Over the last few months i have been leaning out my fuel map for improved gas mileage. Since the burning of more fuel helps keep the combustion chamber cool, could running leaner for extended periods of time, highway cruising, have a noticeable effect on water temps? For reference is am running 15.4-16 afrs at full cruise @ 65mph and between 14 and 15 afr for putting around town.

 

It depends. If you run lean then yes your burn will be hotter, you'll be risking detonation and you'll likely see elevated water temps.

If you're running stoichiometric ~14.7:1 (gasoline) you shouldn't see elevated temps. The ecu dumps fuel if the burn goes lean to make a less efficient combustion and bring temps down. I don't know that i would go any leaner than that for extended periods of time.

Do you have an aftermarket rad?

 

At cruising speeds with very light throttle settings like the OP mentioned, there won't be a lot of combustion pressure so the "risk of detonation" will be extremely low. There's a good chance that coolant temps may be slightly higher at cruising speeds and putting around town, but nothing drastic.
At light throttle settings, the ECU (whether stock or standalone) doesn't exactly "dump fuel if the burn goes lean and bring temps down", because the engine won't run properly much too far beyond stoichoimetric. Again, at cruising speeds/light throttle settings, the difference in the amount of heat generated by differences in a/f ratio shouldn't be too drastic. If you open the throttle up past 50% and start ramping up boost, THEN things will obviously change.

 

thanks, thats what i wanted to hear. i wasn't worried about detonation. just wondering if it could lead to slightly higher coolant temps.

 

More throttle would be altering the map no? Acceleration/boost enrichment would change the a/f.

I'm saying that i wouldn't cruise much leaner than stoich because the gain in mileage isn't significant enough. I also tend to cruise at more than 65- thus more throttle.

Everything i said holds true- it may not affect the car at constant low throttle however.

 

Wouldn't it be a good idea to have a real water temperature gauge and watch it? ..or if your ECU display (e.g. PFC) has water temp, use it. There's no risk of spiking temps at cruise, so you can experimentally answer your own question.

 

Uh...isn't that what the OP was referring to in the first place?

 

I don't really understand the concern about fuel ratios effect on temps while driving easy?? If you are cruising on the freeway, your temps should be under 215 even if it is 100 + outside. If you want low temps in stop & go traffic, you will need to turn on your fans earlier. I don't believe that changing your fuel mixture will make much of a difference in engine temps for the use you are questioning.

 

i have a pfc and do watch it on the commander. im not worried about overheating at cruising. my temps are normal. i was just trying to see if running leaner could make a noticeable difference in water temps. my brother and i were talking about it the other day when he was shocked that i said burning more fuel helps cool the combustion chamber.

 

So you meant, "in theory". In practice, it's not a major factor compared to the overall water cooling system.

 

If your A/F ratio readout comes directly from the O2 sensor, and you are running the secondary air pump, your true A/F ratio is richer than shown on the readout, because of the extra O2 pumped into the output ports of the engine. (Don't know whether you have allowed for that. )

 

Exactly, which is a reason WBo2 gauges are not useful under light load.

 

Actually in the book for my wideband it reminds you to put the wideband before the outlet of any air-pump. It also notes that if you have an exhaust leak you will likely show leaner than you actually are.

Common sense i thought. :/

 

i believe it should be the other way, if u have a air pump, it will defintely read leaner when it's pumping the air in your exhaust... if your O2 is after the air pump.

normally, on cruise, AFR 15 to 16 is okay for our motors. but you need to worry the sudden throttle. your engine needs to drop the AFR from AFR 16 to AFR 12. it needs to be fast else you'll risk of getting detonation during that split moment. there goes your engine...

in term of water temp, no difference i can see here. like kento said... compression pressure is low at the time.

i run AFR 14.3 to 14.7 during cruise and light throttle.

 

another thing of running lean on cruise sometimes gives a little un-easy jerk when letting off throttle going low RPM like 2K and below...running richer doesn't have that problem at all.

 

Regardless, a leaner mixture means hotter exhaust gas temperatures. If you are worried about your water temps being too high during city driving, then I would suggest switching your fans on at a lower temperature. If your running too hot during freeway driving, then I would suggest a more efficient radiator.

 

Done and done. My fans are set to come on at 86 and i recently installed a fluidyne. but again, i was not worried about my water temps. they were normal. just pondering out loud.

 

very reasonable question. i was worried bout this before as well. so far no change to water temps running very similar afr's to yours in cruise.

 

Standard procedure when tuning cruise is to disconnect the air pump. Under boost, it doesn't matter as the air pump is off anyway.