MPG 124 320 Coupe

[Mercky]

Hi,
Recently noticed that my mpg appears to be dropping off, by as much as about 15% with no change in driving patterns etc, mileage is 66k and car runs fine apart from a roughish idle which has always plagued the car. Used to get about 250 miles per tank now its about 220, anyone got any ideas??
thanks in advance
Mercky

 

[kth286]

The lambda probe in the exhaust is considered a service item as the miles get high, so change it, as it is in the main feedback loop which keeps the engine fueling at optimum performance.

 

[kid-jensen]

Mercky,

Check fuel pressure and engine temperature sensor are within spec.
You'll guzzle fuel if either of them are out, and you won't notice any change in performance.
Also check the viscous fan is decouplng as it should, (do the famous carrot test).

 

[kth286]

Mercky

I obviosly do not know what sort of driving you do but even 250 miles per tank is bad.

On a run I aim for 400 miles.

Engine temp sensors do go (as previous post) and are cheap to replace, so do it anyway.

Also the charcoal canister in the wing is a service item at 72,000 miles. If it is not done it can get blocked and cause the petrol tank to gradually collapse.

How many litres can you get into your tank normally ?????????

 

[Mercky]

Cant remember exact litre capacity to fill the car , I'll check at next fill but I guess about 55 to 60. On a long run i will get about 350 miles per fill but average mixed driving used to be around 250. I'll look in to the engine temp sensor, if it was failing would that imply the car would always run too cold? The water temp gauge looks normal. Re the canister..........is that a DIY job and where is it mounted? On that note does anyone know of a good manual for the 320 124's? I have the Haynes version but it's pretty poor. Thanks for your replies.
M

 

[124coupe]

Can you confirm this the HFM 320 (M104 engine)?

(look at the engine number on the back page of the service book if unsure).

Then we can give you some tune-up ideas (the wonky idle tells you that the mixture is off).

 

[shj]

I get nearly 32 mpg on a long run and close to 460 miles on a tank,M104.992 engine.
70 litre tank

 

[124coupe]

I tend to enjoy the performance and cruise around 85-90. Result is 25-27mpg long term so far (it was doing 19 when I bought it).

 

[kth286]

124Coupe

There isn't any other 124 320 coupe - so yes it is in answer to your question.

 

[kid-jensen]

124Coupe,
What did you do to it that gave you such a great improvement?

 

[Mercky]

thanks guys, some tune up ideas would be great, re the engine type.......well I'm not sure.....i dont have the service book but it's a 1994 124 320 Coupe Sportline so I assume it's the standard HFM M104 engine for that year.
M

 

[124coupe]

When I bought the car it was a just-about MOT emissions pass and had a lumpy idle and a big flatspot at about 3,500rpm.

New plugs and boots, cleaned up throttle body (lots of carbon builds up on the butterfly affecting airflow and preventing proper idle control, fixed broken connector to knock sensor (was causing ECU to retard ignition), replaced O2 sensor (was responding slowly- they wear out!), swapped seized viscous fan with 1 from scrappy (carrot test/power drain), removed and cleaned injectors (they were GUNGED and not spraying cleanly - stuff in the tank had done nothing), cleaned MAF (just in case).

Total cost about £100 and the result is inaudible idle and much more torque.

The single best fix was cleaning out the heater drains and then washing through - no more old socks smell

 

[Mercky]

Yes, it's a 1994 124 320 Coupe sportline so I assume HM320 engine, I dont have service book to confirm.
M

 

[124coupe]

Two other (cheap) widgets that hit economy are the Air Intake Temp Sensor ("IAT") and the Engine Coolant Sensor ("ECT").

Test data (even cheap stuff needs to be broken before changing!):

IAT

20c 6060 Ohms, 30c 3900, 40c 2600

ECT

20c 2500ohms, 30c 1700, 40c 1170, 50c 830, 60c 600, 70c 435, 80c 325, 90c 245, 100c 185

If they read colder than reality then rich running results (at colder temps the ECU lengthens the injector cycle.