CAT temp warning

[Stoolie]

Occasionally (usually when I start it after it's been running - but once when it had been left for a few weeks) I get a red CAT TEMP WARNING on the computer display - is this symptomatic of a bigger problem, or just a side effect of the exhaust being hot when I start up?

[MPDano]

Sounds like a clogged Cat to me. They are not expensive and you can get a High Flow one from Magnaflow.

[Stoolie]

Ok - second question - we don't have emission testing here in NZ (yet!) so apart from the computer monitoring it, are there other effects that would need to be considered when removing it completely? Does the performance need to be tweaked / retuned etc?

[Danglerb]

The car will stink, every living creature on the earth will suffer, but otherwise should be fine.

[Stoolie]

Hmm - on further reading, the plugs might be a bit foul - from time to time the wife has mentioned a strong smell of petrol when I start up (I'm long gone in a cloud of dust, so don't smell it!) - that may lead to a hot Cat?

[Normy]

Originally Posted by Stoolie

Hmm - on further reading, the plugs might be a bit foul - from time to time the wife has mentioned a strong smell of petrol when I start up (I'm long gone in a cloud of dust, so don't smell it!) - that may lead to a hot Cat?

OH you better believe it!!

Unburned fuel enters the cat [it runs at 1500 degrees F, I think~] and of course, burns. This can cause the cat to glow red hot. Most cars have a metal guard under the cat so that the car won't set grass, weeds, and perhaps asphalt on fire if something like a coil fails.

My car came without cats from the factory. In 2000 I bought a pair of S4 cats from 928 Intl, and my car, which ran rich for years [finally figured it out], finally melted one of the substrates and partially plugged the device. $500 down the toilet~



Word to the wise: fix your rich condition or your cat will be destroyed.

N

[ROG100]

Heed the words - do something quick about the rich condition before your car burns to the ground - I do not jest.
The warning is there to give you advanced knowledge that there is a serious problem.
Way to many 928's gone to the scrap yard because of cat fires.

[dkkim81]

What do you need to replace to fix the car running rich?? How do you fix it??

[Xlot]

Originally Posted by Stoolie

Occasionally (usually when I start it after it's been running - but once when it had been left for a few weeks) I get a red CAT TEMP WARNING on the computer display - is this symptomatic of a bigger problem, or just a side effect of the exhaust being hot when I start up?

G'day Stoolie, your car is a Japanese import.

Japanese cars have two sensors in the rear of the cats wired up to that warning on the dash. They're there due to Japanese law, and it's the only country to receive that light. Its useful on pre-89 model years, but redundant on 89-onwards 928's as they got an ignition monitoring relay from factory, which uses temperature sensors in the exhaust manifold to detect ignition issues and will shut down fuel injectors and ignition on whichever side of the engine it detects an issue, specifically to prevent unburnt fuel entering the cats.

Your car is a 91 S4, so it has the ignition monitoring relay. Check the clear plastic relay mounted on top of the ECU in the passenger footwell. If it has a red or green LED showing, and the car is idling very roughly, then its detected low exhaust temps on one bank or the other, and shut down the fuelling to that side. The ignition monitoring relay should ensure that no unburnt fuel makes it into the exhaust when the car is running.

You say the cat temp light comes on after you start the car? Does it turn off? Have you hopped out and looked at the cats when the light comes on to see if they're glowing red hot?

The only circumstance I can think of that might cause the cat temp sensors to light up at startup is leaky/clogged injectors spraying fuel into the cylinders after you've shut down the car. The fuel could then get pumped out when you try to start, and ignited once the exhaust manifold and cats are up to temp. Would also explain the smell of fuel your wife mentions.

Does the car crank a bunch before starting when the engine is hot and its been left for a few minutes? If it is hard to start, then that indicates no residual pressure in the fuel rails, due to leaky injectors.

So where does this leave you?

If the ignition monitoring relay says the car is fine, and your fuel pressure remains over 40psi for at least 15 minutes after shutdown, then the problem is a failure of the cat temp probes, rather than an actual warning.

If your symptoms are consistent with leaky injectors (i.e. cat warning goes away and ignition monitoring relay not lighting up), run a few bottles of injector cleaner through your car for the next few tanks of fuel.

If the ignition monitoring relay has an LED on, then odds are a plug wire is shorting, or a coil or ignition amplifier has failed - but you'd notice the craptastic idle quality from the car running on only 4 cylinders, so I'm guessing its one of the above two cases.

[Xlot]

Originally Posted by Stoolie

Ok - second question - we don't have emission testing here in NZ (yet!) so apart from the computer monitoring it, are there other effects that would need to be considered when removing it completely? Does the performance need to be tweaked / retuned etc?

Once you've figured out the issue per my post above..

You can lose the cats - both my 928's don't have them. I have an X-pipe (exhaust crossover) on both my cars with no catalytic converters. The X-pipe adds proven gains (20rwhp according to my local dyno dynamics dynomometer).

You can unscrew your O2 sensor from your existing cats and screw it into the X-pipe, and retuning isn't needed.

[Stoolie]

Thanks for that - from the sounds of it (Yes - the warning comes on at startup, and then goes away) some injector cleaning might be on the cards, but I'll check the others too.

[Mrmerlin]

if you suspect a rich condition while running its always a good idea to look under the car and see if the cats are glowing RED if so then shut off the engine and park the car away from any structures till things cool off and have a hose ready to put out the fire

[Stoolie]

I have been looking for a nice place to mount an in car extinguisher - just as a bit of Murphy's Insurance really.

[Danglerb]

Just below the front of the seat works for some.