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95 SL600
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153 Posts
Discussion Starter #1
I have an 83 380sl that we replanted an 85 euro spec 500se engine into it. Tha car can be driven around town and cruised, but under more than 60% throttle and more than 2500 rpm the car immediately shuts off (like turning off the ignition switch) and soon after backfires. It will resume its running state after that, but don’t know what I should look for. Fuel pump relay, throttle position sensor, ignition switch, or cold solder joints in electronics. I

It appears it may be an ignition related fault, only because it seems to get the hot exhaust full of fuel and gives the “sonic boom” backfire out of the exhaust. Not sure what to start replacing or looking at. It does have the dreaded high idle, and ran perfectly in the 85 500se that it came out of.
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,954 Posts
Some things that come to mind:
  • Ignition Control Unit located behind driver side headlamp on fender liner under hood
  • ECU under floor board passenger side
  • did the 500SEC have two fuel pumps?
 

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95 SL600
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153 Posts
Discussion Starter #3
Some things that come to mind:
  • Ignition Control Unit located behind driver side headlamp on fender liner under hood
  • ECU under floor board passenger side
  • did the 500SEC have two fuel pumps?
It came from a 500se and had one fuel pump. Seems to happen above 3000 rpm and is complete loss of power, but not always consistent. I will see if I can pull out the rest of the components from the 500se (did an ls1 swap in it) and didn’t know what to start with
 

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95 SL600
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153 Posts
Discussion Starter #4
Some things that come to mind:
  • Ignition Control Unit located behind driver side headlamp on fender liner under hood
  • ECU under floor board passenger side
  • did the 500SEC have two fuel pumps?
It came from a 500se and had one fuel pump. Seems to happen above 3000 rpm and is complete loss of power, but not always consistent. I will see if I can pull out the rest of the components from the 500se (did an ls1 swap in it) and didn’t know what to start with.
My 16 year old son drives it and enjoys it, he helped with the engine change from 380 to 500 and the next step is fix the this and the high idle problem. Neither existed in the 500se so I am guessing the problem is with the 380 sl control units or relays or both.
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,954 Posts
For high idle, check the connection on the Idle Control Valve (ICV).
On my 560SL if I pull the connector to the ICV while idling, rpms will noticeably increase.
Putting the connector back on the ICV and rpms noticeably decrease.
If I recall correctly, if you connect a volt meter to the connector you should
read something like 5v.

How about vacuum leaks? Smoke tested? Propane tested?
 

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95 SL600
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153 Posts
Discussion Starter #7
Fuel pump relay can cut out fuel if erroneously computes the rpm is too high.
Petty money to replace
Ordered a new fuel pump relay and we will see how it goes. As for the high idle, possibly a vacuum leak, but didn’t have that problem in the 500se. That’s the next hurdle, but the intermittent cut out above 3000rpm or so is the real dangerous problem at this time.
 

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'80 450SLC Afro RHD Ikonengold
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2,931 Posts
Ignition module can cause the same fault if rpm signal is intermittent.

Ignition modules go for us$15 on Aliexpress but original ones are repairable by educated electronics girls and guys.
If i did not have 2 original working ignition modules, I'd buy 2 or 3 cheap AliXP modules and keep them in the boot. Fuel pump relays are bargain too. I know the likelihood of quality us low but it is a perfect inexpensive piece of mind that will get you home so that you can repair original parts.
 

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95 SL600
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153 Posts
Discussion Starter #9
Ordered a new fuel pump relay and we will see how it goes. As for the high idle, possibly a vacuum leak, but didn’t have that problem in the 500se. That’s the next hurdle, but the intermittent cut out above 3000rpm or so is the real dangerous problem at this time.
Replaced the fuel pump relay, that doesn’t seem to be the issue. On to the ignition. It appears that the issue may get worse when the engine gets hotter, as my son said the problem wasn’t there when the car was colder than operating temp.
 
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