Injector flow affected by fuel/injector temperature?

Hi everyone

I'm experiencing a strange issue where my AFR starts to go lean, most notably after heat soak (I think?), causing the closed loop multiplier to ramp up, often to 10-15% or more. My understanding is generally "heat soak"/high charge temps would cause less air to enter the cylinders, causing a rich AFR. This issue doesn't seem to correlate with any variables I currently have, such as intake temp, coolant temp, map, battery voltage, oil temp, running time, idle time, etc. I do run a surge tank and run the surge pump at half speed at idle/cruise to reduce heat, but being in the engine bay and also AEM 400 lph pump, it does still get quite hot.

Things I have checked:

-Injector deadtimes. Seem pretty good and also car has very good voltage stability - doesn't really change much at all including while this issue occurs.

-Air charge temp table (just a typical decrease as charge temp increases - issue doesn't correlate well with charge temp)

-Coolant temp table - issue normally presents at operating temperature, but coolant temp remains stable +- 2-3ish degrees.

-Relative fuel pressure almost perfectly stable

-Map stable

These tables make up most of the fuelling tables available on my Syvecs S6+ ECU and I don't think there's any other that could be having this effect. Also pulse width doesn't decrease or correlate. It increases as a result of closed loop correction.

I'm running 6 Raceworks INJ-213 (Bosch) 1150cc injectors. I'm wondering, could it be the case that the fuel temperature increase is having this effect on the injectors, at least at very low duty cycles?

I know that the mass flow will reduce as the fuel temperature increases, but my understanding is this effect should be relatively small, even if fuel temp went from 20C to 60C. Would it be worth adding a fuel temp sensor and attempting to compensate for this behaviour or is there something else wrong? Any assistance/ideas appreciated. I'm really stuck on this and have been trying to work out the issue for months. Fuel temp is really the only thing I'm not measuring but I haven't really heard of this being an issue before and OEMs usually don't have fuel temp sensors.

I cannot comment on your specific setup, but fuel does get less dense as it heats up. While it's small, ~0.1%/C, it can multiply out to a significant amount from a lowish ambient to sitting in a hot engine fuel rail. However, nowhere near what you seem to be experiencing, even allowing for the temperature quite possibly reaching 100C, or more.

I assume you have a recirculating (return) type fuel line - are you returning it to the tank or to the surge tank, as the latter will tend to introduce more heat into the fuel feed. If you're running a 'dead head' with no return, you're not going to have the benefit of relatively cool fuel circulating and helping to keep those temperatures down.

If they are based on gaseous fuel injectors temp sensitivity is a known issue, one of the ~1600cc varieties people try to use regularly have problems.

I've had issues with certain injectors previously that had a 20% change in fuel delivery, most notably at low pulsewidth, depending on if they were cold or hot.

Never really had a decent solution for it, other than baking some correction maps ontop of the main fuel table to aid in certain areas.

Seems to be less of a thing now with modern bosch injectors

Thanks all. To answer a few of your questions:

I'm running a return fuel rail which returns to the surge tank and it does seem plausible to me the fuel could be getting to 60-80C, which might account for 4-6 pp of the change.

The injectors are Raceworks INJ-213 which I think is this Bosch https://www.bosch-motorsport-shop.com.au/980cc-min-ev14-injector Doesn't appear to be gaseous?

0x33 - definitely sounds like the issue. Did you ever try a fuel temp sensor? I also tried with various maps to attempt to estimate this but it seemed pretty fruitless as it seems to depend on so many factors, like run time/ect, ambient temp, engine bay temp, etc.

I never had a fuel temperature sensor connected to either car that gave me this issue. Not going to rule out that it could be a factor, but the weather where I am is very mild and on a road car typically dont have issues with overheating fuel.

That said, both cars that had the issue also had the same first generation ID1000's in them, whilst being on two different ECU's. So I figured it was an injector quirk as opposed to anything else.

Haven't tried your injectors that you linked, so can't comment.