running lean after turbo upgrade and overheating

[robbo]

bit of a saga, but here goes. after fitting the repaired radiator and a new turbo equivalent to a high flowed t28 (up from a t28 with t25 backhousing), i took the car to return a radiator on the other side of the city.
It didn't show any signs of overheating on the temp gauge on the dash, but when i left it idling for 10-15 minutes, came back to coolant steam going everywhere from under the bonnet. turned it off and let it cool down whilst having a temper tantrum. the radiator store helped me out, worked out that the thermo fans weren't coming on, and gave me a new radiator cap.
waited for peak hour to end, then drove home with one hand on the microtech controller. temperature went between 72 to 88 depending on whether i had the heater turned on to cool it.
Drove it to work yesterday, and it was ok in terms of temperature, just making a point of avoiding traffic/hot days.
on the way home i turned my attention to the air/fuel ratio. the gauge is all over the shop, reading 13-18 on idle, sitting mainly around 15.0. when it is still under vaccum it is running around 13, but when i down shift it goes to around 17. When it starts to boost it goes to 11.2-11.7 which is ok.
What I want to know, is with a bigger turbo, shouldn't the air/fuel ratio decrease as the engine pumps slightly more fuel considering there will be less air(slightly) at the same revs due to greater boost lag?
Can someone please explain what is going on with the air/fuel ratio, as the gauge wasn't reading like this before the new turbo went in, does it need a retune?

[Damo]

What happened to the thermos to make them not work?
88 deg is where the thermostat would be either just opening or fully opened, so you would be able to let the temp go to 92-4ish before turning the heater on.
With your afr's, I'm assuming that it is a narrow band 1volt sensor, which isn't much chop to tell exactly what afr the engine is running. More of a rich or lean indicator. As for the jumpy reading, is there an exhaust leak pre turbo? Does the engine stumble and fart at idle, or run smoothly? Also, is the engine behavior affected by the readings, or is it just the reading which fluctuate?
Bolting on a different turbo should have minimal effect on AFR's unless the new one flows substantially more exhaust, with a more efficient compressor which will show high revs rather than idle.

Damo

[robbo]

I haven't been able to check why they aren't working yet, other than double checking that the temp sensor is indeed plugged in. Bit disappointed that I didn't check the temp reading on the microtech when it was overheating, because my gut feeling is that it may have only reached 92-4, and it could have even been the old radiator cap that just allowed fluid into the overflow tank at that temperature and ultimately appeared like a problem when it wasn't.

Yep that would be the type, the engine runs fine and idles smoothly. It is possible that I may have introduced a slight pre-turbo exhaust leak although i used new gaskets...

Thanks for the advice Damo

[tassuperkart]

Nope, you boiled it fair and square.

Assuming you have used coolant and not plain water then the boiling point of the coolant will be above 100 deg.C UNpressurised.

Most radiator caps are usually set to bleed off at or around 13psi and this elevates the boiling point of PLAIN water to 125 deg.C

So if it had a half bad cap only holding, say 10psi , it would elevate the boiling point above 120deg.C

Its quite a sobering thought when you actually consider it.........

However, little chance of any harm at all given you let it cool by itself and was not run hard while boiling.

Just unplug the thermal switch and jump the wires together with a paperclip to test the fan. If the fan runs then the thermal switch is knackered.

Oracle