RPM dash problem

Hi, is it a diesel? If so go to the search website tab, type in Rev counter & what do these bits do, scroll down to panel 5 and click on the mb electronics link, this will take you to company in Texas. They should be able to solve your problem. 

Hello Eric,

It is not very clear; when you claim the RPM goes up and down, is that just the rev. counter going up and down or are you trying to say the engine speed also goes up and down?

If the engine and the rev. counter reads up and down; that is the engine is "hunting" at idle speed, it is most likely you are developing a small air leak into the inlet tract. 

It is very common on the Petrol versions to do this in particular with the rubber hoses of the idle circuit perishing. The false air leak into the inlet tract up sets the fuel mixture. 

If you just have the rev .counter reading up and down regardless of the engine speed then I would say you have an electrical fault on the rev counter circuit or the rev counter its self. Having said that, I have not known a rev. counter being faulty to-date.

Ps. I note you have a thread on idle load valve; if your engine hunts then resolving the suspected air leak may solve your idle issue with a/c on. 

Yes that looks like the right one. The only thing is it does not have a fuel drain at the head of the gauge as some kit do; check this site out as notice some heads have a valve to drain fuel and leak off the line pressure.

http://www.ebay.com/bhp/fuel-pressure-tester

Why do want a pressure tester? If your vehicle is running fine then you don't need it. The engine hunting will not be diagnosed by pressure testing.

Hello Eric,

The fuel pressure tester is only good to test the fuel system out as you have mentioned but no diagnoses for piston (i.e. compression testing). I don't see the need to buy one just yet as I don't think you need one at the moment.

To test for pistons or engine compression you need a compression tester. There are two sorts, one for testing compression on Petrol engines and a different one for testing compression on diesel engines. The difference is the testing pressure calibration on the reading head are different (higher readings for Diesels') and fittings for the engine side are different. For Petrol compression testers, the fitting could be a hand held pressure fitting against the spark plug hole or a screw-in fitting into the spark plug hole; the latter is better. The Diesel fittings go into the glow plug hole or could be in place of the injectors its self.

I agree with you; if the engine speed is fine but the rev counter drops its reading then it is an electrical fault at the rev counter or its circuit to blame. I would suspect the probability of a fault could lie at the connections behind the rev counter; if not then move along the circuit. 

I am not operating from my own lap top at present otherwise I could study the wiring circuit to see where to go next. If I do get a chance of that, I will try to find a way to send further information to you on that.

Hi Eric. Certainly for some city driving that fuel consumption is normal. Does your 280 have an automatic gearbox? I have a 5 speed manual and the very best consumption, on a journey, is 200 miles for 45 litres of petrol. My RPM counter behaves the same as yours. I can make it work by hitting the instrument binnacle so I am sure the problem is in the instrument or a nearby connection! What fault /symptoms are you trying to cure in your engine?

If my calculations are correct that works out to be 12 mpg which is acceptable at the lower end of the scale. Fuel consumption between 12 to 16 mpg is what you should be getting. In your case if we look at the top end of the scale you could be getting 211 miles for 50 lts of fuel.

• Check first all wheels are running freely and that you have no binding brakes and the wheel bearings are fine.
• Make sure you run at the correct tyre pressures.
• Do not carry unnecessary load in the vehicle that you don't need.
• Avoid hard acceleration and late braking
• Avoid lots of short journeys i.e. less than five miles; the longer journey will return better fuel economy.
• Check if the vehicle is running at the correct temperature. The coolant temperature gauge should read 90 degrees constant. That is the needle sits on the top part of the digit eight on the gauge.
• Check the air filter if very dirty or over three years old replace this. 
• You may conduct a compression test of the cylinders to have an initial idea of the general condition of the engine. If the cylinder pressures are about similar then recheck after checking and adjusting the valve clearances on a cold engine. But if one or two cylinders read a relatively vast difference compared to the others then there is a valve seating or piston ring condition that  will need addressing and these involve an engine repair or strip down to solve. 
• Check spark plug gaps adjust to 0.8mm gap and check to if the correct heat range have been fitted. There is a sticker or should be one at the bottom right of the bonnet as this is lifted.
• Check the distributor cap terminals; if the terminal have a concave groove in its middle, the cap and rotor arm should be replaced as these have eroded hence having excessive wear through erosion.
• Importantly, check the valve clearance and adjust as necessary.
• Check the ignition timing is set correctly (vacuum pipes off) and check at idle speed and at 3000 rpm, this will give you an indication if the distributor has a problem or not. 
• Finally have the emission checked and adjusted as necessary, have this set at from 0.5 to 1% reading.

The engine tuning side can only be completed if the idle is stable, if you are having engine hunt issues, this does need sorting out firstly before you set the emissions. 

 

Hi Eric, no, I believe we are saying that your 280 probably has no problem based on the amount of fuel you say you are using. Driving into the city will probably take you down to 10-12 miles per gallon (4.54 litres). Hot starting needs a little throttle. Cold starting should not. Certainly have your truck serviced including check valve clearances but otherwise just enjoy it and save your money until something goes wrong!

Big Block, What do you mean "You think....."; you have worked it out as I just have (again) after reading your thread and you are correct. 50 Liters of fuel over 160 miles does work out to be 14.5 mpg. The conversion table I used earlier seemed to be wrong which I doubt; more like my sums were wrong! So we have a correction here. Eric your car is just fine. So be Happy 

Personally I really wouldn't. Those are diagnostic tools rather than regular maintenance tools. You are headed down the automotive equivalent of reading a medical dictionary and finding all sorts of worrying symptoms you might have. Using those tools on an old truck I am sure you will find some results that are a little out of spec and you can spend much money just trying to turn back time! As a g-wagen owner for over 15 years I recommend you drive it and enjoy it until a real fault develops - because one will sooner or later. I hope this doesn't sound patronising, it's meant to be good advice based on my experience.