Slowdown light limp mode with test pipes

I just put test pipes on my 97 355 5.2. First couple times I took it out for a drive no problems. Yesterday my wife went out and on the way home intermittent SD light then limp mode. I did some research on here and people point to a bad ECU. I don't have a bad ECU and I'm positive I don't have a cat overheating. Can someone running test pipes tell me if they have had same problem and how did you defeat it? I don't want to disconnect my ECU's but not sure of another option Thanks

 

well. as known test pipes are not ideal with a 5.2 car.

but some have had success with o2 spacers.
with test pipes on obviously you can't have overheating cat since you don't have any.

if you did not have sdl issues with the cats on prior.
then perhaps the thermo probes are reading higher exhaust gas temps then with cats on
due due to direct flow from the headers.

as a result the probes are tripping, putting car into limp mode.

 

Read some of Dr Bobs posts on this. He went extremely deep into this.
YOu can always go with 02 simulators but you need to keep an eye on the AFR
Not sure if I have anymore but they work amazing

 

I have both Fabspeed primary and secondary test pipes on my 5.2 car. Accordingly I have their SS 90 degree O2 elbows on the primary test pipe rear ports so the O2 sensors don't "sniff" anything wrong. I have no thermocouples on either the primary or secondary test pipes because I don't need them without cats to monitor. Instead, I have Jev's (Fchat member) replacement circuit that sends a normal signal to the ECU so it doesn't throw a code or SL lights. Accordingly, I have not and will never will have any issuers with SL lights, cat related codes or limp mode.

If you have test pipes in place of cats, you need to address both the O2 and thermocouple issues. The rear O2 sensor is easy. Just buy or make an offset elbow to move the rear O2 sensor out of the exhaust stream. Then that problem will be solved.

As for the thermocouple, it's not so easy. It not only looks for too much heat, but its my understanding it also looks for too little. It needs to send a voltage corresponding to temps within the optimum range to the ECU. I don't believe you can just disconnect them (you could try this but, from what I understand, I don't think it would work).

A few years ago Jevs created a small simple circuit in a cigarette package size plastic box mounted in the engine bay that sends a constant voltage to the ECU in the center of this range. This tells the ECU that the cat temps are right where they should be. He has a nice thread with photos describing exactly how to make this for yourself. Check it out.

 

interesting

 

Link to the thread ?

 

Your 02 sensors have zero to do with your SDL.

The sdl is a stand alone warning system measuring cat temps through thermocouples and the sdecu. Period. Don't complicate the issue with 02 sensor things. Unrelated.

If you want to know what's really going on, tap the sdecu outputs and real time monitor them with your DVOM. You may have a bad TC or sdecu or because of the pipes may be sending a voltage to the motronics it doesn't like.

If it was my car, I would throw some Taps on the sdecu outputs using positaps and diagnose the issue intelligently.

 

The slow down light and limp mod have nothing to do with the O2 sensors, especially because you are running test pipes. Your problem is either 1) bad catalytic converter temperature control unit, or 2) bad thermocouple. Since you said your cat tcu is fine then I would suspect a bad thermocouple. You will need to find out what side of the engine the issue occurred. Since you have a 5.2, you'll need to hook it up to a reader to get the info. Once you find the side of the engine that triggered the sdl and then the limp mode, you can test for a bad thermocouple. You can test this by swapping the thermocouples from side to side, then see if the issues jump from one side to the other. The same would apply if testing for a bad tcu.

 

I'm not sure anything is bad. It was fine before I switched to test pipes. I really doubt it just went bad in 2 -3 hours just sitting there

 

Both Bob and Ernie are right. The O2 sensors are not your problem although the rear one will throw codes if left in the exhaust stream with test pipes (its purpose is to monitor the efficiency of the cat and test pipes would give readings the same as a worn out cat). The thermocouples, on the other hand, are there to prevent a dangerously overheated cat. This is where your problem lies.

Exhaust stream temperatures post cat will be hotter than those post test pipe. That is so because a cat works by heating up to foster the chemical reactions within it. That would lead you to believe that post test pipe, the temps would be cooler and therefore pose no problem for a thermocouple to see. However, after trying everything including new thermocouples and their associated electronic modules, Jevs found otherwise.

I don't claim to be the expert here but Jevs (electrical engineer) went through this issue in some depth when he put test pipes on his car. His thread is very thorough and well documented. If memory serves, he could not stop his problems except by finally resorting to his new circuit. That stopped things permanently but is also completely reversible if you ever wanted to go back to cats. Before dismissing it and his extensive research (which is well covered in the thread), you should check it out.

 

Do you have a link to the thread?

 

In playing with this issue quite a bit over the past year or more and also installing three after market temp transmitters (SDECUs for this audience), I found that the motronics has some set parameters it is looking for in order not to throw an SDL.

In the units I currently have in my car for example, the initial voltage is .2V at startup that throws an SDL until the unit warms up and the voltage reads .5V.

We don't know exactly how the ECU is mapped so it's just a guessing game on what it's looking for but the OEM SDECU starts at .5V cold and linearly rises to 2.6V or so in normal operation. Blips, drop offs, low voltages or high voltages will throw an SDL.

If you tap the outputs and watch them real time you will know exactly what the issue is and where it's coming from.

 

The Jevs thread is titled "Thermocouples, slow down lights, related CEL’s, and how to conquer them...". It is a very thorough and an informative discussion. Just search for "Jevs" and find this title (I believe its at the top of his very extensive list).

 

OP, You would be wise to listen to Drbob for specifcs to the 355. Jev's did a good job too and I think there is a circuit diagram of his SDECU creation. I also wrote an old post of generic manual bypassing the SDECU. In that thread cribbj lists an aircraft quality source to monitor and take over SDECU functions and never have a problem with it again. There is also a graph of SDECU function to understand how the SDECU works. Those 3 threads will give you enough info to make your own choice as how to proceed.

 

when I did my engine out I removed the cat thermal sensor (which was working fine prior).....long story short....reinstalled said sensor ...restarted car and immediate slow down light.
the sensor was marginal at best (but working) failed with the re & re of the sensor.
put a new thermal sensor in and problem gone.

 

There was another thread recently that mentioned that unit and if you go to their website it is now NLA unfortunately. It looked like a neat unit.

 

Exhaust Gas Technologies Inc. - Motorsports Catalog

This is the unit that I found which I believe Dr Bob has been using. As he said, on start up you get an SDL because of low output but once the system comes up to temp all is fine.

 

There is guy from the UK on that FBB thread in the technical forum that was posting for awhile on a blue tooth enabled transmitter he was developing that would allow output levels to be read on a phone app. Very cool and he had made some progress on beta testing it.

Nothing more from him in awhile but something like that is long overdue and would be ideal.

 

That's it John. Nothing more from them on their variable parameter model.