winter mileage with near-freezing temp: 19-20mpg on highway. in the summer, 25. that's a 91 LH2.4 stock, tuned up.

i'm convinced that the cause is the airbox pre-heat setup--or rather, the lack thereof. basically, with the preheat assembly missing, the AMM hot wire is cooled more with colder air, and the ECU goes to enrich the mixture. the pre-heat assembly is there to regulate incoming air temperature for better fuel economy in cold weather (flapper goes full hot at 5 degrees C, full cold at 15 degrees C). silly me.

removing the preheat assembly would not affect the florida or california crowd any, but will matter to those who have this thing called winter.

so, any of you flodia/california crowd care to send/sell me your preheat hose, flapper and thermostat?

we in New England have the 10% year round, but I notice a significant decrease in mileage come winter, like on the order of 15-20% decrease. But, my car makes lots of short trips and is just getting warmed up nicely when it is shut down. (I have a 1991 745 but thought this thread interesting)
--
1991 745 & 1993 965 /// RIP: 86 764T; 79 262C

Sorry in California we need the pre heat hose so we can put it on every 2 years for smog inspection :).

Just as a note of comparison. currently driving the wife mobile '06 ford 500. Summer milege (accurate) 23.8 mpg, winter accurate 21.8. The only difference is temperature as tires are the same due to adding air to 32 lbs. cold (30 degrees). Greater wind resistance in cold weather, same gas-a-hole BTW.

The lack of a preheat assembly may indeed be causing reduced mileage, but I think your reasoning may be incorrect.

The air mass meter/LH2.4 compensates for air temperature. It will not enrich the mixture due to cold incoming air at the AMM - it will still aim for a stoich mixture at cruise once the engine temperature is close to operating temperature.

To drive this point home, take a look at a LH 2.4 turbo car like your 1995 940. The preheat mechanism is a little electrical heating element in the AMM to turbo hose. This is downstream of the AMM, so it will not affect the temperature at the AMM.

You can confirm whether the car is going into closed loop by tapping into the connector for the oxygen sensor at the firewall (don't disconnect it, just pull back the rubber insulator and connect the meter). A volt meter set to the smallest range should give you some sense of what the 02 sensor is doing (should fluctuate from 0-1V rapidly).

Now, there are reasons the preheat may help mileage. You say the mpg problem is at cruise, so lets assume the car needs ~10 hp to maintain 65 mph. If the preheat is working, the air should be maintained in the 50-60 deg. F range. The air will be less dense then, for example 20 deg F. air. RPM is the same for both cases since it is determined by gearing, power required is the same, and the ECU will maintain stoich mixtures if it is working properly. Therefore, to achieve 10 hp with the warmer air, the throttle will have to be opened slightly more to allow the same mass flow rate into the engine. Pumping losses will be reduced slightly. This, I believe, is one valid reason the preheat could help your mileage.

Hope this helps - Eric

you are mistaken. the LH2.4 AMM works on the principle that a piece of hot wire is cooled by airflow. more airflow cools the hot wire more, reducing its resistance. with colder air that cools the hot wire more, the ECU is lead to believe there's more air coming in than there really is. the ECU goes to inject more fuel, enriching the mixture.

the problem is that LH2.4 does NOT have an incoming air temperature compensation (even though one of the two wires is kept a certain # of degrees above incoming air temp through a feedback loop). the pre-heat assembly is more of a band-aid fix.

From the Bosch Automotive Handbook, 3rd edition, Page 444, LH-Jetronic:

"Hot-wire mass airflow sensor
The stream of intake air to the engine is conducted past a heated wire (hot wire) which forms part of an electrical bridge circuit. The flow of current through the wire serves to maintain it at a constant temperature above that of the intake air. This principle makes it possible to employ the current requirement as an index of the air mass being drawn into the engine. A resistor converts the heating current into a voltage signal which the ECU processes along with engine speed as a main input variable. A temperature sensor is mounted in the hot-wire mass airflow sensor to ensure that its output is not influenced by the temperature of the intake air. The A/F-ratio at idle can be adjusted with a potentiometer. As contamination on the surface of the hot wire could affect the output signal, each time the engine is shut down the wire is electrically heated for one second to burn-off any contamination. The hot-wire mass airflow sensor has no moving parts, and its aerodynamic resistance in the intake tract is negligible."

I can assure you that if LH 2.4 is working properly it will maintain a stoich mixture at highway cruise, in any air temperature you are likely to see in the US. If it did not it would not pass emissions testing. If you still don't believe me we can take a ride in my LH2.4 940 with a voltmeter on the O2 sensor signal ;) Or you can borrow my A/F gauge that reads the o2 sensor signal and hook it up to your 240. Just 3 wires to connect.

Furthermore, I ask you this: If the heating the air coming through the AMM is necessary, why is there no mechanism to do this on any LH2.4 turbo volvo? Take a look at your 1995 940 turbo and you will confirm that there is no preheat like the 240 upstream of the AMM.

Eric

omg, it's you!

i have your bosch fuel injection book, remember? if that book is correct, the temp compensation is that other hot wire Rk. it stays a constant 100C cooler than Rh. the only quantity the ECU measures is the voltage drop across the fixed resistor in series with the hotter wire. if anything, the ECU will need two numbers (air temp and resistance differential) to look up the correct air mass.

here's the graphical representation of my understanding

Haha yeah, its me! My Bosch automotive handbook (not the same as the injection book you borrowed from me, this is more like a mini-encyclopedia on gasoline injection printed by Bosch themselves) seems to indicate you are correct about the AMM having only two resistors in the flow, Rk and Rh, and also only two signal wires going to the LH ECU. However, it also shows additional circuitry in between these two resistors and AMM output. The two signal wires are not simply connected to Rh, but rather through three additional resistors and the wheatstone bridge driving current through both Rk and Rh.

In other words, the temperature compensation occurs within the AMM itself so that the signal carried by the two wires to the ECU are already temperature compensated. That voltage difference corresponds to air mass flow rate independent of temperature.

I admit I am not great at understanding circuit diagrams, so I don't fully understand how it works, but this is how it appears to me. (Come check out the diagram for yourself if you like)

Regardless of this, hotter intake air should help fuel economy, although I'm not convinced it will be a measurable difference. If you reinstall the system, I would put in a brand new thermostat to control the flap, and check its operation often (every oil change?). I have seen them fail often (I put a new one in my mom's '88 740 non turbo and it failed in the open position within 10000 miles)

I would try to confirm two things before adding the flapper system - a) is the car running in closed loop (O2 signal fluctuating from 0-1 volt) at highway cruise and b) is the ignition timing correct. I have heard of knock sensors being falsely triggered by a noisy valvetrain - a valve adjustment and new hushers could help in this regard.

By the way, the 3rd edition was published in 1993, so LH2.4 was still current technology. You're welcome to stop by my office in MEB 30 and check out my copy. There is a diagram on page 112 showing a separate a hot-wire resistor and temperature-compensator resistor in Bosch LH hot wire air-mass flow sensors.

Eric

I have read your post about the AMM and the air box. You all have a very good ideas about the whole thing.

Band-aid, is really good. The air flap and hose is only good for "faster warm up" cause it is feeding heat back to the engine.

Past that, The air mass is the judged by weight/density. Same for the intercooler's used by turbo's. Got to cool it down to get more packed in.

Temperature and moisture sets density. Throttle controls volume. Human controls throttle because it feels power. Volume /density...can equal the same in this equation.

You might split hairs on the moisture. If you do, then you invoke. Is it good or bad for combustion?

Does it help burn or not?

Does it burn or is it there to rust exhaust pipes with the sulphuric acid fumes from the other band-aid? The catalytic converter, fixer upper. A cup per gallon they say.

Aren't we lucky that we don't see steam bellowing out all the time. Good ole, air density trick! Like the air pumps added on engines to fill the exhaust pipes. Solution to polution, is dilution! Scam? Finish burning it? Why I thought the engine burned up all my money. No, its the inspections and taxes, I almost forgot!

In the case of this band-aid. It the excess heat thats bad news for electronics of AMM's.

The others, Yeah! They all stink.

Tell me again. After 30 years. Why aren't we in different cars by now?

Since we didn't go back to the Egyptian/Chinese transportation era. I guess were are stuck using wind, oceans and solar.

Last time I took a deep snif of those, I didn't smell a thing!

Ah! Its the money..."monopoly game", I missed smelling!
Banks out of money. I thought in the rules, that meant the game was over?
Total the assets? Who wins exactly? Where is that taxpayer with the funny money?

Regards,
Phil

I agree with you, I believe the hotter a redblock is (within limits, obviously), the more fuel efficient it is, and I think the pre-heat system helps to increase mileage.

I have noticed the exact same drop in winter driving, and we use a 10% ethanol blend all year long. The culprit is most likely the temperature.

I'm not sure that is the problem. When the weather turns nice, it gets up to freezing. Usual temps have been -5F at night and warming up to +20F during the day. The warm air hose going over to the air box is gone. My mileage ('87 245, ~230,000 miles) is 24-25 mpg in all round driving.

How do your plugs look ?

Greg

different fuel injection systems. what is your ignition timing at?

I'm with Greg on this one. Don't think your pre-heater is the culprit. Plugs would be an easy first check, however I would be more inclined to blame your O2 sensor...

as i said the car has recent tune up. both plugs and o2 sensors are new.

Many Northern States require a 10% alcohol content in Winter, allegedly to reduce greenhouse gas emissions.

...which is pure BS since ethanol has been proven to consistently decrease mileage by 10-15%, so you net nothing except higher costs and engine damage (alcohol attracts water and 'washes' the cylinder walls bare of lubricant)...it IS good for the economy of the 'corn states', however.

Could your reduced mileage be due to this?

Why it's now been required where I live in Florida is a huge puzzle...until you consider the financial aspect.

i know the winter mix has an effect on the mileage. the thing is i see a pretty obvious correlation between temperature and gas mileage on top of the winter mix.