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Discussion Starter #1 (Edited)
Hello!
I am having an incredibly difficult time trying to figure out how to solve this issue. The car is an ‘08 Mini Cooper S Gen 2 with 64K on it.

Last year the timing chain went out. Did a whole head rebuild. Everything worked great.
Now the problem started a couple months ago, with a rough idle - bouncing between 500 - 1,000rpm, sometimes it would just stall out. The only engine code it was triggering were for the thermostat was stuck open, and coolant temperature sensor. It would not drive at all, so it was towed to my house, and it’s just been my project for the time being, but it would be nice to have the car back..

I’ve replaced the following things:
  • HPFP
  • MAF sensor
  • MAP sensor
  • Thermostat (the whole housing and what not)
  • Valve cover
  • Spark plugs
  • Coils
After replacing all of that, it started rough for about a minute, but then it would smooth out and drive fine after. Every once in a while it seemed to lose power on acceleration.
Now the only code that is triggered is for the coolant temperature sensor (P0118), so I replaced just that sensor, but it’s still showing it’s faulty after clearing codes. At start up (when the engine is dead cold), the sensor is saying the coolant is 290F -- which can’t be right.

Any suggestions on what to look for/check would be greatly appreciated. I’d like to prove my dad wrong and not end up bald.
 

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that your issue with running wrong,, so the ecu see the engine is massively over heating and trims the fueling ie makes it run lean running lean on start up upsets the upstream 02 sensor as it now sees loads of 02 in exhaust so it tells the ecu to add more fuel ,, the ecu then has a little fit because the temp sensor circuit is says less fuel...
this process will keep repeating itself over and over ie the 500 to 1200 revs jumping,,, WARNING DONT DRIVE LIKE THIS IT WILL MELT THINGS AND DESTROY THE CAT,,,
first off what temp sensor is on there it should be a brown one people make the mistake thinking cheap thermo housing are all the same and right for the car ie cheap ones come with green sensor... if its a new brown sensor then its a wiring issue or not unknown for brand new sensors to be faulty,, at 20c the voltage at sensor signal wire should be around 2.9v ish, you could try a new sensor without removing the old one when its cold start it if sensor should run on choke ie 1200revs ish,, need to scan it with a diagnostics that will cover live data and piloted housing see what is reading what,, also deeper scan might help if shows short to positive p-code that only dealer level scanners will see,
if is short to earth or positive chances are its a broken wire somewhere tend to be that wiring back to main ecu,, also note the temos sensor wiring to ecu is switched each via the ecu,, ie the ecu switches the earth return and not the live feed,,,
also bare in mind these engines eat oil this oil over time kills off the cat i have replaced so many of them over the years this can also cause over heating of oil as back pressure and heat cant get away from a engine that is already running hot by design ie 100c-112c fan on,, also if cat is blocked the upstream 02 sensor reacts to it the down stream gets confused as no 02 comes its way so it tells ecu to rich up the fueling etc,, all a nasty little circle that even torments me sometimes as no one wants to replace all thats needs to be changed, ie cat both o2 sensors , around 80k ,,
 

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Discussion Starter #3
Thank you for this heavy loaded response! One thing I forgot to add to the list of things I replaced was the upstream O2 sensor. The new thermostat did have that green temp sensor in it, but that was replaced with the new brown one as of yesterday.
I’ll go ahead and get a new downstream put in, and get the volt reading on the temp sensor.
 

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it tends to be a sign that upstream 02 sensor fails from a blocked cat on these engines and heavy oil use as aswell adds to it also does a timing chain stretched and driven for a long time retarded timing etc, but think you need to find why your temp sensor is reading so high and a short circuit somewhere is common as ecu failure on some cars
 

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Discussion Starter #5
Pardon the potentially silly question - checking the volt reading on that. Stick the prongs into the sensor connector, then fire the car up to check the reading? Or put the key in the ignition and hit the start button, but not actually start the engine?
 

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you can probe the back of connector,, ie get two needles and push in to rear of the plug on sensor,, so sensor is still connected to car wiring and your reading across the sensor as such, can also pull plug off and see what voltage you have from both pins one should have voltage the other nil on open circuit,,
 

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Discussion Starter #8
Perfect! I’ll give that a check today. Which method is better at reading the correct voltage: engine on, or key fob in with button push no engine running?

I also have a downstream O2 sensor on order, but due to the holiday weekend, gotta wait until Tuesday.

I really appreciate your help with this!
 

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would say the 02 sensor is more likely its reaction to the coolant sensor circuit and telling ecu the car is always running very hot,
 

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would read it ignition on engine off first off, then run car until car hot see what the voltage is ie if changes then the circuit is working and prob just the sensor is out of range
 

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Discussion Starter #11
Replaced the downstream O2 sensor, but I’m still getting the P0118 code. Checked the current at the connector, and it doesn’t seem like there’s anything going through it. All other sensors around that area show something.

Here’s a little video of what the engine sounds like: IMG_0198.MOV
 

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02 sensor work like this the smaller signal wires one ecu switched earth the other wire gets its voltage from its self ie 02 sensors generate volts when working correctly and switched to closed loop, ie over 600c heat in exhaust , until this temp they run heater circuit the bigger wires same thing ecu run,
if blocked cat the upstream heats up to fast and to much while downstream still fairly cold this can confuse the system, many people replace the 02 sensor that flags a code first then still has same problem,, on these engines i find most of them need 02 sensors and cat around 80.000 miles if have had timing chain issues or and valve stem seals gone hard after 8 years ish common with the heat my theory to this is because the engines run by design over 100c most of the time oil thins rubber seals get cooked and turn hard,, on turbo engines this is made worst by carbon build on inlet valves and even more heat from turbo and fuelling and driven so much harder,,
stick a small camera in the upstream 02 sensor hole in the cat and have a look at the cat surface for black build residue if have some then most likely cat will need replacing as well, cheap company for cat is CAT2U the non turbo ones are £158 and the turbo n14 £146 ish link below,
 

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Discussion Starter #13
02 sensor work like this the smaller signal wires one ecu switched earth the other wire gets its voltage from its self ie 02 sensors generate volts when working correctly and switched to closed loop, ie over 600c heat in exhaust , until this temp they run heater circuit the bigger wires same thing ecu run,
if blocked cat the upstream heats up to fast and to much while downstream still fairly cold this can confuse the system, many people replace the 02 sensor that flags a code first then still has same problem,, on these engines i find most of them need 02 sensors and cat around 80.000 miles if have had timing chain issues or and valve stem seals gone hard after 8 years ish common with the heat my theory to this is because the engines run by design over 100c most of the time oil thins rubber seals get cooked and turn hard,, on turbo engines this is made worst by carbon build on inlet valves and even more heat from turbo and fuelling and driven so much harder,,
stick a small camera in the upstream 02 sensor hole in the cat and have a look at the cat surface for black build residue if have some then most likely cat will need replacing as well, cheap company for cat is CAT2U the non turbo ones are £158 and the turbo n14 £146 ish link below,
Can’t thank you enough for the help on this!
Not sure if it’s vital information or not, but I am in the US, so I’m not sure what subtle differences there might be with engine components. I’ll definitely get a new cat put in - seems like the logical step per your suggestion.
I shall update with some sort of news regardless of the outcome.
Cheers!
 

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engine wise they are same ,, one other thing that upset them is thermostat housing and temp sensor,, i got one at mo it flags heater circuit on down stream 02 sensor ,,, one of those faults tormenting me,, worked out today the temp sensr and housing was replaced by them with cheap china one, this housing is running a green temp sensor which is wrong this causes idle issue and stalling,, tomorrow i will drop out the green one and put a factory green one in,, and should sort the issue,, these engines are so complicated and burn oil and all gives different issues sometimes i get caught with one as a number of things have failed or worn just enough that when all things add up gives a number of faults etc
 
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