Fuel consumption 350i

[LenChim]

OK I know the 350i is not the best on MPG but 17 to the gallon seems to be pathetic especially when I am trying to be good. I have done all the usual ignition timing, plugs, air filter. What else can I do that could make some improvement? Yeh I know a lighter right foot but I did say I was being good.
Any help would be appreciated.

[mrzigazaga]

Hi mate...Do you have a solid state FPR or an adjustable?...I get around 20 SPG..A few more on a run..Oh and by the way its not MPG..Its SPG...Smiles Per Gallon.....

[colin mee]

I get about 20 mpg.but if I feel the need its single figures

[LenChim]

mrzigazaga said:

Hi mate...Do you have a solid state FPR or an adjustable?...I get around 20 SPG..A few more on a run..Oh and by the way its not MPG..Its SPG...Smiles Per Gallon.....

Not sure will have to have a look. You are right about SMG though.

[LenChim]

Well have looked at the Fuel pressure regulator, it looks quite new (basically it is shiny). It is a Bosch 690 and has a what looks like a part number 0280 160 200, made in Germany. Does that mean anything?

[GOG440]

LenChim said:

Well have looked at the Fuel pressure regulator, it looks quite new (basically it is shiny). It is a Bosch 690 and has a what looks like a part number 0280 160 200, made in Germany. Does that mean anything?

When my fuel consumption was really really poor (as opposed to the normal really poor) It turned out that my car was effectively running on full choke constantly because someone had removed the thermostat to cover up a hg failure.
It was my first v8 so I was expecting it to be bad but I was getting similar mpg to you, now I can get mid 20's on a run

[mrzigazaga]

Sounds like the right one...I take it that it looks like this...

Listing is for a 3500 SD1 so pressure should be around 36-38psi.

Over-fuelling can be attributed to a number of issues...How does it start from cold...If it starts almost instantly then believe it or not it shouldn't...Could be a failing coolant temperature sensor..The engine should crank a few times to allow the oil to get to the top of the engine.

It might be nothing and just be the inaccurate fuel gauge ..

[LenChim]

GOG440 said:

When my fuel consumption was really really poor (as opposed to the normal really poor) It turned out that my car was effectively running on full choke constantly because someone had removed the thermostat to cover up a hg failure.
It was my first v8 so I was expecting it to be bad but I was getting similar mpg to you, now I can get mid 20's on a run

I notice that the engine temperature never gets above 80C and I remember reading some were that it is best to change to a 90C stat, do you think that would help? I also note in the Steve Heath book it says put a lower one in for larger engines but this one is only a 3.5.

[LenChim]

mrzigazaga said:

Sounds like the right one...I take it that it looks like this...

Listing is for a 3500 SD1 so pressure should be around 36-38psi.

Over-fuelling can be attributed to a number of issues...How does it start from cold...If it starts almost instantly then believe it or not it shouldn't...Could be a failing coolant temperature sensor..The engine should crank a few times to allow the oil to get to the top of the engine.

It might be nothing and just be the inaccurate fuel gauge ..

Coolant temperature sensor. How do I test that? It starts from cold after a couple of rotations, very easily really. The fuel gauge is a bit of a liar, full to empty 120 miles, but I filled it up did 100 odd miles and refilled it and that's what it came out to. 17mpg.

[LenChim]

And yes that is what the FPR looks like.

[celcius]

Best just fill it up and drive it , smile a lot ,when it gets low fill it up again ;-)

[400SE Dave]

Never hit 20's in my 400se ever, in fact had it in single figures more than the teens

[mrzigazaga]

400SE Dave said:

Never hit 20's in my 400se ever, in fact had it in single figures more than the teens

Ole lead boots...

TBH i haven't really worked mine out properly...All i know is to fill it up properly...Which takes forever you have to use the "Wedge bonk"...Which looks odd from other peoples point of view but if you don't then you will NEVER be able to fill up the twin tanks...And even if you do manage to then it will pish out of the overflow...Like mine did in an ancient woodland in Belgium, Nearly setting the whole place on fire...

IF...And its a big one you manage to correctly fill up both tanks then you should ideally be getting..60 litres in total in the tanks..So around 200 miles but if you drive it a bit hard knock off around 5mpg...So between 10-12mpg...Jags are about 6....Maybe 8....Then the inaccurate fuel gauge...On my 280i i thought i was going to break down on a choco-block motorway on a very hot summers day ...The gauge was saying under a 1/4 and there was around 5 miles to the next petrol garage...I was sweating on top of sweating...When i finally got to the petrol station and stopped at the pump the gauge went up to half...

If it was excessively high whilst driving Miss Daisy then i would suspect a cam failing....But as i said it more about SPG than MPG....

What i learnt is that the tanks never get filled up properly and they don't empty fully...Although i wouldn't run it too low too often...Its best to keep it topped up to just over the half and enjoy it and don't think about fuel unless you smell it.....Especially if its unburnt....

Edited by mrzigazaga on Monday 15th February 20:08

[ukdj]

LenChim said:

Coolant temperature sensor. How do I test that?

Measure the resistance of the temp sender it should return values like this :-

0C(32°F) 5700-5900 Ohms
20C(68°F) 2400-2600 Ohms
40C(104°F) 1100-1300 Ohms
60C(140°F) 500-700 Ohms
80C(176°F) 300-400 Ohms
100C(212°F) 150-200 Ohms

[LenChim]

ukdj said:

Measure the resistance of the temp sender it should return values like this :-

0C(32°F) 5700-5900 Ohms
20C(68°F) 2400-2600 Ohms
40C(104°F) 1100-1300 Ohms
60C(140°F) 500-700 Ohms
80C(176°F) 300-400 Ohms
100C(212°F) 150-200 Ohms

Thanks for the information, I will take it out and give it a test.
Len

[The Hatter]

I'd say 17mpg on a standard 350i indicates a problem. I generally get 22-25mpg, but it will drop with cold starts and short runs.

The best I've ever seen was 30mpg on a motorway run immediately after new head gaskets, a decoke and a new cam; cruising at about 65mph - I was taking it easy to bed the new parts in.

Personally I'd check the cam (they can wear quickly and the engine still feels OK when they wear); and then move on to the fuel and ignition systems.

You said you checked the timing; have you checked the vacuum/mechanical advance? I found the static/idle timing to be pretty irrelevant in trying to get the engine to perform properly at higher rpms; you can play with the timing until the engine runs best. I also fitted an AFR meter on mine, it enables you to tune the engine much more effectively and pinpoint problems.

[TVRleigh_BBWR]

have to agree 17 taking it easy sounds low.
on my 400SE if I take it easy I can get mid 20's, but on a track single figures.
maybe worth getting a hours on a rolling road to check fueling across rev range, and BHP will let you know a lot about health and fueling.

[adam quantrill]

I used to get 22-24mpg on my 350i in the '90's cruising at 85mph in 5th between cambridge and ipswich, and with the top down (makes a difference above 60).

17 is low for motorway/dual carriageway but average for A/B-road with a few overtakes thrown in, and quite good for town driving.

Other things to investigate - pull a few spark plugs and see what the colour is, that's a good indicator of average mixture.

If the FPR is playing up sometimes the culprit is the fuel recycle tube. If it gets blocked then excess pressure can build up in the rail and it'll run too rich. Easily checked by disconnecting the pipe from the FPR and blowing down it.

[SLB]

I once nursed a 420seac with what turned out to be a broken rocker pad from Bracknell to Blackpool for a Backhome weekend. Two up, loaded with bags and a traffic jam at the RAC building. Got 29.6 mpg but that was as gentle as I could be all the way and nobody would ever normally be as good as that. I'd say normally 16 or 17, maybe 20 on a reasonable journey.

At the same time I had access to father-in-law's 350i which is now mine. That was always way better and although I don't measure it I'd guess low to mid 20's, maybe backing the vacuum valve off a bit to reduce the pops and bangs might help but that wouldn't be any fun!

[LenChim]

Well I have looked at a few of the things that have been suggested:-
I have changed the thermostat from 80C to 88C.
Checked the "Coolant Temperature Sensor" resistance and it is in line with the specification.
The "Fuel Recycle Tube" is clear. E.g. I can blow down it.
Question, what is a AFR Meter?
I have looked at the spark plugs they are NGK R BPR5E. Now looking at them the center is white the are loos grey but the outside of the plug is sooty black. May be suggesting to much fuel? What do you think?

Now why would some of the plugs have the insulator white on one side and sooty on the other?
And last can I measure the cam condition by by measuring the rise and fall of the valves?
Posted 3 pic's but only one seem to come out?