22 mpg, fuel smell

Hi everyone, first post but I’ve been lurking here for a while. 

just picked up a new (well 2003, so M13A non VVT) Jimny, runs great (sounds great, definitely smooth, power feels normal) but I’ve been getting really poor mpg and it smells pretty rich. 22 mpg/13 l/100km on A and B roads driving gently.

i can push the car easily, so no sticky brake 
the front hubs are permanently engaged, I’m sure that’s worth an mpg or 2 so I’m going to fix that but I don’t think that accounts for the rich unburned smell 

looked at the first O2 sensor on live data and it is switching around ok, 0.2 to 0.8 V but the fuel trims are maxed out at 20%, so it thinks it’s running lean but it definitely doesn’t smell lean… second O2 reads high, 0.9 to 1.2 which makes sense if it’s rich 

could first O2 still be bad despite seeming to switch around multiple times per second?

what else to check? I’ve looked for vacuum leaks with unlit propane torch, changed spark plugs etc. 

I did wonder if a faulty evap system could be chucking excess fuel in? Is that possible?

Most modern cars use the MAF sensor to set the basic fuel/air ratio, then fine tune this using the first O2 sensor.

The second O2 sensor is only there to check the catalytic converter and doesn't do anything to set fuel/air or anything else.

Just a few questions

- what are the plugs like?
- what is the air filter like?
- is the exhaust blocked?
- what tyres do you have on?

OK, so assuming, everything above is good, if the MAF sensor is faulty and over-reading air flow, the ECU will think it is running lean and increase fuel, then the O2 sensor will be telling the ECU to reduce fuel, but if your fuel trims are off the scale, it's probably reached the extent the O2 sensor can go and can't adust for the faulty MAF.

If you have a catalytic converter, running rich will also cause damage to the CAT by the rich mixture.

As a test you could disconnect the MAF sensor, then the engine will go on to the basic MAP and set a rough mixture, fine tuned by the O2 sensor.

If the rich smell goes and the fuel trims look normal, you might have found your problem.

Robin

Thanks so much for the detailed reply! Really appreciate it!

The plugs looked kind of like my old Volvo 240 plugs used to look, that had k jet mechanical fuel injection and that definitely ran rich
All pretty equal though

I’ve put fresh ones in and I’ll check them in a few more miles

Air filter looks fine, it’s cheap so I’ll change it
Don’t think the exhaust is blocked, it seems to come out normally, this car did have a new cat and second O2 just before I bought it. It failed last MOT on emissions, they obviously just slapped a new cat on rather than fix the actual running issue

Running 235/75/15 M/T, Speedo is pretty bang on with these but I know the tread will hurt my mpg a bit
It’s lifted 3 inches too
But 22mpg out of a 1.3 seems excessively low when I’m driving gently

I’ll try disconnecting the MAF and see what happens
MAF and MAP both look pretty normal watching the graph on live data but I don’t know what the exact values should be, it could be over reading the whole way up and I wouldn’t know

Has anyone posted standard ranges for the various sensors? As in this is ‘normal’ range at idle, this is normal at full throttle at x rpm etc
I searched but couldn’t find numbers

235x75 mud will certainly hit the fuel consumption a bit, due to the weight and as you say tread.

Locked front hubs won’t make too much of a difference provided the transfer box is still working correctly and is disconnecting the front drive shaft in 2WD.

A stock Jimny of that age will do around 30mpg (+/- a bit), so the big tyres and locked front hubs will suck a bit. However, neither of these should make it run rich or with a fuel trim off the scale and everything you have checked suggests it is actually running very rich.

I used to have Torque Pro on my 2011 Gen 3 so could have told you the airflow etc, but I sold the car 16 months ago and I can’t remember the figures.

MAF sensors can go faulty and under or over read, plus there is a chance someone replaced it with a cheap copy part that has the wrong resistance for your vehicle.

Normally with a faulty MAF sensor you get poor running, especially at tick over, but if the MAF is ok, just the wrong one, I guess you could get the engine to run fine, just very rich.

Good luck


Robin

Have a look in the wiki, about half way down

www.bigjimny.com/mediawiki/index.php/Eng...air_flow_rate_issues

Your engine doesn’t have a MAF, but works it out from IAT, MAP and Throttle position. The wiki mentions running rich.

As the fuel trims are maxed out it thinks it is lean and is over fuelling.
I'd do those simple checks on the MAP sensor (most likely culprit) & IAT sensor and if there is no difference I'd swap out the first O2 sensor- the one nearest the engine.
These tend to go bad and cause over fuelling without triggering any fault codes.

Mine also used to stink of fuel when stationary. Took me ages to spot a tiny jet of petrol coming out of the side of the fuel return pipe where it had gone rusty, right at the bend where it kicks up under the driver's feet!