Fuel consumption variation - sensor issue?

Melvin480 · Post by **Melvin480** » Mon Oct 27, 2008 1:03 pm

Before I describe the 'symptoms', first some information:
B18FT '89 - no diagnostic LED
Engine is running/boosting/idling fine
All I describe is happening with the engine at operating temperature (engine and oil temp >80*C)

My fuel consumption isn't constant at constant speeds. Several scenario's that often happen:

Scenario 1)
I'm driving at a constant speed of 80-90km/h. The fuel consumption adopts every value between 5.5L/100km and 7,5L/100km. For a minute or so the value fluctuates. Sometimes it ends with (for example) 6L/100km, sometimes with 7L/100km.

Scenario 2)
If i'm driving 80-90km/h with a fuel consumption of 7L/100km (or more, but in my opinion too much). I apply a very tiny amount more throttle. The fuel consumption goes up with 0,5L/100km and the car starts to accelerate to a speed 5km/h higher. During this proces the fuel consumption decreases to (for example again, the decimal value can be a bit lower or higher) 6,3L/100km. So I drive a bit faster with a bit more throttle applied, but with a lower fuel consumption.

Scenario 3)
Driving at the highway, the way I reach 120km/h determines the fuel consumption when maintaining this speed. If I drive 80km/h and apply and hold the throttle to accelerate to 120km/h, the fuel consumption at 120km/h is 6,7-7,3L/100km. If I apply much throttle and release the throttle at 120km/h to hold this speed, the fuel consumption stays at 8-9L/100km. Sometimes the fuel consumption drops back to around 7,3L/100km in following minutes, but it can also be that it just stays at 8-9L/100km.

So, a long story, but what can it be? Is it just normal behavior of the engine (i don't think so)? In my opinion it can't be right that (at 85km/h) the fuel consumption first is 5,7L/100km and after slowing down and accelerating again it is 7,5L/100km. Ofcourse both cases in fifth gear

Or is a sensor giving wrong information to the ecu to calculate the correct amount of fuel? Which sensor is to blame then? according to scenario 2 I suspect the throttle position sensor. But is this also possibles looking at the other scenario's?

Melvin480 · Post by **Melvin480** » Tue Oct 28, 2008 8:55 pm

Today I measured the throttle position sensor. The resistance between pins 1 and 3 varies smootly with the position of the throttle valve. Resistance between pins 2 and 3 does the same but the other way around. Should be working fine then. Maybe something else is causing this fuel consuption behavior. Or should I be satisfied with the way it's working now?

Ettore Bugatti · Post by **Ettore Bugatti** » Wed Oct 29, 2008 12:24 am

Check your MAF sensor and fuel injectors.

Although it looks to me quite allright.

Paul240480 · Post by **Paul240480** » Sun Nov 02, 2008 10:38 am

If it helps, my 480 Turbo does about 25mpg I think thats about 9Ltrs per 100 Km.

I think it should do a bit more, say about 28-29 mpg.

Melvin480 · Post by **Melvin480** » Sun Nov 02, 2008 5:24 pm

My car used to be more fuel efficient. For example last summer's trip to Italy. I drove 3400km of which mostly highway, but also some city driving and we took a mountain pass in the Alps twice instead of the tunnel. At the end of the vacantion the average fuel consumption was 7.3L/100km according to the infocenter. Normally the real consumption is 0.2L/100km higher. So say that the car did 7.5L/100km over a distance of 3400km.

I know this was in the summer when it was warmer, but the fuel consumption normally is 7,5-8L/100km every time I calculate it after filling the car up with petrol.

Fuel consumption rised more last week. On a route I drive regularly (60km and another 60km back) the car normally does 7-7.5L/100 if I've reset the fuel consumption before the trip. Last week I did the route twice and the fuel consumption was 8,5L/100km average. I think the increase is to much to be caused by the 5*C drop in temperature outside. Also the idle started to get upset, so probably this is related and I will take a look at this week.

bkf_uk · Post by **bkf_uk** » Mon Nov 03, 2008 12:23 am

Melvin i must remind you that you now have a richmoded ecu, therefore more boost = more fuel used! try not to use the turbo as much and you will get the fuel economy back, however if you send the boost to the red it will gobble petrol as quick as an 850t5r! (booting the hell out of mine i can get it as low as 10mpg! treat it kind and i get 40mpg!)

Melvin480 · Post by **Melvin480** » Mon Nov 03, 2008 7:53 am

I know I have a richmodded ECU. The fuel consumption values I mentioned above are with this ECU, also the lower ones. I know if I use the turbo much then it will use more fuel, but when driving constant speeds it shouldn't be using more fuel than with a 'normal' ECU.

I first need to fix my idle. There is something causing the idle not to be stable at 800rpm anymore when warm. Also the car is running rich. When I park the car I can smell it when I get out of the car. It hasn't smelt like this before. Explains also my higher fuel consumption.

bkf_uk · Post by **bkf_uk** » Mon Nov 03, 2008 4:07 pm

ah, sounds like the lambda sensor is dead! mine did this, i replaced it with a universal one and is perfect now! if the lambda isnt working the ecu automaticaly goes into a closed loop system and runs rich (also causes an unstable idle as well!)

hope this helps!

there is a good test explained in the "how to" section on the forum. but mine was intermitant and so worked sometimes and others not, so just replaced it!

Melvin480 · Post by **Melvin480** » Mon Nov 03, 2008 10:50 pm

I hope it isn't the lambda sensor because I replaced it earlier this year with a brand new Bosch one. I will measure the sensor to be sure.

The idle is already fixed. That was an air leak in the hose between the crankcase ventilation and the inlet manifold. The hose didn't look too well and after replacing it with a new piece of hose the idle is normal again. Don't know yet if the fuel consumption also got lower now.

Melvin480 · Post by **Melvin480** » Sun Nov 09, 2008 7:04 pm

Measured the lambda today. I connected pieces of wire to the lambda connector and the battery negative. After starting the car the value first stays around 0,1V. After warming up (engine reached operating temperature of 80*C for oil and water) the value is 0,45-0,48V when driving constant speeds, but it almost doesn't fluctuate like it should (information from the lambda testing how to). Also releasing the throttle (0L/100km) doesn't result in significant lower lambda voltage. Sometimes it drops to 0,3V but sometimes it doesn't go below 0,4V. It should go to 0,1-0,2V when a lot of oxygen is flowing around it, isn't it? Also it never goes above 0,48V, so it never runs rich according to the lambda sensor, also not when accelerating.

So, I guess my lambda sensor has had better days. Although it has only done 20-25.000km. So, my lambda is dying and I need a new one?

bkf_uk · Post by **bkf_uk** » Sun Nov 09, 2008 11:12 pm

thats exactly how my lambda was behaving, i wasnt getting better than 20mpg. i replaced it with a universal one (although in hind sight ive realised it was cheaper to get the genuine bosch one!) and now it's great 35mpg on town runs and has just about made 45mpg on a long motorway run (bearing in mind mine is an automatic so will do worse regardless!) i think a new one might be on the books sorry to say!

but also do other simple checks that could affect mpg like :-

is the dist cap and rotorarm in good nick (after replacing mine today how the car ran i dont know)?

is the oil in good condition?

are the tyres at the correct pressure?

are the brakes binding slightly?

hope this helps!

Melvin480 · Post by **Melvin480** » Fri Nov 14, 2008 10:26 pm

Fitted a new (not new, but a proven good used one) lambda sensor to test. The 'old' lambda sensor came out really black and full of carbon. So before I installed the new sensor I measured if the heating wires connector was getting power and it was, so the problem wasn't from there. 'New' lambda sensor in and the fuel economy improved a lot. After a 130km trip I got an average of 7,5L/100, instead of the >9L/100km I got before. The car is running and idling smoother now

The 'old' lambda has a semi broken wire to the heating element. If I connect a multimeter to the heating connector and measure the resistance, it is infinit. When playing a bit with the cables this changes to several ohms and shooting to infinit again. Assume that this is easy to fix and I do that, can the sensor be rescued? It is completely black from the carbon in and on it. Can I connect the heater wire to the car battery to burn the contamination off, or is the sensor already dead by now?

bkf_uk · Post by **bkf_uk** » Fri Nov 14, 2008 10:32 pm

bin it, it wont ever work again!

Veegard · Post by **Veegard** » Sun Feb 01, 2009 1:36 pm

My 460 uses 15l per 100km...