1998 355 F1 slow down light

The slow down light will "go away" when the input from the Exhaust Computer removes the signal to the Engine ECU that says (in effect) that the cat is overheating.

There are four items of intrest. A) The Cat, B) The ECU, C) The O2 sensors, D) thermocouple.

If the slow down light comes on before the cats have a chance to warm up, it is likely that the ECU or thermocouple is dead/dieing. In any event, if you have an IR thermometer and point it at the cat and it reads less then 980 dF, the Cat is not overheated, so you are chasing a signal generation/delivery problem.

The 5.2 ECU expects the front O2 sensor to swing from low voltage to high voltage as the ECU moves the mixture a touch rich and a touch lean seaking to optimize the mixture around stoichiometric. After the engine is warm, the ECU expects the rear O2 sensor to read lean and the rear thermoucouple to be above 340dF and below 980dF. So the Exhaust computer unit is detecting a set of signals from the 2 O2 sensors and the thermocouple that it does not like.

The 5.2 ECU also expects a signal from the thermocouple at the bypass valve to change when the valve opens and closes. Although I don't think this ends up in the Slow Down catagory.

 

Not too sure about ultimate resistance values but when I had slow down probs on my 360 I found the bad t/couple had infinite resistance whereas the good one immediately "rang" the continuity buzzer on my meter. I changed the t/couple, light went out and all good since.

Best of luck.

P.s. from what I've read (and assuming your cats are not mega hot and glowing red at night!) the most likely culprit is the small ecu unit and secondly the t/couple. Please do check the cat temp in case it really is a real problem.

 

Get an OBD scanner and it will tell you which cat ecu is throwing the code. Mine threw a P1449 code which told me it was the driver side cat ecu that needed to be replaced.

 

Mike, did you ever find out what was causing your issues? I also had an unusual slow down occurence on Saturday morning.

It was early in the morning and I had been out driving for about a half-hour plus warm-up time, so everything was up to temp. I came to a desolate stretch of straight highway so I decided to open it upt...got to 140 mph near the end of the straight for maybe 3-4 seconds and came back down. As I was coming down to normal highway speed, my slow down flickered for a few seconds then stopped. And by flickered I mean flickered. No solid on and off action, just a flickering for about 3 seconds.

I was a little spooked by this and I listened for any change in the engine performance as I slowed to about 60 mph, but I never felt or heard anything odd and no other warning lights ever came on. I didn't push the car on the way home but everything sounded and felt fine. No oil or water temp spikes or anything of that sort occured.

Has anyone ever had this happen to them? I'm half tempted to believe that the car wasn't used to being pushed that hard and it was a "false" reading (from what I know of the previous owners, none of them tracked the car or drove it hard). But, being an engineer, I know those lights don't come on for nothing, even if it is just a simple instrumentation issue.

And no, I won't be doing it again; my curiosity has been sated.

 

Was it as damp in your area as it was in Winston Salem on Saturday? If you say "yes", or the car was recently washed, I'd guess the sealant on the cat ECU has cracked and allowed moisture to enter.

 

That gives me something to check. Appreciate the response.