Sigma 140 broken down.

[Wodybode]

I've done a few hundred motorway miles today on my way to the Dunsfold handling day and after a brief stop the engine is banging and popping and running as rough as anything. The exhaust is extremely hot too. I'm currently in a layby letting things cool off. The engine will run smoothly after start up for about 1 minute. Any ideas?

[Jonathan Kay]

Any work recently done on it?

Check for loose connections on all the HT bits that are accessible.

Where are you?

Jonathan

[Wodybode]

I'm between Slinfold and Rudgwick on the A 29. Just had its annual service this morning but that was just oil and filter etc. and I've done over 100 miles since then. Been on the motorway all day mostly at 80-90 ish. 

[Wodybode]

I've checked everything obvious under the bonnet and it all looks normal. 

[DJ.]

Might be worth removing the spark plugs to check them. If they are new after servicing, you might have a faulty one?

Duncan

[Wodybode]

Ah ha! It's the lamda sensor. It cuts in 90 seconds after starting. Disconnected it and it runs sweet. I think I'm OK to run without it connected. 

[DJ.]

Great news, enjoy tomorrow

I almost suggested the lambda, but when mine failed (after hitting a badger) it ran fine accelerating hard or on the overun, but just became really unpleasant when trying to cruise at a constant speed.

[Martin J]

Top Tip  Glad you found the problem, I'll remember that one.  When touring I do carry a spare one.

[SM25T]

Happened on my car a while ago. Ran fine without ... But was advised to replace it as soon as possible or it would damage the cat ?

[Jonathan Kay]

Well done.

Anything dubious about the wiring or connection or are you going to buy and fit a new sensor?

Jonathan

[Wodybode]

Hi Jonathan,

the wiring all looked good and there was no damage. I have to say I made a couple of phone calls from the roadside to assist in my diagnostics to Dave at Gemzoe motorsport and Callum and Tim at Sevens and Classics who pointed me in the right direction and reassured me it would be ok to run without it connected. 

The biggest clue was that the engine ran perfectly for 90 seconds after starting as the mapping ignores the sensor for that time period after which it then looks at the input from the sensor and makes adjustments. So I knew the engine was fine and it was a management issue.

I must say I was paranoid that running without it would cause damage to the engine or cat so pestered a number of people at the handling day for reassurance, so to those people thanks for your input.

[huwp]

Sounds familiar. I have a 125 on it's third lamda sensor having only done 13,000 miles. Do idea why it doesn't seem to last...

[aerobod - near CYYC]

An overly rich mixture for too long can kill a lambda sensor (i.e. engine map doesn't act enough to adjust mixture close to stoichometric based on sensor input), water getting into the sensor due to it being mounted too low in the exhaust allowing condensation to shock the hot sensor is another way.

[revilla]

Using silicone exhaust sealant which isn't specifically sensor-safe anywhere upstream of the sensor will quickly kill it too.