Facelift AJ16 Reporting Low Fuel Economy

(Veekay) #1

I have a 1996 XJS, AJ16 engine, about 113k miles.

I do a mix of city and highway driving. I seem to be averaging 15mpg. While I don’t drive conservatively, I’m also not hammering down the gas pedal at each green light either.

My LTFT is zero. Both banks.

The only unusual reading I’ve picked up is that my coolant never goes about 180. With the heat on its at 176F and without heat, it’s at 179.6F.

Would a stuck open thermostat cause that kind of fuel economy?

(BobboSD) #2

Yes I think so. My 1996 XJ6 had a thermostat stuck wide open and exhibited terrible gas mileage until I replaced it.


(Robin O'Connor) #3

Agreed that would cause bad fuel economy, should be above the 20mpg range, thats what I was getting from my AJ6 in the ‘40 and I didn’t pussy foot around.

(Veekay) #4

Excellent. I have a thermostat set at 88C on it’s way! That’s going to be about a 10 degree difference, and more importantly warm up the engine faster I imagine.

(Veekay) #5

Replaced the thermostat with the OEM Waxstat thermostat. The sender is now pointing the needle dead center of the N, but the OBD2 reader is still showing 176-179F.

I wonder if the temp sensor is bad…

(Robin O'Connor) #6

Could be try checking the values for the sensor, there’s a list somewhere on here that gives the ohm readings for hot and cold.
You know its the one with 2 terminals on it located on the thermo housing?

(Veekay) #7

Yes. LHE1600AA

It’s a $20 part, so I’ve put one on order.

(Greg) #8

How are you measuring gas mileage? May be good to do an actual fill tank and count the miles, instead of relying on trip computer.

(Ed Fisher) #9

I’m reading this with great interest as my car, 95 AJ16, never has gotten over 19mpg, as measured by fill and calculate, no matter how easily driven. Most often it is between 17 and 18. The car runs perfectly, I can watch the temp gauge go from cold to mid range just as it should, after a brief period allowing the coolant to get to temp. I had always thought this fuel economy about normal and due to tire size and relative weight of the car. I will be interested to see if this original poster increases the mileage appreciably and by what means.

Ed Fisher
Dallas, Texas

(Veekay) #10

Well, I’ll certainly keep you posted. As I’ve mentioned before, my long term trims are at zero. I’ve professionally cleaned injectors, swapped out MAFs, replaced EGRs, thermostat, exhaust manifolds, oxygen sensors, fuel pressure regulator, etc.

While I have replaced the coolant temp sender, I have never replaced the coolant temp sensor.

Not really all that much else to chase down at this point…

(j limongelli) #11

VEEKAY, That aj16 motor is bullet proof.
The thermostat will help along with fresh ORANGE antifreeze.
Those came originally with a blue which ATE through the heater cores…
Green will work but the orange was the best with all the sensors.
You covered all the bases and didn’t cheapen out.
Always clean down the maf butterfly with solvent on a rag so it closes and doesn’t stick as well.
10W/30 DURING THE WINTER AND 20/W50 in the summer will also help with that mileage.
I was there when they were building them, go that route and you will get to 230,000 miles easy!

(Veekay) #12

Thanks for the help. So after replacing the thermostat, I thought I needed the yellow antifreeze. I bought a jug of Zerex G05, which even states on the bottle that it was good for Ford and Jaguar from 1996 on. I thought it was a match!

So before I added coolant, I compared what I lost…it was green-ish. I know what green coolant looks like, and although it was definitely green compared to the Zerex G05 I purchased, I suspected it is not green coolant.

I purchased a jug of Prestone Universal Coolant. Wouldn’t you know that it seems to match what’s in there.

Why don’t I know what’s in there? After having the car stolen and losing the radiator, I filled it up with what I had on hand (wolfs head universal) just to get it over to a Jag shop. They ended up flushing it and replacing it…I guess I should have asked them what they used?

Lastly, TSB 26-17, dates 4/98 revised the coolant to yellow type D542, which superseded the old blue/green D985. I don’t believe I ever saw a change to orange.

(j limongelli) #13

Its another universal…that’s all.
Anything but the water blue…anything

(Veekay) #14

It was the temp sensor. The car is now showing 192F when driving…I may have replaced a perfectly good thermostat!

I will come back and share mileage after about a week.

(Dzia) #15

Ok, you guys are throwing me for a loop. I’m gathering all the bits and pieces to refresh my coolant system on a 94 4.0. I have everything, radiator, hoses, thermostat and heat control valve.

What my loop is, I’m color blind. I can barely distinguish any colors. Could any of you guys recommend by brand and or number what I should be using?


(Veekay) #16

I don’t know if the AJ6 was also converted to yellow, but if it has, just buy Zerex G-05. Not sure how much youll need, but I think it’s a lot.

If you’re not sure, use the universal and this way you’ll never have to remember what you used!

(Robin O'Connor) #17

I only used green in my XJ40’s with the AJ6 engine. Just ask your FLAPS for green?