Apologies for the very long post but thought it best to be comprehensive!

I’ve got a 1992 Cooper SPi that I’ve had since new. It’s been off the road since 2010 but is now back. Although it passed the MOT it ran like a total dog, hesitating and surging at anything other than full throttle and with a tendency to stall. Thinking this may have been the Lambda sensor, I fitted a new Bosch sensor. Before I swapped the lambda sensor there was always a problem after about 5 minutes running from cold when the engine would stutter and stall (switch from open to closed loop?). It no longer does this which is good! However although it seems to runs better there is now a persistent misfire. It seems to be a misfire on cylinder 3 (remove lead and the misfire doesn’t change much). I’ve swapped leads and plugs around but problem stays with no 3.

I’m not sure if it is linked but MEMS-scan software shows Lambda voltage to be stuck at 1070mV. I have made up a lead to allow me to measure the actual sensor voltage with a multimeter whilst the engine is running; this shows variation between 450-750mV even though the MEMS-scan never moves from 1070mV (if I disconnect the sensor it defaults to about 435mV which I assume is a limp home mode).
I’ve checked the wiring associated with the lambda sensor and relay, all good continuity and 12V in all the right places. I’ve also swapped the relay for a new one. I think this could be a red-herring as the multimeter shows the lambda sensor to fluctuate correctly. Any idea what may cause this and could this cause the misfire?

Other information:
• It has a tubular LCB with the lambda sensor mounted in the Y piece (i.e. cylinders 1 and 4 only). There are no exhaust leaks.
• It has an inlet manifold from a Metro without the pipe for the tank vent so this is not connected. All the other vent hoses fit snugly and are not perished.
• The vacuum hoses to the MAP sensor are new silicone and the fuel trap is not blocked.
• The vacuum take off for the airbox is blocked off.
• Using MEMS-scan and a vacuum gauge I have confirmed the MAP sensor reads correctly.
• Idle switch is good (checked with multimeter and MEMS-scan).
• Throttle potentiometer is good (multimeter and MEMS-scan) and I’ve tried a spare one as well.
• Stepper motor seems to work ok. The idle adjuster has been disturbed but it holds idle quite well and can be exercised with MEMS-scan.
• Air and coolant sensors are good; MEMS-scan readings are relatively accurate.
• Compression (engine warm) is 13.5, 13.5, 14.0, 13.0.
• Plugs, distributor cap and leads are low mileage.
• Plugs are gapped correctly (0.8mm).
• The car has been in a dry garage for 9 years; it is not due to damp!
Any thoughts what may cause the misfire and/or why MEMS-scan gives a permanent high Lambda reading?
Any help, comments, suggestions much appreciated as I have run out of ideas.
TIA
Mike

Possible lamdba if its stuck

What code reader are you using?

Does it have fresh fuel?

Id pop new fuel in, service it with new plugs, filters, rotor arm etc and give it a run, then see how it behaves...

It's got a tank full of fresh Shell VPower (made from unicorn tears judging by the cost) and a dizzy cap, rotor arm and spark leads are on order.
I'm using the MEMS-scan software on a laptop, not a code reader.
The lambda sensor is brand new.The ECU can sense the sensor because if disconnected it goes to 435mV default, but I can't understand why it sticks on 1070mV (even with cold engine) when the lambda sensor shows a fluctuating voltage. IIRC the old lambda sensor also stuck on 1070mV which is why I replaced it. I'm starting to think it's an internal ECU problem, but will try the cheaper stuff first.

Mike

Tonight I started the car from cold. Whereas it used to stall after about 5 mins as it warmed up, it now immediately settles to a constant idle speed without hesitation (apart from the misfire) all the way up to when the fan kicks in. I think this could be the new lambda sensor taking effect as it transitions to closed loop. I've measured the output with an oscilloscope and it varies between about 0.4V and 0.65V with an average of 0.53V, ie a bit rich but in the right order of magnitude (especially with a misfire). I therefore think the 1.075V reading on MEMS-scan is a red herring.

The misfire is definitely cylinder 3. Remove the plug lead and the engine doesn't change at all but the lead sparks to earth so there is definitely energy in there. Swap plugs and leads between cylinders 1 and 3 and the misfire is still on cylinder 3. In the dark with the lights out there is no sign of sparks tracking to earth from the cap or leads.

As I said, compression is good, valve clearances are correct. The only thing left to change are the distributor cap, rotor arm and leads but is it feasible that these could be the problem?

Spark to earth doesn't mean good spark under compression.

Get the new plugs and other service parts and then retest.

The ecus are pretty bulletproof, I'd be surprised if it's that

Have you checked the stepper motor and vacuum lines, specifically the elbows that connect to the vapour trap and manifold. They go hard and crack, very difficult to see. This usually results in rich running, not so much irratic running

All vacuum pipes are new silicone (no elbows) and the stepper motor indexes across full range. TBH I wouldn't describe it as erratic running (although it was before the lambda change), it actually runs very evenly at a constant idle speed albeit with a constant misfire on cylinder 3.
I am pinning it all on the distributor cap!

Madalicesdad wrote:I've measured the output with an oscilloscope and it varies between about 0.4V and 0.65V with an average of 0.53V, ie a bit rich but in the right order of magnitude (especially with a misfire). I therefore think the 1.075V reading on MEMS-scan is a red herring.

I need a little more time to digest your notes of work done so far - but this statement above is a little telling. The standard lamda sensor isn't "broad band", that is to say it's not proportional to stoichiometric, but it just switches high or low above or below stoichiometric. Rather than being 'continuous' it is descrete. With this in mind, it might throw a bit of light on your diagnosis.

When I get a little more time I'll take another look at your notes (hopefully quite soon)

Nozzle

Hi Nozzle,
I thought that lambda sensors produce a voltage (inversely?) proportional to oxygen content where stoichiometric is about 450mV, otherwise how does the ECU fine tune based on a digital 'too much / too little' switching?

Only a wideband sensor does that, they cost a lot of cash so are only fitted in mass produced cars if they really really have to be..

Your description is accurate, "too much", "too little" while in closed loop mode - i.e. coolant warm, throttle at idle. The rest of the load map is pre-programmed and the lambda sensor is ignored, open loop mode. While in closed loop mode, the job of the lambda sensor feedback loop is to keep the catalyst healthy though, to stop it catalysing unburned fuel.

My initial thoughts about your diagnosis were that it is strange about the misfire on Cylinder 3, and suggests there is a different fault, despite all the things you have changed.

Nozzle

Dizzy cap is a good starting point, get all the cheap basics changed.

You might be able to just clean the terminals up in the cap as a short term fix, they fuzz up quite easily which could stop spark.

Bet to get all the serviceable bits - cap, leads, plugs - swapped out for new

Rotor arm, distributor cap and leads have been replaced. Still misfires on cylinder 3.
Tried all the plugs in cylinder 3, still misfires.
Then I remembered I had an old Colortune (wish I'd remembered last weekend) that shows there is a spark in cylinder, but no flame.
So if I've got a spark and plenty of compression it would suggest it's a fuelling issue. if the fuelling was badly timed then I'd expect backfires through the throttle body but it doesn't. It is feasible that the fuel injector may not get a pulse for cylinder 3?

I've checked the fuel injector with my oscilloscope with the engine running there is a regular firing pulse every 35-40ms, ie about 850rpm.

I'm measuring the black wire from the injector. The supply to the injector is a relatively constant 12V when engine is running.

So as well as a spark and compression, I should have fuel. SO WHY WON'T IT FIRE????

Misfire sorted I think.
Out of desperation I squirted WD40 around the inlet manifold to check for air leaks. When I sprayed the brake servo banjo connector on the inlet manifold the engine note changed; not much but it did.
Looking carefully with a torch I could see it wasn't properly seated because the locating lug had jammed against the manifold leaving a gap of about 1mm. With it properly seated and tightened, cylinder 3 fires.
I can't believe that this had such a massive effect on cylinder 3 but did not affect cylinder 4.

Sorry for wasting your time!!

Mike

Sir, in no way a waste of time!!

What we can say with confidence is that the ECU and Throttle body injector as glorified carburetor, and doesn't know which cylinder the charge will get sucked in to (Slightly different for the MPi which has an extra sensor on the cam shaft). The throttle body is a store of vapourised fuel and it goes off in whichever direction the next inlet port opens, the ECU knows not which.

It's been a quality learning exercise for all involved and I'll bet you're thrilled to have it working properly again

Nozzle

Well done Mike, good to hear you solved it!

I've been too busy to update lately, but it's still not running properly.

The misfire due to the air leak has gone but it's now back to it's old ways. Starts fine and idles well until warm, then stutters and stalls. A blip on the throttle seems to cure it for a short while, but it has a tendency to stall.

On the road, the previous symptoms are still there:
Hesitation when flooring the throttle. It's as if the vacuum advance diaphragm on the distributor is knackered. But it doesn't have one so can't be that!
Inability to hold a steady engine speed. If the throttle is kept in a constant position you can feel the engine power surging and falling off. It's difficult to explain but makes town driving very tiring and is especially noticeable in 2nd gear.

I was hoping that the lambda sensor would fix this but it hasn't, however it still shows as 1065mV on the MEMS-scan so I'm thinking I need to look at the ECU next.

Unfortunately, at some point the ECU was wiped with a solvent and all the numbers came off the label!
Can anyone tell me the correct number for a two connector ECU for a 1992 Cooper SPI. Haynes says MNE10027; is this the only option?
Does anyone have or know of one for sale?
Is anyone in the North Bristol area and have one I could swap to see if it does fix the problem?

Any help and advice greatly received as before.

Mike

I think it's sorted (note I've only done 5 miles so things could change). I replaced the crank sensor as the last cheapish part in the system, but no effect.
I then found a ECU (MNE10027) on Ebay and got it within about 30 minutes of it being listed!

Regarding an earlier comment about lambda sensors giving discrete switching signals above/below stoichiometric, there is a bit in the Haynes manual that confirms they provide a voltage proportional to oxygen content. Low voltage = low oxygen = rich mix.
With the new ECU, MEMS-scan gives a lambda reading varying around 450+-200mV, just like my oscilloscope did, and the car runs much smoother - so much so the CD doesn't skip!

It looks like the other ECU wasn't seeing the true Lambda value but didn't register it as a fault and thus wasn't using the default 435mV value when running. My ECU was stuck on 1075mV so presumably running very weak continuously.e incorrect fuelling / timing giving a hesitant, jumpy drive.

If only the rain would stop, I could get it out of the garage again and test my theory!

I'm glad to hear it's running much better now - I hope it stays then same for hundreds more miles to come.

Check out the difference between wideband and narrowband lambda sensor right here:
http://www.enginebasics.com/EFI%20Tuning/AF%20Ratio%20Basics.html

Nozzle

So the Haynes manual tells lies. Who'd have thought it?

Thanks for putting me right!

This is the first time I've heard of a dodgy ecu

Well done for solving it!

It's the second time I've heard of a dodgy ECU (I also had one with a temperamental stepper motor driver circuit)

Nozzle