Am I running rich or got a misfire?

[pdavison]

I posted a little while ago about a machine gun type noise from the drivers side tail pipe. I have found it is knocking on the metal heat shield but I think I may have another problem.

On tickover once warm I get a pop every 5-10 seconds from the drivers side exhaust and I get a machine gun noise at about 2,750 revs in any gear when accelerating gently or holding a constant throttle position. If accelerating hard I get no problems at all (could be a solution in itself I suppose!).

Anyone got any thoughts as to what it may be?

PS - It's a glorious speed six by the way

[GCerbera]

Two things springs to mind.

1) HT leads could be breaking down - that is an
intermitent fault.

When I had this, if I lifted very slightly and powered
down again, it went.

2) Throttle pots need attention?

[pdavison]

Can I be niave and ask for an explanation of what the throttle pots are and what they do?

I'll have a look at the weekend at the HT leads, is it easy to tell if they are at fault?

[arcbeer]

Being a bit sad I saved off little tip-bits like this:



The MBE engine management on the Cerbera uses the throttle angle (how much right foot you're using) and revs to work out how much fuel to inject (via a 2d map), and work out the ignition advance. Most production cars use an airflow meter to work out the fuelling instead, but this isn't quite as responsive, and can restrict the inlet.

The throttle pot is a little potentiometer which measures how wide the throttle butterflies are open - there is one for both banks of the engine, though they're connected by the "link rod".

They're pretty crappy components, so can either fail, or more annoyingly get "noisy", i.e. give an intermittent signal back to the ECU. If it fails completely (or gets very noisy) then the ECU will ignore the dodgy one and use the other one - this is generally fine. If it's slightly noisy, then it just confuses the ECU into injecting random amounts of fuel, which can cause dodgy idling & running behaviour.

I think only the left pot is used for ignition, but I could be wrong about that; anyway, that would certainly cause dodgy running too.

Take along to your dealer - by plugging their laptop into the ECU, you can see the throttle pot readings in realtime - it's very obvious when one of them is noisy generally.

About £35 for a new one, and less than an hour to fit.

[GCerbera]

Another thought - could be a split coil(s)?

As arcbeer suggested, have it checked out with a laptop and it could transform the car.

[kevin63]

I checked that all my throtle pots were taking in the same amounts of air with a cheap and simple to use carb balancer from hallfords and the engine run smoother and seemed to have more power, now will run in 5th at 30mph but you can only do this with a SP6 engine that i know.

[Tatlow]

I had the same problem from the sounds of it, and I had to have a Lamda Sensor changed. Once that was done and tuned up on the laptop it was as sweet as a nut.