Fuel Consumption

[KevinM]

I have a 2001 P38 with a 4.6 which up to last Xmas was fairly gentle on fuel as P38's go. At Xmas, one of the spark plugs decided not to work along with the spark plug leads with fuel consumption leaping to over 50lt/100kms. Since it has an O2 sensor replaced but what is happening now is that when it starts up from cold, the fuel consumption shows over 25lt/100km (prior to Xmas, it used to be around the 17>18lt/100km range) when it first started up. It now takes approx. 10km where it slowly dropped to about 13lt/100km on the highway. The further you drive it, the better it gets - down ino the low 12"s. The motor has only 150K on it and was running quite well up to Xmas. It has been checked on the diagosnosed computer and has come with no faults. Can anyone out there tell me why it is now so high when the motor is cold. i have checked the fuel consumption figures against the dash readout and it appears to be fairly correct.For e.g. I travelled a return broken trip of 180kn which was mostly highway driving at 100kmh and a little bit of urban driving at the start of each trip. It started at 25lt/100kM and then dropped to 12.4lt/100km. At the bowswer, it said that the consumption for the trip came in at 16.16lt/100km.
Can anyone tell me why it drinks do much furl when it starts up. Is there somethig that is not quite right???
Any help is greatly appreciated.
Regards
Kev

[33chinacars]

Have ALL spark plugs & leads bean replaced.

[Keithy P38]

Manufacturers run a slightly rich fuel map when the engine is cold to allow the engine to run smooth until it has warmed up. It's good to take into account that while idling (I.e. At traffic lights or in your driveway) it also adds that fuel usage to your average, so if there's a lot of stop start driving involved it'll put the numbers off a bit. I wouldn't take the dash readout as gospel - a lot of things come into play when you consider that, tyre size, etc.

It's anyone's guess!

[benji]

As Kiethy said, ideling around plays funny buggers with it, as it's using an infinite amount of fuel per km.

I'd be going on the long term average to diagnose something.

[PhilipA]

The further you drive it, the better it gets - down ino the low 12"s. The
motor has only 150K on it and was running quite well up to Xmas

I wonder if your thermostat is remaining partly open causing the warm up to be slower than it was when operating correctly. I suggest replacing it as a first step. Injections add fuel until the engine temp reaches milestones such as 82C to cut enrichment. It could also be the temperature sensor giving false info.
Regards Philip A

[Hoges]

x2 to all the above. The engine will not run efficiently until it reaches proper operating temperature, which means generally about 25-30km. Stop start driving straight after start-up results in heavy fuel consumption. (Wife's ASTRA AH 1.8 averages 11-13L/100km on her 5km journey to work, but less than 6.5l/100km on a dash to Sydney!) Same principle applies to your P38.

Couple of things: the engine ignition timing, fuel injection timing and duration (=volume), when you first start the engine are controlled by the EMS computer according to a set of fixed criteria which has no input from O2 sensors or MAF. This is called "open loop" and as mentioned by others, is a somewhat richer fuel map. (Open loop is also employed under conditions of heavy acceleration). When the engine reaches proper operating temperature, the engine management system goes "closed loop"... the clever software constantly alters the timing and EFI characteristics based on inputs from the O2 sensors, outside temp sensor, MAF, fuel temp sensor, engine load, throttle opening position sensor, and a host of other stuff.

The engine computer then applies the optimum settings for all these conditions by comparing the data being received with values in the database in its memory. It has been programmed to "learn" and adapt.

Because you changed the O2 sensor, the computer will need to adjust to the characteristics of the new sensor. That sensor might be somewhat different to the older remaining O2 sensor on the other side.

Thus, you probably should "Reset" the adaptive values in the EMS ecu with a Faultmate/testbook/other diagnostic device. Essentially this means re-baselining the ecu, enabling it to "re-learn".

The numbers you are seeing now on the instrument panel display are not unusual.

Sorry for the long post...but if you don't' understand what you're looking for then ... (besides, writing this enables me to procrastinate on other matters ...:angel::wasntme:)

[TheTree]

Hi

I have read that if you replace the fuel pump, MAF or O2 sensors you must reset the adaptive values

You can do this by disconnecting the battery terminals and shorting them together for about 30 secs, or use a nanocom, faultmate, hawkeye etc to do it.

Steve

[FANTOM P38]

I recently added O2 sensors to my 95 4.6 and during the process discovered a Faulty Fuel Temp Sensor ( was indicating a temp of -51C) which was probably partly to blame for my dodgy fuel consumption figures! changed it and immediately had reading of +41C!
So worth checking out!

[Keithy P38]

Hey Fantom, while you're here, I have forgotten your name (actual name)! It's for a very important thread I'll be posting in the next week or so *sorry about the hijack*

[FANTOM P38]

Keithy I guess you're referring to Lift thread. I will send info soon.
Martin

[TheTree]

I recently added O2 sensors to my 95 4.6 and during the process discovered a Faulty Fuel Temp Sensor ( was indicating a temp of -51C) which was probably partly to blame for my dodgy fuel consumption figures! changed it and immediately had reading of +41C!
So worth checking out!

Hi

Another very useful piece of info for those of us with thirsty beasts on our hands!

Part number is ETC6661 and they are not cheap :eek:

Steve

[DANMAL]

Hi

I have read that if you replace the fuel pump, MAF or O2 sensors you must reset the adaptive values

You can do this by disconnecting the battery terminals and shorting them together for about 30 secs, or use a nanocom, faultmate, hawkeye etc to do it.

Steve

so you take the battery terminals off and join them together??? how does that work?

[davidsonsm]

I read somewhere danmal, that it clears any static build up. Not sure how it resets things.

[TheTree]

HI

I assume it clears the flash memory area where the adpative values are stored

Steve

[mtb_gary]

I recently added O2 sensors to my 95 4.6 and during the process discovered a Faulty Fuel Temp Sensor ( was indicating a temp of -51C) which was probably partly to blame for my dodgy fuel consumption figures! changed it and immediately had reading of +41C!
So worth checking out!

Did you use OEM or the aftermarket as a replacement? Expensive item as OEM!

Gary

[davidsonsm]

Did you use OEM or the aftermarket as a replacement? Expensive item as OEM!

Gary

I used part number: AMR6244.M8 from LRDirect - GBP112 delivered. Aftermarket, but they include the grey plug and work a treat.

[benji]

I'm wondering at what temp gems starts to lean off the enrichment for cold running. Mine takes 3.2 k to get up to temp (87c) - yes I get a bit bored sometimes- but if it's really cold or takes about 5.

[KevinM]

I started this thread a while ago and apologize for not coming back to you during your comments but I have been checking the things that you have said. The fuel consumption on start up has not improved, actually has got worse and now is 31l/100k. But as before the further you drive it, the better it gets, down to 12.5l/100k after about 15>20km. It has plenty of power and run quite smooth from cold to normal operating temperature. The temp gauge shows normal temp within 5km of start up but it still takes another 10km for it to drop into the 12>13l/100k. I have checked the mileage against when I fill it up and shows an average of 17.1l/100k which reflects what the dash display is showing. I can undertsand that the mixure is richened up when cold but more than double is a very rich mixture-far too rich. Besides it has got worse since i have the problems at Xmas. before Xmas on start up, it was 17>19l/100k. What do your P38's say on start up?There appears to be something wrong on the start up cycle and I welcome your thoughts. It runs well when it is warmed up.

[KevinM]

I post the reply above and just wondering if anyone has any comments.

Regards
Kev

[Hoges]

I started this thread a while ago and apologize for not coming back to you during your comments but I have been checking the things that you have said. The fuel consumption on start up has not improved, actually has got worse and now is 31l/100k. But as before the further you drive it, the better it gets, down to 12.5l/100k after about 15>20km. It has plenty of power and run quite smooth from cold to normal operating temperature. The temp gauge shows normal temp within 5km of start up but it still takes another 10km for it to drop into the 12>13l/100k. I have checked the mileage against when I fill it up and shows an average of 17.1l/100k which reflects what the dash display is showing. I can undertsand that the mixure is richened up when cold but more than double is a very rich mixture-far too rich. Besides it has got worse since i have the problems at Xmas. before Xmas on start up, it was 17>19l/100k. What do your P38's say on start up?There appears to be something wrong on the start up cycle and I welcome your thoughts. It runs well when it is warmed up.

"Apparent" high fuel consumption at start-up is normal. If it is indeed settling down to 12.5L/100 km after 15-20km then you have nothing to worry about. The fuel trip meter on a 2001 Holden Calais 5.7L is far more precise than that supplied with a P38 and indeed can be calibrated. I recall seeing 60L/100km for the first few Km in heavy traffic after starting on a coolish morning here in Brisbane and 99L/100km on a cold Canberra morning (-8 deg C) (wouldn't go any higher:wasntme:). How the P38 calculates its average fuel consumption is unclear from reading RAVE. I frankly pay very little attention to the message centre number.
IF you are still unsure, invest in a Bluetooth OBDII dongle and get a copy of Torque PRO on your android phone (if you have one) and study the short/medium and long term fuel trims for each cylinder bank and watch how they behave as the coolant temperature increases. that's far more informative and accurate measure of how your EFI is working

[Keithy P38]

There is a possibility I'll be taking the Rangie for a town run tomorrow, I'll reset the trip computer the second I start the engine and let you know the initial average (usually higher as the fuel mixture is rich), and what the average dropped to by the time I reached my destination.

Mine is a '99 Bosch 4.6

Cheers
Keithy

[redandy3575]

I started this thread a while ago and apologize for not coming back to you during your comments but I have been checking the things that you have said. The fuel consumption on start up has not improved, actually has got worse and now is 31l/100k. But as before the further you drive it, the better it gets, down to 12.5l/100k after about 15>20km. It has plenty of power and run quite smooth from cold to normal operating temperature. The temp gauge shows normal temp within 5km of start up but it still takes another 10km for it to drop into the 12>13l/100k. I have checked the mileage against when I fill it up and shows an average of 17.1l/100k which reflects what the dash display is showing. I can undertsand that the mixure is richened up when cold but more than double is a very rich mixture-far too rich. Besides it has got worse since i have the problems at Xmas. before Xmas on start up, it was 17>19l/100k. What do your P38's say on start up?There appears to be something wrong on the start up cycle and I welcome your thoughts. It runs well when it is warmed up.

Another problem can be the Catalytic converter. If the cats brocken or crumbled up inside, it can block the exhaust causing your engine to work harder hence the extra fuel issue. Its a hard thing to diagnose, but if you recall bottoming out somewhere and the issue starting shortly after, Bingo!!

Just a thought thats all.

[Keithy P38]

Ok so I started the car, reset the fuel calculator, drove out of my driveway and after 1km it told me I was doing 22.5L/100km.

After 10km (my destination) in town, three set of traffic lights, my consumption was at 18.7L/100km. Had I travelled further it would have dropped, it was dropping quite quickly after about 5km.

Cheers
Keithy

[KevinM]

Hi Keithy

Thanks for the feed back on your fuel consumption on start up. That is what mine used to be like or lower but for some reason since Xmas when i had the problem with the spark plug leads and spark plug, it is now runs at 30+l/100k on start up for the first couple of Km's and then slowly comes down and takes about 15 km before it get to 12>13l/100k.
I just thought that there maybe something else wrong on the start up cycle that is still causing a problem. It would be nice to bring it back to what it used to be.
I do think it has any problem with Cat convertors or likes because it runs well from cold to hot and has plenty of power and returns 12.5l/100k when warmed up.
It appears to be in the start up cycle somewhere that there may be a problem.
Regards
Kev

[KevinM]

Hi Keithy

Thanks for the feed back on your fuel consumption on start up. That is what mine used to be like or lower but for some reason since Xmas when i had the problem with the spark plug leads and spark plug, it is now runs at 30+l/100k on start up for the first couple of Km's and then slowly comes down and takes about 15 km before it get to 12>13l/100k.
I just thought that there maybe something else wrong on the start up cycle that is still causing a problem. It would be nice to bring it back to what it used to be.
I do think it has any problem with Cat convertors or likes because it runs well from cold to hot and has plenty of power and returns 12.5l/100k when warmed up.
It appears to be in the start up cycle somewhere that there may be a problem.
Regards
Kev

[intheozone]

Hi all

I have been away. (Broke/unemployed and unable to play with my p38) but I have a job now (of sorts)

I have experienced a similar issue... I changed a HT lead as I had a misfire. On warm up I get about 25l-100km but even after it is warm I struggle to get below 15l-100km and that is driving at 100kmh for 40kms. How do I see a number less than 13???

[Keithy P38]

Yours is a Bosch 4.6 hey Steve? All I've done to mine is the usual rev-head stuff - quality NGK plugs, quality leads, good air filter, good oil (and the correct viscosity I might add - 5w40 synthetic). If that doesn't net you a fuel consumption improvement, I'd be looking to the exhaust system. A nice free-flowing 2.5" system will get it breathing better.

I also make a habit of running my tyre pressures at 37/40 front to rear. My tyres are 255/70, so one size up from standard. If yours are taller that will throw the fuel calculator off a bit.

Last year on my cape trip I managed 13.6L/100km fully loaded with a roof load, over 400-500km at highway speeds. Both the dash reading and fuel bowser vs GPS were within 0.2L/100km of each other.

Cheers
Keithy

[mtb_gary]

Hi all

I have been away. (Broke/unemployed and unable to play with my p38) but I have a job now (of sorts)

I have experienced a similar issue... I changed a HT lead as I had a misfire. On warm up I get about 25l-100km but even after it is warm I struggle to get below 15l-100km and that is driving at 100kmh for 40kms. How do I see a number less than 13???

Quote in Miles per Gallon :D

Gary

[KevinM]

Hi Keithy

I have the good spark plugs, plug leads, synthetic oil, not quite sure about your good air filter - are you running a K & N ?, the exhaust is standard and still the original one at 155K - I would be interested in what you are running - I am getting old so I do like the nice quiet experience that the P38 gives.
I do run a slightly bigger tyres of 265x60x18 Hercules A/T's but do run a little bit lower pressures of 33/36 front to rear.
I can understand your mileage up to the Cape because it appears that the longer and further you drive the P38 the better the fuel consumption gets. I drove back from Sydney last year and the computer read outs were in the 11's and bottomed out at 10.2L/100k. The fill up at the bowser reflected these figures as being correct. On a long road trip, my fuel comsumption is not much more than my brother's Defender.
It is just so horrible that the fuel consumption during start up is so bad. If you do short trips, do not take the P38 but on a long run they are so good for a 4.6lt V8. What is wrong with the computer starts on the P38's????

[Keithy P38]

I had a K&N filter on my old P38, decided against it after seeing the filtration results came back pretty poor. I'm using Coopers air filters on mine. Exhaust is standard to the centre muffler, from there it's a high flow centre muffler (barrel style) with 2.5" pipe to the rear where I've got a hot dog and 2.5" tip. Nothing special, but enough to open it up a bit. It's quiet in my opinion (loud enough to let you know you have a V8 under the hood, but not intrusive) I would like more note but I'm happy with it so won't change until this system needs attention.

As you say, they are great on bigger drives, not really suited to town runs.

Cheers
Keithy