Strange MPG figures

[championc]

Hi all,

I have a 2000 316i 1.9 Petrol Compact. I have been unhappy with the MPG for a long time but just started looking at it again recently after I noticed that my general overall average was higher than it used to be. It was always in the 18 MPG bracket but I recently saw it up in the mid 20's.

I reset it's value on my OBC and watched with interest. My daily trip to work is through the centre of Dublin City Centre (Ireland). I noticed that after about a half an hour of my trip suddenly the MPG started to rise again.

The car was fully warmed up after about 15 minutes but was AT LEAST a further 15 minutes before the MPG started to rise. I also noted that the car was nippier and freeier.

Can can only assume that some part / sensor is lazy / failing and that it has awoken recently but it's still not at it's best. It has obviously something to do with the engine temperature.

Help and suggestions would be greatly appreciated.


Cormac

[Mallard]

its the way that you´re explaining it wich is wrong

but i get what you mean

MPG means Miles Per Gallon

if its gone form 18 MILES PER GALLON to 20 MILES PER GALLON then you´ve gotten 2 miles MORE than u normally get

but if you mean 100miles per liter and you were on 18 liters to 100 miles , whish has now gone up to 20 liters to 100 miles then you´ve lost 2 liters (1/4 galon)

the usual suspects are

Lambda sensor
Katalysator

but check things like air pressure in tyres / change of wheels and different tread patterns do make things different

[championc]

My usual routing is about 200 miles a week. The milage has been around 18 MPG since I fetro fitted the OBC but it is now around 30 MPG so I have had a sudden 50% increase. I could put a 10% - 15% down to driving conditions etc but 50%.

So you can see why I am suspecting some part has suddenly burst to life. The car runs fine while warming up and then when warm. Given that the idling is fine, I assume this rules out the lambda sensor - any other suggestions ?


Cormac

[cali-ti]

and the temp reads right in the middle of the gauge shortly after startup (usually within 5 mins or so, even when cold out) and stays there, even when cruising at fast speed, braking, etc? wondering if you have a sticky thermostat.

[championc]

Spot on, temp needle hits centre after about 5 - 10 mins and stays bang on centre. How would this effect the MPG and what other effects would a sticky thermostat have ?


C

[cali-ti]

and it was doing that back when you were getting poor mileage as well?

if it doesn't get up to temp, it would stay in enriched mode. one typically notices the temp not getting up to center or it will only do it when moving slowly, sitting in traffic, etc. a lack of heat in the cabin usually accompanies it (or it will fade as you get up to speed on the highway). doesn't sound like it if you get up to temp quickly and it stays there rock solid.

[championc]

Well now that you say this, it takes a good half hour for the heat to really work. Through traffic, my trip of less than 20 miles takes about 1 hour. It only really starts to improve as I go through the very centre of Dublin - from one set of traffic lights to another - the slowest part of the whole trip.

And now that you say this, I just remembered that I used the ring motorway last week one evening. The MPG was crap and I was moving most of the time. I actually thought to myself that I should pull over into the hard shoulder for a minute (toi simulate sitting at a traffic light and then go again and see what happens)

On the motorway I was running at an average 50+ mph for a good period and so the MPG was showing less that 30 when you would expect pretty decent figures


C

[cali-ti]

how long since your cooling system has been serviced? should be done every two years so if it hasn't been done, have the t-stat replaced while that's being done shouldn't add much cost of the job and you can potentially kill two birds with one stone (assuming the t-stat is the issue). the t-stats are somewhat notoriously poor. luckily, they usually fail in an open state and thus don't cause engine overheating (a very good thing).

[mohaughn]

My MPG shows an average since the last time it was reset. If you want to see a current reading, you have to keep resetting, but that makes it less accurate in the long run. After a while you won't even see the MPG in the OBC change that much because it is such a long running average at that point. Highway driving, and city driving will have very drastically different readings of MPG.

[dano670]

Aren't there two coolant temp sensors on these cars? One for the gauge and the other is input for the dme? Or is that on the 95 only?

[championc]

I found the thermostat on the front of the engine - a single pipe into a right hand elbow attached straight on the front of the engine. This is exactly the same t'stat as on my previous '95 1.6 Compact and certainly has a electrical connector plugged into it


C

[dano670]

The point I was making about 2 temp sensors is because 1 could fail and ruin your gas mileage, but you wouldn't know it. Your car, according to your gauge, warms up in 5 to 10 minutes. Which seems normal. There might be a second temp sensor in your car that inputs into the dme (I believe my m42 motor was this way, but M44 motors might be different). That inputs into the dme. If that is reading incorrectly (too cold) it will keep your car engines mixture "rich" so your gas mileage is mediocre.

I am going from memory here but the first temp sensor seem to be between the 1st and 2nd cylinder right near the oil filter. I remember the second one to be further back, between the 2nd and 3rd cylinders.

The look similar, but have different thread sizes.

[championc]

The plan for tomorrow is to run the car for a few minutes. When the needle starts to move a quarter of the way up the dial, I assume that the thermostat should still be shut and the pipe to the rad cold ? I assume that if this pipe has hot water in it then this would prove that the thermostat is broken and is shut open ?

Can someone please confirm so I don't go and waste money (and time) buying and fitting a thermostat. It is damn awkward being on the front of the engine right behind the fan at the back of the rad


C

[dano670]

How warm is your heater? If your thermostat was stuck open, then your heater would blow luke warm on a cold day when travelling down the motorway. If your heater set for full hot and you have to roll down your windows in a few minutes, then your thermostat is fine.

[championc]

I had to take the car out last night and tested a few things. I was traveling in to work this morning in very light traffic so verified things further. It would seem the following happens

After 5 miles (10 minutes or so), the thermostat reached the middle of the dial. However, it was 5 more miles before Avg MPG figure started to revert back upwards.

The strange thing was that the Average headed upwards at about the same place as it always does on my route but it only took me half the usual time to get there


C