cass wrote:

"With the motor spinning over I perceived no build up of pressure on my hand, tho I could feel a little air flow with my hand held just off the exhaust, and the bin bag did not inflate as I would have expected."
I could be wrong but I wouldn't expect to detect any pressure until the engine fired, I'm sure that I've read that engines crank at about 200rpm on the starter? If so then even assuming no leaks or losses that's only about 60L of gas from a 5 second burst of cranking which isn't much to fill an exhaust and all of the other pipework.
does this make any sense?

Liking your thinking.
But just throws up more questions.
If we’re getting x litres induction, but it’s not exhausting, where’s it going ?
(The induction isn’t pulsing, ie sucking then returning. Just sucking)

Unless the turbo has collapsed, and is stopping the air from escaping down the exhaust system...

I'm not saying it's definitely turbo failure, but they are known for it

Takes less than 10 mins to undo the EGR pipes, then we'll know

Kevhdkent wrote:

Liking your thinking.
But just throws up more questions.
If we’re getting x litres induction, but it’s not exhausting, where’s it going ?
(The induction isn’t pulsing, ie sucking then returning. Just sucking)

Appreciate all the input
I will attack the EGR pipes at the weekend, in daylight.

Kevhdkent wrote:

2008 TDV8 3.6

Close to giving up after a month of failed diagnostics.

Cruising at 50/60mph with trailer, uneventful no issues for 150 miles, then suddenly rpm drops to 500 and complete loss of power with no throttle response. BUT IT DOES NOT CUT OUT.
Cruise, NOT coast, 1/4 mile into a lay-by where she sits at a very low 500rpm idle, no throttle response, until I turn her off.
Never to start again
To be clear, there were no pops or bangs grinds mechanics sounds or lumpiness in the running, just low rpm & no throttle response.

This is a great read and believe it or not I went to bed thinking about this and when I woke it was still on my mind--God I am sad But great fault finding and every input made with good intent---great forum!
But what's nagging away at me is the quote above. The Engine never stopped, stayed at 500rpm until driver intervened, never to start again. It does not suggest major failure.

1 I Presume the throttle is a electrical/electronic?
If this failed, surely the ECU will prevent a run away/or a sudden stop while running at speed (possible towing etc) and possibly at night for safety reasons?
In such an event surely ECU will be programmed to ensure enough revs (500 rpm) to maintain system workability lights brakes suspension etc enough to enable driver to intervene and coast to a safe place, then when stopped prevent further start.

2 Also--Surely the crank position sensor will be in the same circuit (another input to same ECU), as it measures engine speed, and if that failed in use while driving, the same program in the ECU would do the same.
So the ECU would monitor these 2 inputs among other things, and when something strange happens it will be programmed to have such a response?

Has any member experience any of this 2 failures while driving? Interesting to know what experience you had

Now I can go back to bed  2018 SE SDV8 4.4 Byron Blue

The throttle is referred to as a driver demand sensor, to which the ecm reacts as it has been programmed. The throttle doesn't have any direct control.

My introduction to failing crankshaft position sensors was many years ago on a relatively new petrol-engined vehicle on which the engine would randomly stop, although more often after half an hour's driving. No fault codes stored so the technicians progressively swapped components with another new vehicle until they swapped the crankshaft position sensor, after which the fault didn't re-occur. I've since encountered faulty crankshaft position sensors in various vehicles, some of which worked when cooled by spraying with a can of soft-drink but none ever recorded a fault code. Hence it's one of the first components that I check when an engine fails to run without having stored any fault codes. The ability of modern diagnostic tools to monitor live values removes the guess-work for this sensor.

Agree with GraemeS. I had a Ford Explorer that either would start or wouldn't. Once started it was fine. One day it wouldn't start. RAC came out and just wiggled the electrical connector on the crankshaft sensor and it started!! So mine wasn't even the sensor. Cleaned connector and no more trouble. Worth trying.

GraemeS wrote:

The throttle is referred to as a driver demand sensor, to which the ecm reacts as it has been programmed. The throttle doesn't have any direct control.

AJGalaxy2012 wrote:

My take would be 3.6 litres of air moved every 2 revolutions (suck, squeeze, bang, blow 4 stroke). so we should have 100 x 3.6 litres per minute, which is 60 litres per second. 5 seconds cranking should have exhausted 300L of gas in the 5 seconds, remember diesel engines have wide open manifold etc. I feel it should have inflated the bag a bit, well, more than a bit.

I stand corrected (partly), my mental arithmetic is slipping, 3.6L x 100 = 360L/min MAX? = 6L/second = 30L per 5 second crank MAX assuming no leaks/losses which isn't very likely, so even less chance of blowing a bag up?

All the more reason to follow the guidance of the likes of Mikey and ignore the rest of us!

cass wrote:

I'm not normally a fan of buying bits and hoping it will fix it BUT when logic and common sense don't work I've repeatedly found that crankshaft position sensors have been to blame where there are intermittent and unpredictable problems.
It isn't expensive or too difficult to fit so that would be my next guess.

You have changed your tune since this post any reason  2018 SE SDV8 4.4 Byron Blue

Yes - I thought I'd sit quietly then look for the "smug tw@" emoji if it turns out to be the sensor

But I always try to follow some kind of logical path when fault finding (or at least something that looks logical to me), it's too easy to take a scattergun approach and end up getting nowhere hence my suggestion to follow Mikeys plan.

Kot wrote:

Kevhdkent wrote:

2008 TDV8 3.6

Close to giving up after a month of failed diagnostics.

Cruising at 50/60mph with trailer, uneventful no issues for 150 miles, then suddenly rpm drops to 500 and complete loss of power with no throttle response. BUT IT DOES NOT CUT OUT.
Cruise, NOT coast, 1/4 mile into a lay-by where she sits at a very low 500rpm idle, no throttle response, until I turn her off.
Never to start again
To be clear, there were no pops or bangs grinds mechanics sounds or lumpiness in the running, just low rpm & no throttle response.

This is a great read and believe it or not I went to bed thinking about this and when I woke it was still on my mind--God I am sad But great fault finding and every input made with good intent---great forum!
But what's nagging away at me is the quote above. The Engine never stopped, stayed at 500rpm until driver intervened, never to start again. It does not suggest major failure.

1 I Presume the throttle is a electrical/electronic?
If this failed, surely the ECU will prevent a run away/or a sudden stop while running at speed (possible towing etc) and possibly at night for safety reasons?
In such an event surely ECU will be programmed to ensure enough revs (500 rpm) to maintain system workability lights brakes suspension etc enough to enable driver to intervene and coast to a safe place, then when stopped prevent further start.

2 Also--Surely the crank position sensor will be in the same circuit (another input to same ECU), as it measures engine speed, and if that failed in use while driving, the same program in the ECU would do the same.
So the ECU would monitor these 2 inputs among other things, and when something strange happens it will be programmed to have such a response?

Has any member experience any of this 2 failures while driving? Interesting to know what experience you had

Now I can go back to bed

cass wrote:

Yes - I thought I'd sit quietly then look for the "smug tw@" emoji if it turns out to be the sensor

But I always try to follow some kind of logical path when fault finding (or at least something that looks logical to me), it's too easy to take a scattergun approach and end up getting nowhere hence my suggestion to follow Mikeys plan.

Good luck  Pete

__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
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