"IF" Economy gauge works, it pulls perfect why??

[ReSuL]

My dear friends,

I have been working on the car for about 2 years and I almost renew everything..

The last thing I realized was the economy gauge on the clock that shows mile per galoon..

when I first start the car in the morning, it works for one minute that it stops working.. then like 30 or 40 km later the gauge starts working again

The important point is the car also starts pulling well with the gauge starting to work..

The questin is "From where this economy gauge gets its information?? where should I look to make it work all the time??

Thank you in advance..

[colin86325]

It gets its info from the ECU.

Sometimes if a car has bveen sitting for a while the econometer can be unresponsive.  Does it start working after you smack the dash?

[ReSuL]

no actually I tried hitting the dashboard for a couple of times but no response again..

I use my car everyday.. and in the first start in the mornings, the gauge comes and goes and hesitates before I move.. when I start driving the car it stops at the left side and doesnt come until it wants to work again..

I have learned that it gets the info from ECU so, if there is nothing in the harness, the problem is from the ecu.. do you know the pinout it gets the info from??

another thing is if the car gets hot after like 30 km, and if you stop and restart the car it starts working... may it be about the cold or hot start??

[DesktopDave]

Does your speedo go out at the same time?  If so, check the speed sensor on the differential. That cluster econogauge just calculates speed (from the cluster) and divides it by injector bandwidth (from the DME) to give an approximate instant efficiency number.

So it'd have to be one of those two, either the injector signal or the speed signal.

I've heard rumors that the speed signal from the diff is used by the DME for proper fueling.  I'm not sure how that happens though.

[ReSuL]

I know that issue about the sensor on the diff but everything is fine I mean the speedo works well.. the only thing is the econo gauge even the lights on the gauge works fine...

the point is if that gets the info from the ecu and think about the times it does not work, is this a sign that something goes wrong in the engine or Ecu part? and may this be about my power loss when the ecogauge does not work??

[monty23psk]

Anything with the gauges not working sometimes is related to the cluster. Either a ground is loose or the boards are going bad. If the SI board is going bad due to bad batteries or the board itself having contact issues, you can see symptoms everywhere on the instrument except the temp and fuel. Those are independent and receive the signal directly from a sensor. So it can be the ECU but my bet it is due to the something in the instrument cluster. Mine does it once in a while and I know my SI board is bad as I can't reset any of the SI lights. I replaced the board over 4 years ago and out of warranty, so now waiting for $$ to fix it again.

[DesktopDave]

I'd be tempted to take a quick look at the injectors to be sure they're working well.  for a quick & easy test I use a long screwdriver - put the tip on an injector and then put the handle into your ear.  You'll hear a very distinct clicking if the injector is working perfectly.  Alternatively, you can pull injector pigtails to see if you have a dead injector, but that's very difficult to do on an M42.

I'd also be wondering about the injector drivers in the DME, wiring faults, dead O2 sensor, that sort of stuff.  It might also be something simple but hard to track down like a bad DME ground or failing main relay.

Did you try a stomp test?

[ReSuL]

Anything with the gauges not working sometimes is related to the cluster. Either a ground is loose or the boards are going bad. If the SI board is going bad due to bad batteries or the board itself having contact issues, you can see symptoms everywhere on the instrument except the temp and fuel. Those are independent and receive the signal directly from a sensor. So it can be the ECU but my bet it is due to the something in the instrument cluster. Mine does it once in a while and I know my SI board is bad as I can't reset any of the SI lights. I replaced the board over 4 years ago and out of warranty, so now waiting for $$ to fix it again.

everything is fine with the cluster I mean the IS bord should be fine since I can reset the lambs.. and my bet is on the instrument cluster or maybe a ground since sometimes it comes and goes..

[ReSuL]

I'd be tempted to take a quick look at the injectors to be sure they're working well.  for a quick & easy test I use a long screwdriver - put the tip on an injector and then put the handle into your ear.  You'll hear a very distinct clicking if the injector is working perfectly.  Alternatively, you can pull injector pigtails to see if you have a dead injector, but that's very difficult to do on an M42.

I'd also be wondering about the injector drivers in the DME, wiring faults, dead O2 sensor, that sort of stuff.  It might also be something simple but hard to track down like a bad DME ground or failing main relay.

Did you try a stomp test?

there is no fault code recorded in the DME... and the thing you recommended about the injectors is really hard to control for me but I use LPG on the car so if the injectors do not work this will affect my LPG system since it gets the informaton from the first injector that applies it to others..

but thank you for your advice.. if I find a way I will also check the injectors..

[Nick_318is]

I thought the economy gauge worked off of vacuum pressure.  Could it be that there is a leak in one of the hoses and after the car warms up the hose expands enough that it's not leaking as much?

I would think any bad leak would make the car run poorly though.

[ReSuL]

I have an AFR and it says the mixture is OK.. around 14,6..  but what you said is true.. when the car warms up, it accelerates wonderfully..

and I think if there is a leak in somewhere most probably the idle will not be straight and I have a perfect idle both when its cold and hot..