What causes me to have such good mileage in hot weather?

[12voltsolar]

Originally Posted by TestDrive

MAF needs "cleaned" much more often than it needs "replaced."

Changing MAF implies a reset of the ECU. Typically an ECU will take a few miles (eg 150 mile) to fully recalibrate itself. If you're FE drop is based on a scan-guage or similar reading, give the ECU a little time to tune itself in.


If you've already gone 150 miles or more and the FE is still down more than colder weather would account for, you might want to double check that the mechanic didn't introduce a spec of lint or crude on the new MAF when he installed it.

Hey TestDrive, not sure that it needed to be cleaned only, may have been, but as I say it already was replaced (past tense)...
I'm fortunate in the A4 to not have to run a scan-guage or similar product the car has a nice built in system that tracks and displays Ave MPG, Ave Speed,
realtime MPG, engine run time in hours & mins., and approx miles left until the fuel runs out (which makes for a fun ecodriving game to squeeze out more miles then the factory computer thinks you will get)
Thanks to the EM Garage I can tell you that I've driven 1,899.9 miles since the service (Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com) my lowest FE per fill up was on 9-27-08 first full tank after the service. I think the engine control unit (ECU) has had plenty of time to adjust...
I'm thinking secondary air injection system has an issue, this is very common on VWs and Audi's. the pump's plastic housing is just riveted together and they rust and break... also the combination valve has issues in these cars.

it really is a great car...

[TestDrive]

Originally Posted by Compaq888

A couple mpg. I got 31.4mpg driving 200 miles at night on the freeway with a warm air intake. When I put the stock intake and filter on I would get 38-39mpg. Huge difference if you ask me. This is why i'm keeping everything stock on this car.

Definitely sounds like your WAI modification plumbed into the system after the MAF instead of before the MAF.

IF no air flows through the MAF the ECU is forced to operate in open loop. Hence your 17% decrease in fuel economy.

[Metrosexual]

Originally Posted by cfg83

Metrosexual -



Isn't this the logic behind the CAI (cold air intake)? The goal is to get cool "humid" air into the engine in order to increase the oxygen and thereby increase the amount of fuel injected.

Question: How come some people report better mileage on cold wet days? This sounds like a contradiction to me.

CarloSW2

Yes this is what a cold air intake does, and I have found that I get significantly worse gas milage in cold wet weather. Cold being 30-40degs f.
I am thinking of installing a block heater to alliviate some of this

[PaleMelanesian]

I get better mileage in humid weather.
I get lower mileage in cold weather.
I get lower mileage in wet (rain) weather.

The best combination is warm and humid. Like Florida.

[Compaq888]

Originally Posted by PaleMelanesian

I get better mileage in humid weather.
I get lower mileage in cold weather.
I get lower mileage in wet (rain) weather.

The best combination is warm and humid. Like Florida.

My car behaves exactly the same way.
I used to get 50mpg on the freeway but now it dropped down to 45-47mpg
time to do another maintenance.

[eco_generator]

Originally Posted by TestDrive

MAF needs "cleaned" much more often than it needs "replaced."

Changing MAF implies a reset of the ECU. Typically an ECU will take a few miles (eg 150 mile) to fully recalibrate itself. If you're FE drop is based on a scan-guage or similar reading, give the ECU a little time to tune itself in.

If you've already gone 150 miles or more and the FE is still down more than colder weather would account for, you might want to double check that the mechanic didn't introduce a spec of lint or crude on the new MAF when he installed it.

A new MAF sensor might just give the engine the correct airflow so it can fuel properly. Running lean with a bad signal will increase your gas mileage, but your piston tops might complain if that happens under load...

[TestDrive]

Originally Posted by 12voltsolar

Hey TestDrive, not sure that it needed to be cleaned only, may have been, but as I say it already was replaced (past tense)...

How long since it was replaced?

Every time the air filter is inspected is an opportunity to introduce a spec of lint/dirt. It only takes a tiny spec of gunk on the small heated wire that is the guts of the MAF to cause inaccurate air flow reading for the ECU.

Also if you've installed a K&N or other "high performace" filter that requires periodic cleaning & oiling, even a slight over oiling will lead put a film on the guts of the MAF and cause inaccurate readings sent to the ECU.