1988 Volvo 760 Turbo Wagon (Estate) Preheat Hose

lummert · Post by **lummert** » 25 Oct 2009, 13:33

Will having the airbox preheat hose disconnected cause my car to use considerably more fuel while driving in the rain with outside temp of 66 degrees F? 1988 Volvo 760 Turbo Wagon (Estate) 214,000 miles.

Post by **billofdurham** » 25 Oct 2009, 13:59

That would depend on whether the flap in the air box is stuck or moving and if stuck is it open or closed. This is where confusion can arise - what is open and what is closed?

My view on the flap is that when it is closed it is drawing hot air from the manifold and when it is open it is drawing cold air from the outside. If the flap is closed, even with the hose disconnected, all of the air will be coming from the manifold, increasing petrol consumption. If it is open then all air will be coming from the outside resulting in a minimal increase in petrol consumption if the air is particularly cold. However, at 66F the flap would normally be open anyway. These are rough figures for the operation of the flap: closed at +5C (41F), +10C (50F) half open, +15C (59F) only cold air.

The answer then depends on what the flap is doing.

Bill.

lummert · Post by **lummert** » 25 Oct 2009, 21:14

So, maybe I should have removed the flapper thermostat and sealed the warm air inlet so the airbox would only get cold air. I would probably have to warm the engine a little before driving in very cold weather. But, now I get acceptable highway mpg (27) in good weather but the city mpg is about 14 mpg. Is this normal for the difference between the city and highway mileage to be so broad? My average on a monthly basis is about 21-22 mpg. A recent 70 mile trip while raining heavily took about one more gallon of 89 octane fuel. Last winter I used 93 octane (by US standards) gasoline and I did not notice any mpg difference from using 89 and 93 octane. It did use more fuel in colder weather but I thought that was to be expected.

Post by **billofdurham** » 26 Oct 2009, 13:58

A lot of owners do remove the thermostat and wedge the flap to allow only cold air in. I am not a great believer in this as Volvo spent a lot of money on research and development and came up with a system designed to give optimum performance whatever the temperature. So the flap/thermostat gets a little temperamental at times - it is repairable.

I don't like commenting on fuel consumption figures as I don't know all of the factors involved. Highway driving is usually easier to figure out but one of the major factors that is missing is how the driver drives. Foot hard down to get to the desired speed as quickly as possible or gradually build up to the desired speed using the weight of the vehicle to assist? Leaving braking to the last possible moment necessitating heavy braking or easing off as soon as a situation develops where braking may be required? Town or city driving is very difficult to analyze as traffic conditions will vary considerably. Long waits in traffic queues will burn a lot of fuel.

As our gallons are different I converted your figures. 27mpg US is about 32mpg UK which is what I was achieving with a 940 turbo. 14mpg US is about 17mpg UK which I wouldn't have been happy with on the 940 BUT it depends on the traffic conditions in your towns/cities. On my three visits to the US I found a lot more traffic in your urban areas than would ever be seen in the UK. More traffic = more holdups.

Bill.

lummert · Post by **lummert** » 26 Oct 2009, 21:37

Bill, I drive about as normal as one can get. I live in a rural county and the furthest that I usually drive without leaving town is 1.5 miles one way. I make a 60 mile trip to an urban area at least once a month. So probably the poor city mpg can be blamed on the engine only getting warmed up by the time that I get back home.

About three years ago I was getting 29-32 MPG highway and probably 18 mpg city. On one of my oil changes I noticed that the vacuum hose from the throttle body to the fuel pressure regulator had a hole in it. It was after replacing that vacuum hose that my mpg went down. I have removed the hose and it does not smell like fuel. Could vacuum from the intake pull enough fuel from the fuel pressure regulator to cause poor mpg and not cause the inside of the hose to smell of fuel? Also could a leaking fuel pressure regulator pull more fuel at low speeds than at highway speeds?

lummert · Post by **lummert** » 30 Oct 2009, 08:36

I think I may have found a reason for my recent decrease in MPG. I recently had the tires and wheels rebalanced and rotated front to rear. I told them I run 36 psi in all the tires. Yesterday I checked my tire inflation and the rear was 36 psi but the front was 29 psi. Today I have a 60 mile trip to make maybe it will be enough to tell if the inflation pressure was the problem.

Post by **billofdurham** » 30 Oct 2009, 12:15

It is possible.

Bill.