As the title says, my gas mileage stinks, and it has ever since I purchased the car last August. My mixed driving mileage has consistently been in the 13s, and by mixed I mean 50/50 or even 60/40 with an emphasis on highway miles.

When i bought the car it had approximately 80k miles and was in stock form. My first trip in it was about 500 miles at about 70 mph and I got in the 18s. OK, I thought, it's a high performance car and mileage is going to stink. No big, because it's so darn FUN!

Over the ensuing months I made modifications to the intake, replaced the upper pulley, and installed nameless 200 cell cats. My mixed mileage continued in the 13s. Two months ago I blew a head gasket and replaced the engine with a 2002 unit with 33k miles from a XKR. After installation, my mileage continues to be in the 13s.

After checking with other Forum members who drive XJRs, I found that they were getting 2-3 mpg better in mixed driving (and, no, they don't drive any more sedately than I do ). That got me wondering why my mileage is so poor.

Current engine modifications: 1) 92mm Pro-M maf with cone filter, 4" into 3.5" intake, catch can for full load breather, cold air divider (Waterdragon's old set-up), 2) 3# steel upper pulley, 3) 200 cell nameless catalytic converters.

According to my OBD2 reader, I have NO codes showing and NO pending codes. I checked my long term fuel trims yesterday morning and both banks show +~8-9%. Yesterday afternoon I carefully went through the intake to verify that I had no air leaks. I then did a hard reset for the computer to clear the long term trims. I am driving the car in a reasonable manner for the next 100 miles to see where the long term trims go.

*The O2 sensors are the same ones that were on the car when I purchased it in August 2013. The engine and it's sensors were changed when I replaced the engine, but NOT the O2 sensors.*

I am requesting advice from knowledgeable Forum members as to what I need to be looking for the next time I hook my OBD2 reader up, ie., what specific readings do I need to focus on and at what rpms, etc.

Thank you in advance!

 

Given you've got the same results with two different engines, there are only three things that have remained constant, your intake, your ECU, and your exhaust.
The intake shouldn't have any adverse effects, you've done is remove a restriction, but the AFM could be sending an incorrect flow rate to the ECU?
The ECU will only do what it's told... you could swap it for another one, but it's higly unlikely there's anything wrong with it.
Then finally, the exhaust. Is it possible the cats aren't as effective as the old ones, and giving unusual readings between the pre and post O2 sensors, causing the ECU to dump in more fuel?

The only time I found my car running too rich was when my post-cat 02 sensor died. I ended up with two large black soot marks on the back wall of the garage, but I think it also caused the car to stall a couple of times, which was the trigger to get the local garage to check it over. If I had had a code reader back then I'm sure it would've shown up there though, and as you've checked the codes, I would expect your sensors to be ok.

 

There is always a possibility of dragging brakes or bearing problems, but to affect mileage this much, something should be getting real hot! AC system demanding too much load...?

If your oxygen sensors are reading, but are not giving correct readings, your STFT will be correct, but the actual lambda might be way off. Obviously, the modified MAF would be suspicious, but the trims seem to discount that UNLESS you are at WOT a lot!

I assume you are sure you are getting the right shifts in your transmission.

The other thing that comes to mind... with all those hot rod mods, did you lower the thermostat temperature? That heat you are getting rid of... remember where it comes from.

Have you monitored whether you are getting into closed loop engine control?

The fuel trims will be right although the engine is not performing if your egr valve is leaking. Did it get changed in the swap?

BTW, I got about 16.5 mixed in my MY 98 XJR, but only 14.5 n my MY 99. No explanation!

This is an interesting issue. I hope you keep us posted with your results.

 

Grandell and Ross - thank you, both!

The mileage was just about the same before I began making any modifications to the engine. The thermostat is a standard temperature unit - my coolant temps yesterday morning were 190-195F. There have been 2 different EGRs: the one on the original engine and the one that was with the 'new' engine. I suspect the upper O2 sensor(s) - maybe not faulty enough to throw a code, but also not giving entirely accurate readings.

What should I focus on when I next hook up the OBD2?

 

The lambda precat lambdas in an AJ27 engine are wide band, linear units, so be careful with advice on how to test sensors to be sure it is specific to widebands. I, myself have very limited experience with them. My AJ26 engines both use the narrowband units

You mentioned your trims, and noted that the LTFT was +/- 8 or 9. That seems like a little high variation for LTFT in my experience, depending on the period of variation. You did not mention the STFT, so I assume they are cycling around zero, plus and minus. Just remember, a total STFT and LTFT of 7 % each in the same direction is a 14% fuel trim, and 2 mpg out of 14 mpg can be explained that way. If you can, run some charts of LTFT and STFT for different load conditions, ( while driving) and then do a scatter chart relating trims to engine load and rpm. You might be surprised that something funny happens above idle.

 

Update:

Since the hard reset:

Miles - 55
Gas used - 4 gallons
Avg mpg - 13.6
Avg mph - 21

LTFT1 - 8.6%
LTFT2 - 8.6%

PENDING code - P0447 Vent Control Circuit Open

The pending code may be an indicator of my problem. I will investigate!

 

Okay, I looked up P0447.

The ECM canister close valve (CCV) drive inactive - valve open.

Possible causes:

CCV B+ power supply circuit fault
CCV to ECM drive circuit open circuit, high resistance or short circuit to B+ voltage
CCV failure

I don't understand (yet) how the whole EVAP system works, but perhaps this is the source of my problems. I recall that when I bought the car and I went to fill up, it burps up the filler neck and trips the auto-off on the pump. I disconnected the valve attached to the charcoal canister so that I could fill up. I replaced the valve on the canister a week later.

All right, time for more research!

 

Something I just remembered:

After I replaced the CCV I still had a gas smell. I rooted around in the trunk and discovered a line (I assume it was a vent hose) that was disconnected from the fuel tank. I reconnected it, which eliminated the fuel smell, but maybe it was disconnected for a reason.

I have to think that my issue is somehow related to the whole EVAP system.

 

I am having an issue with gas mileage...cleaning my MAF sensor helped...but the mileage is still off...

 

LTFT and STFT ???

 

Sorry, been distracted by other things and heading off on family vacation today. I'll look back into it when I get back.

 

Wild *** guess: clean the EGR

 

Sorry for the delay.

While on vacation I disconnected the battery for 8 days. I have driven about 120 miles since reconnecting it.

• My STFT's are running 1-2% +/-
• My LTFT's are running near +12% for both banks
• The car does shift into 5th gear
• My fuel pressure at idle with the vacuum hose attached to the FPR is 38psi, which is correct I believe
• The car is running in closed loop
• I sprayed around with carb cleaner while the car was idling to see if I could detect any air leaks - none noted
• 13.8 mpg mixed
• Although modded, I really get into the throttle maybe every other day for a quick blast or two. Otherwise, I tend to drive rather leisurely

Any suggestions on where to look next? Thank you!

 

The odds are that you either have a vacum leak, a bad MAF, of clogged fuel injectors. Charting the STFT + LTFT vs engine load may help define what is going on. A vacuum leak at idle will wind up the fuel trim map, then when you are under load, the mixture tracks the same ratio of extra fuel and you burn rich.

 

A big part of the answer is highlighted above. My wife's car spends most of it's time in commuter traffic and has a higher avg.
If you are constantly accelerating a two ton car from a stop your mileage will suck, period. A lead foot will obviously compound the matter.
Driving style, proper tune, tire inflation in that order will help but you can't argue with physics.


Fuel trims are at least single digits but higher than you'd like to see.

 

OK, using my OBDII reader I recorded about 80 snap-shots of what was going on with the engine load and fuel trims during my drive to work this morning. I entered them into a spreadsheet and have attached it below.

 

I sorted the data by load and plotted the total trim versus load. Unfortunately, I think there might be some bad data. Notice that there are many samples with exactly the same data at load = 30.2 and total trim = 30.4 However, there are also many samples with load between 30.6 and 35 with much lower total trims. Maybe a sampling anomaly?
Anyway, with the data available, I would say it is still inconclusive what might be causing the problem, but you have a serious trim problem.

I would like to see more data, collected with time stamps so transitional data can be excluded, and I would like to see the results with the stock MAF. Also, to exclude crankcase leaks, you could plug the air plenum ports on the part and full load breathers, venting to atmosphere, and capture data. Also, a smoke machine might find a leak you cannot see. In my case, it was the front main oiul seal, believe it or not!

Good luck.

 

Last time my fuel trim and milage went wacko I found a leaking brake booster vacuum line collet o-ring at throttlebody followed by a dirty air flow meter. after repairing leak and cleaning MAF i have in creased 2-3 mpg

 

Ross and Randy - Thank you! for your suggestions. I will start walking through the engine bay today looking for vacuum leaks.

I also updated the spreadsheet to include additional data and comments that I believe will provide a better picture of what was happening when the data was recorded. The additional data and information includes RPMs, MPH, MAP psi, and what was happening on the road, i.e., sitting at a stop light, etc. The first thing that jumps out at me is that there is a huge amount of extra fuel being added at idle!

Thanks again for any and all insights provided!

 

Just to point out- there is a high ratio of fuel at idle, but because the air flow is low, it is not as much fuel as the "extra" being added at higher loads. Where did yoiu get MAP? Is there really a MAP sensor?