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AFR, pop bang and I wish I knew what I was doing


The Pikey

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Right, I spent millions of pounds getting my engine mapped but I have never been 100% happy with it because there really is a lot of popping and banging going on on over run. So thinking it was running rich but after connecting my AFR meter only to find that it's running really lean (17-18-19-) So I take a look at the TPS voltage and it's reading 3.1v which should be fine. I check the fuel pressure and it bang on where I set it before it's mapping session at 3 bar. As not to mess with the map I decide to up the voltage on the TPS to increase fuelling across the range. I set the AFR at about 14 at tick over which took the TPS voltage to 4.1 which is silly high and I took it for a blast. It now goes better than ever with no popping on over run at all but still slightly lean (15 ish)

 

 

I can see I will have to go for another mapping session but why would giving it more fuel stop it popping and banging on over run? I though it would be the other way round??

 

thanks

Jason

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I'm no expert, but I have experienced popping and banging due to lean mixture.

 

I had a Strada Abarth in the eighties and it suddenly started banging on the over run, and it was split rubber inlet manifolds allowing air in when the throttles closed. I changed the manifolds and it was cured completely.

 

Duncan

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Thanks Duncan, I just had in my mind that un-burnt fuel for a rich mixture detonates in the collectors-silencer.

 

Been thinking, I think I'll put the TPS back to 3.1 ish and up the fuel pressure until I get a AFR of 13 ish at tick over and hopefully the map close enough for it to run well.

 

Jason

 

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I'm no expert either but if your AFR meter is correct then you are far too lean. Cutting the fuel on over run might help. I think there is a theory that a lean mixture on over run at higher revs 'misses' the spark and just goes straight through to the exhaust. A richer mixture actually helps at it ignites.
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Quoting Dave Walker: 
After the rolling road you need to test under actual driving conditions. You often find things in the real world that never show up in the test environment. One example is that when setting up, you never go into an overrun condition for more than a few seconds. But when you are driving, you are on overrun a lot of the time - especially when braking from top speed for a hairpin bend. You may find that on overrun you are getting a lot of explosions in the exhaust, with flames exiting the tailpipe.

 

What is happening is that you have the mixture too weak on high rpm/light load sites. The weak mixture passes through the engine , and misses the spark. The fuel builds up in the exhaust until it is rich enough to explode, triggered by the heat in the silencer. Richen up the mixture across the light-throttle range, and the explosions disappear. The better method is to use the overrun fuel cut-off in the software.

 

Edited by - Tony Whitley on 23 Apr 2011 17:47:58

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Hi Jason,

My "k" series was popping and spitting flames after having an emerald fitted and it was running very rich. Dave Walker has just mapped it and it is perfect.

As I had a switchable map facility I got Dave to add pops and bangs on to one map because I am childish. It is nice to be able to switch it off though.

I don't have a perfect seal on my 4-2-1 exhaust.

My point is that it doesn't necessarily mean that you are running too lean.

Regards

Dignity

 

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Jason,

 

Set your fueling/rpm map so that at anyting abouve 1500rpm there is no fuel on load sites 0 and 1 increasing to about site 5 as teh rpm goes up.

 

Your system is injecting fuel with zero throttle. Eventually it builds up and ignites, hence the pop/bang.

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Yes but it's definatly running lean throughout the range and I think the poping on over run is just what happens when it's lean. If I drive at a steady 60 it's reading 17 ish on the AFR and I would think It wants to be in the 13 to 14.7 range. So something has happened to make it run lean. There is no leaking T.B's as far as I can see, I can see the sensors are ok, I don't have a MAP sensor. I can't understand it.

 

Jason

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Jason, the main issue is that you are running far too lean. The pops and bangs is secondary and may well go away once the AFR is sorted.

 

First check should be the TPS calibration. If you are running open loop your mapping is dependent on that. Connect to the ECU and check that it reads 0 to 100% across the throttle range. Ignition on and engine not running, display the injection map and the highlighted cell should move smoothly from 0 to 100% as you open the throttle. Make sure you can reach the full load cell ie 100%.

 

Steve

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Sorted it out. I was down to two things. The TPS had moved slightly from it's initial setting and this is what made it pop excessively. Also my AFR meter is reading way way off, at 14.7 it's reading 19, I only found this by stuffing the probe up the Wife's Mini exhaust. Anyway, with a lot of help from Steve Greenald I have it running perfectly. There is still a very small amount of popping but I like it *cool* so no problems.

 

Jason

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