41 replies to this topic

[Mayfair94]

Hi

 

Happy to have the engine running now after over 4 years since I purchased the my Mayfair as an MOT failure (mainly due to rust at the rear). As per my previous posts, I have a 95 Mayfair SPI. I have recently replaced the Vacuum hoses with 3mm silicone hose after reading other threads and finding two broken off. I have also just changed the oil, oil filter, fuel filter, spark plugs, coolant, thermostat and aux fan temp switch. I have also just adjusted the valve clearances.

 

When running the engine it starts with no issue. After reading other posts I would say my idle is high, normally around 1100 to 1200 rpm. I have also noticed that when accelerating slowly, the engine builds up revs nice and smoothly. If you try and accelerate quickly, the revs drop and the engine sounds like it wants to stall. If you do the same at higher revs, moving the throttle quickly can lead to a mis-fire/popping sound coming from the manifold.

 

I have been reading through the relevant SPI threads. I managed to get some readings through Rover MEMS Diagnostic. Coolant Temp started at 14 and warmed up to around 87 nice and steadily. Intake temp started at 12 and climbed steadily to around 26.

 

Lambda:

The LAMBDA Voltage did fluctuate as described in other threads. It seemed to go quite low though, down to 40mv and even 0mv briefly and then up to 880mv. I plotted the results with the engine running:

 

 LAMBDA Graph.PNG   223.61K   1 downloads

 

MAP Sensor:

With the engine at idle it seemed pretty steady at around 27 to 30 kpa. When accelerating quickly it would go up to 100kpa and then would gradually settle back down (sometimes briefly dropping to 20 kpa):

 

 MAP Graph.PNG   33.78K   2 downloads

 

Throttle Position Sensor:

Now this sensor gave me the most concern with the readings. I have read the other threads and plan to get a multimeter out to check the voltage readings. When running the engine I noticed throttle angle would not go any lower than 36 and the voltage and angle would remain steady for about 1/3 of travel of the pedal and would then start to change. The voltage would be around 0.5v at idle and at full throttle it got up to about 1.1v. It was like there were dead bands in the potentiometer. I have also seen a thread on cleaning the TPS so will get the multimeter readings and see if these tally up and if so see if a clean helps. I plotted the throttle angle and voltage:

 

 Throttle Graph.PNG   177.11K   3 downloads

 

Questions:

- Do the MAP and LAMBDA sensor values seem reasonable? I suspect a TPS issue and want to confirm this, but looking at the graphs should I also be looking at the LAMBDA and MAP sensors still?

 

- Can the Throttle Position Sensor cause my issue with the high idling and mis-firing/stuttering or is it more likely to be MAP/LAMBDA related?

 

Any extra advice appreciated as always.

Edited by Mayfair94, 17 December 2020 - 09:24 PM.

[MiniMadRacer]

I am no expert on these Minis at all, but is it worth checking the fuel pump pressure?

[brivinci]

Interested to hear more about this. My 94 SPi does the same thing when you mash the throttle down at idle. If you hit it to the floor at idle, it will stumble badly. If you keep it there, it can clear up and rev or it can stall. Obviously, I do not need to stab the throttle at idle for any particular reason BUT the fact that is doesn't rev freely, means there is something up.

[brivinci]

I am no expert on these Minis at all, but is it worth checking the fuel pump pressure?

This is would most helpful to see what the pressure is across the rev range, no? Under load.

[sonscar]

I am not knowledgeable on Mini injection but what triggers acceleration enrichment?The MAP seems to behave properly,if the throttle is only going to 1.1v Max I would suspect this.The reference voltage to sensors is usually in the range 0vto 5v.Steve..

[FlyingScot]

From the graph and your own investigation looks like the TPS is worn. You should have a smooth transition from idle position to max throttle. Regulated sensor voltage comes from the MEMS ECU and is 5V.

 

FS

[MiniMadRacer]

My comment on checking the fuel pump pressure was based on an old 1993 BMW 730i V8 E32 I had, which ran but developed a lack of power suddenly. To cut a long story short it turned out the fuel pump did not reach the required pressure (3 bar i think) that it should do.. it would get up to about 1 bar and that was it.. it meant the car ran, and accelerated (eventually) but the injector spray pattern could was not optimum for acceleration but cruised great (eventually),,, I know not a mini but may be something to check

[Mayfair94]

From the graph and your own investigation looks like the TPS is worn. You should have a smooth transition from idle position to max throttle. Regulated sensor voltage comes from the MEMS ECU and is 5V.
 
FS

Thank you for having a look FS, I know there are many posts for the SPI but it’s nice to get a 2nd opinion. I was looking at the logs again tonight and the TPS doesn’t fill me with confidence. The readings have dead bands and are far from smooth. As a novice on cars I sometimes doubt what I am seeing on the diagnostics but having read some of your posts I will be trying to get some voltage readings to double check and then try and find a TPS (seem to be very rare!).

Thanks again all

[xrocketengineer]

High idle should mean a vacuum leak. Did you check the fuel trap for leaks? Brake servo and hose? 

[Mayfair94]

High idle should mean a vacuum leak. Did you check the fuel trap for leaks? Brake servo and hose? 

I crudely checked the fuel trap for leaks while it was out (I blew through it and blocked the other port). At the moment at idle the MAP reading is around 30 kpa using a laptop program. Is this reading within limits? Is it likely to be causing my high idle?

[Mayfair94]

As an update, not sure if it is causing my high idle or stuttering but today I removed the Throttle Position Sensor. Before removing I had a look at the values again through the laptop. I was getting the same voltage readings again of about 0.5v to 0.95v at full throttle (I believe this is low after reading other threads). With the sensor removed, I got 5V between the outer pin and earth so supply voltage seems good. I believe with the TPS removed I have found a fault with it in that when you move the TPS lever full throttle and then release it, it only springs back about 2/3 of the way and not back to the stop. You can move the lever the rest of the way back to the stop. I suspect this is why before I was getting a throttle angle minimum of about 30% as the sensor sticks there. From this, I suspect I will need another TPS (new ones not available) or try and disassemble this one and see if it can be cleaned up. Not sure if this is my only issue causing the high idle and stuttering, but one thing to get sorted before moving on?

 

TPS input lever springs back to here:

 A93BF753-EF43-47CF-A39D-1229E9F7C618.jpeg   38.45K   2 downloads

 

TPS input lever can then be moved back to the stop by finger pressure:

 

 4573DACF-5E60-4D90-865C-225867BCCBCD.jpeg   33.82K   2 downloads

Edited by Mayfair94, 21 December 2020 - 05:33 PM.

[tmsmini]

I thought this product was made as a replacement, but looks much different:

http://classicminisj.../potentiometer/

 

Appears it also needs a harness adapter of some kind.

Maybe someone who has used one will respond.

[brivinci]

I am going to have to remove mine and check it out as well. I just hooked up the car to a phone running MEMs Diag and the TPS voltage was sitting a little high. I VERY slowly pushed the throttle down and the voltage seemed to rise gradually, to a max of just under 4v, never higher then 3.96v...and pushed it as hard as I could to the floor! I wonder if mine is not returning fully on its own. I believe it was resting at .8v when I switched on my ignition. I stabbed the throttle and it went down to .5v. I feel like I have other issues going oh. I am hitting a flat spot that is most noticeable under full load in 3rd, between 3-4k. 

 

Mayfair94, as yours clearly has an internal issue if it not going back to full stop and reading low, you might as well open it up and have a look. Buying a used unit might end up giving you the same issues unless you can test them first. Japan has a few options, not cheap, but neither were the last few TPS's I remember seeing for sale on the UK vendor sites.

 

http://classicminisj.../potentiometer/

Edited by brivinci, 21 December 2020 - 08:05 PM.

[Mayfair94]

brivinci, I think I have found a used one from someone I know. I will probably get it off them and slave it in place and see what the readings are like. If they are good, will just buy it and then maybe open up the one I have currently when I get time and see if I can make a possible spare for the future. It may not just be the fact it doesn’t return to the idle position, as the voltage it was reading was 0.5 to 0.95v and I believe it should be nearer to 5V at full throttle. I am getting 5V from the ecu on the wiring harness side.

[FlyingScot]

Okay I have some brand TPS NOS  I shall check them out and let you know. Please check using ohms reading as I’m not able t9 connect to the car. 
I have recently had a club member with a loom which had burned through connections at the bulkhead which showed low voltage due to the loom - might be worth checking continuity from ECU to TPS connector.

 

FS

Edited by FlyingScot, 21 December 2020 - 08:35 PM.