smnc

I've noticed my car seems to be burning a LOT more gas on days when it's cold vs. days when it's warm.

At first I thought it was just my imagination, but I've been tracking my fuel consumtion fo 4 weeks now, and there is a patten.

Could this be caused by a faulty temp sensor?

13B4port

the colder your engine is the more fuel you burn. it's simple. when you start your car andthe engine is cold the car runs in "open loop" the sensors need to heat up to get proper readings. all open loop is, is a prefabed fuel map in the ECU that is realy rich and safe for the motor. after the sensors heat up the ecu will make it's own fuel map based on sensor input.

also think about this. how do cold air intakes work and why do they make a few more hp? the colder the air is the more fuel is injected and the better the fuel will atomize in the air to a certain extent. there are limits to this (the air being too cold). so what your saying is perfectley normal. unless it's a 5 -7 mpg diff from a hot day to a cold day. then there's a problem, most likly a bad O2 sensor.

smnc

I understand how the EFI system works.
Perhaps I phrased my question poorly.

When it's colder outside, my car runs rich and burns signifigantly more fuel then a warm day.
I'm averaging about 18-20l/100km (12-13mi/gal) on cold days(10C or 50F or lower), and 13-15l/100km (16-18mi/gal) on warmer days (20C or 68F or warmer). (I haven't found the exact temp cut-off between a 'cold' day and a 'warm day' so I basically discount the days between the two aforementioned temperature ranges).
The car doen't seem to make any signifigant power gains, but the exhaust smells VERY rich on cold days.

Basically it seems to run in open-loop mode all the time. Thats why I'm wondering if the air-temp sensor may be the culprit.

Tech_Greek

I don't know about you but I'm having the same problem and so I'm going to change the O2 Sensor soon enough because if it was faulty then it would be in open loop 3/4ths of the time.

- Tech

13B4port

the easy way to test it is to unplug it on a warm day and see if it runs the same as a cold day. ussually the O2 sensor is the culprit. this may sound retarded but i've seen it. the fuel on my 88 toyota celica alltrac was going out. on warm days it got real good gas milage but it didn't seem to have the power i was used to. on cold days it would run great. i found out the pump was over heating on hot days. replaced it and it worked great. check your fuel preaure maybe.

smnc

Pump has less than 20,000kms (12,500mi) on it... I doubt that's it...

But I'll try unplugging the o2 this week on a few warm days and see how it goes...

Rx-7Doctor

iTrader: (13)

Ok all you EFI SE owners. The O'2 sensor on the SE's only work when in 5th gear.
Remember it is only a 1 wire sensor, not like the newer 3 and 4 wire heated systems. The O'2 is only a small portion of the fuel management system on the SE.
I have run without it hooked up and driven spirited, avg speeds of 55-90. Combination of city/highway and still got 18.5mpg.
The main sensor that gives the most info to the ECU is the coolant temp sensor not the air intake temp sensor. This is the one that will make the car dump fuel like a tanker. You can either do the pin test at the ECU or just replace it. It's around
$25-30. I recommend replacing it because it takes more time to do the pin test than to replace. Just remove the alternator. The sensor is located at the back of the water pump housing just alittle below the t-stat housing.

smnc

Thanks Doc!
That's EXACTLY the kind of help I was looking for.

I actually have a spare sensor on my parts car (used to get about 19-20 mi/gal in that car) that should be fine.
I'll pull it this week and try it...

Now, why would that only affect my car on cold days?
Once the engine is warm, it's warm! So cold day or not, my coolant temp should be fairly close... no?

Plus (and I know it's innacurate) my water temp guage shows about normal either way, cold or hot.

That's why I was thinking air temp. I figure that's the variable that WILL change with the outside air temp...

Tech_Greek

Well,

I unplugged mine and ran it around for a little bit.

I unplugged the connector to the air box and as soon as I hit the gas it died.

- Tech

smnc

The O2 sensor?
Which airbox? The Air filter housing? As in the AFM? You kinda need that one...
Or the intake plenum/dynamic chamber? There's a couple plugs around there...

Tech_Greek

The one that's hooked into the Air Box right below the Air Filter, it idles without it plugged in just step on the gas.

Yes, I ran around without the O2 Sensor Plugged in just testing it out, my girlfriend noticed a slight pickup in power (she's in the passenger seat).

This sensor behind the alt, will it cause smoke on start up? Or would that be the O2 sensor?

Somethings making my car run rich when I start it up on a cold start.

- Tech

smnc

The one below the air filter is the Air Flow Meter... it's the one sensor actually required to run the EFI. If gives the ECU a rough idea of the ammount of air going in to the engine.

Anything past idle and it dies from fuel starvation.

And your car SHOULD run rich on a cold start. It's running open-loop or a sort of "fail safe" mode. The two temp sensors and the o2 sensor only work when warm, so when the engine is cold, it runs rish, just to be safe. Once the sensors are warm, it can fine tune the fuel mix.

-Spencer

Tech_Greek

Well,

I'm getting a lot of smoke on start up like its flooding (it smells like fuel) and the oil doesn’t smell like coolant and compressions fine and yadada.

Oh, when it starts up, and its smoking, it acts like it wants to die (the rpms will drop down to about 200-300 RPMS [No **** Here] and then after it sits there for about 3-4 seconds it'll SLOWLY start to raise the rpms to about 1,300 RPM and then I have to gas it up to 2,100 RPM to burn the gas off I'm guessing and then the idle smoothes out and there’s no smoke on WOT or anything like that.

- Tech

Rx-7Doctor

iTrader: (13)

SMNC. The reason that it has more trouble on cold days is because the air is denser when it is cold outside therfore telling the ecu from the ATS that more fuel is needed. WArm air is thinner requiring less fuel. So if you have a problem with the CTS also then on cold days it will really dump fuel.
Tech Greek.
Your problem may be one of the following.
1) Either the injectors are leaking. Which means all the residual fuel that is in the fuel pressure line is not going back to the tank but going thru a injector and saturating the combustion chambers.
2). The AFM is faulty.
What you need to do on the first is after shutting down the car for the night. Take a rag and wrap it around the pressure line and pull the hose off to release all the pressure and excess fuel. Rehook up the hose and see how it starts in the morning. No smoke, smooth idle = leaking injectors.
On the AFM, shut the car off or before you start listen for the fuel pump if it comes on when you turn the key just to the on posistion. If it does the AFM is defective and giving the injectors fuel before you start. It should only come on when you start to crank.
SMNC/TECH GREEK-AFter doing repairs or in your case TECH report back and let me know what you find. Good luck.

Tech_Greek

Sir Yes Sir!

Thank you!

- Tech

Rx-7Doctor

iTrader: (13)

LOL

Tech_Greek

Can you point out the hose I'm supposed to disconnect in this picture of my engine?



Thanks!

- Tech

Rx-7Doctor

iTrader: (13)

It's the one closest to the oil filter.

Tech_Greek

Roger Roger!

Tech_Greek

Alright,

I just took off the hose, man talk about a fuel shower, it went everywhere when I took the hose off (which I figured something bad had to happen when you said hit it with a rag).

So, I leave it unhooked until I go to start the car in a few hours and if it doesn’t smoke then I have leaky injectors and they need to be rebuilt but what if it smokes?

- Tech

Tech_Greek

Nope,

Still smoked!

- Tech

smnc

You took it off right after turning the car off?

Tech_Greek

Yep,

It spewed fuel everywhere.

Put it back on after about three hours and tried to start the car and smoke came.

- Tech

Rx-7Doctor

iTrader: (13)

Have you tried giving her a nicotine patch,lol.
Did you listen to see if the fuel pump was coming on before you started to crank?
Did she sputter, cough then cleared up after you revved it up?

Tech_Greek

I need one, and I don't smoke...

Yes, it comes on, my dad swears up and down that that's normal, but I think since it's a rotary engine it might be different, anyone care to settle that argument?

Bingo, this is what happens every time, after I get on the gas a little, the smoking stops and is gone for the rest of the trip until I go to cold start it again.

- Tech