620 with possible lambda sensor issues

[HeMan]

Hi,

I have a 3 year old 620S which has had an intermittent fault since new.  I have seen many similar reports on here, and from 420 owners too, but basically, under very light throttle, the engine runs rough. When you push further it suddenly comes on song and is fine again. It makes driving slowly (even I have to sometimes!) really rough and going from off-throttle to a balanced throttle to accelerating out of a corner impossible in the wet!

Every time it has been back to Caterham they fiddle with it and more often than not tell me they replaced a faulty lambda sensor and it is fine for a while.  But before the next service it is playing up again, so we rinse and repeat.

I have noticed it smells a bit of pertrol, and I am only getting about 20MPG, even on motorway driving.

During the fish & chip run it was getting slowly worse throughout the day (which suggests its not a simple component failure, but maybe the lambda sensor coking up?). By the evening it suddenly got worse and it was as though there was nothing for the first quater of the throttle pedal travel then after that it was fine.  So in the end I disconnected the lambda sensor at the roadside and from then on, it was perfect again!

I also got 25MPG out the next tankful and I don't think it smelled of petrol anywhere as much.

So I guess the question is, do I need the lambda sensor connected at all? Can I remove it and bung the hole?  Or do I need to get back on to Caterham for a permanent fix, or do I need to consider lambda sensors as consumables!

Is it needed for emissions MOT testing?  As when it went for the last MOT the guy didn't even know if it had a cat or if he even needed to do the tests, so I get the feeling if I had said no he would have just skipped them.

There is a nagging part of me that wonders if the failed lambda sensor is actually caused by something else causing over fueling which is coking the sensor up and although disconnecting the sensor solves the symptoms, it doesn't fix the root cause.

I am also aware there are many similar issues reported on here and much talk about Easimap and the like, but I have read so much I am a bit lost, so wondered if someone could provide some more basic advice (we can get on to Easimap later if required...).

Thanks

[BigCol]

I was at PGM earlier in the year for them to set my car up and, amongst other things, another rough-running 620 was a topic of conversation.

My understanding is that the Caterham set-up/map is a flawed compromise and is never going to be optimum as its ECU map doesn't use a full/ideal set of inputs/parameters to determine the ideal fueling.

PGM's solution as I see it (worth having a chat with Andy - he'll explain better and given his character and how busy they are, you won't get a hard sell!) is to take a step back and remap based on different/additional engine inputs.  

The result is the car the 620 should be - no over-cooling, no over-fueling (and so potential bore score) just a proper, smooth, bonkers-fast car.  Whilst it wasn't their objective, their solution in some cases has also released getting on for another 20% of power!

This may or may not be where your problems come from (how many faulty lambda sensors are there out there? ) but certainly worth investigating...

 

[HeMan]

Ah, so PGM do a remap for the 620 do they?  I took my last 7 to them for a flat-floor some years ago, so I know PGM. But a quick look at their site and it seems to start from £3,418, which seems a lot of money to solve a problem.  I might give them a call, bit it's not like I need more performance or anything  , so it's going to be a hard-sell unless there are some other benefits to the kit?

[aerobod - near CYYC]

The lack of coolant thermostat in the 620R is a primary cause of over-cooling in gentle use, that leads to too much operation below 60C, leading to over fueling (as the ECU is in warmup mode below 60C coolant temp). This then leads to four problems that will kill the Lambda sensor quickly:

1. Raw fuel in the exhaust that cools/shocks the sensor

2. Fuel potentially igniting in the exhaust and shocking the sensor

3. Too rich fuel mixture causing carbon clogging of the sensor

4. Overall operating temperature too low for too long, creating increased condensation in the exhaust that cools/shocks the sensor.

Harder driving or a thermostat are really the only good solutions to the problem.

[HeMan]

OK, that's interesting to know.  But I assume installing a thermostat isn't as easy as it sounds, as it would require wiring, plumbing and an ECU re-programming to use it? Is that what PGM effectively do for the £3.5k

I already drive as hard as I dare 

So, in the absence of £3.5k, and back to my original questions; is it OK to simply do without the lambda?

 

[aerobod - near CYYC]

You can do without the Lambda sensor if you don't need to pass an MOT, as emissions will be almost certainly out of spec in the closed-loop Lambda zone (typically up to about 5000RPM and half throttle). If your coolant temp doesn't get above 60C within about 2 mins of engine start from cold and stay there until it has been shut down and then cooled down again, you risk a too rich mixture creating bore wash and high risk of scoring, though.

[John Vine]

It seems to me that you're suffering from the well-documented problems of the 620 being over-cooled due to shortcomings in the design of the cooling circuit.

There's a deal of history here.  PGM (and maybe others?) now offer a permanent fix by means of an upgrade to the cooling system.

JV

[DW199]

It would be useful to know if the car is reaching normal operating temperatures.  Assuming it has an MBE ecu, do you have access to easimap?  I purchased the adapter to try to sort out my poor running problems and set the tps correctly.  You are welcome to use mine if you fancy a trip to the south coast.  

[HeMan]

Going by the dial on the dash, it runs cold most of the time when it is moving (i.e. the needle hovers around the bottom of the dial). In traffic it quickly rises and the fan comes on and off every few minutes.

Is the Easimap required the free software from here:

https://sbdmotorsport.co.uk/product/easimap-6-pc-software-easimap-6-pc-software-download-for-free

And then is it the £192 cable (!) here:

https://sbdmotorsport.co.uk/product/mbe-map-kit-3-can-basic-can-mapping-kit-for-mbe-can-based-ecus

Or would I need the £600 one (!!)?

Or a trip to the seaside... 

 

[aerobod - near CYYC]

It is the £192 cable (MBE985), plus the free Easimap 6 loaded on a laptop with a USB port, running Windows 7, 8, 10 or 11.

If your dash coolant temp dial is dropping below 60C (i.e. to the bottom) when in normal operation, that is a problem, though. A short term solution may be to blank off part of the radiator or grille when not in traffic to ensure you maintain that 60C minimum coolant temp.

[pgm]

Hi HeMan If you would like an explanation about the issues that your 620 is suffering please feel free to give us a call during the week days. There will certainly not be any hard sell at all.  Best wishes the PGM team.

 

 

[DW199]

Would it be possible to connect a resistor in parallel with the temperature sender?  This would have the effect of raising the temperature that the ecu is "seeing" so keeping it running in closed loop mode even if water temp is a few degrees lower than 60C.  It would be considerably cheaper than the plumbing mod.

[aerobod - near CYYC]

The only problem in doing a parallel resistor is that it would have a disproportionate effect at lower temps, causing difficulty in starting and running initially from cold. As an example, say that the ambient temp is 10C, the desired "faked" coolant temp is 70C and the actual coolant temp is 50C, the temp sensor resistances at these temps (using Bosch NTC curve) would be:

10C - 3792 ohm

50C - 834 ohm

70C - 436 ohm

A 1000 ohm parallel resistor would cause the ECU to detect a temp of about 69C when the coolant was at 50C, but about 52C when it was only at 10C during cold start.

 

[DW199]

I see your point.  I had assumed the change in resistance with temperature was more linear. It would be useful to log the temperatures to see how far below 60C they go in normal operation.  If only a few degrees of faked increase was required, then a 5 -10K resistor might be sufficient without affecting the cold end of the curve too much.  If it was mine I would probably do a few experiments.

[ScottR400D]

#14 James, if it were mine I'd fix the issue not just mask it.