Chimaera rough idle confusion

[Steve_D]

Belle427 said:

Is cross firing really a thing these days with better quality insulated leads?

Debatable but in our case it would only show itself between 5 & 7. They are the only cylinders that both live nextdoor and fire close together.

Steve

[Tim_C]

Thanks again all for taking the time to post on this.

My latest updates on trying the suggestions so far are below...

> have you routed the HT leads correctly?
I have now. Unfortunately problem still present.

> spark arc voltage depends on the combustion chamber condition
Great suggestion, but I believe this was ruled out by observing the spark when not connected to the plug. I will definitely pull plug 6 out and take a look at the condition though. – please see photo of plug #6


> Short term will also spike at 100% adding fuel during a misfire
Makes sense, but didn't observe that – This was observed after leaving at idle after the 15min run to gather the RoverGauge logs for Frank.

> have you put a test meter on the coils 12v ignition supply to make sure its not dropping
Not while running, no. I'll give that a try too, thanks! – The voltage across the coil terminals was ~13.93V, it did increase during the periods where the engine dips and runs rough, but only to ~13.98V

I haven't had chance to look at the lambda sensors yet, although I'm not sure what to test on them.
I have generated RoverGauge logs for Frank and will send for analysis.

[blaze_away]

RoverGauge data shows everything behaving normally, specifically OP has a goof MAF sensor (often the cause of many running issues that I've had the pleasure to look at data for over past 5 years of doing this for owners).

On that basis this does indeed look more likely to be an ignition issue......although just waiting for one bit of info from OP regarding the 'rough running'. As its intermittent maybe RoverGauge caught it and I can take a closer look at the data at that time if it was captured.

[blaze_away]

Having looked at an older file collected during period of rough running it very much looks like this could be a vacuum leak issue.

During the issue at idle the data supports this postulation.
Revs are elevated at 1800 ish
ie Very high vacuum strength drawing in unmetered air.
RG data shows ICV closing to pull them done to target 900.
As ICV closes Vac strength declines causing vac leak to dimish rapidly so the Revs drop dramatically to 500 ish. ICV then begins to try to pull Revs up so vac leak gets more and Revs climb again.........and repeat from the beginning.

Op also states base idle cannot be set further supporting the above.

Have advised op to go look for vac leaks

[blitzracing]

One thing. That plug 6 is really sooty. Now a misfire on one plug dumps unburnt fuel into the exhaust and because it's full of oxygen the ECU must reads this as a lean mixture so adds lots of extra fuel. But it can't just add it to one cylinder at a time so it adds it to the whole bank on one side. This means all 4 plugs on the failing bank should all be sooty. So have a look at the rest and also the other " working" bank to see if the plugs are cleaner. Grade 6 plugs should run OK in a TVR but the 7 grade ones fitted to the 5LTRs tend to run sooty in day to day use. By the photo it looks like you have the 7 grade shrouded tip plug type ? Personally I'd be running the BP6ES type plug unless you track the car as it will be less prone to sooting up. If only plug 6 is black then you are looking at failing injector or oil burning.

Edited by blitzracing on Tuesday 18th June 21:30

[blitzracing]

blaze_away said:

Having looked at an older file collected during period of rough running it very much looks like this could be a vacuum leak issue.

During the issue at idle the data supports this postulation.
Revs are elevated at 1800 ish
ie Very high vacuum strength drawing in unmetered air.
RG data shows ICV closing to pull them done to target 900.
As ICV closes Vac strength declines causing vac leak to dimish rapidly so the Revs drop dramatically to 500 ish. ICV then begins to try to pull Revs up so vac leak gets more and Revs climb again.........and repeat from the beginning.

Op also states base idle cannot be set further supporting the above.

Have advised op to go look for vac leaks

The issue you have here is dropping a firing cylinder as the engine approaches idle has exactly the same symptoms as an air leak because the engine RPM response to the stepper motor position now falls outside the expected range as engine RPM drops far faster than expected, as it misfires. This leads to the same unstable high RPM idle state. I'd also be pointing an IR temp sensor across all the exhaust headers to see if cylinder 6 is unique. I would triple check my diagnosis of weak spark on just 6 seeing as the whole system has been replaced with the 123 system yet the fault remains.

[indigochim]

blitzracing said:

Here is your HT routing:

Thanks for that I was only vaguely aware that was a proper route that the cables should run.

Hope you don't mind but I borrowed the image from your site and got the crayons out as I found it hard to trace where the wires went when it was all B&W.

[PabloGee]

That's excellent crayoning.
Are you a designer of some description?

[blitzracing]

Belle427 said:

Is cross firing really a thing these days with better quality insulated leads?

I think it's more to do with radio transmission and reception along " aerial wires" with spark transmitters more than jumping across physical insulation. Better check with Marconi if you can find him.