Low rev misfire

Low rev misfire

Author
Discussion

gruffalo

5,937 posts

167 months

Wednesday 15th May
quotequote all
Jhonno said:
Plugs from the bank with a miss..

https://photos.app.goo.gl/RuoeCBLmgMRr9hfSA
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?


ukkid35

4,742 posts

114 months

Wednesday 15th May
quotequote all
Jhonno said:
Plugs from the bank with a miss..

https://photos.app.goo.gl/RuoeCBLmgMRr9hfSA
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?

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
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.

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

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
ukkid35 said:
Jhonno said:
Plugs from the bank with a miss..

https://photos.app.goo.gl/RuoeCBLmgMRr9hfSA
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?
It ran superbly on them pre-rebuild. Other than the smoke on overrun it was a very rapid/smooth 4.2.

Plugs have sat for 2.5yrs and potentially been knocked/bashed. So have ordered a couple of spare ones.

Leads are showing low resistance.

Thom

1,673 posts

188 months

Thursday 16th May
quotequote all
For what it's worth I have been running NGK Iridium plugs on a pre war engine which runs accordingly quite a bit richer at idle than any correctly-tuned modern fuel-injected engine would, and so far they have resisted fouling a lot better than the previous NGK copper plugs, all other things being equal. In fact the engine has not shown to misfire (yet hehe) even if the plugs look remarkably sooty after having the car idle in traffic for "too long"... However they are only a year old and the engine has run perhaps only about half a thousand miles with them.

Also, since your engine has a higher compression ratio, shouldn't plug gap be decreased a little or do the coils provide enough of a strong spark already?

Edited by Thom on Thursday 16th May 09:23

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FarmyardPants

3,446 posts

159 months

Thursday 16th May
quotequote all
It looks like the plug on the far left is missing the centre electrode, maybe it’s an illusion?

Jhonno

Original Poster:

3,459 posts

82 months

Thursday 16th May
quotequote all
FarmyardPants said:
It looks like the plug on the far left is missing the centre electrode, maybe it’s an illusion?
It is there if you zoom in, obviously easier on a PC..

ChimpOnGas

8,585 posts

120 months

Thursday 16th May
quotequote all
Jhonno said:
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?
You don't say if this was an engine rebuild or a chassis restoration, or re-wire ect? but for fear of stating the obvious if the engine didn't misfire before the rebuild yet misfires after it you should first suspect the components replaced during said rebuild, also suspect any wiring that may have been disturbed during the rebuild with a special focus of attention being placed on your engine grounding cables that may have not been connected or poorly tightened.

Many people fall into the trap of assuming just because they've replaced something with a new component that component can't be the issue, actually this couldn't be further from the truth and is a classic mistake that can have the uninitiated chasing their tail. New components can be faulty, this is actually a lot more common than you may assume and if you think about the new components should be your first area of suspicion if the misfire only appeared after something was recently replaced.

If as suggested you can drive around the misfire by gently rolling on the throttle but find a moderate prod of the throttle will stimulate an occasional miss it is almost certainly a high tension ignition fault, this is even more conclusive if the misfire becomes more consistent with an aggressive stab of the throttle. The reason for this is because when you get on the throttle you are momentarily stressing the high tension side of your ignition system, if anything on the HT side (coils, leads, or plugs) are starting to break down it'll first show up when you get on the throttle.

You will also likely find a high tension ignition misfire is worse at lower engine speeds, or at least it feels worse, the truth is the engine is still misfiring when you get back on the throttle at higher engine speeds but because things are happening so fast you will very likely not feel it at all. Before condemning actual physical components in you ignition system do keep in mind your high tension spark should be jumping across the plug gap and your spark plugs are grounded through your engine block. A poor engine ground can and will therefore cause all manner of hesitation and misfire symptoms, the connections and integrity of your ground straps are vital and need to be careful checked before condemning coils ect.

With engine grounding tested and confirmed sound if the misfire persists turn your attention to your HT leads, even if you've recently replaced them your HT leads should be the first point of suspicion, and the easiest and simplest way to test for leaky HT leads is to use a little hand held plant sprayer to wet the leads and especially their plug ends.



Wet one bank at a time and go for a drive, if the misfire is more pronounced you've found your issue, repeat the wetting in the dark watching for sparks and listening for the telltale tick to find the lead thats leaking (grounding to the block).

Now move onto your coil packs and once again don't discount the very real possibility your new coil packs came faulty! What make of coil packs are they?, there's a lot of rubbish out there so I'd strongly recommend using genuine NGK coils as they are known to be very consistent and reliable. Everyone knows coils can fall over at high RPM but thats not the be all and end all coil failure symptoms, far from it in fact, as covered above the throttle prod misfire is another clear indication your coils may be failing under load.

Avoid the temptation to get involved in 'Part Darts' diagnostics, the discipline of automotive diagnostics is no different to medical diagnostics. If you go to the doctor with chest pains you wouldn't expect or want him to immediately refer you for a heart transplant, your doctor will run a barrage of tests, he is looking for two or more results that point conclusively to the true issue which may be something very simple to correct like gastric acid reflux thus saving you from having that invasive and now proven pointless heart bypass wink

To solve your misfire systematically test each component in the chain that makes up the system you suspect is a fault, in this case your the ignition system. Moving on from HT leads the way to check coil packs properly under variable load conditions is to use an automotive oscilloscope, I was trained on a Sun MEA 1200-4 diagnostic machine which was a brilliant bit of diagnostic kit that gave you your essential HT KVs and coil behavior as wave form on the inbuilt oscilloscope screen but cost thousands and was the size of an industrial washing machine, so big in fact you had to wheel it about the workshop.



These days for just £85 you can buy a Hantek 1008C 8CH thats the size of a book, it connects to you laptop by USB and using the provided software your PC becomes something even better than a Sun tuner machine. At just £85 the Hantek 1008C 8CH oscilloscope is without any doubt the best bit of diagnostic equipment you can have in your tool kit, and unless you've got a rolling road it's better than those old but excellent Sun/Krypton tuner machines because you can put the Hantek and your laptop on the passenger seat and go for a drive to simulate the same load conditions that stimulate the misfire while logging up to 8 coil outputs.



https://www.ebay.co.uk/itm/Hantek-1008C-8CH-PC-USB...

Other things to check relate voltage:

IE battery condition, coil supply voltage, grounds (as discussed above), and your alternator output, all must all be checked using the correct testing procedures and all must meet spec. As with the HT leads, plugs and coil packs don't just assume because you have a new battery or alternator these components are functioning correctly, a weak battery or a poor alternator output are classic causes of misfires. Keep in mind Ohm's law is a relationship thing, ie for a given resistance current is directly proportional to voltage, so misfires are inevitable if your battery voltage or alternator output voltage is low or simply unable to support a spike in demand such as when you tip in on your throttle. In such conditions the coils may well be unable to deliver a spark at the plug at that moment and we are talking milliseconds here, this all happens so fast you will not see it on a normal multimeter, you need an oscilloscope which will soon show it up wink

We have spoken a lot about ignition misfires because this is the most likely scenario in your case, however the very same symptoms of misfire on tip in could also be attributed to a fuel misfire.... or more specifically a sticking fuel injector. Of course injectors are just little electrical coils themselves so just like testing your ignition coils correctly demands an oscilloscope the very same oscilloscope can be used to diagnose injector issues, hopefully you can see now why I'm suggesting spending just £85 on a Hantek 1008C 8CH is such a good investment?

Hope this helps you in some way?

Dave.

Jhonno

Original Poster:

3,459 posts

82 months

Thursday 16th May
quotequote all
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.

ChimpOnGas

8,585 posts

120 months

Thursday 16th May
quotequote all
Good luck with it Jhonno, misfires can be a biatch but once you've eliminated them throttle response will feel like it's come alive driving

Jhonno

Original Poster:

3,459 posts

82 months

Thursday 16th May
quotequote all
CerbWill said:
What are the adaptive doing at idle (whatever idle speed you can currently get)?
Both about +10 iirc..

Jhonno

Original Poster:

3,459 posts

82 months

Thursday 16th May
quotequote all
ChimpOnGas said:
Good luck with it Jhonno, misfires can be a biatch but once you've eliminated them throttle response will feel like it's come alive driving
I've had glimpses of it amongst it all, as it does run well intermittently.. It is a beast hiding behind this issue.

ChimpOnGas

8,585 posts

120 months

Jhonno said:
ChimpOnGas said:
Good luck with it Jhonno, misfires can be a biatch but once you've eliminated them throttle response will feel like it's come alive driving
I've had glimpses of it amongst it all, as it does run well intermittently.. It is a beast hiding behind this issue.
The truth is most TVRs are beasts hiding behind a series of issues wink

Releasing the beast just takes:

1. A good understanding of four cycle spark ignition internal combustion engine fundamentals

2. A systematic approach using good diagnostic equipment

3. Patience!

The golden rule with TVRs is most idle and drivability issues can be traced to poor earthing and low voltage conditions, however never discount other critical elements such as fuel pressure, if you are not recording a spike in fuel pressure when you stab the throttle suspect issues with your FPR.

In your case you absolutely must analyse and log your coil voltage and injector behavior during misfire or you'll be going nowhere fast with a blindfold on, and the only way to do that is with an oscilloscope! This was all very well understood some 40 years ago which is why we had Krypton and Sun tuner machines, these days any guy off the street can access the same diagnostic functionality for just £85, for geeks like me running an alternative fuel and anyone like you with a misfire issue this a wonderful thing.

I have no idea what control features you have with your ECU but I can offer you food for thought in the fact I can literately dial in and dial out tip in misfire on LPG just by altering my coil dwell time and or timing numbers in certain rpm x load cells where hard to strike propane becomes highly intolerant of combustion initiation.

Somewhere in your ECU software there will be a dwell vs voltage compensation strategy, don't mess with it but do take time to study why the ECU is calibrated to increase your coil dwell as battery voltage drops. Understanding this simple strategy will help you to appreciate why you need to capture coil voltage at the split millisecond misfire occurs.

At this point it's important to understand all spark ignition internal combustion engines misfire to some degree, even the most modern and engines using the best ignition systems have an accepted misfire rate which the sophisticated OEM ECU is closely monitoring. The OEMs know misfires are inevitable so they set a threshold before the ECU throws a fault code, this will be long after the engine has started to misfire which is considered normal but long before the misfire can be felt by the driver.

A misfire that can be felt by the driver is a very severe misfire indeed!

Study and understand the fundamental first principles of internal combustion engine ignition systems, then use the correct diagnostic equipment to understand what's really going on when you experience the misfire. This is the way to fix your issue because armed with the right knowledge and kit..... the fault will be sure to jump out and slap you square in the face redface

That's got to be way better than the 'Parts Darts' approach, right wink

Jhonno

Original Poster:

3,459 posts

82 months

Saturday
quotequote all
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.

Jhonno

Original Poster:

3,459 posts

82 months

Saturday
quotequote all
ChimpOnGas said:
Jhonno said:
ChimpOnGas said:
Good luck with it Jhonno, misfires can be a biatch but once you've eliminated them throttle response will feel like it's come alive driving
I've had glimpses of it amongst it all, as it does run well intermittently.. It is a beast hiding behind this issue.
The truth is most TVRs are beasts hiding behind a series of issues wink

Releasing the beast just takes:

1. A good understanding of four cycle spark ignition internal combustion engine fundamentals

2. A systematic approach using good diagnostic equipment

3. Patience!

The golden rule with TVRs is most idle and drivability issues can be traced to poor earthing and low voltage conditions, however never discount other critical elements such as fuel pressure, if you are not recording a spike in fuel pressure when you stab the throttle suspect issues with your FPR.

In your case you absolutely must analyse and log your coil voltage and injector behavior during misfire or you'll be going nowhere fast with a blindfold on, and the only way to do that is with an oscilloscope! This was all very well understood some 40 years ago which is why we had Krypton and Sun tuner machines, these days any guy off the street can access the same diagnostic functionality for just £85, for geeks like me running an alternative fuel and anyone like you with a misfire issue this a wonderful thing.

I have no idea what control features you have with your ECU but I can offer you food for thought in the fact I can literately dial in and dial out tip in misfire on LPG just by altering my coil dwell time and or timing numbers in certain rpm x load cells where hard to strike propane becomes highly intolerant of combustion initiation.

Somewhere in your ECU software there will be a dwell vs voltage compensation strategy, don't mess with it but do take time to study why the ECU is calibrated to increase your coil dwell as battery voltage drops. Understanding this simple strategy will help you to appreciate why you need to capture coil voltage at the split millisecond misfire occurs.

At this point it's important to understand all spark ignition internal combustion engines misfire to some degree, even the most modern and engines using the best ignition systems have an accepted misfire rate which the sophisticated OEM ECU is closely monitoring. The OEMs know misfires are inevitable so they set a threshold before the ECU throws a fault code, this will be long after the engine has started to misfire which is considered normal but long before the misfire can be felt by the driver.

A misfire that can be felt by the driver is a very severe misfire indeed!

Study and understand the fundamental first principles of internal combustion engine ignition systems, then use the correct diagnostic equipment to understand what's really going on when you experience the misfire. This is the way to fix your issue because armed with the right knowledge and kit..... the fault will be sure to jump out and slap you square in the face redface

That's got to be way better than the 'Parts Darts' approach, right wink
Read, acknowledged, taken on board and appreciated btw Dave..

Thom

1,673 posts

188 months

Saturday
quotequote all
Sounds good. Will you replace the remaining 6 plugs too?

Jhonno

Original Poster:

3,459 posts

82 months

Saturday
quotequote all
Thom said:
Sounds good. Will you replace the remaining 6 plugs too?
No, I will leave them. They are Iridium and only done 2k odd.

Jhonno

Original Poster:

3,459 posts

82 months

Is it possible to have a lazy/intermittent lambda?

Noticed my Lambda 1 keeps dropping to about 0.01-0.05 and staying stationary for a bit, coinciding with the low rev stutter, when it starts flicking the values jump about a lot more and it pulls cleanly from low revs. It also seems to be inactive when cold/for a period after lambda 2 comes in also.. With my symptoms being worse when cold..

Thom

1,673 posts

188 months

Yesterday (13:33)
quotequote all
Could the fouled plug(s) have let too much unburnt fuel into the exhaust and damage the corresponding lambda sensor?

I would also replace the other plugs, they may have done only 2k miles but then the failed ones also did...

Jhonno

Original Poster:

3,459 posts

82 months

Yesterday (14:35)
quotequote all
Thom said:
Could the fouled plug(s) have let too much unburnt fuel into the exhaust and damage the corresponding lambda sensor?

I would also replace the other plugs, they may have done only 2k miles but then the failed ones also did...
I am swapping them all out for NGKs.. I am having to a do some parts department diagnostics as I don't have time to order diagnostics and then order parts. Plus these were a bargain @£18 for a full set.

I have heard of other AJP's not liking Iridium plugs, copper ones being more powerful. The iridium plugs are 1.1mm gap, NGK's are 0.7mm, so it might not like the larger gaps/weaker spark.

I have also ordered a spare lambda, in case. No harm in having any of these bits as spares, but I only have this week to solve it.