Flat acceleration at low revs - 1987 SL420

[psteynberg]

Hi All, I seem to have a problem with lack of acceleration at low revs. From just getting going if you hit the gas, the car seems to not respond AT ALL but if you back off and allow it to accelerate slowly once you get past a certain point she becomes quite responsive. It reminds me of the old days on my farm bikes when you flooded the bike. Any clues on where to start looking?
Regards,
Paul

 

[mattkh]

What happens when it is idling and you press the accelerator, do you notice a flat spot or do the revs go up straight away..?

 

[psteynberg]

What happens when it is idling and you press the accelerator, do you notice a flat spot or do the revs go up straight away..?

I revved it up last weekend when I got back from a run and while out of gear it seems ok. Revs freely from low to high and drops back to idle with no problems.

 

[mattkh]

Manual or Automatic..?

 

[psteynberg]

It's an automatic.

 

[mattkh]

One thing to try
1. Let the ATF warm up while idling, then drive

 

[psteynberg]

I Googled that and am now educated ATF = Automatic Transmission Fluid. Does the car have to be rolling in order to warm this up. I took the car out after allowing the engine to idle until the temp came up past 80C. I then drove around 2 miles to a garage to pump the tyres up. The car never switched off fro about 10 minutes while doing this so kept it warm. Then travelled around 2 miles home. Is this enough time to get the ATF warmed up?

 

[00slk]

Is the transmission selecting first or second when you drive it, could be driving off in third??

 

[psteynberg]

Is the transmission selecting first or second when you drive it, could be driving off in third??

How do I work out which gear it started in and which gear it is in? I have not noted a LCD display anywhere. This could be what is happening as when I put my foot down it kind of feels like it is flooding then when it kicks down it takes off.

 

[00slk]

I Googled that and am now educated ATF = Automatic Transmission Fluid. Does the car have to be rolling in order to warm this up. I took the car out after allowing the engine to idle until the temp came up past 80C. I then drove around 2 miles to a garage to pump the tyres up. The car never switched off fro about 10 minutes while doing this so kept it warm. Then travelled around 2 miles home. Is this enough time to get the ATF warmed up?

Yes I am fairly certain it will have to be driven to warm up the ATF
The transmission fluid does take a while to get up to normal temperature.
I had a service don on my 5 speed transmission and the serviceman wanted the trans at the recommended temperature before he could do a complete start reset. I drove for about 20minutes around town and dual carriageway.

 

[00slk]

How do I work out which gear it started in and which gear it is in? I have not noted a LCD display anywhere. This could be what is happening as when I put my foot down it kind of feels like it is flooding then when it kicks down it takes off.

Drive it as you would with the selector in D.
Try it again with the selector in L or 1 depends on the type you have, I'm sure you will have 1 2 D N R P on the selector.
If it accelerates quicker in 1 or 2 than if you select D would indicate transmission issue.
Or the engine might just need a good service?

 

[psteynberg]

I do a couple of 100 miles a year and have the engine FULLY serviced each year. The Mercedes mechanic thinks I am mad for spending the money but he does as asked. New plugs, filters, oil etc etc. So that is not an issue. I will try and run it in various selected modes and see what happens. Now that you mention it, it does sound like it is in a much higher gear for the speed.

 

[mattkh]

...........................Then travelled around 2 miles home.

What was the response like on these 2 miles back ?

 

[psteynberg]

Not noticeably different to be honest. If this weekend is OK, I will run a battery of tests and run her much longer to see what happens.

 

[LostKiwi]

I'm assuming this is a 3 speed auto?

 

[psteynberg]

I'm assuming this is a 3 speed auto?

This is a 1987 SL420 so I believe it is a 4 speed auto. It is right hand drive so UK spec.

 

[mattkh]

I have engine FULLY serviced each year. The Mercedes mechanic does as asked. New plugs, filters, oil etc etc.

I doubt if the mechanic changes the ATF every service.
Normally people get it changed say every 50000 miles.
Can you ask him when he changed the ATF last.

 

[star]

Is the petrol old and gone off?
How high is the idle and does it start well? Possible ovp fault.
Possible sticking fuel governor fault or metering head fault.

 

[LostKiwi]

If it's a 3 speed it will be marked PRNDSL on the shifter. If 4 speed it will be PRND32 and possibly a L position to the right of 2.

 

[psteynberg]

If it's a 3 speed it will be marked PRNDSL on the shifter. If 4 speed it will be PRND32 and possibly a L position to the right of 2.