Bad fuel economy

[CMF_detach8]

A big hello from Singapore. (GMT+8)

I just bought myself a second hand Micra 10.A 1996. I've saw the throttle body soldering guide but I don't seem to have a rough idle and engine stall problem. My engine idles around 900rpm fine even with lights and air conditioning and brakes applied.

BUT I am getting a really bad fuel economy of approx 8km/l. My tank of 30l brings me only about as far as 240km city driving. I've tried various driving style (accelerating really slowly and getting the engine up to 2.5k rpm before releasing the foot pedal a little to get the gears to shift up) didn't help.

Tyres are properly inflated as it looks. I don't know what's the exact pressure to put in since the owner's manual was lost a long time ago.

What could be the problem here?

 

[CMF_asdame]

tyre pressure values r usually labeled on the chassis when you open the drivers door. mine says 2.2bar front 1.9bar rear for light load.

things to check are air filter, spark plug condition, all electrical ground connectors are clean, valve gaps, cylinder compression, binding brakes.

check the exhaust lamda sensor cos if that's faulty and sends no signal back to the ECU, the computer reverts to an alternative mode where the mixture runs very rich = injects lots of fuel = bad economy

haynes says a way of checking the sensor is:
*drive around to warm engine up

*turn off engine

*ignition on

*open your fusebox cover

*below the collection of fuses is a diagnostic connector.
using a paperclip, short the 2 lower pins on the bottom left hand for 2 secs then remove

*ecu's on diagnostic mode 1, the dashboard engine warning light should now flash. 5 long flashes then 5 short flashes means all fine, anything else means a problem.

*now to check the lamda sensor, start the engine and hold it at 2000rpm. the ecu's in mode 2 where the engine warning light now represents what signals the lamda sensors sending to the computer.

*at 2000rpm the warning light should normally be flashing on & off quickly bout 0.5 inbetween. if it's in one state quite long, then the sensor may be nackered.

*turn engine & ignition off returns the ECU back to normal mode.

 

[CMF_Brendon]

maybe you just got dodgy fuel.

 

[CMF_detach8]

thanks asdame for the tips. I'll try it in a while.

Meanwhile does leaving the battery disconnected for a long period reset the ECU?

 

[CMF_nz_aj]

bad temperature sensor can cause bad fuel economy like this
They're generic, used on many nissans
On the CG motor, it's locaed below the distributor. The single wire sender under there goes to the gauge, need to check the 2 wire sensor.

The fault code may or may not identify this.

I would think you'd show other symptoms aside from bad fuel economy if it was the AFM, like stalling, hesitant starting, cutting out etc.

 

[smidge]

generic temp sensor, got any more info on this? genuine nissan only or are there alternatives?

things like this are worth replaceing once the car ages anyway i reckon

 

[CMF_detach8]

Thanks for all the feedback. I'm not so inclined towards car components so I just sent the car to mechanic about half an hour ago. For about S$100+ he'd do a full service, change the engine oil to Mobil fully synthetic (not sure what grade tho, maybe 5W40) spark plugs, oil filter and air filter replacement. I told him to check if my injectors are choked.

The throttle body seems good, no engine issues with stalling and rough idle, so I think that's not the problem now.

I tried resetting the ECU by leaving the batts unplugged for about an hour and it did seem to "reset" somehow - I noticed just after I reconnected the batt the engine RPM was really low and after like 2 seconds it went back to 900rpm idle.

 

[CMF_detach8]

Next does anybody have a scanned copy of the original Micra manual .. please?

 

[CMF_asdame]

ooh detach, on your last comment, also clean you Idle Air Control Valve when your car returns. over the years it tends to stick with carbon buildup.

the device is like an auto choke where during cold start it opens to allow more air in causing the rpm to rise above 1k rpm, once engines warm the valve closes and rpm drops to whatever the idle adjustment screw is set to, should be 650rpm

now when mine use to stick when i first brought it, the engine always started off slow then speeded up above 1k rpm once its warm.
after i removed and cleaned it with brake/carb cleaner, the engine worked perfectly

here's a pic of mine

 

[CMF_JKL]

nz_aj WROTE:

"bad temperature sensor can cause bad fuel economy like this<BR>They're generic, used on many nissans<BR>On the CG motor, it's locaed below the distributor. The single wire sender under there goes to the gauge, need to check the 2 wire sensor.<BR><BR>The fault code may or may not identify this.<BR><BR>I would think you'd show other symptoms aside from bad fuel economy if it was the AFM, like stalling, hesitant starting, cutting out etc.

This is where I would have looked.

Also the carbon problem with the ISC is sooo common in many makes and models..

Good post asdame. 10/10.

 

[CMF_asdame]

btw reading back on my own posts, a worn piston ring would cause blow-by which results in a loss of power as well as economy too cos all that air/fuel would seep past the rings and wasted.

this could be tested with a dry & wet compression test. maybe the garage can do it?

 

[CMF_JKL]

Would also be noticed on the dip-stick wouldn't it? Unless you got some oil consumption going on....

 

[CMF_asdame]

aye over time oil would appear blacker with soot

 

[CMF_Raman]

hi detach8,

change your lambda sensor, go to woodlands ind park E and source for 1. should be able to get one for abt S$80.

I use to have your problem recently and i change mine..it is all sweet now..

 

[CMF_detach8]

Raman WROTE:

"hi detach8,

change your lambda sensor, go to woodlands ind park E and source for 1. should be able to get one for abt S$80.

I use to have your problem recently and i change mine..it is all sweet now..

Do you happen to know of any mechanic competent to do the job? As you know our city is small and I don't have (the luxury of) a personal garage hee and I am not very well educated with car engines, even tho I fiddle with RC helis.

I need a mechanic. I just sent mine to Sin Ming drive.

 

[CMF_detach8]

Educate me please, wher is the lambda sensor and how do I replace it?

I tried the ECU tests pointed out by asdame, it all seems fine.

BTW asdame, my tyre pressure markings have worn off and I can't see it. I'll try around 32psi the next time I visit a pump.

 

[CMF_asdame]

a lamda or "oxygen" sensor is screwed onto the exhaust manifold with a cable trailing off it.

if you can use a spanner, this is real easy. only do this with the engine cold to reduce burning fingers

the sensors cable trails out to a connecter, unplug the connecter, seperate the connecter from it's metal support bracket (may help to unbolt the bracket from the engine block), now using a C-spanner or adjustable spanner unscrew the sensor off the manifold anti-clockwise (may need a little nudge from a rubber mallet to loosen it off)

screw on the new sensor, tighten it with slight tap from mallet, slip the new sensors connecter onto the support bracket, bolt it back onto engine and connect the cables back up. done

 

[smidge]

thanks for the post and pic of the ICS, have been meaning to get in there and clean it as JKL said, its a common problem of crap building up in there and a good clean out (with carby cleaner and a brush) i found did wonders for the idle on some of my other cars

my idle is pretty crappy on the micra so i'll have to check it out

now to get my hands on a tacho so i can set it properly!

 

[CMF_detach8]

Wow i love the pics. Thanks a million!

As smidge said, can I just take it out and clean it up instead of buying a brand new one?

 

[CMF_asdame]

of course you can. when something mechanical is not working properly, cleaning them up is bound to improve their operation and appearance(one of the gr8 things bout overhauling bikes or cars). if nothing has improved then it's either faulty or caused by something else.

i'd advise the use of an impact driver when possible in removing the 2 screws holding the IACV to the TB cos they're pretty tight from manufacture. an impact driver reduces the chance of rounding of the screw head.

what i did extra in my case was i marked the position of the IACV motor (grey plastic body) with the valve mech it controls before unscrewing and taking it apart.
this gives access to the inner shaft that moves the valve.
i spray some brake/carb cleaner on the valve end whilst turning the shaft to loosen off any crap then wipe it dry clean. i do this till the shaft moves freely without any grittyness.
then reassemble it back and lining the motor up with the valve using the 2 marks i made earlier.

 

[CMF_detach8]

Woah slow down asdame I am very very new to vehicle parts so I need some time to digest all the terminologies, let alone know where to find the component in the engine compartment.

The veh mechanic said my brakes was faulty such that after a test drive it was hot, so I guess a binding brake did kill my fuel economy a little. I also noticed my exhaust deposits being grey in color, which means the engine is probably running rich.

I want to give cleaning the lambda sensor a try. Now that I know where it is, next is how do I clean crap off that thing? Will a brush and some alcohol/water do?

 

[CMF_asdame]

oops sorry bout that

aye sticking brakes reduces economy. did the mechanic not fix that?

my 1.3 SLX use to have a seized left rear brake and i replaced it with a reconditioned one. i overhauled me other brakes too. brakes no longer bind now

i'm not sure about using any solutions on the lamda sensors internals cos it's a delicate thing. most i'd do is wipe the outer shield with damp cloth and dust off any soot from the openings with a soft paint brush.

 

[CMF_detach8]

OK i will find a suitable time to remove the lambda sensor for an inspection. I do have some soft brushes around and a cloth. Only thing is it looks like a really HOT! place to get near to, so the car must be left for hours before I can actually reach for that.

Yea the brakes are under replacement now should be done within the next 24 hours and costs me a few hundred bucks! If that and a regular servicing of filters and plugs fixes the FC of my car then I guess i'll be a happy man once again. (Check the time, i can't sleep damnit! Hee)

 

[CMF_asdame]

hope it all goes well for ya ;P

 

[CMF_detach8]

Right, I just got back from the workshop. The engine is a little quieter and smoother now and idles around 800rpm instead of the previous 900-1000rpm range. Brakes fixed. I just got a full tank of fuel into the car and so far I've traveled about 30km and the fuel meter hasn't dropped a notch. It's really too early to tell now, maybe another 50km will tell.

Once again thanks so much for the excellent tips. I love this forum guys.

 

[CMF_detach8]

Just did about a quarter tank city driving, and that's only about 80km. I was doing about 60km on a quart tank, so the binding brakes and change of filters and sparks did help a little.

Guess this issue is only about half resolved. I will try cleaning up the lambda sensor once the engine has cooled off tomorrow morning.

Just curious, how many km are you guys doing on a quart tank (city driving). Mine's an auto transmission btw.

 

[CMF_detach8]

asdame WROTE:
*now to check the lamda sensor, start the engine and hold it at 2000rpm. the ecu's in mode 2 where the engine warning light now represents what signals the lamda sensors sending to the computer.

asdame, looking at your post, i figured this means the lambda sensor only works above 2k rpm. (mine starts going off at approx 2100+ rpm readong on the tacho)

I've been trying to stay below the 2k rpm reading, i.e. max gear, min RPM - which means the lambda sensor ain't sending any signals at that range? so is my driving technique wrong?

 

[CMF_asdame]

my manual does bout 30mpg city driving.

i thought the lamda sensor works throughout the rev. just says 2k rpm in haynes. i guess cos it's a closed loop system, at idle the exhaust gas flows through sensor so slowly that the systems reaction time is too long (light only blinks every few secs) so haynes says hold 2k rpm so it works faster, light blinks faster so you could see that it's working. notice the light blinks faster as rpm increases.

i find that most the torque begins round 3k rpm.
2k is ok for flat cruising but struggles on hills
i aim for 3k when accelerating or climbing.

tip is build speed/energy before a hill, maintain set speed going up a hill and try cruising bout 57-60mph (any faster wastes fuel)

 

[CMF_Raman]

detach8 WROTE:

"Woah slow down asdame I am very very new to vehicle parts so I need some time to digest all the terminologies, let alone know where to find the component in the engine compartment.<BR><BR>The veh mechanic said my brakes was faulty such that after a test drive it was hot, so I guess a binding brake did kill my fuel economy a little. I also noticed my exhaust deposits being grey in color, which means the engine is probably running rich.<BR><BR>I want to give cleaning the lambda sensor a try. Now that I know where it is, next is how do I clean crap off that thing? Will a brush and some alcohol/water do?

if you are running rich, it got to do with your O2 sensor. mine has the same problem too the other time. now i got abt 13km per liter of petrol. mine is an auto too.

as for workshop and mech.. you can visit www.k11club.todayhow.com to get more details.

 

[CMF_detach8]

asdame WROTE:

"ooh detach, on your last comment, also clean you Idle Air Control Valve when your car returns. over the years it tends to stick with carbon buildup.

the device is like an auto choke where during cold start it opens to allow more air in causing the rpm to rise above 1k rpm, once engines warm the valve closes and rpm drops to whatever the idle adjustment screw is set to, should be 650rpm

now when mine use to stick when i first brought it, the engine always started off slow then speeded up above 1k rpm once its warm.

after i removed and cleaned it with brake/carb cleaner, the engine worked perfectly

here's a pic of mine

hey man , my mechanic said the idle on this engine can't be set but reading back on your post you said there's an idle screw, where would it be?

 

[CMF_economy_guy]

I know this is an old thread, but it has been very useful to me. The answer to detach8's question (where the idle screw is) is here:

https://micra.org.uk/cmf/message/37918