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I own a '91 Ford Sierra 2.0 8V SOHC. Fairly simple engine, utilizes a Bosch L-jetronic fuel injection system. (EFI has following components: MAF, MAP, TPS, IAC, narrow-band o2 sensor, TFI module for ignition control, AIT, CTS, hall on dizzy, and ECU)

I get higher fuel consumption, then I should. It should be around 23-24MPG (10L/100km), and I get ~19-20MPG (12L/100km).

I've already:

  • had injectors cleaned

  • replaced computer and harness

  • replaced o2 sensor

  • checked all the sensors (replaced MAP and CTS)

What I've haven't replaced and can be related are basically the fuel pump and the fuel pressure regulator. FPR looks fine, but not sure. Is it possible that a larger-capacity fuel pump was installed by previous owner, and that causes higher MPG? How should I make sure, that FPR isn't faulty? (I've already tried removing vacuum hose, does not spit fuel or anything suspicious.)

  • Before looking at the expensive stuff, have you given it a proper service : oil air fuel filters plugs etc? – Solar Mike Feb 15 '17 at 10:33
  • Have you checked causes of high consumption outside the engine? Tire pressure, excessive brake drag, extra weight, etc. And do you use standard tires, or higher/wider? And how did you conclude consumption is high? Is it higher than in the past, or higher than Ford's specification? – Hobbes Feb 15 '17 at 10:48
  • It would help to know where you live. If you are in northern europe at the moment, your fuel consumption will be affected by the cold weather. – Steve Matthews Feb 15 '17 at 11:25
  • I've had the car for more than 2 years, it's been like that since I've bought it. I regularly service it. It's high compared to other examples of the same car with the same type of engine. It's a restoration project, most of the parts have been rebuilt/replaced, so it's most likely the engine, I'm in central EU, yes, temperature has an additional impact on MPG, but it's high in the summer as well. – galingong Feb 15 '17 at 12:31
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Sounds high for a car with a 2.0 liter engine. I used to have 1989 Opel Vectra with 2.0 8V SOHC engine, and the consumption was around 8-9L/100km. But then again, if you drive mainly short trips in stop and go city traffic, the consumption can actually be fine.

If it's really consuming too much fuel, you should consider where the fuel goes. It can go to a fuel leak. It can turn into heat (and possibly motion, propelling the car forward). Or it can go into hydrocarbon emissions.

If the difference between normal and your fuel consumption is so high, I don't believe it can turn into heat anywhere else than in the engine. If your engine is in proper working condition, it will turn into motive power as well. If it turned into heat in the catalytic converter, it would be overheated and destroyed very quickly.

So, you should be looking at fuel leaks (very common in cars of this age, especially if the rubber fuel lines haven't been changed) and hydrocarbon emissions. For example, a bad O2 sensor can cause the engine to have a rich fuel mixture, thus making in impossible for all fuel to combust, and causing hydrocarbon emissions. Bad spark plugs also can cause poor combustion, causing hydrocarbon emissions.

I don't believe a bad fuel filter could cause increased fuel consumption, because where does the energy go? If more fuel is consumed, it has to be turned to heat, and a fuel filter won't withstand much heat. Lack of power, yes, but lack of mileage -- no! A bad air filter, on the other hand, could reduce the efficiency of the engine. In this case, the heat will be produced in the engine, and the engine can certainly withstand it.

So, in summary: you probably either have a bad engine, a bad air filter, a fuel leak or a hydrocarbon emission problem (can be caused by e.g. bad spark plugs or bad O2 sensor). Engines are quite durable, so I would be looking at other things first.

About your list of things you have done:

  • Injector cleanup may give more power, but probably not more mileage (consider the "where does the energy go" argument)
  • Computer and harness probably don't help mileage unless it's for some reason continuously giving rich fuel mixture, which is unlikely -- that's not the typical failure mode
  • O2 sensor replacement may solve mileage issues
  • Checking other sensors may be a good idea, as during cold start the car is relying on these other sensors because closed loop control of the combustion process is unavailable due to low O2 sensor temperature
  • Thank you, really extensive. A fuel leak is highly unlikely, since I've inspected all the fuel lines thoroughly. Spark plugs and O2 sensor have been replaced. Air filter is replaced regularly. The car has no catalytic converter (previous owner removed it). So that leaves bad engine or bad mixture. My guess is bad mixture (exhaust smells a bit like gasoline), but since I've replaced/checked all the sensors, I thought it can only be the fuel pressure regulator. What do you mean by bad engine? – galingong Feb 15 '17 at 12:39
  • Bad engine as in poor compression. If this is the case, more of the energy of the gasoline goes to heat and less of it is turned to useful mechanical power. You could obviously check the compression to check the engine is good, but I don't really think this is your problem. If the exhaust smells like gasoline, bad mixture is the problem. When hot, it is usually caused by bad O2 sensor; when cold, the other sensors are in use. I don't believe fuel pressure regulator could cause a rich mixture at least when hot in closed loop control mode. When cold, it perhaps could affect the injected amount. – juhist Feb 15 '17 at 13:33
  • Thanks! Is there a way to test a narrow-band (4-wire) O2 sensor in car or on a bench? – galingong Feb 15 '17 at 13:56
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    @galingong yes there is. Here's a Q&A – Zaid Feb 15 '17 at 14:24
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I think juhist has a very good answer, but for completeness sake:

Do you have the properly sized (read: manufacturer spec) tires/wheels? Do you have some exotic tire compound? Are they alligned and balanced? Do you keep them inflated to the recommended pressure?

What are your driving habits like? What alititude are you at? Do you drive primarily city or highway?

Since the car is old and largely rebuilt, are you sure it has the correct ECU with the correct fuel maps?

  • Tires are fine, regular size properly balanced. I drive city/highway 50-50 (in terms of distance), city-only MPG would be even worse. Alt. is ~300m/1000ft. ECU/harness is 100% original. As I've stated earlier, mixture is probably rich. – galingong Feb 15 '17 at 17:34

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