Fuel Consumption

MBlake · 06-19-2001, 01:20 AM

Okay, I have been pushing the car hard, and cruising at high speeds, but on my recent journey, I did not get good fuel consumption. I know the reason why I am getting poor consumption, but was wondering what sort of range I can expect from a stock 2.4S if I rectify the problem.

I think that I got about 18mpg (UK) over the 700 mile journey (UK Gallon = 4.58 litres, US Gallon = 3.78 litres)

My main problem (so I am told) is the fuel metering unit has worn. I took the car to a specialist for set-up earlier in the year, and they were unable to get a good setting for both idle and mid-range. The compromise they could give me was that is too rich in the midrange. The only solution they could offer was to get the metering unit re-built. They said that it was not that common, but in this case, was the way forward. I have read the posts on the board and the tech articles which state that all other things should be checked first. I have spoken with the garage, and they confirm that that they tried everything on the list. Therefore, I think I am going to attempt to remove the unit myself, and send it away. Once returned and re-assembled, I hope to be able to drive it to the garage where they can set it up with gas metering equipment. Is this realistic for a novice to try?

Do you think this could also help with a cold start problem, where the car will not idle / run properly until warm. If you try to rev it slightly before warm, the engine has a tendancy to slow, or stall. (The cold start enrichment valve works, the heat exchanger is okay, and the inter-connecting pipe is new.

I really appreciate any help you guys can give.

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Marcus

'73 S Targa

MBlake · 06-19-2001, 02:39 PM

I know you guys are bored with MFI posts, but I would appreciate your thoughts. If this is old hat, I am sorry - pls point me towards a link!

Should a novice mechanic attempt to remove the fuel metering device (to allow re-build by professionals)? Is it easy to remove the injectors? (to enable a overnight soak in cleaner) Withou a gas analyser, will it be possible to get the car running well enough for me to drive to a garage?

And also, what fuel consumption can i expect to get (can I tell the wife) to help me justify this work - remember UK prices are just a bit higher than the US :-)

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Marcus

'73 S Targa

Early_S_Man · 06-19-2001, 03:24 PM

Marcus,

The board is so cluttered with non-technical posts these days ... it is hard to catch all of the questions before the 'chaff' covers them up or they get scrolled off.

Your gas mileage does not sound like that serious a problem to me. The typical gas mileage I get is 14-16 mpg (USA) in town and 18-21 mpg (USA) on highway trips.

Have you printed/read all of the resources on Pelican's MFI Tip's page? The only thing I can suggest about the warm-up problem is that maybe the warm-up thermostat on the injection pump is dirty/clogged/binding. It can be easily removed and cleaned, as per the instructions on pp. 35-36 in the factory publication 'Mechanical Fuel Injection: Check, Measure, Adjust' ...
the Berryman B-12 Chemtool mentioned may not be available in the UK, but it is in the category of a furl system cleaner liquids that may be poured in the gas tank, or used as a solvent for cleaning parts off the engine as well!

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

As far as removing/replacing the injection pump yourself, yes, you can ... metric flare-nut wrenches are needed to remove injector lines from the pump and injectors, but the task isn't that difficult. Good luck!

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Warren Hall
1973 911S Targa

MBlake · 06-24-2001, 02:26 PM

Thanks Warren,

Between my tinkering and the garage diagonosis, the finger of blame seems to be pointing at worn internals on the MFI pump.

I gave the other 'cheaper' alternatives one more chance today - plug gaps, injector cleaner in the fuel, checked and cleaner ALL the electical contacts I could find, and cleaned the injectors (except No.6 - I couldn't get in past the AC pump, and was pushed for time!). The dwell, timing etc I am trusting the garage on.

There was some improvement in the crispness of response (or maybe it is in the mind), but it is still popping and cracking a bit on a trailing throttle, and it can stutter when rolling on more throttle at around 3000rpm.

I started to removed the pump, but fell at the first hurdle - How do you determine TDC for #1? Can I check the mark on the fuel inj camshaft pulley? I guess this only makes one turn for every 6 engine revs? Is there an easier way?

Thanks

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Marcus

'73 S Targa

jsmarcus · 06-25-2001, 04:42 PM

I might be way off, but...
I was getting between 20-22 on my Carrera and then it dropped to 17-18 for a while there. Changed out the fuel filter and plugs and now I'm back up to 20-22 again. Try cheap first.

campbellcj · 06-25-2001, 06:25 PM

Resident guru Warren said:

Quote:

Your gas mileage does not sound like that serious a problem to me. The typical gas mileage I get is 14-16 mpg (USA) in town and 18-20 mpg (USA) on highway trips.

My experience echoes that: I've owned my 2.2E for 11 years now, and have always gotten somewhere between 12 and 22 mpg (US). It can even drop to 10 or so with especially lead-footed driving, i.e. canyon carving or track events, but the overall long-term average is around 16, which is almost 100% street driving (vs. steady highway cruising).

I did not notice much if any change in consumption after the pump (and virtually everthing else on the car) was rebuilt in 99-2000. I'm sure there was a change, but not a drastic one. I don't pay much attention to the consumption anyway, as this car is only driven around 2-3000 miles a year, mainly just for fun. (And our gas is "only" US$2.25 for crappy oxygenated 92 octane these days.)

Chris C.