Chimaera rough idle confusion
Discussion
I kind of already tried that (from my original post): “My confusion really set in when I disconnected #6 HT lead and held it to the engine block to see the spark and I saw the same behaviour i.e. rough running when the spark quality dropped or disappeared - even though it wasn't connected to the plug!”
I just know from many years of trouble shooting that sometimes a mistake diagnosis early on leads you up the garden path, so double checking is never a bad thing, but assuming the fault is as seen on just one plug line you have very few options as on the surface. All you have is any HT item from the dizzy cap output output to the relevant failing plug or less likely something on one lobe of the 123 trigger that corresponds to that lead / plug.
But as all the obvious has been suggested or checked you will have to get creative. Next step would be move the HT leads around one position on the dizzy cap BUT you would have to retime the engine for the new number one position for the rotor arm and lead leading to number one by rotating the body of the distributor. I know the 123 uses the LED for it's basic timing for number one TDC but this would now correspond to the wrong cylinder, so you need to use a strobe to reset it. I don't know physically if you can rotate the dizzy to get things to realign as it needs to go through 45' but it's what I'd try to do next.
The current firing order should be 1,8,4,3,6,5,7,2, for alignment. Then after all this see if it's the same plug dropping out as logically from your initial tests the failing spark would move as it's not combustion chamber conditions and has to be the 123 or HT leads / plug cap. See where I'm coming from?
Given an old school HT display from a Crypton tuner or Sun engine analyser would make things a while lot easier but these tools are really obsolete in this OBD age, let alone finding some 30 year old working electronics.
But as all the obvious has been suggested or checked you will have to get creative. Next step would be move the HT leads around one position on the dizzy cap BUT you would have to retime the engine for the new number one position for the rotor arm and lead leading to number one by rotating the body of the distributor. I know the 123 uses the LED for it's basic timing for number one TDC but this would now correspond to the wrong cylinder, so you need to use a strobe to reset it. I don't know physically if you can rotate the dizzy to get things to realign as it needs to go through 45' but it's what I'd try to do next.
The current firing order should be 1,8,4,3,6,5,7,2, for alignment. Then after all this see if it's the same plug dropping out as logically from your initial tests the failing spark would move as it's not combustion chamber conditions and has to be the 123 or HT leads / plug cap. See where I'm coming from?
Given an old school HT display from a Crypton tuner or Sun engine analyser would make things a while lot easier but these tools are really obsolete in this OBD age, let alone finding some 30 year old working electronics.
Problem is with HT, you cant measure whats going on without a HV scope or specialist tool. Such DIY tools did exists and Actron and Gunson did a unit that displayed the HT voltage in an LED bar graph just by plugging a clip over the HT lead. Brilliant bit of kit for the V8 long since out of production unfortunately and you can only tell so much looking at the spark. It may be well be possible to make something similar using an audio LED display and some interface electronics- its something I should look into as I shifted 30 of the peak testers along with RoverGauge cables in no time as old end of line stock came onto the market.
It may have been mentioned before but the long obsolete Crypton tuners can give a useful display of HT spikes from the 8 cylinders together.On its oscilloscope display.
Maybe someone near you has a working Crypton ( having mended several) the Oscilloscope is generally not working) But an easy fix is to put the EHT unit in a warm place for a while as they tend to be affected readily by damp.
Maybe someone near you has a working Crypton ( having mended several) the Oscilloscope is generally not working) But an easy fix is to put the EHT unit in a warm place for a while as they tend to be affected readily by damp.
Belle427 said:
I think at this stage it may be beneficial to hand it over to a specialist to help out.
Not wishing to doubt your abilities but some will have access to good spares and the appropriate test equipment needed.
Diagnosing on a forum is always difficult.
Thanks all. My next step is exactly that. I'm booked in with Lloyd Specialist Developments next week. They know about TVRs/RV8s and 123 ignitions, so I'm really hoping they can sort this out. I'm also hoping the car will make the 2hr+ drive over there ok!!Not wishing to doubt your abilities but some will have access to good spares and the appropriate test equipment needed.
Diagnosing on a forum is always difficult.
Apologies for the delayed reply.
All sorted now.
So... it was a combination of things:
1. #6 injector was slightly blocked/dirty so spitting a bit - this was swapped for a known good one.
2. The brand new MAF I fitted when the car wasn't starting was reading low, hence the 100% +ve trim on both banks. This was also swapped for a known good (old, original) one.
3. The timing and advance were out, which was contributing to the rough idle - I knew this would be the case and it was my original reason for planning a trip to Llyod's to get it set up on the dyno. I set it to 8' BTDC, but didn't appreciate that there should be some advance loaded in at idle. I left it at the default of zero advance at lower revs. Lloyd's set the timing to 10' BTDC and loaded in some advance (14 degrees initially, but I took that down to 10 @1000rpm).
After these changes all was well. Although I'm still left confused as to why #6 spark was weak.
BTW, Lloyd's were excellent. Lots of great gadgets for diagnostics. Knew what they were doing 100% and nice friendly guys too!! Many thanks to them for getting to the bottom of the problems.
Many thanks also to all those on this forum who offered suggestions and tried to help out.
All sorted now.
So... it was a combination of things:
1. #6 injector was slightly blocked/dirty so spitting a bit - this was swapped for a known good one.
2. The brand new MAF I fitted when the car wasn't starting was reading low, hence the 100% +ve trim on both banks. This was also swapped for a known good (old, original) one.
3. The timing and advance were out, which was contributing to the rough idle - I knew this would be the case and it was my original reason for planning a trip to Llyod's to get it set up on the dyno. I set it to 8' BTDC, but didn't appreciate that there should be some advance loaded in at idle. I left it at the default of zero advance at lower revs. Lloyd's set the timing to 10' BTDC and loaded in some advance (14 degrees initially, but I took that down to 10 @1000rpm).
After these changes all was well. Although I'm still left confused as to why #6 spark was weak.
BTW, Lloyd's were excellent. Lots of great gadgets for diagnostics. Knew what they were doing 100% and nice friendly guys too!! Many thanks to them for getting to the bottom of the problems.
Many thanks also to all those on this forum who offered suggestions and tried to help out.
Great news that you've had it sorted!
Sounds like some of the ideas on here were not that far off, which is reassuring, but glad that Lloyd's reputation stands firm!
Interesting point on the idle advance, I recently adjusted mine (the previous owner had set it with 19degs mechanical advance by virtue of the distributor position), and swapped the vacuum advance signal from the ported nozzle on the throttle body to one of the ports on the side of the plenum to make it consistent.
This required me to set the mechanical advance to around 8degs, and the consistent vacuum pulls it to 20/22degs at idle.
The engine is much happier, and picks up slightly more readily from lower revs.
Actually very easy to do, but I was lucky enough to have access to a friend's decent timing strobe gun.
Sounds like some of the ideas on here were not that far off, which is reassuring, but glad that Lloyd's reputation stands firm!
Interesting point on the idle advance, I recently adjusted mine (the previous owner had set it with 19degs mechanical advance by virtue of the distributor position), and swapped the vacuum advance signal from the ported nozzle on the throttle body to one of the ports on the side of the plenum to make it consistent.
This required me to set the mechanical advance to around 8degs, and the consistent vacuum pulls it to 20/22degs at idle.
The engine is much happier, and picks up slightly more readily from lower revs.
Actually very easy to do, but I was lucky enough to have access to a friend's decent timing strobe gun.
Edited by PabloGee on Thursday 22 August 10:54
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