Low rev misfire
Discussion
Got my Cerb back on the road finally, but got a few teething issues (as they all do after rebuilds)..
Biggest one is a misfire at low revs (sub 3k), worse when cold. It stutters and almost dies kind of misfire. It will only take a slow smooth throttle opening, again more so when cold. It is on all cylinders.. Once warm it will take a heavier throttle poke, although still a minor blip at times, and above 2.5/3k (ignoring lambdas?) it ups and goes as it should.
Coilpacks are new (had them on for 500 miles or so before it came off the road), same with plugs and leads. Lambdas are relatively new and working, and TPS' also renewed within 5k. Injectors cleaned also. Clearances are fine too.
It is on a best guess chip to run it in, so a remap with Uncle Joolz is imminent, however, don't want to turn up with a running issue that isn't map related.
Do 4.5 RR's run different plugs to the others?
Biggest one is a misfire at low revs (sub 3k), worse when cold. It stutters and almost dies kind of misfire. It will only take a slow smooth throttle opening, again more so when cold. It is on all cylinders.. Once warm it will take a heavier throttle poke, although still a minor blip at times, and above 2.5/3k (ignoring lambdas?) it ups and goes as it should.
Coilpacks are new (had them on for 500 miles or so before it came off the road), same with plugs and leads. Lambdas are relatively new and working, and TPS' also renewed within 5k. Injectors cleaned also. Clearances are fine too.
It is on a best guess chip to run it in, so a remap with Uncle Joolz is imminent, however, don't want to turn up with a running issue that isn't map related.
Do 4.5 RR's run different plugs to the others?
CerbWill said:
Get the software on it. Lambdas aren't deactivated until 4k or so (assuming you're using the standard MBE ECU). My 4.7 with Red Rose spec heads ran fine with std 4.2 injectors and spark plugs on a 4.2 chip, obviously being sympathetic with the throttle to keep it in lambda control. So in your case it may be the 'best guess' map, especially if you've changed injectors, but lambdas should bring that back to 14.7:1 anyway.
I've got the software running on it, and the adaptives suggest the fueling is about right when in lambda control.. I say best guess, it is just a safe fuel and spark map based on 4.2 fuel and 4.5 spark iirc, with some extra fuel added outside lambda control. I will have to check it again later when cold and missing.It is still on the standard 4.2 injectors currently..
Also, crank sensor and coolant temp sensor are new.
I am wondering if it is fueling related, as it won't hold idle with less than 20% TPS, and struggling to get it to idle below 1150rpm, and every time I drive it it comes back idling higher than when I had it at when setting it up.
Thom said:
The misfires suggest a problem with the crank sensor and the problem being worse when cold suggest the coolant temp sensor could be faulty.
It is not uncommon these days to find that new sensors do not actually work, even from Bosch. On some other cars I had to revert to used sensors due to defective new ones.
These items had crossed my mind, but.. The miss is only from heavier throttle openings sub 3k, getting worse the lower the revs are.. It is not uncommon these days to find that new sensors do not actually work, even from Bosch. On some other cars I had to revert to used sensors due to defective new ones.
Edited by Thom on Monday 29th April 12:05
Thom said:
On cars running a MAF this sort of thing happens when a vacuum leak messes up with the enrichment and the engine is temporarily drown in fuel, but since the fueling here is controlled with an Alpha N strategy, could a throttle pot be faulty or any fault in the wiring?
Could even just be a faulty ground messing with the various sensors.
That is what it feels like.. Could even just be a faulty ground messing with the various sensors.
Maybe! Although I did check and refresh earths where required.
I could put the standard chip back in also.
CerbWill said:
Noisy throttle pots or wiring can cause the ECU to believe you're constantly moving the throttle and therefore accel enrichment is permanently occurring.
As mine ran OK on a standard 4.2 map it'd be worth giving it a go if you've still got the original chip.
Would that show in the TPS figures jumping about? As mine ran OK on a standard 4.2 map it'd be worth giving it a go if you've still got the original chip.
You have just reminded me the TPS' do seem to wander from what I set them too..
ukkid35 said:
Jhonno said:
I am wondering if it is fueling related, as it won't hold idle with less than 20% TPS, and struggling to get it to idle below 1150rpm, and every time I drive it it comes back idling higher than when I had it at when setting it up.
Unless your throttle bodies are in perfect condition, I would expect it to idle with idle screw and all throttle links removedAre you sure there are no air leaks around the TB insulators, perhaps an O ring hasn't seated properly
The flow figures at idle are as I would expect, and even across the banks.
Thom said:
Whilst the closed-loop lambda control is enabled below 4k rpm it might be covering an issue with low-speed enrichment and/or cold start enrichment, as you said the problem was worse with the engine cold.
It's perhaps time to try with a standard chip and/or with the closed-loop fuel control disabled and/or cold start enrichment disabled.
This of course assumes everything in the hardware is fine including sensors, which I am still not convinced about.
Not sure how the ECU reacts with lambda sensors unplugged? Is it then just running on the main fuel map?
Yes, I will try the standard chip next, before throwing money at sensors.It's perhaps time to try with a standard chip and/or with the closed-loop fuel control disabled and/or cold start enrichment disabled.
This of course assumes everything in the hardware is fine including sensors, which I am still not convinced about.
Not sure how the ECU reacts with lambda sensors unplugged? Is it then just running on the main fuel map?
Edited by Thom on Tuesday 30th April 09:43
spitfire4v8 said:
Curing misfires on cerbs reminds me of the instructions on a brand of paint aerosol you could get in the 80s ..
1) shake can
2) shake can again
3) shake can again...
cerb version
1) replace ignition components
2) replace ignition components again
3) replace ignition components again
then move on to anything else you might think of
but steps 1,2 and 3 is always replace ignition components.
1) shake can
2) shake can again
3) shake can again...
cerb version
1) replace ignition components
2) replace ignition components again
3) replace ignition components again
then move on to anything else you might think of
but steps 1,2 and 3 is always replace ignition components.
So true! Misfire roulette!
Tried unplugging a TPS (odd bank) as I believe it should run on 1 and it wouldn't run..
I had it balanced and idling strong at 1k, turned it off, took it out later and it started hunting at idle, but holding (ie not dying), stutter is below 2.5k, although seems better sometimes than others. Adaptives are generally doing roughly the same, but occasionally one will show rich the other lean.
I don't want to just throw money at it, but I need it right before the 28th..
The hunting seems to indicate lambdas? But at £150 a pair, I don't want to do it for fun, plus mine are relatively new.
I had it balanced and idling strong at 1k, turned it off, took it out later and it started hunting at idle, but holding (ie not dying), stutter is below 2.5k, although seems better sometimes than others. Adaptives are generally doing roughly the same, but occasionally one will show rich the other lean.
I don't want to just throw money at it, but I need it right before the 28th..
The hunting seems to indicate lambdas? But at £150 a pair, I don't want to do it for fun, plus mine are relatively new.
Right..
Took it for a longer run and part way it started to really lack power and miss. So I reset the adaptives to see if it made a difference and boom. Ran perfectly again, pulsing idle became smooth etc. Lambdas are the correct way round. So I am thinking lambdas need replacing? Even though they seem to read as they should? The do flick back and forward. Generally together, although sometimes one will go lean, the other rich.
Took it for a longer run and part way it started to really lack power and miss. So I reset the adaptives to see if it made a difference and boom. Ran perfectly again, pulsing idle became smooth etc. Lambdas are the correct way round. So I am thinking lambdas need replacing? Even though they seem to read as they should? The do flick back and forward. Generally together, although sometimes one will go lean, the other rich.
spitfire4v8 said:
Also try:
On the next outing drive the car until it runs rough, then without stopping the engine reset the adaptives and then drive away immediately .. does the engine behave smoothly initially (until it starts learning values again ..)
if it does drive smoothly then you now know it's the trimming of the mixture that's causing the issue.
You've then got to work out why.
Is there one or two cylinders which are naturally slightly weaker anyway (higher airflows, air leaks, poorer injection / spray rates) and the trimming then puts those weaker than lambda=1 which would be very bad for smoothness.
This is a long drawn out diagnosis period and you really need to know what you're looking at.
As an aside is it ok under load, ie larger throttle openings.
report back your findings
Thank you for taking time to respond Joolz!On the next outing drive the car until it runs rough, then without stopping the engine reset the adaptives and then drive away immediately .. does the engine behave smoothly initially (until it starts learning values again ..)
if it does drive smoothly then you now know it's the trimming of the mixture that's causing the issue.
You've then got to work out why.
Is there one or two cylinders which are naturally slightly weaker anyway (higher airflows, air leaks, poorer injection / spray rates) and the trimming then puts those weaker than lambda=1 which would be very bad for smoothness.
This is a long drawn out diagnosis period and you really need to know what you're looking at.
As an aside is it ok under load, ie larger throttle openings.
report back your findings
This I've done.. And it runs perfectly for a short while after resetting them.
Parking up after a 130 mile round trip yesterday, I noticed the even bank was now reading rich on idle, when I had it setup running even across the banks (roughly 0 adaptives +/-3) previously.
My plan is to recheck airflows this evening. I will also try the other things you have mentioned.
Well my injectors were refurbished recently, my heat barriers and o-rings are new..
Why might a cylinder be flowing enough to cause a weak mixture?
I shall report back with lambda traces and having checked the airflow balance again this evening.
Why might a cylinder be flowing enough to cause a weak mixture?
I shall report back with lambda traces and having checked the airflow balance again this evening.
Edited by Jhonno on Monday 13th May 12:16
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