Low rev misfire
Discussion
spitfire4v8 said:
Sounds like youve done pretty much everything reasonably possible so far
I'd like to think I try and exhaust the "obvious" before I start bothering others with these things.. 
I should have brought it in to work, I could have played with it at lunchtime! Not that I'm impatient to sort it..
Right, I can't get Evo Olli's software to work on my laptop, well I thought it would work with a USB to Serial Adapter but no luck.
So.. I held throttle part open on the idle stop so it would sit about 2k.. The banks were out of balance again, at this point at least. So I've balanced them the best I could. I have 2 cylinders that flow more. No.1 and No.8.
No.1 has a hollow screw in it. So if someone (Joolz? ) could tell me the screws fitted (allen heads), I will replace the hollow screw.
I need to take it for a run now and see how it behaves.
So.. I held throttle part open on the idle stop so it would sit about 2k.. The banks were out of balance again, at this point at least. So I've balanced them the best I could. I have 2 cylinders that flow more. No.1 and No.8.
No.1 has a hollow screw in it. So if someone (Joolz?
) could tell me the screws fitted (allen heads), I will replace the hollow screw. I need to take it for a run now and see how it behaves.
Driven it twice now.. It is currently holding a slightly fast idle, and the stutter is still there, but much less, if it is doing it you can back the throttle off slightly and it will go through it. It also gets better as it warms up. When I first drove it after setting it up it was a bit of a dog, however it seems to have settled into something driveable. I will tweak the throttles again, to get the correct idle and see what occurs.
I think to be anal about this, I need to get hold of the 5 hollow bolts, to up the flow of the other 3 cylinders on the bank with the highest flowing cylinder.
I think to be anal about this, I need to get hold of the 5 hollow bolts, to up the flow of the other 3 cylinders on the bank with the highest flowing cylinder.
Thom said:
Not sure I understand how it seems to go out of tune rather quickly after each time you have adjusted the throttles.
Isn't there just some excessive play in the linkage(s)?
Perhaps there is something I am doing wrong.. BUT.. I have done this before very successfully! I am the closest I've been currently, however there was always a couple of higher flowing banks, which I hadn't realised could be such an issue until Joolz pointed the potential lean misfire issue out. It is a high comp engine. My linkages are pretty tight. The banks stay in sync (to how they are set) across the throttle range.Isn't there just some excessive play in the linkage(s)?
Perhaps it is partly as the newly built engine evolves and beds in..? Grasping at straws perhaps.
Edited by Jhonno on Tuesday 14th May 10:57
Thom said:
Perhaps this is stating the obvious but it's important to adjust the linkages with the engine fully warmed up. I feel that quite a bit of thermal expansion may take place in the very hot engine bay of a Cerbera, and most classics using several synchronised carbs should always be adjusted when warm, in my experience.
Yeah, it is being done whilst warm!https://photos.app.goo.gl/gbwKNLePrBwBJFHs6
A video of diagnostics whilst running..
I have noticed the odd bank TPS tends to hunt a bit, also, not fully shown in the video, but the adaptive hunting is!
A video of diagnostics whilst running..
I have noticed the odd bank TPS tends to hunt a bit, also, not fully shown in the video, but the adaptive hunting is!
FarmyardPants said:
From my limited knowledge if it runs ok after resetting the adaptives but then “learns” to run poorly, it must be a defective lambda sensor, since that is what the learning process uses for input. Even if you have a throttle or two flowing more air, it should learn to adjust for the bank overall. The fact that it runs ok over 3k rpm and wider throttle openings corroborates this. Bad lambda sensors can also cause high idle problems.
Yes, I am lead to believe this also, and have found others with the same symptoms/issues. However, mine seem to switch as they should! Hence asking if they can give wrong/misleading readings..Right.. Yet more tweaking and tuning after work, and for the 6 mile trip home it holds an idle in or around 1100rpm, and very little hesitation from low rpm.. Some but not that much, it could be those 2 high flow banks throwing it slightly.. I might let it work things out for a minute before any more tweaking.
I think I have found a source for the hollow bolts M4x6 I believe. So will order those up and go from there if it continues to run as is. I'm thinking it should be running lean (as in adding fuel in the adaptives) under lambda control as it is running 4.2 injectors still, so maybe I have been running the throttles too far advanced for the rpm.
I think I have found a source for the hollow bolts M4x6 I believe. So will order those up and go from there if it continues to run as is. I'm thinking it should be running lean (as in adding fuel in the adaptives) under lambda control as it is running 4.2 injectors still, so maybe I have been running the throttles too far advanced for the rpm.
Edited by Jhonno on Tuesday 14th May 17:50
2nd drive update..
Still much more driveable, and holding idle after a few seconds of throttle when starting, and sometimes it will pull hard from low revs, however it is still missing. It has a inconsistent miss you can feel if you put your hand on the end of the driver's bank exhaust, which is also the bank with the highest flowing cylinder/greatest flow difference.
Still much more driveable, and holding idle after a few seconds of throttle when starting, and sometimes it will pull hard from low revs, however it is still missing. It has a inconsistent miss you can feel if you put your hand on the end of the driver's bank exhaust, which is also the bank with the highest flowing cylinder/greatest flow difference.
Thom said:
I doubt that a runner or two flowing a bit more (how many % are we talking?) than all the others would cause the engine to run so badly rich, according to the video, which also suggests the throttle pots are functional.
Anything richer than around 0.80 to 0.85-ish on a cold start up should not be needed for such an engine.
Edited to add : does the rear exhaust runner you mention feel cold compared with the others? A fouled plug could cause the low rpm misses you describe. Does the MBE ECU feature some "limp home" mode enabled when detecting faulty sensors, and then deliberately injecting more fuel than under normal running conditions to protect the engine?
The fueling now runs leaner than the video, and adds fuel in, in it's new fairly driveable state. I perhaps had too little air, too much fuel at idle before.. It is a 4.5 running 4.2 injectors currently, so under lambda control you would expect it to be adding some fuel in.. Anything richer than around 0.80 to 0.85-ish on a cold start up should not be needed for such an engine.
Edited to add : does the rear exhaust runner you mention feel cold compared with the others? A fouled plug could cause the low rpm misses you describe. Does the MBE ECU feature some "limp home" mode enabled when detecting faulty sensors, and then deliberately injecting more fuel than under normal running conditions to protect the engine?
Edited by Thom on Wednesday 15th May 09:40
I tried to get to the runner last night, but it was late and would have involved taking the airbox off. Front 2 felt even. The high flow cylinder does run a significant amount more than the rest. If it was a little bit I wouldn't be that worried about it. I will pull the spark plug tonight to see what it looks like. Of course it has to be the most awkward one.
spitfire4v8 said:
Sadly the video tells us little about the running of the engine, the warm up timer won't have cut out and the adaptive values will just be what was stored from the last time it was run. Also the constant moving of the throttles/revving doesn't allow us to see anything long enough to make any kind of judgement anyway.
Airflows : it's not uncommon for one cyl to flow say 5kg/hr and the neighbour 6 .. that's a 20 percent airflow difference. When you consider that a poor combustion chamber design already struggles on lambda=1 to have smooth combustion then it doesn't take anything like 20 percent weaking of the mixture to have one cylinder not firing at all. In fact even a good chamber design would struggle with that.
Just a 5 percent mixture change weaker has noticeable effects on the running smoothness, it's only the fact we have 8 cyinders that means it's as good as it is.
Sorry, it kept dying, so I was having to manually keep it running.. Airflows : it's not uncommon for one cyl to flow say 5kg/hr and the neighbour 6 .. that's a 20 percent airflow difference. When you consider that a poor combustion chamber design already struggles on lambda=1 to have smooth combustion then it doesn't take anything like 20 percent weaking of the mixture to have one cylinder not firing at all. In fact even a good chamber design would struggle with that.
Just a 5 percent mixture change weaker has noticeable effects on the running smoothness, it's only the fact we have 8 cyinders that means it's as good as it is.
Yeah, my 2 high flow cylinders are more than 1kg out, so definitely worth trying to bring the others in line!
spitfire4v8 said:
^^ also worth pointing out that the effects of drilled screws is at its greatest on closed throttle .. as soon as you start to open the throttles then that percentage in-balance decreases, so the influence of mixture in-balance decreases with throttle opening, but it makes sense in your case to at least try and get them somewhere near. It eliminates the closed throttle and very-close-to-closed-throttle airflow variable at least.
Yeah I did see somewhere you said about 0.5kg per screw at best.. Seeing as my issue seems to be at idle/smallest throttle openings it is worth a try! And I will be happier with them closer anyway.gruffalo said:
Is the car breathing heavily or have you done a compression test?
Are the injectors in good nick, can't really make out if the black plugs are coked or oily?
It doesn't breathe at all really, no oil in the breather system, no smoke etc.Are the injectors in good nick, can't really make out if the black plugs are coked or oily?
Plugs to be fair are from pre-rebuild. So they have done a couple of k with worn valve guides/stem seals.
Plugs are coked.. No 4 smelt of fuel/was a bit damp.
Compression is plentiful and even across the cylinders.
Edited by Jhonno on Wednesday 15th May 22:39
Edited by Jhonno on Wednesday 15th May 22:39
ukkid35 said:
Jhonno said:
Does the AJP8 like Iridium plugs?I assumed it was more attuned to old skool spark plugs
Either way they look very different to each other, which is not ideal
Is there any way to dial it back towards standard, and then add complications gradually?
Plugs have sat for 2.5yrs and potentially been knocked/bashed. So have ordered a couple of spare ones.
Leads are showing low resistance.
All of that Dave..
Thank you for the detailed response!
I have checked resistances of the leads and all low. I have however rerouted them to have less tension/heat.
For the weekend I have ordered a couple of new plugs to try in the cylinder that was showing a plug smelling of fuel, and probably put the other in the fouled cylinder. I have been trying to narrow it down without throwing parts at it. Apparently if coils are failing you lose 2 cylinders, due to the wasted spark. I can borrow a coil off a mate to confirm. My vented throttle screws are with me tomorrow..
If I have no success I will invest in that diagnostics machine, I can also borrow a heat gun to check primary temps. I am reluctant to keep running it with this misfire now I have found a fuel smelling plug, as it could undo all my hard work.
Pre-rebuild so tested working, it had new plugs/coils/leads/injectors cleaned/heat barriers/o-rings.. Perhaps the extra compression is showing a weakness. Or something has degraded whilst sat for 2.5yrs. As you say, nothing can be discounted, and going back to basics is the answer.
Thank you for the detailed response!
I have checked resistances of the leads and all low. I have however rerouted them to have less tension/heat.
For the weekend I have ordered a couple of new plugs to try in the cylinder that was showing a plug smelling of fuel, and probably put the other in the fouled cylinder. I have been trying to narrow it down without throwing parts at it. Apparently if coils are failing you lose 2 cylinders, due to the wasted spark. I can borrow a coil off a mate to confirm. My vented throttle screws are with me tomorrow..
If I have no success I will invest in that diagnostics machine, I can also borrow a heat gun to check primary temps. I am reluctant to keep running it with this misfire now I have found a fuel smelling plug, as it could undo all my hard work.
Pre-rebuild so tested working, it had new plugs/coils/leads/injectors cleaned/heat barriers/o-rings.. Perhaps the extra compression is showing a weakness. Or something has degraded whilst sat for 2.5yrs. As you say, nothing can be discounted, and going back to basics is the answer.
Small update..
Throttles finitely balanced.. Now all flows are within 0.5kg of each other, and banks the same.
2 new plugs. One in the coked cylinder, the other in the plug that smelt of fuel.
Quick tweak on the throttle bar to roughly correct the known difference. Fired up and settled into a smooth idle, albeit a bit high. Backed off and throttles turned back, tweak on the link bar to balance banks perfectly, and we have a smooth idle at about 1050-1100rpm, a bit higher than I'd like, but airflow won't let it go any lower. Linkage end hitting on the throttle back plate. No miss felt anymore when covering the even bank tailpipe.
Took it down the road and it isn't missing but it is hesitant on part throttle. Checked the values as driving and the even bank TPS is lagging by as much as 10% for some reason. Odd bank will be ~30% and the even bank will still be at idle value. It catches up but part/light throttle there seems to be an issue. I will try my new TPS which arrived. New issue that one.
Throttles finitely balanced.. Now all flows are within 0.5kg of each other, and banks the same.
2 new plugs. One in the coked cylinder, the other in the plug that smelt of fuel.
Quick tweak on the throttle bar to roughly correct the known difference. Fired up and settled into a smooth idle, albeit a bit high. Backed off and throttles turned back, tweak on the link bar to balance banks perfectly, and we have a smooth idle at about 1050-1100rpm, a bit higher than I'd like, but airflow won't let it go any lower. Linkage end hitting on the throttle back plate. No miss felt anymore when covering the even bank tailpipe.
Took it down the road and it isn't missing but it is hesitant on part throttle. Checked the values as driving and the even bank TPS is lagging by as much as 10% for some reason. Odd bank will be ~30% and the even bank will still be at idle value. It catches up but part/light throttle there seems to be an issue. I will try my new TPS which arrived. New issue that one.
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