My car has developed a large misfire/fuel cut at 5500/6000rpm and its stopping it from having a remap finished as the mappers cant work out whats causing it
I'm after suggestions as to what the best action of trying to track it down are. I have a couple of specialists I need to ring, but would appreciate any input. I've tried searches on here and google, but not found anything too similar.


This is not something that has happened only since they've been mapping it, but I had originally thought that it might've been something that was caused by how lean it was running before hand. (as it was very lean and detting on boost) Since the remap, the fueling is fine to that point and beyond, but obviously jumps about a bit at the those revs. Its not overboosting or factory fuel cut as i'm only running on actuator pressure at the moment, (10/11psi) i havent turned the boost up yet.

When it happened before it felt like a fuel cut and i hadnt noticed at what revs it was doing it, but i knew it was when giving it full beans so could well have been at 5500/6000rpm.
When it happens the rev counter drops momentarily to zero, and it does it both on or off boost!

I read up on this stutter and found that the Vos chip I had fitted could make similar issues more apparent and the suggestion was the coils. I swapped all the coils but the problem remained. (The chip is no longer in the car so thats unrelated anyway)

I'm not sure if its related, but the rev counter has been a bit twitchy at times and has had moments when it has stopped working or bounced around or dropped to zero on boost then jumped back up when you lift off again. It has been behaving recently though, so its quite intermittent. I had wondered if it was a dry solder behind the dash, but am now wondering if its something more sinister and related to the miss. The rev counter gets its signal from the ECU which apparently gets its readings from the cam and crank position sensors, so could they be at fault?


Things its not:

New plugs
All coils swapped with good condition non cracked ones
Compression is good
Fuel pump swapped
Fuel pressure spot on
FPR tested and working
Maf swapped
Injectors swapped
There are no fault codes stored on the ECU/no CEL's

Car is a 1994 Legacy single turbo (mechanically very similar to early classic)



I have a couple things I could try but I dont want to just throw money at it pointlessly as its already cost me a small fortune recently!

The guys mapping it have suggested I borrow another ECU to see if mine has developed a fault of some sort and that maybe I should swap my o2 sensor, yet i'm not sure how that could affect things. They say its almost like the ECU is shutting off momentarily, which is weird.

I'm wondering whether to try changing the cam/crank position sensors due to the issues i've been having with the rev counter incase its all related? Though having read up on the two sensors, it doesnt look likely that the cam sensor can create a misfire unless anyone knows different.

It seems to be some sort of electrical fault though rather than mechanical or fuel related, which is going to be hard to track down! Could it be something as simple as an earth of some sort? I have an earthing kit, so the engine bay should be good at least. I have heard a suggestion about running an earth directly from the ECU to the negative battery terminal has solved someones misfire in the past so could try that too I guess.

Has anyone any other suggestions as its doing my head in and is very nearly a nicely set up car. Once the misfire is sorted, it'll be great and i'm eager to be able to enjoy it again!

 

is this happening consistently, every time you get to 5.5-6k revs ? also who is doing the remap and on what ecu ?

 

have u checked the map sensor is workin fine this would cause similiar symptoms

 

Yes its all the time specifically at those revs, on or off boost. Its being remapped on an AEM F/IC (piggybacked to my original ECU) by my local Jap specialists as i'm not going for big power or anything, it was more a way of sorting the fuelling out and being able to upgrade the injectors.

is there an easy way to check the map sensor, or is it only possible to swap it with another? I had read a bit about them and had also been informed that they controlled overboost mainly? and that this is happening off boost too.

 

sounds like fuel cut in the original ecu. Piggy back is a bad move imho

Simon

 

When you say fuel cut, do you mean somethings gone wrong with the ECU? As I thought fuel cut only happened on boost. If it doesnt, is there a way I can stop it from happening - why is it at this specific rev range?

Standard fuel cut on boost of my ECU is 15 or 16psi to my knowledge.

Out of interest why are piggybacks so bad? when i was researching my options, it appeared to do most of the same things as a standalone, but still let me keep things like the error codes and knock sensor which can be useful at times. If I was going for big power and had changed significant components then I would have gone standalone, but otherwise unsure of the benefits?

 

I have had experience with something similar on 2 V1 cars which I chipped. The problem was due to a dry joint at the addon ROM socket - where you would plug the VOS chip to.

However on both those cars re-connecting resistor J1 (the one you cut to fit a chip) would remove the problem. Also the problem only happened due to shock (bumps etc.) and not RPM. I think swapping the ECU is a good troubleshooting option.

Piggybacks are usually frowned upon since many of them only control fueling, however in my opinion the AEM F/IC is ok since it allows the control of timing as well. But for similar money, you could probably get an ESL daughtboard which basically allows remapping of the standard ECU.

 

Question. Does it cut out at 5.5K revs when the engine isn't under load (i.e just revving in neutral) ?

Secondly are you sure its fuel cut and not ignition cut?

Ignition cut is usually noted by puffs of smoke out the exhaust whilst its cutting.

Eitherway, its certianly a possibility of being a electronic fault, be it the drive cicuit for the injectors or the coilpacks (or ignitor module). If it is ignition cutting, its worth swapping the igniter module with a known good one if possible, just to rule it out.

 

thats interesting to know, thanks. The original ECU has a switch mounted to the resistor so i could turn the chip on and off so that joint could be an issue.. I have now got my hands on another ECU so hopefully going to get across and test that tomorrow.

The guys mapping it did comment that the AEM was a better piggyback than most, due to being able to control the timing too so it sounded like a good option. it also comes with a lead and a cd so i can plug my own laptop in and change things if i wanted (not that i really know what im doing) and that if i changed a couple of things later (i.e like a better flowing chargecooler) then it would be very easy to tweak it to that, where as the ESL I believe (i could be wrong) you would have to have a full remap at full remap prices..

I havent tried revving it in neutral actually. I would guess it does cut from how its acting, but could have a go and report back.

I'm not entirely sure whether its fuel or ignition cut. The guys said that it didnt seem to be ignition cut, i.e plugs or coils as it was too large and like almost all of the cylinders cutting together, but im not sure of the effect the ignitor module would have?

have you any idea where it normally is or what it may look like as i've done a Google search but not much has come up?

 

The igniter is located on the same bracket that holds the boost control solenoid, you have to remove the bracket to see it, since it's on the back of the bracket.

FWIW, I have never heard of a case of a failed igniter

 

seen this loads of times on uk turbos , one last week it was the vehicle speed sensor plug on the gearbox wasnt pushed all the way home , pushed it in , problem solved

u get to 5500rpm and the car sounds like its got a bee-r rev limiter lol also pops out the back a lot when happening basically a rev limit

would defo check speed sensor

 

thanks, i'll have a look at that too

 

Ok so an update on this,

Having driven the car today, it is apparent that it does not happen off boost like first thought. When revved in neutral it doesnt miss, likewise slowly increasing the revs off boost it carries on through.

I tried swapping the ECU today and that made no difference. I tried it with the Map sensor unplugged and that made no difference. The speed sensor is snuggly fitted so i dont think its that either.
i am half way through swapping the Crank position sensor, but it has become stuck in the hole and looks like it will be a complete **** to remove as i've already broken the plastic bit on top, trying to get it out. i'm starting to wish i hadnt started as im not sure this is the problem, it was just something else to cross off the list and now its going to be a right pain! Grr..

If it was an issue on boost, has anyone got any other suggestions to try, baring in mind i have done:

New plugs
All coils swapped with good condition non cracked ones
Compression is good
Fuel pump swapped
Fuel pressure spot on
FPR tested and working
Maf swapped
Injectors swapped
There are no fault codes stored on the ECU/no CEL's
Map sensor tested by unplugging
ECU swapped
Speed sensor plugged in correctly

currently trying to test Crank sensor, and could get hold of an ignitor unit but a lot of people have told me thats very unlikely.

Its a very violent cut/miss but i havent been able to distinguish whether its fuel or ignition cut but I dont think I can carry the revs on after the cut.

 

Your problem looks identical to the one I have had.
I have done pretty much all that you have except changing the injectors.
I also changed the ignitor - made no difference.

I still haven't managed to find the cause of problem so am currently just keeping the revs below 5000 when driving. How frustrating is that.

I would really like to get to the bottom of it too so am intensely watching this thread.

Here is a link to my original thread:

https://www.scoobynet.com/general-te...lp-please.html

 

Ok so the crank angle sensor didnt do anything other than be a pain to swap!

I have been for another drive and taken a video this time of what its doing so hopefully it'll be easier to understand. It does it on or off boost, but not if you rev it in neutral. (in my last post i said it didnt off boost, but it does, it just is higher up than 5.5k sometimes as high as 6.5/7k)

I was thinking the ECU might be shutting off because it seemed like it was dying, but at the same time the speedo etc still work which might indicate that its not the ECU itself, but more an ignition failure. The reason i thought it was the ECU before was because the CEL went off when it went funny and came back when it cut back in (like the second video shows).

The first video is taken on a duel carriageway, gradually increasing the revs to 5500rpm in third gear. When it cuts, I am just holding the throttle in the same position. You can hear as the ignition cuts the revs start to fall before it violently cuts back in again and without any change from the throttle I let them increase again where it happens again.



This video is on boost in second and is more to show the CEL going off and on again.



In first and second on boost it cuts around 5.5k and off boost it basically makes it to 6.5-7k before cutting. As i said earlier, the CEL is unrelated to my knowledge as its for the Purge valve solenoid which shouldnt affect running.

My next thing I'm going to try swapping is the Ignitor module incase thats gone a bit funny but I'm not holding out much hope.

So any ideas please? has anyone seen anything like this before?

 

Had a similar fault years ago, on 5gt turbo. the exaust was blowing, very slightly, something about back pressure, and turbo spooling up, dont know!!,might be easy fix?

 

That is utterly bizzare, I very much doubt its a sensor nor a actuator issue. To me that's more of bad/dodgy electrical wiring/relay or internal ECU fault. IMO.

The randomness of the check engine light may point to a supply/earthing issue to the ECU (or indeed a bad PCB track/solder joint inside it)....i.e why does it turn off when the engine loses power and turn back on when power is restored?...This to me maybe a clue, and my first port of call will be the main engine loom connector and checking the resistance of every wire to its associated pin on the ECU connectors...along checking the security of the loom...if the engine has been messed with or removed/rebuilt and the loom has not been propely fixed in the correct places, it can fray the wiring internally from the engine moving on its mounts....all for the lack of a few cable ties.

Just to rule it out; You haven't got a clifford blackjaxx by perchance? I only ask as the way these are wired in to sense RPM and cut power could pose issues if the unit devoloped a fault.

 

It is very weird, i'll give you that! Its got to be something electrical, but trying to find it is proving difficult!

Its not an internal ECU fault as i have swapped ECU's with another identical one and the problem remains with no change.

I dont have a Clifford, no. Its got a Autowatch on it, but I dont think that is wired in to detect the rpm at all.

After a bit more searching online i'm going to try measuring the voltage across the terminals as the problem occurs to see if it either drops or spikes at that point. Apparently if it dropped by a certain amount then the coils/injectors can shut down due to lack of power?

Im not sure now whether the ECU can be shutting down, as other gauges such as the temp sensor dont move or drop to zero, its only the rev counter that seems to be affected. But if its ignition only then i cant explain the CEL or the rev counter. I hate this car sometimes, haha! The engine has been out 3 times in the last year, so due to its age there is chance that something has become damaged/mis-positioned etc, so the loom wires are a good suggestion to go and check, thanks.


thanks rob for the suggestion, as far as i know the exhaust is all good with no leaks.

 

The other gauges are not as quick to react as the rev counter, so you will not see them moving for a momentary ECU power loss like the one you have.

If you still don't mind swapping items, try the main relay as well, it's located somewhere to the right of the accelerator pedal and supplies power to the ECU.

 

ok thanks for that suggestion, definitely one to try! cheers.

 

Right, update.

I have now tried swapping the Ignitor unit that controls the Coils and its not that. At the same time I tried a different Map Sensor and Atmospheric pressure sensor. No difference.

I have tested the voltage across the battery terminals whilst the problem happens and there is no change, it continues to hold a steady 13.7V. There is no drop which would suggest the battery and Alternator are ok.

With regard to the main Relay, I had a look for it and found this one? Is this the main ignition/ecu relay or have i found the wrong one?




If its the right one i could try swapping it for another, but im not convinced it could make enough of a difference, especially as its at such specific revs?

I have had the cam belt covers off to double check the timing is correct and not jumped a tooth (and it appears to be fine), though i would expect that to cause a misfire rather than shut things off quite so abruptly.

So where do I go from here?

I'm going to try cleaning up every earth that i can find and ALi-B, how would i go about checking the resistances of the wires from the main loom plug to the ECU? As it only goes wrong at the high revs, would the fault be apparent when checking? - would you expect it to have a different resistance to the others?

The only other option I can think of at the moment is data logging the signals from the individual sensors etc to see if any go a bit funny at those revs.. but this is going to cost me a lot of money in labour so I'm willing to try anything else first if anyone has got any brain waves?? I hate this car sometimes!

 

is the relay above the main one for the ignition/ecu? Anyone know?

 

Just a quick update on this so far, I swapped my main relay (not the one above - but a brown one mounted up high behind the dash) and it made no difference.

I have now moved on to data logging the ECU.

I havent yet found the route cause, but we have found the the ECU is indeed cutting out and resetting itself hence why everything stops.

What is hard to get to grips with is why is it happening at these revs in particular? What is changing or happening specifically that could cause the processor in the ECU to reset itself? Its proving difficult!

 

Wow, this issue must be so frustrating for you.

A fairly far-fetched theory to try out: A wrongly wired/damaged sensor might be shorting out when load/RPM gets above a certain point. The car should be able to run (off boost of course) with only the crank and cam sensor (TPS might be required though). So disconnect MAF, MAP, IACV and knock sensors at the same time and see if the problem disappears - make sure you stay off boost.

Also try logging RPM, load and MAFv up till the problem happens in another run in case you can see any anomalies that might point you in the right direction.

Measure the input power pins, GND pins and the IGN input to the ECU when the problem happens so that you can really be sure the cut is happening within the ECU.

 

It certainly is frustrating! I've barely driven the car over the last 2.5 months!

Thanks for the suggestions, i'm really appreciative of anything which will help sort this out!

I'm not entirely sure the first theory will work as last time i accidentally drove without the MAF connected it barely made it down the road and round the corner, let alone to 5500rpm, but i may give it a go and will certainly look at the other things to data log

 

Ive had a similar problem to my scoob,

But it only happened after fitting a de-cat pipe,

Turned the boost down slightly and the prob was sorted (Dawes device)

Try turning the boost down slightly and see what happens

just a thought?

 

+1 you'll need to measure power input to the ECU and see if the ECU is loosing power at this RPM.

Anders

 

That was overboosting leading to fuel cut, it does not drop the revs to zero like the OP is experiencing. His ECU literally switches off when this happens.

 

I've been following the thread, I had something similar but not exactly the same, happened in fourth and fifth at high revs and bang the power drops out, but my rev needle didn't zero like yours. Changed the spark plugs and the MAF but it still happened. Getting the clutch changed on Friday as some suggested it was that... Doubt that helps you, but just thought I'd share as you've tried so many different things! Hope you get it sorted.

 

that sounds a lot to me like your over boosting and hitting fuel cut. Mine used to do exactly the same for a while when i first chipped it.

Do you have a boost gauge or any other way of monitoring the boost? fuel cut is at around 16-18psi iirc.

I'd be surprised if its to do with the clutch, but i dont really know.