Author Topic: Fuel Cut-off Solenoid Trigger Method?  (Read 295 times)

rbouch8828

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Fuel Cut-off Solenoid Trigger Method?
« on: October 16, 2025, 23:35:55 »
My 4-Speed Manual 1969 280SL backfires when I am down shifting or just take my foot off the gas and brake. I have tested the solenoid and it is working properly when I apply 12 volts.

Without having to rebuild the emissions control boxes is there a way to trigger the solenoid just when needed in downshifting or braking?
RB

wwheeler

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #1 on: October 17, 2025, 01:39:50 »
I like to have everything working on my cars but the fuel stop solenoid is one that I have disabled. The advice from most injection pump rebuilders is to disable it. If the fuel stop system has a failure and engages at the wrong time, your engine will stall immediately. Kind of a safety issue and is difficult to diagnose. That is probably the last item you would troubleshoot when your engine stalls. It was installed as a US emissions device to prevent unburnt fuel from being reignited in the exhaust during deacceleration. That causes the popping noise. If I remember correctly, the fuel stop solenoid engages when the throttle is at idle and the RPMs are below 1200 or so. Maybe different on manual versus automatics.

It may be also that you are slightly rich and you can check that. 
Wallace
Texas
'68 280SE W111 coupe
'60 220SE W128 coupe
'70 Plymouth Roadrunner 440+6

rbouch8828

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #2 on: October 17, 2025, 14:20:55 »
My problem is that i get more than just a “popping” sound. I get backfires that can sound like gun shots.

As you say, maybe i am running too rich. I will have to check that and see if i can lean it.
RB

roymil

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #3 on: October 17, 2025, 21:53:43 »
I had a similar experience to Wallace and also have disabled the fuel shutoff solenoid.   Originally, without the solenoid, I noticed the slight popping when letting off the gas and my motivation to get it working was mainly to improve the gas milage and it did help that by about 10%. 

The downside when the solenoid activates and cuts the fuel is that you can feel a noticeable jerk due to additional engine braking.  This was really annoying if you are barely on the gas just trying to maintain a low speed in 3rd gear and it kicks on and off.     This is because of the two switches on top of US manual transmissions that activate in 3rd or 4th gear only and are in a circuit that activates the solenoid when you completely let off the gas. These are not shown in the regular schematics.  I have a special addendum sheet for 1968 that just shows the circuit with the 3 switches.   

The clincher for me was figuring out it will sometimes kill the engine when slowing down in 3rd or 4th gear. You can easily roll to a stop with a dead engine, even pushing the clutch in plenty of time.

Technically the design works and achieved something to satisfy the new vehicle regulations that were rolling out in the late 60's, but it is more annoying than its worth in my case, but maybe if you are running rich the fuel in the exhaust is more annoying for you.   

The other issue is that solenoid can leak oil.   When I disconnected it my oil leak went away.   

In any case, just try it for a while, it's certainly easy to disconnect if you dont like it.
« Last Edit: October 17, 2025, 22:03:19 by roymil »
Mark Miller
1968 280SL
Rode in his pagoda's first mile.

wwheeler

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #4 on: October 17, 2025, 22:05:08 »
Another possibility if the backfire is that strong is ignition. Not an expert here but seems like if the ignition is retarded too much, the engine will backfire upon deacceleration. Maybe one of your advances has failed - vacuum or mechanical. Might check that. Has it always done that or just started?
Wallace
Texas
'68 280SE W111 coupe
'60 220SE W128 coupe
'70 Plymouth Roadrunner 440+6

rbouch8828

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #5 on: October 19, 2025, 17:53:21 »
It is something that is relatively new behavior. That is, the backfiring is something that I have begun to notice this year. I do not remember hearing it previously.
RB

Mike Hughes

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #6 on: October 19, 2025, 20:54:15 »
Are you running points in the distributor? Timing is altered as the rubbing block wears down.
- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havana Brown (408)
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rbouch8828

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Re: Fuel Cut-off Solenoid Trigger Method?
« Reply #7 on: Today at 18:22:44 »
No points. I have Pertronix.
RB

 

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