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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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Discussion Starter #15
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
Well, the new cat has been installed (what a nightmare those things are to replace), and I’m still getting the same symptoms. Runs smooth for the first 30 seconds or so, then it starts it’s usual rough/sluggish/coughing idle. If I give it some gas, and get the RPMs up around 2.5k it’ll smooth out again, but then return to its usual thing. The only code being trigger is still the P0118 temp sensor. Fuel pressure appears to be normal. Would the fuel injectors be an issue here at all?

Thanks again for all the insight and assistance!
 

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From a third party point of view, have you checked the engine temp sensor is correct (not its colour but it’s reading)? If it’s not then that has to be sorted first, otherwise the engine will never run correctly and continuing to run it is likely to cause damage.
Sorry it wasn’t clear to me that you’d sorted that issue, apologies if you have.
 

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i had a n14 a few days a go same sort of thing from a garage down the road they had spent a load of time on it and most to tend to be cat and 02 sensor related,, the temp sensor needs to be correct one for car ie brown and needs to read correct temp,, i was going on the temp sensor is fine and circuit reads correct voltages 20c 2.8-3.2volts normal , hotter car gets lower the voltage ,
anyway this n14 everyone had a look at it and gave up and like usual i seem to get them to repair after everyone has messed around with them lol,, i done same thing has misfires on 3 cylinders worse on idle,, what i done is i have a in boot tyre pressure pump that i have cut end off and fixed a old compression gauge adaptor to the end of it so can put air in to the bores as then dont need to drag a compressor around with me,, can use car 12v socket,, i added air in all cylinders with inlet manifold off and exhaust manifold off found when added air all valves closed on a cylinder and sprayed seafoam in each port and had bubbles form nearly all of them ie valve seats were causing a problem,,, pulled head done all valves and stem seals ie was stem seals went hard was 2008 87000 miles this one also had high pressure fuel pump issue as well,, and the oil cooler on the oil filter housing was blocked caused a cooling issues car always ran very hot ,,, this car would sound better over 3k sort of drivable but could feel the misfires ,, and when it was compression tested it would show 160psi but when a leak down test was done it failed,,, due to carbon build up on exhaust valves as well as inlet valves added valve stem seals ,, trouble when like this even wall nut blasting will only fix it until the leaking stems seals coated it all again in a few thousand miles,,
you need to sort temp sensor out first off,, after that worth doing the above test or ask a garage to do leak down test on it,
 

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Discussion Starter #18
I checked the current rolling through the temp sensor, and this is what I got:

Key in, car on (no engine running), the radiator fan kicks on immediately. Turn the engine on, coolant temperature is reading 290F at startup. You mentioned a few posts back to wait for the engine to get up to its optimal temperature before checking the volt reading, so I kept the engine on and hovering around 1,500 rpm. Checked the temp sensor, and it’s showing no current going through it. I checked all the other sensors around the thermostat, and they have current in them.
When the engine code (P0118) gets cleared, the car does smooth out and run fine, but after a minute or so, it’ll trigger the same code again, and idle drops to its sluggishness.

I got the car back from a mechanic back in August for the failed timing chain/head rebuild, so carbon buildup shouldnt be an issue here because all of that was cleaned during that time. It was driven for about 6 months before this issue popped up.
 

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what happens when no volts at temp sensor the ecu sets a default ie what you have, as you have no volts at sensor your next step would be pulling wiring harness apart to see the wires trace the two wires back to the under bonnet ecu ie mevi17.4 ,, both the wires from sensor are direct back to ecu ie switched earth return and 5v feed wire,,
find the two wires then you need to read the wires with ohms ie the multimeter when connect the ends together it bleeps to show a closed circuit etc,, use this on both wires with ecu disconnected and temp sensor if wiring bleeps and shows its not broken then plug ecu in again ignition on check for voltage at ecu plug,, if no voltage here you are in a whole world of pain might be ecu gone south,, the ecu its self generates the 5v rail all input voltages are 12v... you could try a power latch if its a internal glitch in ecu this should reset it all but not seen one come right ,,, seen many ecu failures on mini and psa cars use same ecu and wiring
 

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Discussion Starter #20
Brought the stubborn mule to a mechanic today, and they suggested a couple things:

  • Fan controller could be bad(?). Something I forgot to mention is the cooling fan seems to run a lot longer than usual. Sometimes it'll kick on as soon as the key is put in (engine off), and it'll stay on for about 5ish minutes after the car is off (even if the engine has been started and is cold). This would require the entire fan assembly, correct? It isn't a relay/fuse type of situation?
  • Like you mentioned in a couple posts, the ECU could be bad.
The idling issue has been solved. Apparently it was a faulty camshaft sensor.
 
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