Better milage when car is cold?
 

I recently installed a MPGuino in my 1992 Saturn sl1. Although I don't have it completely calibrated yet (I know my ppm is 10000, and I have the usec/gal close), the Mpguino is showing that I get much better mileage in my first 5 miles of driving. Driving at 60mph I start out getting 40+ mpg and as I drive my mileage dwindles down to around 36.5.

Based on what? Dont trust instant consumption, it jumps around too much.
Better test would be:
start cold engine, accellerate to 60mph and reset current trip. Drive 2 minutes or 2 miles or something and check current trip consumption.
When engine is warm, reset current trip again and drive same distance, check current consumption.

Yes, My instant mileage jumps around, But when I start driving it jumps between 39 and 41, After the engine is warm it jumps between 35 and 37 and never gets close to 40. When I drive home tonight I will take a couple current trip samples to gather some more concrete evidence.

Alright here are my current trip samples. Two miles engine cold @ 60 mph 40.7 mpg, Two miles engine warm @ 60 mph 36.8 mpg. So now back to my first post, What could be causing this? I have heard the opposite about Saturns, that they get better mileage when they are warm, and a warm air intake can boost their mileage. My car has 225000 miles on it, the rings are worn so it has a lot of blow by and burns oil (common among saturns) Any ideas?

Terrain issues maybe? beginning of trip is downhill maybe?
Normally engine takes more fuel to warm up engine.

Sounds really good car for people who make a lot of small trips :)

The basic theory is that when cold the engine enriches itself which means lower mpg.

When warm the choke is off resulting in higher mpg.

I would think you have some issue causing your gauge to not show this.

Hi Highjumphero,
I think your oxygen sensor has failed.
-mort

That is the kind of answer I was looking for!
I will look into that. I know that the car burns a lot of oil maybe the soot affected the O2 sensor.
Thanks Mort

Although it is yet too early to tell for sure, I think I have solved my problem. Ever since I have got my mpguino its readings were all over the place. It said I got better mileage when the car was cold, the mileage did not decrease when the engine was under load, and even though I calibrated it often it never matched my actual mileage. I thought that the mpgiuno just did not work with my car. For the last 6 months or so I have noticed my actual mileage was getting worse, it has dropped from 40+ mpg down to 27, but the mpgiuno without calibration was still reading in the upper 30s. This again led me to believe the Mpguino was not compatible with my car.
I have created a thread for my lost mileage also, http://ecomodder.com/forum/showthrea...omy-19870.html
I think I have solved both problems with one fix, Replacing the fuel filter. With only 60 miles after the change I appear to be getting better mileage, but I cannot verify that. Today on my way into work I noticed that my Mpguino actually responded to engine load. Also I turned off the rear defrost and heater fan, my instant mpg increased!

This is my hypothesis. I am mechanically minded, but I am not an auto mechanic so bear with me here! First I had to realize fuel injectors do not forcefully inject fuel into the engine, they are a plunger valve that opens and releases fuel pressurized by the pump. In turn low fuel pressure from the clogged filter caused a bad spray pattern from my injector. The poorly atomized fuel did not burn as completely, and the O2 sensor detects unburned oxygen. This then prompts for more poorly atomized fuel on the next cycle.

What do you think, am I missing anything here?

The MPGUinno is ACTUALLY only looking at how long the fuel injectors are open.

Lets say your fuel filter is partly clogged. You start your car, and it starts out with 40 psi pressure at the fuel rail.

After a while crud inside the fuel filter stirs around, and partly plugs the filter. Pretty soon your fuel pressure at the rail is only 32 psi.

Your O2 sensor says the engine is lean, so the computer compensates by leaving the injectors open longer. Longer injector time means the MPGUino says you are getting less mileage.

FWIW, I am a mechanic, and see a LOT of fuel pump issues. Plugged fuel filters cause the fuel pump to work VERY hard, and plugged filters end up burning up fuel pumps. Fuel pumps cost 100 to 300 dollars, and I charge 200 bucks to drop a tank and put it back up because it is a MISERABLE job.

All for a 20 dollar fuel filter. I tell my good customers to regularly replace the filters, and my best customers I don't even ask them, I just do it every year or 3 when I see the car.