AFR changes when things heat up - one for the gurus

[PL]

Hey guys,

Just wondering if anyone has seen this themselves? I was tuning an RB25DET using a Z32 ECU the other day. All went well except for one unnerving thing. As heat built up the AFR's went richer and richer. To the point where I couldn't trust my adjustments anymore.

I don't think this is a particular problem with this ECU or engine, as I've seen it before when tuning SR's. All seems well and then as heat builds up in the dyno cell the mixtures start going rich. I always allow everything a break to cool down between runs but you know how it goes - sometimes heat buildup does occur. Admittedly it was 42 degrees C in the dyno room - far from ideal! But coolant temps were well under control (generally under 90 degrees).

What gets me is OK, the ECU is seeing the higher temps and adding fuel (which is fine by me - at least I know it will try to save an engine under stress), but how is it doing it? The coolant temps weren't sky-rocketing and there's no separate inlet air temp sensor. About the only way it could do it would via the AFM, which doesn't have a separate temp sense output, but does allow for temperature in its measurement.

Unfortunately I didn't have the time or patience to log a few runs and examine the AFM voltages - due to a raft of other problems I was having. Maybe next time...

I've been pondering this and just thought I'd throw it open for comment by others who may have seen the same thing.

Just for the record it was an R33 RB25DET running GT3076R on 18psi with Z32 AFM and 740cc inj.

PL

[raddy]

interesting probelm...at least, you should consider change of fuel pressure as result of fuel temperature change. It looks that gasoline change its volume/viscosity noticeable with temperature, what lead to pressure change, as FPR hold same mechanicall restriction to fuel return, but fuel is thinner with temperature. But, this lead to decerease of fuel pressure, what should give opposite syndrome that you noticed... but I agree, that if coolant temperature is normal, thare should not be other way how to ecu shoudl/need to know outside temp and adjust fueling eventually. AFM is temperature compensated device, maybe if its compensation system doesnt work correct (or you reached some kind of temperature limit of it), it lead to change of AFM output value which is dependent to ambient temperature. Best way should be repeat test with more reasonable mabient temp. for comparison...

[PL]

Yeah, I'd like to do some more testing but time is money on the dyno. And there are always too many other things to do!

PL

[Legionnaire]

Is this car using Eric's new .bin with disabled fuel temp routine? Maybe this "feature" is somehow related to some temperature input that's been unaccounted for?

[gunni]

You´ll need to review the load between runs to successfully make sure that the load is not changing from the MAF but inside the ecu.

[PL]

No, this is using a std image. I actually used that image that Trent (Status Tuning) posted elsewhere. Thanks Trent - apart from some tweaking of the K constant and fuel map figures (to account for 740cc inj) it was pretty good.

I got screwed up by dodgy fuel pressures but after that (except for the extreme temps causing the richening) it was all pretty good.

Fuel pressures were dropping off above about 4500. It had a GTR pump in it, but std wiring. Pump was seeing 9V! Strapped one side to chassis GND and got it up to 11V. Then replaced positive wire and got decent voltage at pump. Pressure still dropped off! Replaced filter - same. Replaced pump - same! Finally replaced FPR and all was good. I opened up the FPR later and I can't see any evidence of problems. Cars....

PL

[FHCRSky]

I gather you test FPR by disconnecting vacuum hose to FPR at idle and see if fuel pressure rises.

Or higher pressure fuelpump required adjustment of base fuel pressure via adjustable FPR???

With engine running richer, have you tried to divert air from engine fan away from podfilter???

[PL]

Fuel pressure was tested by monitoring pressure throughout the rev range at full load. It was fine at idle, fine at 0psi boost and even fine at full boost. But as revs increased and fuel flow increased, fuel pressure would start to drop off dramatically. This is why we suspected fuel pump.

A Nismo FPR was then fitted and adjusted to 43psi with vacuum hose disconnected. Fuel pressures were then perfect throughout the range.

We didn't try anything to fix the richer mixtures as it was 42 degrees and I was a bit over it all...

PL

[Eric]

maybe the AFM was defective ? have seen similar behaviour on a 300ZX before...replaced AFM and all was fine.
Also make sure it is/was a genuine (Nissan) unit...chinese copies of the Z32 AFM are available nowadays and you can probably guess what happens if you install one of these.

Also an often overlooked item is fuel temperature during tuning...especially if you have been testing for a couple of hours.
the pump(s) can heat up fuel quite a bit and the fuel that has flowed through the fuelrail and is returned to the tank has been heated up even more by the engine.
All this will cause the (decreasing level of) fuel in your tank to become gradually hotter and hotter.
Hot fuel can have strange effects on AFR's as well.

[PL]

I got to have another look at this car/problem the other day. I did several runs back-to-back and it went from normal 12:1 to 11:1 next run to 10:1 after that. I logged all runs in NIStune and it turns out the injector pulsewidths were the same every run.

It was a dodgy wideband sensor on the dyno!!!!

PL

[Fusion Ed]

I think there is some sense in using two probes just to keep an eye on things... Especially when you don't know what abuse the dyno one has been subjected to!

[PL]

Yeah, good point Ed. I might even start taking my Innovate along and use it just as a reference.

It doesn't do that much work so hopefully the sensor will last.

PL

[Fusion Ed]

Yes that's what I do to (were possible) some cars cant get two in the pipe or if you do it causes a big restriction (gained 10hp once by pulling the probes out of the exhaust lol)

[PL]

Yeah, I've had the same. I might make up a skinny adaptor just for the purpose...

PL

[skyline_stu]

Or do as I do, put a M18x1.5 bung in the front engine pipe..

Stu