Red oil in coolant and vehicle not starting when hot!

Hi There, I'm a long time lurker of this forum and more than 10 years owner of a 1998 Porsche 996 Carrera Tiptronic car. For the last few years, I could barely use the car (apart the covid) thanks to some issues escalated because of some incompetent mechanics that I choose so in the end, I decided that it would be wise to get some advise from such a great community in here. There's a long story of multiple times of engine rebuild and than a transmission rebuild on the car, which I don't want to bother you but in the end, I've 3 main issues which couldn't solve for a while : 1) Last time I drove the car, the rev counter was bouncing a few hundred rpms up and down while driving and we found a fault code on Transmission which says "Speed Signal <- DME Control Unit". As far as I know the speed signal is received from the crank sensor which we replaced but still wanted to know whether there is any other sensor related with car's speed where the transmission might be getting data from? 2) When the car gets into the operating temperature and you shut it down, it's nearly impossible to restart it. At first it was happening rarely but after changing the crank sensor, it started to happen all the time and we see no signal from crank sensor errors as well. After replacing the crank sensor (2 times) we replaced fuel filter, Alternator Y cable, spark plugs, Ignition Switch and now will change the Fuel Pump as well. The mechanic says they suspect from ECU as a last option which I'm not that sure although had some similar issue like this happened because of engine air blower going out like 10 years ago with similar symptoms so who knows . May any of you have any other idea that we should check? 3) There seems to be a red colored oil cumulating in the coolant, which I suspect is the transmission oil getting into the coolant. When the transmission was rebuilt, the transmission oil cooler was replaced with a new one as well but since those guys did manage the broke the engine as well (overheated because they couldn't fill the coolant properly to the car), who knows whether they somehow broke the brand new oil cooler as well . My question in here would be whether there is any other part that may cause transmission oil to get into coolant other than the "transmission oil cooler"? I see there is a part called "basic plate" which holds the oil cooler but didn't find any information whether it can be a cause for this. Sorry for the inconvenience and thanks for your time! Regards, �zden

1) no 2) do you see crank signal at all times after latest replacement of sensor? If not, focus on wiring. If that is bad the signal will not reach the ECU. Other cause could be a bad⁹p coolant temp. sensor but even better check what resistance you see when measuring at the big connector to the ECU after you disconnected it whilst engine is hot and not starting. If it's ~200ohm it is fine as is the wiring. If it is greater than 2000 Ohm it is bad or the wiring is bad. You need a wiring diagram for that. 3) no, only source is your trans oil cooler

Quotes source post: 19110028

First of all, thanks a lot for taking time and answering my questions! Items 1 and 3 are pretty clear but would like to discuss a bit further about item 2, apart checking out your suggestion, as it reminded me something I forgot ; We had already changed Coolant Temperature Sensor but swapped it back for some odd reason I observed with my own eyes. After we replaced it, my mechanic thought they solved the issue so I went there to pick the car and after we started the car and while talking, I noticed the water temperature gauge was falling down after a while, even sharply when you push the gas pedal, like it moves from (Celcius) 70ish degrees to 50ish when you give gas so thought it's broken and ended up putting the old one back. Also, we could replicate the car not starting with this new sensor as well. Does that give any clue to you whether the issue is related with Water Coolant Temperature data whether the sensor is broken or not delivered properly to ECU because of wiring maybe? Otherwise, were you suggesting checking wiring in between crank sensor to ECU? Cheers!

Does that give any clue to you whether the issue is related with Water Coolant Temperature data whether the sensor is broken or not delivered properly to ECU because of wiring maybe? If your mechanic cannot answer this question, you need a mechanic who can... If a new, known good(!) Temp sensor does not solve the issue, then you still have not found all the causes, even if you have ascertained that the old Temp sensor was indeed bad (which your mechanic hasn't...) It is so easy to check a Temp sensor: at 20�C they all have 1-3 kOhm resistance. At 80-100�C this drops to 10-20 Ohm. So measure at 20 deg C, then put the sensor in a pot with water and wait till that boils. Take sensor out and measure resistance. Compare to spec and KNOW whether it is OK or not. Have this done and come back with the result.Quoting my own post : check what resistance you see when measuring at the big connector to the ECU after you disconnected it whilst engine is hot and not starting. This is also where your mechanic should check the signal coming from the CPS using (at the very least) a Volt meter set to AC or ideally an oscilloscope. Measuring at the plug ensures that any bad contacts in connectors and/or the wiring loom between sensor and ECU will show up. It is heat releated. With heat things expand and could short out chafed wiring to ground rendering a signal from a functioning sensor useless. Look for chafed wiring. With heat electric resistance increases and voltage drops thereby changing sensor signals. Look for loose and or oxidised ground connections and oxidised terminals inside connectors.

If you have "red oil" in the coolant the Tiptronic's ATF cooler has failed. I would suggest fixing this asap. The ATF will ruin all the hoses in the cooling system.

Quotes source post: 19111405

Thanks for pointing this out. We are aware that it may occur and my mechanic let me know they even changed one of the hoses already because it became too soft. I ordered a new ATF Oil Cooler as this is the priority to change in here to see whether it solves the issue and than will keep an eye on related secondary items

Quotes source post: 19111197

Thanks for further clarification. I'll get back to my mechanic with these information and than will get back to here with our findings, hopefully with a news that everything is ok! Thanks once again, cheers

2 things. 1. Did u replace the starter. When hot a suspect starter will not turn fast enough to turn over the motor. 2. On the red in the coolant. Yes check the cooler but the temp drop and uncontrollable tempn s also associated with the trans cooler temp valve right off the cooler and can also be a contributing factor. There r several threads on the 6-gt3 board about this as they have a similar device.

Quotes source post: 19111818

Thanks for sharing your thoughts! Apologies in my previous reply I mentioned starter as changed but I ment Ignition switch but I think the starter was tested which was ok, but will still consult with the mechanic about this. I'll also check about the trans cooler temp valve, again thanks for the tip

I hate to bring it to you but It's next to impossible to give relevant info if you're mixing up ignition switches with starter motors or if you can't remember exactly what was done... Perhaps you can convince your mechanic to read this thread as the risk of not relaying the info back and forth with the necessary precision with you as the man in the middle is just too great and a waste of time and energy...

Quotes source post: 19112675

I need to correct a misunderstanding in here ; mixing starter motor and ignition was a language confusion, since english is not my native language, rather than get confused about the parts themselves. After I posted I just thought, "Starter may not mean what I first thought" and after doing a quick googling I realized that and fixed the thread entry, which in total was a minute from start of posting the original to correct and re-posting it. I can assure you that for more than 10 years I source %98 percent of all the parts to my mechanic(s) to do the fixings, which includes 3 times engine rebuild so I know what is inside out at the car better than anybody else in here, even from the current mechanic as he's not a porsche specific one but still does a good job when guided. Thanks again for all your time and effort, much appreciated.

Thank you for pointing that out. Based on your experience I'm confident this thread will continue to be valuable to all of us. Ideally, after you've found the root cause we get to enjoy a final post to this thread in which you share your success and tell us what caused it. Check this thread perhaps it contains something relevant to your case too https://rennlist.com/forums/996-forum/1349967-sensors-that-can-cause-turn-over-but-no-start.html?ispreloading=1

Hello Everybody, After a whole 2 years, it was time to resurrect this thread as all this time, the car has been sitting on 2 different mechanic shops and finally I decided to step on it and solved some of the problems that I described but there is one item still remaining which causes the car not to be %100 back to life again. I don't want to list everything what happened in those 2 years as it's really nerve racking but in the end here're the state with the problems I mentioned originally : 2) I found out that the original Crank Sensor to ECU cable was cut and another wire has been put (from a different location) in between the sensor and ECU. Just connecting the original wire back eliminated %90 of this issue. I don't say %100 because there are still some rare cases the car cranks and the engine doesn't fire as it should but the current state is much better where at least at 2nd or worst 3rd try (whether the engine is cold or hot), the car starts and that's enough for me for now. This also eliminated "Can't get signal from Crank Sensor" fault we saw on diagnostics. 3) The red oil was indeed transmission oil. The whole issues started with a supposedly reputable transmission builder and it seems those reputable guys even did blow the brand new transmission cooler I bought for them which caused this whole mess. After another new transmission cooler installation and tons of cleaning on the coolant side, there is no more any oil in the coolant. And that leaves us with item 1, which still does exist in the car. To recap, when you drive the car (not when stationary), you see the rpm jumps up and down constantly where you also feel the car is shaky. The transmission can go through all the gears whether it's automatic or manual (This is a tiptronic car). Today I connected iCar Por v2 to see the fault codes and it still shows Fault code 21 : "Speed Signal <- DME Control Unit" fault in the TCM unit side. Related to speed signal, my researches show that there is a speed sensor in the transmission as well like seen here : Speed sensor 5HP19, 5HP19FL, 5HP19FLA G-SPS-5HP19 I'm tempted to give a try to this, anybody may have any other ideas at this point? Thanks a lot again!

Wow! I just have no words. 2 years have passed, the engine has been rebuilt three times, faults and codes still exist. Again, and of no help, Wow! I'm going to church and will say a prayer for you.