Back on the road! Major, Guides, Full Exhaust! New Codes & Slow Down

My car is back on the road! Fresh heads (scintered guides used), all new aftermarket exhaust (except my tubi muffler that was already on), full major service with cams degreed etc. Runs great! Put 80 miles on it last night. Checked everything, monitored temps etc. No issues that are real.
So along with the new exhaust I do now get a flashing slow down light. The funny thing is my slow down light is really a speed up light . If I drive harder it goes off. So I suspect the thermocouples are actually running a little too cool now sunce their are no "restrictions" in the exhaust to make things hot. Anyone else get this?
I am considering my options. I think I can probobly put a couple ohm resistor in series with thermocouples so it looks like they are a little hotter or get rid of the modules and thermocouples completly and put a fake voltage in place.
Any advice before i get time to expiriment?
Check engine light came on also after the learning period (about 50 miles). I think this will just be a mod to the rear o2 spacers since everything is flowing even better now. I have not pulled the codes yet. It could also just be the slow down if that gives a code? I suspect they will be cat effeciency related though and require tweaking of the rear o2 spacers.
Anyway, other than these couple annoyances, she is running and driving great!

 

Maybe o2 sensors reversed front-back.

 

Nope. Checked that just to be sure.
I think it will be the same codes as before that the rear o2 spacers solved. I suspect i will need to move them either further in or out of the stream. I will know more when i pull the codes. As for the slow down. I never got that before, but i was running stock headers and stock bypass pipe. Now I have fabspeed headers, fabspeed bypass pipe, top speed cat pipes, and tubi muffler.

 

If you are running highflow cats I'm gonna guess the CEL will be for the secondary O2 sensor/s being below the thresh hold. I've heard of people putting spacers in to get the second O2 sensor out of the exhaust flow more.

As far as the cat ecu's go, I had the same thought cross my mind, about running a resistor in place of it. I have been running my 348 cat ecu-less for years, but it does bug me that the Motronic stores silent codes for that. Plus on the 5.2 Motronic 355s the CEL will stay illuminated if the cat ecus are removed. So you would have to either fix what is causing the cat ecu to get tripped, or by pass it.

 

I am one that used modified 90 spacers to solve cel with empty stock cats. This is why i suspect the new setup is just going to need a tweak of those.

As for the resistor, it would not replace the thermocouples, but go in series with them. This will still allow them to work normally but appear "hotter" therefore still appearing to the electroncs to be in range. It will just raise the reisistance a little through its temperature range. These are usually pretty linear, so there is a good chance this simple solution will work. The resistance climbs as the temp increases, so adding a little reistance appears hotter.

 

I am one that used modified 90 spacers to solve cel with empty stock cats. This is why i suspect the new setup is just going to need a tweak of those.

As for the resistor, it would not replace the thermocouples, but go in series with them. This will still allow them to work normally but appear "hotter" therefore still appearing to the electroncs to be in range. It will just raise the reisistance of the thermistor a little through its temperature range. These are usually pretty linear, so there is a good chance this simple solution will work. The resistance climbs as the temp increases, so adding a little reistance appears hotter.

 

Did you change your coolant temp sensor? it is the ECU's main reference for engine temp, at which it also uses to meter fuel.

it is #40 on the pic

Did you participate in my 355 poll? read my last post!!

http://www.ferrarichat.com/forum/showthread.php?t=377706
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No. I have not had any issues with that prior to new exhaust. It runs great. Not rich or lean. The exhaust is cooler and much more free flowing now. I would not suspect the thermistor in the coolant sensor changed at the same time the exhaust changed.

 

Does that sensor also control the gauge or is there a separate one for gauge and ECU?

 

The #40 is for the ECU and #39 is for the gauge.

 

No. I have not had any issues with that prior to new exhaust. It runs great. Not rich or lean. The exhaust is cooler and much more free flowing now.

 

No. I have not had any issues with that prior to new exhaust. It runs great. Not rich or lean. The exhaust is cooler and much more free flowing now.

 

Ah, I see.

Well then give the resistors a go. I'm curious to know how well they'll work. Hope you post a DIY thread with plenty of pics, so that if they are successful the rest of of can do the same.

 

Maybe the thermocouples were swapped from bypass to RH cat?

And just to be certain, your coolant temp gauge (between speedo and tach) after full warm up, is always between 8-11?? even under very aggressive driving??
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I will have to double check thermocouples. I did replace one due to bad threads, but i tested it before using it. Does the bypass one use a different value thermistor than the other two?

I will report on temps when i get to drive again. Was over 100 degrees last night when test driving.

 

Well it will be harder to go below the 8 when ambient is at 100°F .......... but as I posted on my poll ........ make sure under steady highway cruise, that it never goes below the 8 position on my gauge pic above. Anything under that and the thermostat is not working correctly. I have seen that happen, even with a new thermostat.

 

Please post the Pcode. I wouldn't rush the diagnosis. It is entirely possible (and random) that you simply have a bad catalyst ecu or thermoprobe.

 

That won't work because you can't tirate the system to work when you really need it when something is hot. This system is so lame since the early days and ferrari refuses to fix it. It is so lame it is not worth trouble shooting because it WILL break again so....


http://ferrarichat.com/forum/showthread.php?t=328364&highlight=sdecu

We got a solution for that as fancy as you want to be. Go manual or go fancy aircraft spec and put the problem to rest forever.

 

I can no longer provide any meaningful comments on a solution to this until I get more time to mess with it. I need to at least pull the codes. I chose to take my boat out instead today. Something more fun than working on these cars.

My car had no issues with this system before the major service and the new exhaust. I did have to replace a thermocouple though during this process and I got a used one. One of mine had bad threads. I did test it and at room temp it was about 1 ohm and rose to 4 ohms under my faucet as the water got hotter. I assumed it was good to go because it is working as expected (it is not open or shorted). However, I did not get it near as hot as exhaust would. My exhaust should actually be even cooler now than before. I never had a slow down light. My flashing slow down light actually goes away if I run the car harder and then comes back on if I just go to regular cruising. It seems as if it is coming on because the exhaust is too cold, or when I push the RPM's up the exhaust is somehow getting cooler?? It also never gets to a point that it affects the motor running. I ran it in 100+ ambient degress outside with the AC on and it never did anything more than flash when driving slow. As soon as you would drive harder it goes away. I also got a check engine light once the ECU got through the learning phase (at about 50 mile). This is the same as before when I was getting cat codes due to empty stock cats. This was fixed with 90 degree o2 spacers in the rear. I suspect things have just changed enough that they are no longer giving the same output. I will not know until I pull the codes though. I never had a slow down light though, so that is what is more annoying.

I still have the thermocouple that was working fine, but had bad codes. I will see if the resistance is any different than the repacement I got.

In reading some other posts no one every mentions that the slow down light can come on with too low of a temp. Anyone know if this is possible, or is it always looking for hotter? I also seem to have read about some people just leaving them dangling out of the exhaust with no slow down. Anyway, I just need to get my hands dirty and work on it rather than typing here. It is possible I just got a thermocouple that is out of tolerance. It is still wierd that slow down really means speed up and then it goes away on my car

On a side note, the 355 has 10x1mm threads for the thermocouples. I notice the other thread said that car has 8mm.

 

I just checked the original thermocouple just by running it under my faucet and it reads about the same as the one I put in (about 1 ohm at room temp and just over 4 at hot water).

I was just thinking of something else.....

First of all, does anyone know if the bypass valve thermocouple is the same value as the other two? I could not find the bypass one in the parts list for some reason. Is it possible my problem could be because they got mixed up in their locations, or maybe a main cat thermocouple is plugged into the one for the bypass and the bypass one is plugged into where a main cat one should be?

Just a thought... I need to get it on my lift and check this stuff. Anyone got a link to some pictures of where everything should be plugged in, or remember if it is in the manual? It could be something as simple as this.

 

Thermocouples are a thermovoltaic device (not a thermoresistive device). Measuring the resistance with a mulitmeter may be unreliable (since the multimeter is applying a small voltage, measuring the current, and calculating the resistance), and may be different for thermocouples that would give the same voltage vs temperature behavior (because that's related to the metals used), but are made from different sensing wire diameters so have different resistances. Would be better to compare voltages -- just a thought...

Attached is some useful type K thermocouple data/information that a 328 Owner previously posted.
View attachment Thermocouple data K-type.doc

 

True, resistive readings won't tell the whole story, but it is a first quick step in diagnosing a bad thermocouple. If they are shorted, then you are done testing. If there is no response to temp you are done, if they are completely open you are done... etc. This was merely a quick comparison really and they both did the same thing and had about the same reading over that small range of temp so I assumed the used one I got was probobly ok to use. I never had a reason to diagnose thermocouples prior to this, so I would need to do more reading. I have been in the electronics field for about 18 years but never had a reason to mess with thermocouples. Everything temp based has been thermistors for me....With that said I do see now how looking at the voltage will be much more reliable, expecially if you actually put some heat on it. I only went to maybe 110ºF just to see that it was doing something and not DOA. At the time I only had the replacement and wanted to make sure I was handing off a working part to the guy putting my exhaust in for me.

It is still odd that the light goes off when I drive harder. This was making me think that maybe the light comes on if the system is too cool, but from my reading and others leaving them dangle, that makes me thing they dont care how cold they get?? It makes me think something is just not hooked up right or if there is a thermocouple difference from the bypass valve one to the others that they could have got put in the wrong spot. Need some hands on time to look things over.

"Words to the Wise
Temperature transducers usually fail in a big way. Rather than drifting, they usually just stop working."
David Pereles, Fluke Corp.

I believe mine are all still working or the light would not react to temp changes the way it does. It seems unlikely that they just drifted a bit and from sitting during a service and then suddenly there is a problem. Supposedly the used one I bought came out of a good running car that was being parted also. Running the car harder should make them hotter and keep the light on I would think... I have to rule out something being installed wrong first thing......

 

I got a little more info on this problem this morning (still no time for hands on).

I did find out from Daniel at Ricambi that all 3 thermocouples are the same part number, so moving them around should not affect anything as long as they are good and plugged into the proper electronic module plug.

I also found out that they were getting a slow down light at idle on my car at first and then swapped the connectors between the main thermocouple and the bypass thermocouple and it stopped doing it.

So, I need to know for sure what plugs in where and make 100% that is correct? Anyone have a diagram or pics?

My next theory is this.......

Is it possible the bypass thermocouple is getting hotter now when the bypass valve is closed due to the fabspeed Bypass pipe not having any restriction? Exhaust gases more easily hitting the bypass plate area and heating it up more? Thus barely tripping the flashing slow down light? Then when the bypass valve opens up the computer expects to see a hotter temp on that thermocouple and the light goes off?

I think I can test this by simply removing the bypass thermocouple and leaving it dangle and plug the hole? All temps were checked on the exhaust and everything was good and temps on the header tubes all even etc. There is not a real temperature problem. Something is just different that the electronics don't like.

Anyone found a good 10mmx1mm cap for the thermocouple fitting to close it up with the thermocouple out?

 

No argument about that -- just thought that you were getting somewhat beyond looking for opens or shorts so thought I'd mention it. If you want a higher test temp, I think you could stick the thermocouple end in, or above, the flame of a natural gas stove for a while -- just a thought...

Don't know for a M10 thread, but on the M8 thread of the TR thermocouple bungs, they use a steel ball bearing to do the actual sealing (contacts the same seat where the ferrule on the thermocouple snout contacts) and then the acorn nut just pushes on the back side of the ball (i.e., no true sealing at the threads). A similar thing should work for you unless the seat for the ferrule is something much different.

 

Your post definately sparked me to look into thermocouples more, so it was good. I did some reading on them and have a much better idea how to test them if I need to go down that path. I may have continued on the thought of treating them like a thermistor had you not said that.

Good idea on the ball bearing and acorn nut. The 355 does use a ferule and compression type fitting, so the same thing should work with 10mm stuff.