Black Smoke And Bad MPG

[andy-w]

Hi all...i ahve been through the old forums and had a bit of a search and know i am going over old ground.

I took my car down south at the weekend (Newcastle to Brighton) a round trip of 700 miles and used £200 of petrol...this means it was doing roughly 15mpg. The car is a 4ltr and there are no petrol leaks!!!

When i did the trip about a month ago it cost me £130 and worked out about 24mpg.

I was only doing about 80mph so it was not being driven hard and for some reason the temp gauge never went above 70...

The inside of the exhausts are really sooty and when i start it up there is quite a bit of smoke until it is warm. If i use an ECU reader i get the error 'fuel sensor out of range' and i have changed this but still get the error.

I am using Optimax, have cleaned the air filter, stepper motor, new leads rotor arm and dizzy cap.

Any ideas would be greatly appreciated.

[rfisher]

Hi Andy,

I think you are going to get the prize for the highest number of fault codes in one session!

Think the fuel sensor is a pressure valve or regulator measurement so this could suggest a leak somewhere.

[andy-w]

Thought i would supply as much info about the way the car was running in the hope it would give people a better idea of what was going on with the car during the trip...!!!

[GreenV8S]

Black smoke and poor economy, sounds like it's running rich.

Fuel temp sender warning suggests that either the fuel temp sender, or the wiring to it, are faulty. The fuel temp sender is screwed into the front of the fuel rail, high up at the front of the engine. Don't know what the ECU will do if it loses the fuel temp sender input, but it might have to make a conservative guess about the fuel temp (and hence viscosity). A conservative guess would lead to overfuelling.

Cruising at 70 also implies that either the gauge or the stat are faulty. If the stat is faulty then the engine really is running cold, this would prompt the ECU to richen the mixture.

There's nothing immediately catastrophic about running rich, but over time it can lead to accelerated bore wear due to the excess fuel washing the protective oil coating off the bores.

[andy-w]

thanks GV8s...
that has given me a few things to look at...i did change the temp sensor not so long ago but it wasnt a genuine lucas part...it was a halfords equivalent...i shall put the original one back in reset the ECU and check the wiring to the fule temp sensor...and see what happens

cheers
Andy

[GreenV8S]

Check the resistance to see if you are getting the correct reading, shame to change it again if it is OK.

[andy-w]

not sure how to check the resistance on the temp sensor as i dont know what it is meant to be..it does change if i put a meter on it and put it into hot water so it should be working...i shall have a trawl through the forums and see what the resistance should be...for some reason i am just not confident that the halfords part will be any good...dont know why i just dont trust them!!!!!!

[GreenV8S]

If memory serves, its 5K at 0C, halving per 20C increase. (That's only approximate, but it's enough to show whether the sensor is more or less working.) I can confirm the figures tonight if you want.

Edited to add: the fuel and coolant temp senders have the same characteristics. If the senders themselves seem OK, try measuring them again at the ECU plug to see if the wiring is OK and they are connected correctly. I had a right old game with mine when somebody got the connectors crossed over ...

>> Edited by GreenV8S on Tuesday 19th July 14:34

[andy-w]

Cheers...i shall try the above and see if everything is as it should be...i shall have to look in the bible tonight and see if there is a mention in there as well...need to get this sorted as i am getting stick from work colleagues with the "told you they were unreliable" statements, i get them back with "...but does it look good from behind???"

[shpub]

2nd edition has several pages including a temp vs resistence chart. Just a warning I did have a sensor that worked fine out of the engine but failed when in the engine. Worth testing in and out as they say.

[andy-w]

I had a look through the second edition last night but didnt have time to go through it all...so i shall have a look tonight and do some probing with the multi-meter....should be interesting!!!