctly, the ECU will be chasing a strange mixture. Weak, causing more overheating (viscous circle) leading to obvious troubles, or rich - I have no idea how long an egine needs to be run very rich to cause enough dilution of the oil to cause it to break down (borewash etc.) but thought I'd bring it up anyway. Just to show I have no idea

[Edited by nom - 9/6/2002 11:34:27 PM]

EvilBevel

This picture shows more or less how the sensor starts to behave at higher temps



A) Exhaust gas at 500 C B) Exhaust gas at 750 C C) Exhaust gas at 900 C

Complete article : http://www.ffp-motorsport.com/tuning/o2meter.php



[Edited by EvilBevel - 9/7/2002 12:02:43 PM]

dowser

Lambda sensor undereads when hot - meaning it reads leaner than it really is.

However, the ecu is only in closed-loop operation in low load conditions. If you've got DD you can this clearly as it swings back and forth across stoich. As soon as there's load, it reads from the maps.

If you're worried about a car going bang for a second time, the MAF sensor would be a much more likely culprit. I've just mapped a car which has always run much better than any other similarly modded car for a given level of boost. His MAF is slightly under-reading, leaning out the mixture a bit.

He reckons it's always been like it and has chosen to do nothing, except buy lambda and knock links

Richard

nom

That's the key bit - 'load'. At what point/speed does it actually stop using closed loop & switch to open? I've never reached a cruising speed at which is running open loop, but then I've never cruised faster than 110ish. If it's still open loop at 130, say, there will be significant heat - possibly enough to overheat the labda sensor, at which point the engine starts running lean (thanks for the info on which way it goes ). Meanwhile, AFR instruments read OK as they are misreading the same information. There's unlikely to be det at low boost, so nothing there either.

This may only effect cars with slightly iffy lamdba probes - for general use (i.e. not tuning, etc.), as the only effect a dodgy labda probe would have AFAIK is bad (or good!) fuel consumption - again because most of us only worry normally about what's happening under boost - so is easy to go unnoticed. It may be that some probes are more affected by heat than others.

I only brought this up because there are quite a few cases of cars with effectively a new engine each time blowing again & again. So it's possible/probable that the cause of this problem is elsewhere. It may only be "the spark that starts the fire" but is still the cause. Whatever it is. Possibly

nom

Hmmm, from looking at the graph, it's actually going to run rich when the EGTs are higher - the ECU presumably will be searching for a value betwen 0.3 & 0.8v - at 750C that's 14.7:1. If it continues to search that range, it'll get bewteen 13:1 & 14.7:1. Which may cool the engine down a tad...
Well, there we go!
I shall go & pack my bags now

EvilBevel

Taxi !

Funny you mentioned the big end thread... was talking to my dealer 2 weeks back, about MAFs, big ends etc... told him the only constants we found was a) shortly after a service and b) high speed run and c) only noticing when you slow down or start up again. He looked at me as if I was crazy ... (well, I am but that's not the point)

His MY97 had a big end gone. The day before it went, he ... had serviced his own car, then did a high speed run on Sunday to the Ring. Did one lap. Parked car, went for coffee, came back, turned on engine, rattle rattle...

So it fits in with previous observations.

Butty

iTrader: (2)

Hmmm....
services.. fast runs.. BANG
My tale is similar but only that mine didn't go bang.
I have an MY97 and have noticed that on doing long, fast runs (500 miles round trip) straight after a service (usually major for some reason), the oil has disappeared to below min. level.
This puts the frighteners on me and I top up, thinking that some expensive damage has been done.
The car then proceeds to keep all its oil in until the next service....

Nick