Throttle

[Mykul]

1993 C220
Problem is, I think a throttle operation problem.

1) jerky pulling away
2) fuel is being cut off as I lift off the throttle - but only in one position - ie almost closed anyway. Travelling at a steady speed uphill, (ie fairly open throttle) things are normal. Steady speed on gentle downslope & then lift off produces these symptoms
3) "feathering" the throttle on a downhill causes the RPM to hunt 50 rpm either way.

One possible problem is the stepper motor - but that's about £300 to replace, so I would have to be sure that the problem is there.

Can anybody advise on either how to test the stepper motor, and the role of potentiometers in the throttle linkage?

Many thanks

 

[Andy]

Hi

I am assuming that the vehicle is fitted with LPG. As the signiture says. Are you sure that is not causing the problem.

Have you had any fault codes read.

You could try clean the throttle actuator flap.


Regards

Andy @ www.mercedesservicing.com

ps. If the LPG has been fitted a while. DONT assume it cant be that!!!!

 

[Mykul]

Hi Andy,

thanks for your reply. Yes the car has an Autogas2000/Tartarini system on it. The LPG is convinced that there is a problem elsewhere, although the car behaves OK on petrol. The "Merc" side of things have been checked by a merc specialist who can only find that the peptrol injectors show a fault (of course, the LPG switches them off) & by MB in Exeter who came to the same conclusion.

The problem does seem to be related to throttle position - not RPM or road speed.

Many thanks

Michael

 

[dieselman]

Yes the car has an Autogas2000/Tartarini system on it. The LPG is convinced that there is a problem elsewhere, although the car behaves OK on petrol. The "Merc" side of things have been checked by a merc specialist who can only find that the peptrol injectors show a fault (of course, the LPG switches them off) & by MB in Exeter who came to the same conclusion.

The problem does seem to be related to throttle position - not RPM or road speed.

I think this confirms Andys thoughts. It's your LPG system shutting down on over-run.

"although the car behaves OK on petrol." Think about it!

 

[Andy]

Hi Mykul

The thing is. When LPG is thrown into the equation. No body ever finds the faults. Then you play ping pong between the mechanics & the LPG man. If the LPG was not fitted I would venture to say that the problem would have been solved by now.


Regards

Andy @ www.mercedesservicing.com

 

[Mykul]

Timesheet

Hi Andy & Dieselman,

thanks for your comments. Andy you are right about the ping pong between the LPG people and the others. Any suggestions about how I could proceed? Either to prove or disprive my theory about the throttle. Is there a potentiometer in there somewhere that could be faulty?

I hear dieselman's comment about the "obvious" situation that if the problem doesn't occur when running on petrol, then it must be a LPG problem, but is it not also true that problems can exist (eg plugs/leads etc) that are there all the time, they are not bad enough to show themselves on petrol, but do on gas because LPG is less tolerant of problems?

Michael

 

[guydewdney]

after a long chat with one merc man i found out that there are TWO throttle sensors - one at the pedal, one on the end of the cable to the 'pot. if the two get out of sync (by, for example, revving the engine from under the bonnet by twisting the second pot) thing sgo spoons.


dont know how to cure this though... :-/

Guy

 

[Andy]

Hi

I am almost certain the problem will be with the LPG. Take it off. Remove all the relays that are choped into the loom etc.

But any MB tech worth his salt will tell you MB & LPG just don't go. Sooner or later you WILL have big bills.


Don't say I did not warn you.


Regards

Andy @ www.mercedeservicing.com

 

[neileg]

I know Andy doesn't like LPG, and he's at least half right. To solve this type of problem you need to understand both the standard engine management system and the LPG management system. On the best systems, the two talk to each other.

At work, we do LPG conversions. We have experience of factory fitted systems that do not work when delivered, from two very large motor manufacturers. If they can't get it right, it's no surprise that your chaps are having problems.

The fault may well be in the standard MB system, but, as you suggest, petrol systems are more fault tolerant than LPG systems so it only shows up when you run on LPG.