Thread: tuning problem
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Alan L Alan L is offline
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Join Date: Apr 2006
Location: New Zealand
Posts: 3,261
tuning problem

I've been struggling with this one for ages (months) in between a million other jobs to get this car back on the road(track).
I am sure it is a leaness problem, but I can' t isolate the cause. It runs rough and intermittently on various cylinders on cold start up. Eventually, if you are lucky, after a short period all 6 will kick in (backfiring too). I managed to get it going well enough to take it for a few squirts on the road a day or so ago. There is hestiation and slight bogging down under throttle before boost comes on. Under boost it seemed to go very well, pulling away in 4th like a rocket up to around 100-120 mph before I backed off on the wet greasy road.
I now have the advantage of an AFR, and the numbers tell me under boost I was running mid 11's - 12. But off the throttle, or low throttle cruise14's +.
It seems by the numbers and feel, the car is running lean at low throttle openings, I keep tuning it richer - and then it is running a bit rich under boost. If I lean the boost end off - it is too lean at idle and low throttle and runs/starts like a pig.
My best guess is an air leak. I have spent hours chasing leaks and replaced most everything that looked like leaking. I am about to start over again - I am still loosing about 20 liters/min leakage at 0.5 bar boost pressure. These leaks I presume are air leaks under vacuum (leaness) and boost leaks under pressure. The ones I know of are at the throttle spindle and air screw. Whether they add up to 20 l/min,I don't know.
I have checked the AAV, and will check again it is still functioning (was broken first time I checked - fixed). I have checked the cold start valve, and will check again (was broken first time I checked - fixed). I have calibrated the injectors (twice and they are within 1-2 %).
I am at the point of having to go over everything I have already been over - often many times, and still not fingering the source of the problem. Have I missed something?
Checked WUR pressure function too.
It does not seem to respond properly to the mixture screw tuning - there is no sweet spot between rich and lean - normally I can tune to the point where either range just becomes eveident and hit a mid spot which gets you very close. There seems to be no steady mid spot - just a point where it is better than either way from there.
Old 05-25-2008, 02:11 PM
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