Discussion
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.
Of course, there are plenty of other things to go wrong!
danny
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.
Of course, there are plenty of other things to go wrong!
danny
PS Hesitation or misfire under load - does it get worse with higher revs/wider throttle? I had this very recently and it was *entirely* due to a dodgy HT lead somewhere; I replaced the lot and it was instantly better again! Some dealers like to change the HT leads every 6k miles. I think mine were only around 5k old, so if you can afford it I think that's a wise plan.
danny
danny
I'm told that the other issue is that the two throttles for each bank are linked by an aluminium rod. Under the hot bonnet the cheap nasty rod expands and coarses the slave throttle to open ever so slightly more that the master throttle. This discrepancy is relayed back to the EMsystem that gets confused and either opens up the butterflies or closes them down out of sync to compensate leading to dodgy idling or missing of the engine.
I'm no expert but for £70 i think its well worth changing the ht leads ,my cars not running perfect but loads better than it was.I drove it yesterday about 100 miles in town and on motorway etc, and its far more pleasurable to drive than it was, more throttle responsive and consequently feels quicker too.Best £70 so far.
P.S. had the cerb 5 weeks now,done a thousand miles and its not blown up,i think i'm even getting about 20mpg!
>> Edited by mlumb on Saturday 18th May 11:09
P.S. had the cerb 5 weeks now,done a thousand miles and its not blown up,i think i'm even getting about 20mpg!
>> Edited by mlumb on Saturday 18th May 11:09
quote:This is all a bit confused. The ECU doesn't open or close the butterflies, it just fuels based on angle. You're right that one of the throttles opens slightly as it warms - but that doesn't cause misfiring or dodgy idling. If it did, then ALL cars would do it, and they don't. As for a "cheap and nasty" rod... well, what would you use? There's plenty of cheap and nasty things there, but I'd hardly pick on the poor rod. Doesn't steel expand more anyway?
I'm told that the other issue is that the two throttles for each bank are linked by an aluminium rod. Under the hot bonnet the cheap nasty rod expands and coarses the slave throttle to open ever so slightly more that the master throttle. This discrepancy is relayed back to the EMsystem that gets confused and either opens up the butterflies or closes them down out of sync to compensate leading to dodgy idling or missing of the engine.
danny
yep you're right to say one thorttle opens up as the engine gets hotter .. I exploit this with my cold-start fast idle modification. the rod is alloy so it will lengthen as it heats up, as will the engine / manifolds grow slightly and it's probably a combination of these that opens the throttle. Part throttle driving can be improved by modifying the throttle assembly to cure some inherent faults in the design.
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