36 replies to this topic

[Mini40UK]

I only did the wet test on the lowest cylinder which initially read 95. After my teaspoon of oil it jumped to just under 150. Now i'm looking at options for engine strip and look at the rings and bores. Thinking positive I can paint the engine and change the clutch at the sametime......every cloud.

[Deeppockets]

If you have not done so, double check for vacuum leaks and/or exhaust leaks.
Vacuum leaks can be quickly checked for by spraying carb or brake cleaner at various hoses (being careful to avoid the exhaust manifold), a change in engine pitch will indicate a leak. The exhaust is less obvious, a small joint or gasket leak anywhere before the cat or soon after will skew the O2 reading on the lambda sensor and cause over fueling.

[NLinPEN]

I only did the wet test on the lowest cylinder which initially read 95. After my teaspoon of oil it jumped to just under 150. Now i'm looking at options for engine strip and look at the rings and bores. Thinking positive I can paint the engine and change the clutch at the sametime......every cloud.

I would suggest that you repeat the test on every cylinder: measure the dry and wet compression ratio.

[68+86auto]

If the compression was too low, we'd see different emissions readings such as low CO2. Your CO2 is good.

Edited by 68+86auto, 12 December 2025 - 03:54 AM.

[Mini40UK]

Ok thats interesting thanks, so are you saying there is no need to have a re bore even though there is such a difference in the barrels? And also thanks I will go ahead and check again around those hoses.

[68+86auto]

Ok thats interesting thanks, so are you saying there is no need to have a re bore even though there is such a difference in the barrels? And also thanks I will go ahead and check again around those hoses.

 

Yes, I wouldn't be rebuilding the engine unless there was a definite reason to such as oil consumption. I don't normally do compression tests, a leakdown test would give a better idea. There are many variables to compression results. I would only use them relative to each cylinder. I just measure cylinder performance electronically.

Edited by 68+86auto, 13 December 2025 - 05:48 AM.

[NLinPEN]

 

Yes, I wouldn't be rebuilding the engine unless there was a definite reason to such as oil consumption. I don't normally do compression tests, a leakdown test would give a better idea. There are many variables to compression results. I would only use them relative to each cylinder. I just measure cylinder performance electronically.

 

I do agree with you that a leakdown test gives a more detailed analysis of were it is leaking toward, if there is leakage. On the other hand: a compression test is simpler to execute. I therefore recommend it as a first step, especially on an engine where you haven't done it before. Here we have a case where, at least on one cylinder, there is a large difference between dry and wet results (95 -> 150 psi). On top of that, a dry result of around 100 psi is not very impressive.

 

However, to keep the discussion closer to the topic title: even if the engine has leakage from one or more cylinders, it will probably not explain the emissions results.

[genpop]

if you still search for coil measurements

 

https://www.mini-for...089e2-zündspule

Did you check the IACV?

[Mini40UK]

 

 

Yes, I wouldn't be rebuilding the engine unless there was a definite reason to such as oil consumption. I don't normally do compression tests, a leakdown test would give a better idea. There are many variables to compression results. I would only use them relative to each cylinder. I just measure cylinder performance electronically.

 

I do agree with you that a leakdown test gives a more detailed analysis of were it is leaking toward, if there is leakage. On the other hand: a compression test is simpler to execute. I therefore recommend it as a first step, especially on an engine where you haven't done it before. Here we have a case where, at least on one cylinder, there is a large difference between dry and wet results (95 -> 150 psi). On top of that, a dry result of around 100 psi is not very impressive.

 

However, to keep the discussion closer to the topic title: even if the engine has leakage from one or more cylinders, it will probably not explain the emissions results. 

 

Thanks, yeah everytime i get the emission retested all I get from my Spanish Mechanic is 'comida! comida! mucho comida! basically eating too much fuel and running rich.

[Mini40UK]

if you still search for coil measurements

 

https://www.mini-for...089e2-zündspule

Did you check the IACV?

 I havent done but now I have these figures, thank you so much, I will check. I had visually checked and cleaned all plugs but I will get the multi meter back out and see what I can find. 

[Mini40UK]

If anyone is interested I bought a cable and plugged it in, heres my findings:  

[Mini40UK]

I haven't really made sense of it yet but how hard can it be

 

Attached Files

•  PG.1 Mini Test.pdf   1.17MB   11 downloads
•  PG.2 Mini Test.pdf   870.86K   6 downloads
•  PG.3 Mini test.pdf   983.58K   5 downloads
•  PG. 4 Mini Test.pdf   185.53K   6 downloads

[Mini40UK]

I'm not one for AI but this is another good test of how good it is maybe??? Would you agree with any of this report that its came up with once I dropped those test findings in?

Attached Files

•  ECU DIAGNOSTIC REPORT.pdf   349.81K   15 downloads

[Deeppockets]

Looking at PG4, the O2 sensor is giving erroneous readings - jumping from ~500mV to 65523mV (which I assume is a null value). This is then likely to cause overfueling. The 02 sensor, connector and wiring to it from the ECU should be checked.

I would also be suspicious of the O2 sensor's general behaviour as AFR consistently above 14 which will also cause overfueling as the ECU tries to compensate. In addition to checking the wiring, as above, if you have not already done so run a bottle of fuel system cleaner though the engine to ensure muck and deposits are not contributing to the issues. If the odd O2 sensor readings persist then you may be looking at a faulty O2 sensor or exhaust leak.

From my own experience, in closed loop control (engine at full temperature) the ECU adjusts the fueling such that the O2 sensor is frequently stepping between low (~0.5v) and high (~2v). The AFR typically sits between 12-14.5 (the perfect air/fuel ratio lambda is 14.7).

Edited by Deeppockets, 23 December 2025 - 03:47 PM.

[Mini40UK]

Looking at PG4, the O2 sensor is giving erroneous readings - jumping from ~500mV to 65523mV (which I assume is a null value). This is then likely to cause overfueling. The 02 sensor, connector and wiring to it from the ECU should be checked.

I would also be suspicious of the O2 sensor's general behaviour as AFR consistently above 14 which will also cause overfueling as the ECU tries to compensate. In addition to checking the wiring, as above, if you have not already done so run a bottle of fuel system cleaner though the engine to ensure muck and deposits are not contributing to the issues. If the odd O2 sensor readings persist then you may be looking at a faulty O2 sensor or exhaust leak.

From my own experience, in closed loop control (engine at full temperature) the ECU adjusts the fueling such that the O2 sensor is frequently stepping between low (~0.5v) and high (~2v). The AFR typically sits between 12-14.5 (the perfect air/fuel ratio lambda is 14.7).

Firstly Merry Christmas! Secondly, thank you so much for your reply, really helpful. I have already replaced the Lambda sensor but one thing I will say from my research is I only have one sensor on the exhaust and thats up on the down pipe from the manifold, which is the one I've replaced. I have read in certain places that I should have one either side of the Cat but I haven't and never have had, I wouldn't even know where that wires into. Anyway although I have replaced all gaskets on the exhaust I will search again to see if I have any leaks an maybe have a dig around the other sensors for another inspection and clean. Thanks again, all the best for 2026.