W124 E280 Correct idle speed?

[Richard Hamilton]

I have a 1994 W124 E280 Auto with HFM-SFI injection. What are the correct idle speeds in neutral and drive? Mine seems a bit low - in neutral it reads about 650rpm, which dips below 500 when you engage drive, after which it recovers to about 550. I find that it can stall when you are moving slowly and wind on a lot of lock. Is there any way to adjust it up by 100rpm or so?

 

[Terminator]

This is a common problem with the HFM engines when the bio degradable wiring harness breaks down. The ECU is very good and will compensate up to a point. Best way of checking is to remove the air cleaner assembly and the spark plug central cover. You will see three coils which sit over the plugs with two sets of leads running off each of them with caps onto the coil. The wiring from the LT caps passes within a rubber gasket and then out into the wiring harness to the ECU. The wiring tends to lose its insulation and causes the current to fluctuate which causes your problems. The degradation will be obvious if your pull back the black cable cover near to where the LT leads plug onto the coils. The bad news is a new wiring harness costs £500 plus fitting. The good news is buy the LT caps that push onto the coil (3 x £30 from Mercedes) and get an auto electrician to wire it up for you.

 

[PINBALL]

Scuse me for bein a bit dumb but ...

whats an HFM engine please?

 

[Terminator]

Hot Film engine Management. Bet you wish you hadn't asked now!

 

[PINBALL]

Thanks Terminator.

Sounds like I'm better off not knowing!

Sounds expensive n sooper dooper.

Keith.

 

[svetor]

This is a common problem with the HFM engines when the bio degradable wiring harness breaks down. The ECU is very good and will compensate up to a point. Best way of checking is to remove the air cleaner assembly and the spark plug central cover. You will see three coils which sit over the plugs with two sets of leads running off each of them with caps onto the coil. The wiring from the LT caps passes within a rubber gasket and then out into the wiring harness to the ECU. The wiring tends to lose its insulation and causes the current to fluctuate which causes your problems. The degradation will be obvious if your pull back the black cable cover near to where the LT leads plug onto the coils. The bad news is a new wiring harness costs £500 plus fitting. The good news is buy the LT caps that push onto the coil (3 x £30 from Mercedes) and get an auto electrician to wire it up for you.

Hello Terminator !

It's been two years since there was any activity in this thread but I hope you are still around!

I have an E280 1993 Sportline with a similar Idling problem. That is to say I need to press the gas when it is started cold and some times when driving the engine won't go below 1100 rpm.

You said "The degradation will be obvious if your pull back the black cable cover near to where the LT leads plug onto the coils."

Could you please explain where this is in the engine and how the coils and LT leads/caps look like, I just can't find it now

 

[Myros]

svetor, you have to take the heatshield

covering off. It's the metal plate in the middle of the top of the engine held on by about 8 small bolts. the ignition stuff is underneath.

 

[svetor]

Thanks Myros!

As you can tell I am fairly new to DIY's

 

[eric242340]

whats an HFM engine please?

It is all about Air mass and its measurement there are several types of Air Mass Sensors and several types of Hot Air Mass sensors (AMF) and (HFM) there also other types, but Benze uses Both dependant on model. HFM stands for Hot Film Mass. They are air intake systems.

 

[television]

These threads are going off in the wrong direction as the HFM is not responsible
for the high idle.

The biggest cause for this is in the bottom airflow sensor housing, it is a potentiometer built in to the housing that wears around the idle position, if you have the 3 wire idle control valve the further contact that switches over to idle may also be worn.

 

[Thegemmesabogie]

Since when did a HFM managed engine have an 'idle control valve'?

 

[eric242340]

Since when did a HFM managed engine have an 'idle control valve'?

On HFM? Never its all controled by the throttle valve-ECU.