GChat

Thanks for all of the info and it is very interesting... Garth, my air pump is still pumping and the manifold back to the cats is original. From the cats back, if it is not original, it is OEM.

Ken, I am definitely a lay person on this topic. Very intersting .. so what should be done? Would the MAF be starting to go south? Or is it another issue?

Greg

Garth S

True for the air pump at WOT, which is most likely Greg's street fun case; however, everything I've read re. false rich conditions as seen by catalytic convertors due to poor ignition system performance ( fouled plugs, etc..... ) suggests that cat temps will rise dramatically from a 1100-1200F norm to ~2200F+ due to oxidation of the excess fuel.
Cooling by latent heat is not an issue as the fuel is already 'atomized' and at ~ stoichiometric by the intake valve - it simply doesn't get consumed in the cylinder if , say, the plug is deficient: the residuals are then used to cook the cat. At WOT, AF goes to ~ 12.5:1 and the ignition is more heavily strained.
One might speculate that this would be the more critical time for the cat were plugs, dist cap, coil, wire(s) ... marginal. Even a dead O2 sensor creates a false rich condition by telling the LH that it 'sees' lean, although they are normally ignored at WOT. Just a guess ...

heinrich

If you say so Ken. However, cat fires happen from unignited fuel, igniting in the cat ...... lay that

GChat

Hi Garth,

Again, very interesting. Caps and rotors are new, but the plugs might be the likely culprit. I have not done those. O2 Sensor could be an issue also. So, in your opinion, not likely MAF related?

Greg

Garth S

A MAF can cause a rich condition, but it would be a stretch to point that way just from this incident: John & Rich could better address that. MAFs do strange things at times - we just replaced one on Jeffs '89 which had an idle that (unsafely)surged to ~1200-1400 rpm. It now seems cured, but was not typical failing MAF stuff.
How's fuel economy on the 120Kmph/2400 rpm highway runs? - ~ 10l/100Km? - if so, the MAF is likely OK.

PorKen

Greg,

I was just ribbin' heinrich. I only pretend to be an expert.

If your car is running normally otherwise, I would say it might be as simple as a loose/chaffed wire.

Randy V

Garth -

GChat

Hi Garth,
Yes my fuel consumption is as you quoted. All seems to be running fine - so I would imagine the MAF should be ok (for now).
Ken - I'm still a relative newbie at this 928 stuff - and am learning more and more. I appreciate the thoughts. Good idea to check the wires.

Thanks,

Greg

John Speake

MAFs can fail and give an overrich condition, but this then applys at all airflow rates fron idle to WOT.

The most typical MAf failure is ageing, producing a progressively weaker mixture with time. The most noticable feature of this is a off ilde stumble or flat spot as the throttle is opened. ALso a lack of power at WOT, because the mixture is too weak.

Garth is correct, when at WOT the O2 sensor value is ignored. The mapping parameters are just rpm and MAF volts, with an offset for temperature (temp sensor 2)

The problem with Greg's car may be a leaky injector ?

Regards

Shark_gts

Could be just a bad/lazy temp sensor in the cat.

Get a laser pyrometer on the exhaust and check the temp.

We'll have a car in here today, from Japan with the Cat alarm 100% of the time and you can't cancel it.

We'll be looking to bypass the thing.

Alan

Pretty sure I don't have the Cat Temp Warning - its certainly not listed - how many other different warning messages are there across years and regions?? I always assumed everything was the same for all models on the digital dash...

Alan