I can watch fuel trims while I drive with my PLX OBDII Multi Gauge.
Just seeing if I can get a concencus on what is normal. In cool weather my LTFTs are around +5 to +8 at idle and light load. There seems to be two load ranges before going to open loop. After the calculated air flow exceeds 6.0 the LTFTs go to about +1 to +3/+ 4.

HOWEVER, during this heat wave I've been watching my LTFT climb to +17 to +20 at idle and low load and +5 to +8 before switching to open loop.
My IATs are around 125+F, it's 100F outside latley.

From my motorcycle jetting days you would lean a motor out for higher temps/altitude.
For ice racing you would fatten your jetting up to keep the motor from leaning out due to the cold dense air.

Why does it seem that the ECU is enrichening at higher temps?

I have seen RUMORS on the interwebs that Mercedes Fuel Trims are displayed backwards. ie +20 would be PULLING fuel not adding. This would make some sense in that it's pulling fuel for the hotter IATs.


If I had an adjustable fuel regulator I could just crank the pressure and see which way the trims go, but that is on the mod list of dreams for now.

WHATDOYATHINK????

 

the hotter the more fuel. At least in my case as the SRT6 Albino is drinking, drinking and drinking. Or is it that I drive faster during summer?

 

Seems that way but that doesn't make sense. Hotter air is less dense and needs less fuel. I can see the ECU dumping in more fuel to prevent detonation while UNDER BOOST. But closed loop doesn't deal with much boost at all.

 

It is to control pre-ignition and compensate for heat soak. The flame front travels faster with hotter air temps and leaner mixtures.
It would make more sense for it to happen under boost, but I guess its a certain factor of safety.

From megasquirt's megamanual:

IAT-based Fuel Correction
Some users have found that the ideal gas law calculation of air mass does not work especially well for them. This could be because the air entering the cylinder does not reflect the measured temperature. The IAT fuel correction table lets the user define a 6-element table of IAT (intake air temperature) corrections top the fuel amount.

• File: This offers the usual fetch, burn, and exit options.
• Tools/Curve Generate: This allows the user to generate a linear, exponential, or logarithmic curve automatically to fill in the 6-element table.

IAT-based Timing Retard
This is a table that is used to reduce the ignition advance at higher intake air temperatures. Because hot mixtures (typically from the heated air from a supercharger or turbocharger) burn more quickly and are more likely to detonate, reducing spark advance at higher intake air temperatures can help the engine to run better and avoid destructive detonation. The ten element MAT-based retard table lets you specify this reduced advance.

 

Thanks man, even at idle? If so then I'm adding a lot of fuel just for idle and low load cruizing. If I could crank up the fuel pressure and watch the LTFTs I would know for sure if + was indeed adding fuel.

Anyone else watching their fuel trims in this heat?

Edit: Downward I re-read your first sentence. My AFRs are at 14.7 as they should be in closed loop. So I am thinking it's just adding (?) to keep stoich, not a safety measure.

 

Could also be that emissions levels are different for hot vs cold?

 

From what I understand, you do have one, just don't know it

According to posts by waldig (IIRC), the vent port on the regulator will double as a reference adjustment port. So if you were to connect a rubber hose to it, then the other end to a pressure source say a bicycle pump with a gauge, and put 10-15 psi air on it, the rail pressure *should* rise accordingly. Then you should see some movement in the trims.

 

You read my mind, I'll set my air compressor regulator for a few pounds and and raise my pressure. I all ready have split my fuel rail and have -6 pushlock hoses run but have run out of mod money for a while GRRR.

I'm sure my unusual fuel setup is changing things, I'm trying to decipher what my gauges are telling me.

 

This thread:

https://www.crossfireforum.org/forum...on-church.html

is what I am basing this on

 

I'll see what happens this evening. Thanks for your help guys!

 

I haven't watched the short term fuel trims but at idle I see the spark advance changing as well as throttle position. If I can remember to look, I will.

Les

 

I re-read your original post a couple times too lol.. That is strange, and it does seem as though the higher the positive LTFT value, the more fuel is added. But I am pretty confused now myself after reading a couple posts on mercedes forums saying contrasting things. However, I did notice a couple places saying trims should not exceed +/- 10%.. and that trim issues tend to be connected to O2 sensor or MAF issues. Assuming positive means more fuel.. If your MAF was dirty the heated element would stay hot, especially when the air in the intake track is moving slowly.. leading the engine to beleive less air is entering it.. So less fuel would be injected. If the engine is in closed loop, and 14.7 is desired, the amount injected would not be enough and the trim would increase to compensate. Once the car was in motion and under load, the increased air velocity would theoretically cool the sensor and the trim would decrease. I am thinking of it like a car overheating in traffic due to the radiator being deprived of moving air.

 

I may be off base here, but wouldn't fuel serve as a coolant under those conditions?

 

Yes, it would.. but the strange thing about it is that the amount of fuel added decreases as load increases. When the engine isn't under load, it can run very lean.. I run at 15:1+ under light load in my mustang, but don't like to see less than 13's under heavy load. It shouldn't need that much fuel at idle.

 

Good info guys. You can learn a lot by watching this gauge, it shows lots of OBD info all at the same time. Talk about distracted driving!! It is interesting to watch the STFT and LTFTs react, and for months now I've been paying attention. The Long Term trims are like an offset and add or subtract over time. Not very long they change with the temperature. If you start out on a cool morning, and drive home on a hot afternoon, the LTFTs will have adapted within an hour or so. Say your car needs more fuel for outside conditions. You can watch the short term trims stay in one direction and climb higher after staying higher in the + than negative it will add a positive to the LTFT then reset to zero. If the Short terms are still consistantly + then they will add another + to the LTFT and go back to zero again. So the LTFTs are an offset added to the short term trims.

Anyway, I think it's interesting and this post should probably be in the three drink minimum thread. LOL!! Goodnight.

This is all happening t keep the AFR at 14.7 not the open loop.

 

Great troubleshooting but the SRT has no MAF. It takes throttle position and MAP to calculate air flow. Speed density.

 

It's not the MAF, maybe dirty o2's? Do you have any datalogs?

 

I can watch what's going on while I drive. I didn't get to apply pressure to the filter/reg combo last night. Finished up some other stuff.

The car runs great, and is maintaining stoich like it should. I have a wideband so I can see my AFRs are correct.

I'm just really curious if + fuel trims are adding or subtracting fuel on our cars.

I'm also curious what other modded SRTs are seeing for LTFTs and if I am normal. (no I'm not very normal, my car that is).

 

I'll post some closed loop datalogs this weekend.

 

I quess my last install will have to be a Zeitronics Dashdaq...but, I am not sure I even want to know at this point...lol...