I have a 1997 F355B. I drive it often...it never sits for more than a week. About a month ago, I noticed a difference in the operation of the bypass. It started opening with less frequency. Whereas in normal operation the bypass would open dependably at about 4000 rpm in 1st gear under any driving conditions, it started "lagging", to the point at which it wouldn't open much until 2nd gear, or sometimes not at all. Now, it won't open in 1st gear unless I engage 1st gear in a downshift, say, in a corner, in which it functions perfectly. It seems like it needs a "head of steam" to open properly in 1st gear... So, since my vacuum lines were getting a bit crispy, I replaced them all, including the check valves, all the way from the intake manifold. I checked everything and vacuum is definitely good. That improved a lot of things...idle is smoother, very few "drop outs' etc. I took out the bypass valve, and aside from having just a touch of "rattle", both the valve and the vacuum actuator for the valve are good. But...still the same situation...the bypass won't open in 1st gear without a "head of steam" coming into 1st gear use, such as a downshift. And NOW, I occasionally get a BLINKING slow-down light, but no thermal issues are at play with either of the Cats. Is it possible that the bypass thermocouple and/or control station (ECU) are causing the problem? I ask this because I know that the CAT Temp thermocouples/ECUs will generate a slow-down light, but not sure if bypass thermocouple/ECU will do the same thing. Any insight?
Is there a proper procedure for checking the valve? Maybe it works, but is just a little tired or is getting sticky at high temperatures? If it's not opening, perhaps this would the temperature build-up in the exhaust system at high rpms cause the SD light. The ECUs put on the SLOW DOWN lights when they get a signal from the thermocouple ECUs. They may be able to tell you which part of the exhaust system is generating the SD light. The bypass valve thermocouple ECU will generate faults in the ECU if the valve is not operating properly. Do you have an OBD2 reader? (I'm assuming you have a 5.2).
Thanks very much for your post. Your comment regarding whether or not the vacuum actuator is not working properly due to age/temperature is an interesting one. I have checked the actuator and have found it to be leak free at ambient air temps, but not when elevated to operating temperature. I threw a thermocouple on the shell of the bypass pipe (muffler side), mounted an air temp sniffer right next to it, hooked up a data recorder, and I will go and drive today. Since I have access to an enviro chamber, I will put the valve in the chamber with switchable vacuum supply and test it at the operating temp which I will capture during today's drive. Meanwhile, I've sourced one new ECU and one new Thermocouple, since my car has the infamous "black" ECU in the Bypass position. The price was good enough that I can do a quick swap and, at the very least, have a spare on hand. As soon as the parts arrive I will get to work on that. Thanks again for jumping in.
We can always swap mine out with yours to trouble shoot it. It's just that mine hasn't even been out yet this year but it will be soon in the next few weeks, hopefully.
