Help! Very violent misfire under load!

[Shardach]

Hey guys! Newbie here.

Hoping someone could help me out here, I have a 96 Galant VR-4, EC5A 6A13TT.
Ever since I brought the car it's a had a very violent misfire under load, I haven't had time to look into it until now and was hoping someone could give me some leads as to what to look for.

It happens when I put a load on the engine and get up to around 3500-4500rpm and then the whole car slams violently losing power and makes a loud smack noise but continues fine afterwards. As far as I know it happens only well driving as I haven't had it happen well revving out in neutral, but then I again I don't really do that very often. It has also happened once well launching.
I haven't checked anything yet but I know some possible problems could be:
Lean air/fuel mixture from injectors, dirty or faulty MAF
Bad spark from coils, plugs or leads
Carbon build up on the EGR valve

I have recently replaced the fuel tank due to a crack (What a c**t of a job) and well doing so I did the fuel filter and pump so I know fuel delivery at least to the engine is not the problem.
I'm hoping to go into Waiariki polytechnic later today with a mate who is studying there so we can use the scanning tool so hopefully I'll have some codes soon for you fallas to look though.

Any help would be very much appreciated :)
Cheers guys!

[SEAN-NZ]

Most scan tools won't work as these cars aren't obd2 compliant even thought they have the plug, it sounds like fuel cut, usually caused by high boost, does it have an aftermarket boost controller? If so, try and back the boost off a bit


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[Atik]

Sounds like boost cut. Usually kicks in at that rev range if you are running boost too high. Feels like hitting a wall.

If you accelerate at medium throttle, do you get the same issue?

[Nick Mann]

A 3rd vote for fuel cut. (The car cuts fuel if the MAF reading is too high, it doesn't directly monitor boost.)

Fuel cut is not necessarily high boost - it could also be a small leak in the air pipes after the turbos, which would increase the MAF reading at low boost.

[Davezj]

Yes I would go for fuel cut as well.
You need to know what boost pressure you are running to diagnose the issue. You might have animal boost control lot tucked away under the bonnet that you don't know about. And this would cause. Fuel cut if the boost is to high for the ecu settings. But as nick says it could be standard boost levels and a leek. This is why you need a boost gauge.
Buy the way as a diagnostic tool you need evoscan ($25) and a VAGcom 409.1 cable (£5 eBay) to monitor the engine and other stuff.
Cheapest full diagnostic setup you can possibly get.

Have fun


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[foxdie]

You might have animal boost control

He's got gremlins then? :D

@Shadrach I too believe this to be fuel / boost cut. Another way to verify this is to try the following;

Despite what your instinct says, try keeping your foot planted through the violent loss of power, if it recovers, builds boost up again, then violently cuts again, it's definitely boost / fuel cut.

If however you feel the car hesitating or you hear rumbling / burbling / popping / banging noises from the engine bay (specifically from the exhaust extractors under your feet) then that's a severe fuelling issue. If that's the case, my first suggestion here would be to take it to a dyno tuning house and get them to give it an examination / power run with both AFR and knock monitoring.

To add to what my other friends here have said, I don't think you'll see any fault codes if you've simply hit fuel cut, but you might see fault codes for the MAF sensor or the O2 sensor. Do you have a check engine light on at all?

[Shardach]

Sorry I've been quiet, I've been at work.
Thanks for all the replies guys, I went and talked to some experts well scanning the car and they too believe it's over boosting. The car trys to run 12psi and well watching the gauge it does peak at 12, I'm aware the ECU cuts ignition above 12psi so this must be the issue. I don't know how the previous owner cranked up the boost but he did say the ECU was reflashed however didn't give any detail on what was done. Anyone have some good suggestions on what to look for to find how the boost was wound up?

Cheers, Shadrach

[Shardach]

No codes at all, completely clear and no lights either

[foxdie]

Hi again Shardach

Glad to here you're getting to the bottom of it.

Unless the person who tuned your car is a complete joker*, I'd say it's more than likely that either there's a manual boost controller under the engine bay, or far less likely, your boost solenoid has stuck open.

If there is a manual boost controller fitted, it's most likely going to be near the stock boost solenoid located under the black hose here;

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Search Google images for examples of what a manual boost controller looks like.

If there is a manual boost controller, I would recommend removing it and plumbing the stock one back in. Boost control via ECU is far safer than aftermarket control, because the ECU can turn the boost down if there's a problem.

If you're still using a stock boost solenoid, I would consider replacing it, either with an original one (part code MR212636) or an aftermarket 3-port Evo 3-9 compatible solenoid (such as the Perrin 3-Port EBCS or a Mac solenoid) and then get the car tuned to suit.

It could even be as simple as a plumbing issue if it's all been removed at some point.

Hopefully this will help :thumbsup:


* I say joker because if they have just upped the boost without changing the boost cut table too, they're an idiot

[Confused]

I'm late to the party, but as soon as I saw the thread title, I was coming in here to write "fuel cut", just as everyone else has.

Not that it means much to say "me too" but it's another opinion matching the rest!!


The ECU does not, and in fact, can not, read the boost level. Fuel cut may hit at any PSI, based on a large number of factors, so ignore people who say blanket statements like that! ;)

[Davezj]

you are perfectly correct gary it is load level that trigger fuel cut and not boost level.

but i was trying to dumb it down for him to understand, lol.

however again you are correct and we do have to maintain standards around here don't we, So well said.
Technical is Bestnical.


i believe the following is the actual trigger condition for feul cut,

higher load than in the fuel cut limit table in a given rpm band for long than the fuel cut ignore window is set to (which is factory set to 1000ms) brings on fuel cut.

[Shardach]

foxdie
I checked in the location you pointed out and found nothing. I also checked the turbo in the rear and the nut the adjust wategate is wound completely to the left, could this be not allowing the wastegate to open enough or is this the stock position of the nut? Also if I have to adjust the nut on this waste gate will I also need to jack up the car and adjust the nut the front turbo?

Cheers, Shadrach :)

[Davezj]

Ok so you have 12psi boost which is about 0.9 bar.
Standard boost in PFL is about 0.55 bar
Standard boost in a FL is about 0.7 bar

So you do have raised boost level if you are seeing 0.9 bar.

If you adjust the waste gate actuator rod and nut, you are not going to get correct control of the boost.
To raise the boost via this method you would lengthen the rod. So if you rod on the wastegate actuator is at its max length then this might be you issue.
However the nut and the rod are usually rusted in place and will not undo or do up.
Plus modifying the boost in this way can be done on a single turbo setup but a twin turbo setup you have to ensure the boost from each turbo is ballanced.
Which is why they use boost pressure and wastegate actuators with the same spring load in each actuator.


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[Atik]

If the ECU was reflashed, it might be worth trying to find a stock ECU to use to see if you get any issues with that. The reflashed ECU may have had its boost or fuel settings changed to run higher boost. Its a tricky thing to get right and a small difference could result in the fuel cut.

[Kenneth]

Is the car fitted with a pod filter?

[Nick Mann]

Pod filter is a good question. I still think that a boost leak should not be overlooked.

[Shardach]

Yes it is fitted with a pod filter, could you explain how this could be the problem? I have the original air box I can put back on to see if it will work

[SEAN-NZ]

Screws with airflow and can make maf read incorrect, so it might be reading to much air


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[Davezj]

he did say his boost gauge is peaking at 12psi

what does the boost settle down to after the peak?

the pod filter will not help, remove it and put the original one back on go for a drive see if it changes the situation.

[Shardach]

I will put the original air box back on on Friday and let you know how it goes

[Shardach]

I put the air box back on this morning and took the car for a test drive and everything seem fines. I will test more when I have time later in the day and let you guys know later tonight, cheers :)

[Shardach]

It turned out to be the pod filter after all. Everything is running sweet as now, cheers everyone!

[Davezj]

Here is a little additional info for you.
You can still use pod filters but you have to have a length of pipe in between the filter and the MAF to allow the air to stabilise before entering the MAF.
I have see people using curved about 45cm length of pipe the same diameter as the MAF entrance with no bad effect.
I don't think there was any positive effect either.


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[Kenneth]

There is one problem, but 2 possible causes, its important to identify them and understand what cause you are addressing with which solution.

1) The adaptor is incorrectly sized or shaped for proper distribution of air across the MAF
2) The air mass is incorrectly distributed across the MAF due to turbulence directly preceding the MAF entrance due tot he pipe work attached.

As you can probably tell, the problem is the air distribution across the MAF.

Here is a picture of the MAF. I have drawn 2 different colours to outline important sections of the MAF.

75967
RED
This box is the section of the MAF where the air is measured. It is approximately 1/3 or so of the actual frontal area of the MAF and it shaped so that the MAF can get a stable reading on the air speed. Our MAF uses Karman Vortex (you can google it) principle which measures pulses caused by air speed. This requires the air to be laminar so that there are not pressure artefacts which will result in over reading.
However, being only roughly 1/3 of the frontal area, the measurement formula used in the ECU takes this into account and assumes that the air distribution across the front of the MAF is all the same. With the standard airbox, this is the case as the airbox has a much larger cross sectional area which results in slow moving air.

Blue
This shows that the edges are curved to create a bell-mouth type shape to smooth the entry into the MAF at the sides. This promotes laminar air flow through the side sections of the MAF so that the reading is accurate. Laminar flow will result in more air mass entering the intake at a slower air speed. Because the MAF measures air speed, this is important. If you are interested in how this works, google Velocity Stack.

If the air flow through the centre section is either faster that it should be (due to poor distribution) or it has disturbed flow, this causes over-reading. Because the ECU measure air volume rather than boost, this results in it thinking there is more air flow (and so, more boost) than there really is. As a result, you hit fuel cut earlier than you should.

Cause #1 - Incorrect MAF adaptor
75966
Here is a picture of a "BAD" MAF adaptor. The air has to travel through a hole which is effectively smaller than the MAF intake. This means the air speed is higher and concentrated more through the centre section of the MAF.
I have never seen one of these work correctly.

75965
As you can see, this adaptor presents a much larger opening area. This results in slower moving air and allows the features of the MAF to control air flow much better. I have seen major improvements in MAF readings just by swapping from the top type to the lower type.

This adaptor problem is the primary cause of incorrect MAF readings from Pod filters.

Cause #2
This is less of a problem than #1, but also an issue. It usually results in less over reading so can be tuned out somewhat better. Still not ideal though.

Air is a fluid and behaves like water, it is just a lot lighter and usually invisible. Because of this, we can visualise what happens with water to understand what is happening with the air.
If you think of a water (hydro) slide, as the water goes through a turn on the way down (because it is travelling quite fast) the water rushes up the side and sticks to the wall of the slide. If it is fast enough, it will go over the roof of the slide and swirls chaotically until gravity causes it to travel along the bottom of the slide again.

The same happens in the intake. The fast moving air goes around a bend and swirls chaotically. The MAF isn't able to get laminar air flow across the sensor and the vortex street is disturbed causing it to read incorrectly. Fortunately, this seems to always cause over reading rather than under. I do wonder if this is a major advantage to Karman Vortex style measurement. If the air distribution is concentrated more on the outside, then a hot air style MAF may under read, causing lean conditions and possible engine failure.

Adding a length of straight pipe will give the air time to straighten out somewhat and improve the distribution across the face of the MAF by the time it is reached. Combined with the MAF adaptor above this can result in pretty decent readings.

Ideally of course, using the correct MAF adaptor with a large pod filter (slow air speed through the filter) connected directly to the MAF is the best solution.
If you need to relocate the filter, consider relocating the MAF with it.