1989 328 GTS. I purchased the car in late March this year and am unfamiliar with its service history. Mileage was 46,500km and is now just over 52,000km. Motor is strong and everything is going well. The red "slow down" warning light came on about one month or more ago whilst driving on the freeway. I pulled over and light extinguished after two or three minutes. I resumed driving. Now, light usually comes on when I start the car from cold - it is the middle of our winter. Say 10°C in early morning. Light stays on for ~five minutes then goes off and stays off. Light does not come on if I stop the motor and then re-start. Such as for fueling. There is a noticeable dusting of black carbon on the rear of the car after a drive. For example, after yesterday morning driving 250km mainly on the highway. For a few minutes the speed might have been ~100mph. I suspect it is time to replace the cats but I really don't know. I'm planning a major service in September - cam belts, all fluids, etc. - and am thinking now is a good time to order any parts should I plan some specific servicing such as cats. Would appreciate some advice on the above. Many thanks, Pete
From what you have written, I'd suspect a rich mixture. My 328 that I have owned for 17 years, never has had carbon on its rear. Purpose of cat is to clean up a car's exhaust with a proper mixture. If you are rich, you will get the warning light indicator. I'd pull out the spark plugs and have a look at them to see if they indicate a too rich a mixture. If it is just one plug or all of them. It could be your mixture enrichment for cold starting is not shutting off.
isn't the dust from the brakes? The OEM pads seem to produce a remarkable amount of the stuff. I swapped out mine with porterfields to reduce the crud accumulation.
Thanks very much, guys. From your comments, Lawrence, running rich "fits the bill". Thanks too for mentioning the cold starting mix enrichment. Wolftalk, I doubt it is the brake pads. There is not much brake dust around the wheel rims, I wasn't using the brakes much and the deposits wiped off easily. I think brake dust would be different and have smeared when wiped. regards, Pete
Pete -- A "bad" cat converter really can't be the root cause of an exhaust over-temperature condition (but if you have a true exhaust over-temperature event, it can permanently damage the cat converter). Your comment above is not a good sign for your warning light ECU if you mean it stays on immediately (zero delay) after a cold start-up. Have you been observing the self-test behavior at all start-ups? (Check your OM for the self-test description). Although what you're getting (a false indication) gets your attention, no light at all during a restart is just as bad a result (i.e., if the self-test is unreliable, you have to assume the whole system is unreliable). Thermocouples are very rugged devices IME, so if the thermocouple shows continuity, it probably is OK and you need a warning light ECU-ectomy. If you're driving and the light comes on, IMO it's a good idea to just stop and look at the cat converter (and the nearby coachwork/paint) -- if it's really overheating, it should be fairly obvious to even the non-mechanic (glowing, smoking, creaking). Good luck with the fix.
Thanks, Steve. Yes, the warning light is on when there is not much heat. Certainly, the warning light can be on in cool temperatures just after start up. I haven't observed if it is on instantly at start up, but suspect it is. If I understand your comments correctly, first look at the ECU, and secondaly the less-likely-to-fail thermocouple. (Will look at mix richness before that.) regards, Pete
