Backfire on decelleration

[johnshenry]

First of all, I did search on backfire, and see that some people experience this, but wanted to just post and get a better feel if this is normal behavior.

I bought a '69 280 last Nov and have just now really gotten it out and am driving it after a long New England winter.  It backfires a bit/burbles on deceleration, mostly low RPMs coasting  up to a lights (its an automatic).  Not really loud, "afraid the muffler's going to come off" or anything. In fact, I think it sounds kind of neat.  Some grumbling with a few pops in between describes it best.  I really like the exhaust tone of these cars (having driven vintage VWs for some 30 years now).

Is this common/normal?  Car seems to run very well otherswise, but as a new 113 owner driver, I really have no point of reference.  I notice a little black soot on the tailpipes, that normal too?  Or running too rich?

TIA for any help...

JH

[Richard Madison]

My understanding is that a "black box" was installed on later Pagodas to reduce or eliminate backfiring.
I also believe that Euro version cars may not have this fix.

My '69 Euro backfires on downhills when the accelerator is not pressed.

Richard M, NYC

[jeffc280sl]

I think most of the back firing on decelleartion is due to the early emissions systems installed on our cars.  There is a fuel shut off solenoid on the rear of the fuel injection pump.  Try disconnecting the wire to it and see if the back firing goes away.

[johnshenry]

I read about the fuel shut off valve in another backfiring thread when I searched.  How would that work on trailing throttle?  How does the engine know to cut fuel completely vs keep an idle feed going in? It's basically a completely mechanical system, so I don't see how this could work for trailing throttle situations.

Interestingly, my '04 VW Jetta has a coasting cut out that shuts down the injectors below 2300rpm, or after a a period of coasting with no pedal depression.  It is so pronounced in the engine drag that you can feel a "bump" when you coast down through 2300 rpm.  I used to wonder what it was until I hooked up a diagnostic VAG-COM system in my laptop and could see that the injector interval when to zero when coasting down.  No doubt an emissions reducing feature there....

[jeffc280sl]

I assume you have an automatic transmission.  The fuel shutoff solenoid is active when the transmission is in 3rd or 4th gear and a micro switch on the throttle linkage indicates the gas pedal is at an idle position.  A pressure switch on the transmission also has to come into play. 

I don't know for sure if the fuel is completely shut off or if it is set to an idle condition.  The only way to know for sure is to put 12 volts on the solenoid when the engine is running and see what happens.  I have removed the solenoid on my car so I can't perform this test.

[hauser]

My "69 Euro had this problem.  I took it to the shop everything was adjusted along with a new Crane and has never backfired since.

[dtuttle123]

I have that issue on my '67 250SL (US Version)....but only in the summer, not in the fall or winter!  It's not really bad - sounds cool!

[rogerh113]

On my '66 manual the backfire on deceleration happens when I get lazy and rest my foot on the accelerator pedal.  Maybe my linkage is overly tight, but it seems that just a little pressure on the pedal causes a little extra fuel and I get the effect.  Not really unpleasant, but when I lift my foot it goes away immediately.

Regards -- Roger

[dean]

i too have the backfire on deceleration (68 Euro Auto).  I kind like the sound of it.  Is there any harm in not fixing it?  or i should get it fixed?

[bpossel]

Are you guys actually having back firing, or are you referring to the nice Pagoda "burble" sound, like a Harley?
Bob 

[thelews]

I'd call it a "popping," not a backfire.

[johnshenry]

I'd call it a "popping," not a backfire.

True, that is probably a better description. 

[jeffc280sl]

That is the sound of the fip going to a lean condition.  When I coast down a hill I get the same sound and I have watched via my air fuel meter the ratio go to 16 or so.  It is the way the 3D cam is designed.  I think Bosch did what they could emissions wise with the cam and then added the solenoid as emissions specs became tougher.

[santropezblueSL]

Are you guys actually having back firing, or are you referring to the nice Pagoda "burble" sound, like a Harley?
Bob 

I have just bought a car that REALLY does "backfire on decelleration".  No burbling or popping here.  Solid blow-the-muffler-off BACKFIRING.  It runs very strongly and sounds great in all other conditions so it will be interesting to see what the cause ends up being.  (BTW it has a cheap chain-store-brand rear muffler...) As another point of interest I am selling a 1961 300SL that also has some trailing throttle burbling which is a new thing on that car.  The car has been stored a lot and came out of storage this year with a new burble on trailing throttle.  It runs perfectly well in all conditions, has no items worn, or missing, or changed, but has that popping or burbling on trailing throttle in the right conditions, and never did before.  Of course the w198 300SL from 1961 is in mint condition, and has no emission controls, and has no new work done on it in the last few years, so I am interested to see what the cause of that burble will be and how that relates to the backfiring problem on my 1969 280SL.  It seems to me that the issue on both of my cars is the same and is somehow related only to the storage of the cars, and the low miles which they were driven in the last few years.  So I expect it should not be a mechanical linkage setup, or timing issue, but maybe a dirty fuel filter, or a plugged jet, or corrosion on the points; something related to the storage.  I would be interested in comments from anyone with experience on fixing the backfiring caused by storage on either of these cars.  Perhaps someone can save me some time by pointing me in the right direction   

[Benz Dr.]

You need fuel, high exhaust temps, and air to make this popping sound. I would check your throttle stop screw and make sure the plate is fully closed at idle. On 250's and 280's, when you lift off on the throttle to the idle position, the distributor should go to full retard which may aggravate this popping condition.
Unburned fuel in the exhaust pipes mixed with air will cause popping. I would also check that there are no leaks in the exhaust, especially around the flanges where the pipes hook up to the manifolds.

Black soot around the tail pipes is fairly normal with unleaded fuel. Back when we used lead in the fuel you would look for a bone white or ash grey colour.

The 300SL has a number of jets in the throttle housing. Changing these jets to larger or smaller sizes makes the IP run leaner or more rich. The IP is controlled only by a vacuum line that runs from the throttle housing to the back of the IP. The diaphragm in the back of the IP could have a pin hole leak in it. Disconnect the vacuum line at the throttle housing and put a vacuum on the line. It should hold perfectly or you have a problem. Check that all of the connections are tight as any drop in vacuum signal will make the pump go rich.

I was driving a ' 61 Roadster that I was working on when the diaphragm broke. The engine would still run but it was eye burning rich. I replaced the diaphragm with a proper chamois one and threw the cheap rubber one away.
The engine was still running rich and fuelling down the oil which I had to change frequently - all 12 liters. I found the cold start wasn't adjusted properly and not closing fully. I adjusted it with the engine fully heated up and a small mirror. All of the axillary air holes should be fully closed by the slide valve/pin with a warmed engine. After I did this the fuel millage almost doubled and the oil had a nice smell after a long run. I would take a sniff at the oil tank. If it smells like fuel, there's a good chance the cold start needs adjustment.