Low rev misfire

Low rev misfire

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Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
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?

CerbWill

401 posts

59 months

Monday 29th April
quotequote all
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.

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
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

1,673 posts

188 months

Monday 29th April
quotequote all
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.

Edited by Thom on Monday 29th April 12:05

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
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.

Edited by Thom on Monday 29th April 12:05
These items had crossed my mind, but.. The miss is only from heavier throttle openings sub 3k, getting worse the lower the revs are..
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Thom

1,673 posts

188 months

Monday 29th April
quotequote all
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.

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
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..

Maybe! Although I did check and refresh earths where required.

I could put the standard chip back in also.

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
Why do I have a horrible feeling this could be a throw money at stuff until it stops job.. rolleyeslaugh

Need to be ready for Joolz!

CerbWill

401 posts

59 months

Monday 29th April
quotequote all
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.

FarmyardPants

3,446 posts

159 months

Monday 29th April
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Sounds like the map you’re running is not advancing the ignition timing properly.

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
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?

You have just reminded me the TPS' do seem to wander from what I set them too..

CerbWill

401 posts

59 months

Monday 29th April
quotequote all
The original TVR software has a box to show whether it's active, so does the EvoOlli software. dThrottle or dSpeed.

Jhonno

Original Poster:

3,459 posts

82 months

Monday 29th April
quotequote all
CerbWill said:
The original TVR software has a box to show whether it's active, so does the EvoOlli software. dThrottle or dSpeed.
I am using RS-AJP..

ukkid35

4,742 posts

114 months

Tuesday 30th April
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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 removed

Are you sure there are no air leaks around the TB insulators, perhaps an O ring hasn't seated properly

Byker28i

17,110 posts

158 months

Tuesday 30th April
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Misfire at low revs isn't coils, that tends to be over 3k revs

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 30th April
quotequote all
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 removed

Are you sure there are no air leaks around the TB insulators, perhaps an O ring hasn't seated properly
Yep.. New insulators and o-rings, all seated well. I've had them off a few times and they never move, and the idle symptoms never change.

The flow figures at idle are as I would expect, and even across the banks.

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 30th April
quotequote all
Byker28i said:
Misfire at low revs isn't coils, that tends to be over 3k revs
Well, potentially rules one thing out then!

Thom

1,673 posts

188 months

Tuesday 30th April
quotequote all
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?

Edited by Thom on Tuesday 30th April 09:43

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 30th April
quotequote all
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?

Edited by Thom on Tuesday 30th April 09:43
Yes, I will try the standard chip next, before throwing money at sensors.

CerbWill

401 posts

59 months

Tuesday 30th April
quotequote all
What are the adaptive doing at idle (whatever idle speed you can currently get)?