What causes exhaust popping on tip-in (lean)

I am running Gen 3 3SGTE MR2 with a Link G4+ plugin.

After getting a re-tune, my car have started to experience exhaust popping while driving hard. I first noted it when I was attempting to a few 3rd gear pulls on the highway after getting the new tune. As soon as I have shifted from 3rd to 4th gear and slam my foot back on the throttle, the exhaust would pop.

I thought afterfire is only supposed to happen on overrun when you are off the throttle but popping during tip in is definitely not normal.

My tuner asked me to take in a look at tip-in and add in some fuel which was initialy clamped at zero above 4500 rpm by the accel clamp table. Looking at my logs I could see things leaning out on tip-in as I guess it makes sense to add in some accel enrichment but I don't understand is how a lean condition could cause the exhaust to pop. I know a rich mixture and/or retarded ignition timing may cause exhaust fires but I am running lean.

Anyway, I've progressively added more accel enrichment at the high RPMs up to an additional 25% according to my logs but it seems to be doing very little to correct the lean condition and the exhaust is still popping.

I guess I can try adding in more fuel again but what really baffles me is why would a lean condition cause the exhaust to pop?

Here are the logs after adding in enrichment.

Is the FPR manifold presure reference line connected correctly?

Yes, I am still using the stock setup for the fuel pressure regulator so it should be fine.

I also took a look at the fuel pressure sensor readings from the logs and they are spot on.

Since adding fuel for tip-in does not seem to be making a difference at all, could the lean spike on tip-in actually be misfire? I did have some misfiring problems when initially converted to COP. Reducing the spark plug gap seem to have solved the problem but maybe it is still misfiring under certain conditions? Does the log look anything like a misfire?

Thank you!

Can you upload you current map so we can see all your accel settings? It's unlikely that it's a misfire that you're only seeing on accel due to the low cylinder pressure as the turbo is just starting to spool. My best guess right now is that it is accel related but it's quite unusual since you normally need little enrichment at higher rpm.

Thank you for the offer to help Andre.

Map1 - attached is the original tune when I first got the car back from the tuner. I was experiencing major problems with accel enrichment at low rpm/loads that made the car barely driveable at rolling speeds. The settings looked wrong with abnormally high values so I had to make broad changes to fix it for the time being to make it driveable. It works, but I definitely have to fine tune it. Your webminar on accel enrichment have definitely provided great insight on how to do it properly and will follow your methods once I find the time.

Accel1 - is the map after I have corrected the drivability problem mentioned above (also corrected the fueling table a little for the light loads as certain values seem to be off in case it will distort the accel enrichment settings) and also added in some accel enrichment for the higher RPMs after I discovered the popping. Logs of this map were represented by the first run in the log file previously posted.

Accel2 - I simply added a little more accel enrichment at higher RPM over Accel1 for this map to see whether it would make any difference. It's the second run in the log file (it was only a short run).

Hopefully that will provide some insight to the problem. Thank you so much!

I think I know what happened. Just took a look at GP RPM limit 2 which was supposed to be my fail safe on fuel pressure setup by my tuner.

It is a fuel pressure vs rpm table and looking at the table, it limits the RPM to 4000-5000 rpm when fuel pressure is at 200kpa.

When I close the throttle during a shift, my fuel pressure falls to around 200kpa due to the manifold vacuum which will satisfy the limit condition in the GP RPM limit 2 table. Looking at the PC Link help files, the fuel cut is also depend on the throttle position. Since my TP is at 0% there will be no ignition cut at the beginning. However, as soon as I get on the throttle, it will trigger the ignition cut until manifold pressure recovers causing the exhaust to pop!

Just took a look at the ECU statistics, GP2 RPM limit is at 101 counts.

I'm still busy at work right now to carry out any testing, will give it a test in the next few days to confirm my theory.

Anyway for a proper fail safe on fuel, I think I should be changing the X axis to MAP instead?

Hi lad, your problem may be that GP limit but that fuel map is wrong on so many levels and your tuner does not know what he is doing. It is just plain wrong. I can see so many errors in your maps it is a joke.

Wow, you need to pay attention to the fuel IAT table as by using the modeled fuel equation that your engine is set to, it already uses Boyle's universal gas law in adding and subtraction fuel in regard to IAT and he has the IAT table doing a double dose, could be big trouble on high intake temps as if your main fuel table is not bad enough. The ignition table is a joke as well in lots of ways but at high load it looks to have the most timing around the peak torque area which is just wrong.

Thank you Grant!

Indeed the tune does look very weird even to someone with little experience like me.

I was shocked when I looked at the graphic view of the initial fuel map which had so many sharp spikes and deep pits. This goes against everything I have learned. A was a little more relieved when I realised most of the spikes where on areas for conditions that my engine will never hit but having such a mess in such areas really does make it difficult to make out the shape on the rest of the graph. As you can see I have begin to flatten out such areas in the changes I've made to the map but after flattening those areas out, there are still some really weird spikes on areas where my engine will actually hit. Bit by bit I am trying to fix them too. I did not take a close look at the ignition map but at least the graphic view of the table does not look as chaotic as the fuel map but from your comments, looks like there are still many problems.

This is very surprising as the tuner who did this map is very well known here locally in HK and actually tunes for professional race teams in China taking part in CTCC and TCR China series. I was trying hard to convince myself to trust him but after having to clear up the messes he made and now the feed backs about his tune, I am really starting to doubt the tune.

Safety is the main concern here as do track the MR2 and runs the engine on its limit for extended time at the track. Given how bad the map looks, I am now surprised that the engine survived when I ran it last time at the track after getting it tuned.

Just disabled GP RPM Limit 2 and popping is gone.

I really have no idea why the tuner would use a Fuel pressure vs RPM table while the fuel system is MAP referenced. The way it is setup, it gives ZERO protection at high boost where I need it most! I'll be changing the x axis to MGP and setup my own protection.

Seems like there is going to be a ton of work up ahead for me to fix this tune.