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Discussion Starter · #1 ·
Hi,

I'm hoping someone else has had this same issue and can give me some ideas.
Firstly my R33 GTR has new GT2860-5 turbos, an Apexi Power FC (mapped by myself) and possibly more important the dump valves blanked off.

When driving the car pulls like a train, all datalogging shows perfect AFR's (AEM wideband fitted) and using datalogit with the Power FC all readings are fine....I run a very conservative tune with knock readings never going above 20.
The issue I have is that if I bring the revs upto around 2,800rpm without any load so just sitting in neutral, the fuelling is perfect around 14.5 and the engine runs sweet but if I hold it there for a few seconds suddenly the wideband reads 10.0, the revs drop and the engine starts misfiring from excessively rich mixture, it then clears after a few seconds and the revs climb back to around 2,800-3,000rpm and the cycle repeats.

I recently fitted the new turbos myself and fitted all new gaskets, I have smoke tested the engine and have no leaks, I have fitted new RB25 MAF's with new Apexi filters and the issue is still there.
It doesn't normally bother me as I drive to the track, spend the day at mostly around full throttle and then drive home but I went for an unusually leisurely drive earlier and every time I was trying to cruise at around 3,000rpm with a very slight amount of throttle I could see the wideband switching to rich and then clearing and I could feel the engine surging slightly.

It isn't a major issue but one I would like to get sorted and after driving backwards and forwards up and down the A5 today datalogging etc I can se no reason for it so hoping someone else may have some ideas I haven't thought of.

Thanks,
Edward.
 

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What does your injector duty read when that happens? Any fuel pressure readings?
Are you running the fuel pump controller?
 

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Discussion Starter · #3 ·
Hi,

Thanks for the reply.
I can't remember now what the injector duty was, I know it didn't change by much... Not enough to warrant the sudden over fuelling but I will check it again on Thursday when I get it back out.
I don't have any fuel pressure readings, it is still running the standard regulator and still running through the pump controller, are you thinking the pump controller could be causing the issue? I hadn't even considered it but it would certainly help to explain the behaviour.
One of the next jobs is to fit the new uprated pump that's been sitting on the shelf for ages along with the new MAFs, Link G4, 720cc injectors etc etc and also bypass the controller and fit a gauge on the fuel rail.
I am slowly fitting all the bits I have purchased over the years and so far none of them has affected this strange behaviour.

Edward.
 

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Are you running narrow band O2 sensors?
 

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Discussion Starter · #5 ·
No, O2 feedback is switched off.
The original narowbands were mostly flatlining on the original ECU so they are dead.
They are still in their original position but turned off in the Power FC, I have always had them switched off and setup my map using the wideband.

Edward.

Are you running narrow band O2 sensors?
 

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Discussion Starter · #6 ·
Okay, a little more testing done today.
The problem is there from around 2,400rpm upto around 3,000rpm. Above 3,000rpm the issue doesn't seem to be there and below 2,400rpm it is also stable.

The injector duty when it happens jumps from around, 4.5% upto around 6.5% also the averaged air flow voltage jumps from 1400mv to 1700mv.
Also strangely if I let the revs fall to around 1500 it does not clear from the rich condition this only happens once I have come completely off the throttle.
Watching the Map tracer as the trace jumps around, all of the boxes only have very slightly different figures so it is not even as if the revs/load stray into a box that causes a very rich condition.
Very strange.

Edward.
 

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Discussion Starter · #7 ·
A bit more of an update-

Carried out a compression test yesterday and all cylinders were 170 psi.
Fitted new NGK BKR7EIX plugs (old ones were fine)
Can confirm it is not injectors, fuel pump, fuel pressure regulator etc as the issue is the same when I switch to run on LPG and obviously this bypasses all fuelling issues other than the MAP/ECU driving them.

I have ordered a new set of coilpacks more of a maintenance thing as I am doing numerous track days as I do not believe these are the issue due to the fact that on boost it pulls fine.
I have uploaded a video showing exactly what happens (ignore the knock warning light in the video, I had set the threshold at 5 to see if it actually was doing what it should)

Really hoping for some ideas as it is now irritating to say the least when trying to cruise around 2,800rpm.


Edward.
 

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What does your air flow/voltage curve look like?
I would suggest that injector duty is a function is AFM voltage. So if the AFM voltage jumps, so will your injector duty cycle.
 

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Discussion Starter · #9 ·
Hi,

The airflow curve is unchanged from the standard Power FC settings, I have only changed the settings from the standard RB26 MAF's to RB25 MAF's when I changed the MAF's over (the issue didn't change when I changed the AFM's)

The readings between the 2 airflow meters does change when the engine starts misfiring from being too rich but I would expect that to happen.
Puzzling the hell out of me.

Edward.


What does your air flow/voltage curve look like?
I would suggest that injector duty is a function is AFM voltage. So if the AFM voltage jumps, so will your injector duty cycle.
 

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Can you try a different ECU?
To me, from my armchair, i cannot see when the AFM voltage should rise. Can you plot TPS with AFM voltage and injector duty?
 

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Discussion Starter · #11 ·
I think I will try another ECU, I might put the standard one back in just to test, although the O2 sensors are dead so that will misbehave but at least it will be a different behaviour.
I have a Link xtreme that one day I will get round to fitting and another Power FC if I can find it.

With the small amount of throttle required to get the engine to sit just above 2,500rpm, the TPS reads 0.65v (0.42v at idle) and the TPS stays fixed when the fuelling changes on its own (one of the first things I checked)

From reading the hand commander, the TPS reads 0.65v, the AFM's read 1.4v and injector duty is around 4.5%..... then suddenly the AFM goes to 1.7v and the duty jumps to 6.5% for no apparent reason.
I will try a different ECU as soon as I get time and report back.

Thanks,
Edward.

[QUOTE=R32 Combat;2942090]Can you try a different ECU?
To me, from my armchair, i cannot see when the AFM voltage should rise. Can you plot TPS with AFM voltage and injector duty?[/QUOTE]
 

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Have you read the AFM voltage at the AFM rather than relying on that the ECU thinks it is?
 

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Discussion Starter · #14 ·
New Power FC ECU = No change.
New MAFS + filters = No change.
Tweak mapping = No change.
Remove all TPS enrichment = No difference.

Voltage at air flow meters is the same as showing in the commander/datalogit. MAF 1 shows 0.10v higher than MAF 2 but switch them and MAF 2 then shows 0.10v higher so that is a difference in the MAF rather than a difference in the actual airflow between turbos.

Just read the "Does your Skyline shuffle" thread and I think that may be the issue I am seeing as if I hold the revs around 2,800rpm while standing outside the car when the fuelling suddenly goes rich, the turbos start chuffing like crazy.
Strange that I haven't noticed anything before though and could easily cruise at under 3,000rpm before whereas now I can't.
Edward.
 

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Good observation about shuffle. Mine did that too. I have been told you can reduce the effect through mapping. Not sure how. Fitting the anti surge twin turbo pipe will fix it.
 

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Discussion Starter · #16 ·
I have a full hard pipe fitted (very similar to HKS) so the turbos are separate until they enter the main intercooler pipe.
Not sure either on how to map out the shuffle, I mapped out the cutting out since I blanked off the recirculation valves. Every time I came off the throttle the car would try to stall so I traced the cells on the map it was hitting and adjusted them but can't really see how to get over the shuffle in the mapping.
I will persevere and at least one day l get the G4 fitted so ditch the MAFs and hopefully eliminate/lessen the issue.

Thanks again for your opinions.
Edward.
 

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If your turbos go into surge conditions the air can pass the airflow meter both ways, basically re-measuring what's already been measured. This could cause your spike in AFM readings?.

The ecu is a slave to the airflow meter so will increase injector duty cycle.

An open to atmosphere dump valve can make the issue worse.

Some people have made a system to vent the air back into the pre turbo pipes at part throttle to stop surge before it happens on other platforms (on my GTO-TT it was 2500 revs and 30% TPS..).
 
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