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-   -   380 sl jumping on idle (http://www.peachparts.com/shopforum/mercedes-benz-sl-discussion-forum/78271-380-sl-jumping-idle.html)

lynn tassin 10-28-2003 05:58 PM

380 sl jumping on idle
 
I have an intermittent problem here. ON idle at red light or stop the car actually jumps. I've had full tune up rotor replaced all new plugs and wires everything. The car actually moves up and down. Have any of you guys had this problem? Is it possibly a vacuum leak problem?

Robert Mowbray 10-29-2003 11:29 AM

I recently had an intermittent problem at idle, where the revs would go way up and down, sometimes stalling out. The last time it happened, it stalled and would not restart. It ended up being the fuel pump relay. The car did NOT jump or move unusually for a rev problem. Does the tach indicate flucuating revs? Does the car want to go as soon as you take your foot of the brake, before you hit the accelorator? If so, I'm guessing the idle is set too high and it's trying to engage the transmission. Good Luck BOB

lynn tassin 10-29-2003 01:21 PM

car runs fine with normal idle. then all of a sudden when you stop. it will kinda jump like it wants to kill. Mechanic just called to tell me he thinks its the fule pump. Its has very little pressure. Would this make it miss and jump?

Robert Mowbray 10-29-2003 05:15 PM

Lynn, The fuel pump or pump relay will cause it to missfire, but I don't know about the jumping part. My revs would go way up and down when sitting at a stop light. When it finally failed completely, it stalled and wouldn't restart. It was the relay. Hope this helps. BOB

ctaylor738 10-30-2003 11:25 AM

In addition to the idle relay or idle air valve, it could also be a vacuum problem, especially if it is occurring when the brakes are on. Check the vacuum booster for leaks, and look for disconnected vacuum lines, cracked connectors, etc.


If you send me an email, I can send you the procedure for testing the various components of the idle system.

RickM 10-30-2003 03:23 PM

Chuck,

Can you tell us where the Vacuum Booster is?

Thanks

ctaylor738 10-30-2003 05:59 PM

You can't miss it.

It's the larger round cannister on the left firewall. It has a large vacuum line leading to it. The brake master cylinder and fluid resevoir are bolted to the front of it.

lynn tassin 11-04-2003 11:27 AM

Ok no vacuum leaks, they replced the fuel pump, some glide thing that adjusts the idle and the car is still jumping. Now he tells me he needs to replace the timing chain and guides. I dont think this has anything to do with the jumping. CAr has also had full tune up new rotor all new plugs , wires etc Somebody help Im up to $6400.00 in repair and making no progress. What makes a &%$##( engine jump?

ctaylor738 11-04-2003 03:16 PM

The timing chain and guides may need to be replaced, but it's hard to see how that would affect the idle.

What it sounds like is happening is that for some reason, the idle is dropping and the relay is telling the air valve to pump more air to bring it up. Then it gets too high, and the relay calls for less air, the idle falls, and so on.

So there can be something causing the idle to fall, like a vacuum leak or a bad injector. Or the air valve or the relay can be bad.

There are very straightforward tests for fuel pump delivery, injection pressures, vacuum leaks, and idle relay/air valve function. You should ask if these were done, and what the results were.

Steve Gutman 11-04-2003 05:40 PM

Control Module?
 
The idle is controlled by the Idle Speed Control Module. That is what gives the signal to the "air Slide Valve" which sounds like it was replaced. When the Air Slide Valve fails it draws too much current and burns out the Idle Speed Control Module.

The Idle Speed Control Module is also fed inputs by the climate control, fuel pump relay and can't recall what else.

It is probably your Idle Speed Control Module.

I replaced the Air Slide Valve and Idle Speed Control Module and corrected the jumping of idle. I also had some eratic idle after that and it was cleard up by new fuel injectors. It still cycles up and down 600 to 650 RPM or so which could be a slight vacuum leak or something but a couple mechanics think it's excellent. I'm not satisfied until it's perfect.

I doubt it is your valve timing and a new chain may not help at all. How many miles are on your car?

lynn tassin 11-05-2003 11:17 AM

I sent the mechanic all of the information that you guys gave me yesterday. He says" Well I still think its the belt" so I told him to just go ahead and keep changing parts, but I am not paying for the belt or labor if you change it and my car still jumps. He wont even consider checking the things you guys told me. He is a certified MB mechanic at a dealer in Louisiana(this may be the problem... a little inbreeding I suspect.) Ill keep you posted

lynn tassin 11-05-2003 12:42 PM

Sorry he is replacing timing chain not belt

Thierry07 11-06-2003 07:42 PM

380 SL Jumping on Idle
 
I have fixed the same problem by replacing the main catalyst and the two small one on each header. it stopped the jumping and improved the performance.
I eventually remove the two small one and the car is driving better than ever.
:)

fredddd 11-06-2003 10:17 PM

In park with the motor running, move,twist,or gently pull on the throttle control switch wire to see if you can change the speed at will. The wire comes out of the front center of the motor and goes to the right side. There should be a connector somewhere near the alternator. Move that around also. Good Luck.:)

lynn tassin 11-07-2003 11:19 AM

Ok guys ready for this. Now MB mechanic is calling to tell me that one of the guides has fallen into the engine and it will cost $3000.00 more to remove it. Never mind that I told him the timing chain was not causing it to jump as you told me and he ignored me. I think while he was changing the chain he probably caused this damage and is trying to pass the cost on to me. DOes anyone know if he would be able to determine this without ever having removed the old chain, or is it more likely he caused this damage while switching the chain.


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