[Adam G RRC_92]

My 92 SWB has a 4.6 swap from a later RR. I've adjusted the timing, but I'm still getting average about 9 mpg. I drive SUPER conservatively. This last test after my timing advance fix was about half highway, half in town.

Engine was rebuilt, well maintained, 60K miles on it or so. New cam shaft, and runing with this: Tornado Systems | Tornado 14CUX 4.6 Ltr upgrade chip

Anyone know what a proper checklist would be to chase down? I don't have any codes thrown on the engine, so I haven't though much about O2 sensors or fuel pressure stuff.

I thought I'd see something like 12-14 MPG on average.

Thanks

 

[LanceL]

This is a conundrum, because I have a very similar setup (4.6 swap with a crower cam and the Tornado chip). My gas mileage is nothing to shout about for sure, but then again I don't tend drive conservatively and I pull a trailers occasionally. I recently pulled a small camper trailer (~1600lbs) from sea level up to the mountains (~4,500ft) and averaged about 10-11mpg on that trip, which I thought was acceptable considering the truck was loaded up too. If you're getting 9 around town with no load, that's definitely a bit low.

My timing is set to 10 degrees BTDC per the instructions I got from Tornado (they said 9-12). I would think any O2 problem that would cause that significant of a fuel usage change would thow a code for sure. Perhaps leaky injectors? I feel that would throw a code too.......

 

[RRLondon]

To put some basics on the table for investigative consideration.

Presumably the tail pipe is running black? And can you smell unburnt fuel there while running?

What colour are the spark plugs if you turn it off promptly after a decent journey then examine them?

Do you know what gain voltage the MAF is set to (measure at blue/red wire on multi connector at MAF) ?

Presumably the air intake filter is not compromised/dirty?

 

[JS5D]

Realistically and driving conservatively you should get about 15mpg with 18 mpg on long steady runs possible. These engines are not very efficient even with the best tune. Head and piston design is dated simple as that.

 

[Adam G RRC_92]

Great start guys. Tail pipe runs clean as far as I can tell. Def not black... and really no smoke at all. Maybe a little unburnt fuel, but nothing crazy.

Here's what I'll do:

1: I'll set the timing to about 10 first. That's easy. Maybe even up to 12?
2: I'll check the air filter. It may be dirty.
3: Then I'll measure the MAF voltage.
4: Spark Plug color? I don't know what you mean. Would they be a different color after a long haul?

If I were getting 15 mpg I'd be thrilled.

 

[RRLondon]

Plugs, yes the colour will give a decent indication of combustion mixture and temperature.

Longer run as they'll just be black (rich running) after a cold start. Ideal is a milk chocolate colour when running at normal mixture and temperature range. If it's too rich consistently, then they'll just show too dark all the time and confirm fuel waste.

If you can get a clear picture up of the combustion end of a couple (one from each cylinder bank) it'll give a pointer to what's happening.

 

[LanceL]

This is a decent guide to plug colors, nice tan/light brown is considered optimal.

 

[Adam G RRC_92]

Harness was sending 14V, 5V, and a .02V at idle on the harness. But, when I pulled the harness, the engine dipped but didn't shut off. Is that ok?

I need to measure while plugged in, I think, right? But the harness has a black wrap so I cant get to the back of it. Gotta figure out if I can remove it, or just jumper it.

The exhaust is not black, but I do smell fuel, mostly under the hood while running. Just, a very rich gas smell.

 

[Adam G RRC_92]

Ok, here are my results:

MAF sensor seems good: I jumped between the sensor and the harness so I could get live readings. Power is 13.5V, 1.87V is one that doesn't move on the far end of the harness. The one that moves goes from 1.75V at idle up to about 2.3V when I hit the throttle. All that said, it's a Lucas MAF sensor dated manufacture 1991. I took it apart to inspect the sensor. Kinda hard to see the wires, but it's not dirty or anything. Are those values ok?

Air filter is clean.

The pic is of sparkplug 1. Looks great judging by the guide you posted. Very helpful! So, it doesn't seem to be running rich.

I adjusted the timing to 12 BTDC in line with the Tornado. Doesn't feel much different in a drive around the block.

Last tank of gas took me 195 miles. I filled up at the gas station 23 gallons. Less than 8.5 mpg this tank.

???

 

[RRLondon]

Plug colour does look good. As you, I'd agree it looks like combustion is accurate.

Which plugs do you have fitted, the full identification?

The ignition advance setting of 12 degrees has more to do in relation to torque peak near 3000 rpm and upward to the red line as it's fundamental to the fuelling the chipset is designed to work with. You'd not ordinarily detect much in the way of behaviour at the lower rpm range.

RPi Engineering - V8 Engines link gives a clearer indication of measurement and setting of MAF voltage, also data for Tornado chip too. (The throttle pot in the link is the old type, so ignore that description) but your TPS should be verified as working well to remove it from suspicion.

Presumably you are running catalysts?

Certainly odd that something doesn't stack up, more thought needed about it to help you.

 

[bagospanners]

Just need to watch out for the difference between US and Imperial Gallons here as well guys (3.78 ltrs compared with 4.54 ltrs) it makes a difference when measuring against a full tank of fuel. I got caught like this once before when comparing mpg across the pond!

 

[JS5D]

Just need to watch out for the difference between US and Imperial Gallons here as well guys (3.78 ltrs compared with 4.54 ltrs) it makes a difference when measuring against a full tank of fuel. I got caught like this once before when comparing mpg across the pond!

Yes indeed! my quoted MPG (15) was USGal still twice the OP consumption. Other things to watch for odometer calibration. If any changes to wheels/diffs/transfer case this can misread the distance. Also the mechanics inside the odometer can 'miss' causing lost distance. Worth checking the distance driven on a journey with the Google Map distance.

 

[Adam G RRC_92]

I hadn't done the math on the updated tire size, though it doesn't make a massive difference. I think stock vs the goodyear wranglers I put on show a 5.2% increase in diameter. That would make my take of 195 miles actually be 205. Better, but not great.

@RRLondon I do have a catalytic converter, and the ceramic is rattling a bit now and then on the right side. That's a newer problem after my orig driveshaft/BW mix began hitting the anti-roll bar. Different post/issue... but the cat doesn't seem to be restricting airflow. The mileage was bad beforehand . I have a new Y pipe on order, so... we'll see if that contributes, just in case.

I do note that it smells fuel-ish. If it's not a rich mix, does that eliminate concern?

 

[LanceL]

You said in an earlier post you smelled fuel under the hood, I wonder if you have a leak somewhere in the fuel lines or the rail. Fuel leaks can be hard to find since it evaporates and doesn't leave a puddle, but a steady leak could have you just dripping away your fuel rather than burning it.

 

[RRLondon]

The cats question, I was just confirming as the MAF gain value differs to non cat. But it looks like you are in range as stated for that chip+cat, plus the plug's colour.
The only thing I'd check is a plug from the other bank as that set of injectors is fired separately from the one checked.

Comment on mpg, mine is best of 19.7 mpg, so 16.33 (I think) to a US gallon.
Range (plastic type fuel tank volume) is pretty much 300 miles average, 330 long run and 270 around town "fussing"

Long run is usually fully loaded and running it fairly hard with consistent 80 / 90 mph pace.

 

[LanceL]

Steady 90mph in a RRC??? You're quite brave sir!

 

[Adam G RRC_92]

Fuel Leak : Maybe so. I mean, 9 mpg vs 16 is big. I feel like there is something big here. And, I do smell fuel under the hood... and maybe the exhaust too (though the mix seems good). Would a slight leak in the fuel line/injector cause that much of a drop? I have no visible signs of a leak.

 

[RRLondon]

Fuel Leak : Maybe so. I mean, 9 mpg vs 16 is big. I feel like there is something big here. And, I do smell fuel under the hood... and maybe the exhaust too (though the mix seems good). Would a slight leak in the fuel line/injector cause that much of a drop? I have no visible signs of a leak.

I think you'd notice it more than a cursory smell, even with a small leak. Mine had been affected by corrosion of the fuel line where it comes across from centre of vehicle exiting tank toward rear wheel arch. It was initially only a small hole and didn't make any mark on the ground (more evaporating as it dripped onto chassis) but it reeked of neat fuel, even approaching it to get in you could tell.

Certainly worth a look, but very different unburnt (rich) to neat fuel smell. Check round the injector hoses and the regulator at left rear (looking from front) if you feel it's odd in engine bay to see if any evidence is immediately there.

 

[Adam G RRC_92]

I may also pull all plugs out and check to see if maybe one injector is faulty?

 

[jbaker1121]

Steady 90mph in a RRC??? You're quite brave sir!

My RRC is steadier at 90MPh than my 2017 Wrangler is.... with the same BFG K02 tires!