I've had my MS running for a while and I've not got rid of a problem where after a couple of seconds of overrun, when I come back on the accelerator I get a load explosion in the exhaust. It doesn't happen when changing gear. I'm pretty certain I've got no leaks in the exhaust - I did have a small pinhole in the 4-1 collector which is now welded up and I've put sealant on the join to the silencer - the manifold it tight onto the head and there are no signs of soot around the edge of the gasket (but i guess it could be a one way leak).

I've most recently tried lowered the overrun bins in my ve table to 0 but this hasn't solved it. It only happens when the car is up to temperature. The engine is a 2l 16 Vauxhall XE unit with 42mm GSXR throttle bodies.

Any ideas?

Thanks,
Chris

Tickover set a tad too high?

Backfire on the overrun is a good thing. Try searching this has been discussed a lot before.

It's not popping on overrun - its when I come back on the gas after slowing down with engine braking (eg coming into a 30mph from a nsl) so i don't get down to idle type revs. I get one big bang as soon as i crack open the throttle.



Chris

I was thinking that when you are off the accelerator, your fuel is governed by the tps and if the idle speed was set too high it might be letting too much fuel through on the over run (filling up the exhaust with fumes) and igniting when you open the throttle again.

I don't fully understand ms so I might be talking nonsense ..... but its a theory

I have some fuel cut on decel set but i'm on the standard ms2 code so there isn't a fuel cut on overrun - hence me putting zeros in on overrun (20kpa) above 2500rpm.

I was thinking about putting fuel back in to see if that helps.

My load control is all from map not tps. My accel enrichment is based no a combination of MAPdot (60%) and TPSdot(40%) but its not finalised yet.

Do you actually get below 20kPa on overrun with your setup? Seems to be a bit low. Also, if you set the 20kPa bon to zero, be sure to include a 21kPa bin with the normal figures. If you don't and MAP is just above 20kPa, the system will interpolate between 0 and the actual VE value and the engine will run lean.

Also, check if your injectors aren't leaking

Chris
Did you manage to resolve this problem?
I have the same problem on a mates car that we are trying to set up.
Paul.

Two things spring to mind for me. First would be to try temporarily turning off accel enrichment by changing the threshold values to artificially high levels, and see if it still happens. If so, look at reducing the size of the first couple of accel bins (from the description it sounds like it's when you're getting gently back on the throttle). The second would be to look at your spark advance at the transition from overrun to light load at the rpm's it occurring at (at a guess about 2-2.5k rpm and 20-40kPa?), you may well find that more advance is required. For what it's worth I have run about 45deg at very light load/overrun on a similar engine without problem.

just IMHO of course

I'm running ms2e v2b7at the moment and have overrun fuel cut on and its better than before but still does it when the exhaust is hot.

I suspect timing would help but I've not been brave enough to play around with advance yet.

I also want to have a go with EAE as I can't get normal AE to behave properly across the rev/map range. The requirements change too much.

This discussion inspired me to try a little more advance at low load and it seems to have improved matters somewhat. It only took a degree or so - need to do the same at higher revs now.

We also put some more advance in at the low load bins and some extra fuel in at the bottom line, it is set up for fuel cut on overrun but think it is when it goes from light load weak mixture, to back on load .
My own car is running a similiar fuel map and doesnt do it to the same extent which lead us to think about the ignition settings.
Paul.

quote:

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Originally posted by chriscook

This discussion inspired me to try a little more advance at low load and it seems to have improved matters somewhat. It only took a degree or so - need to do the same at higher revs now.

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