Engine "popping" (small backfires?)

[TR]

My 4-speed 280SL produces small intermittant popping sounds from the exaust when I take my foot off the accelerator and the car slows under its own compression.  These noises are occassional, and sound sort of like small backfires although are muffled way down.  I'm not sure if this popping is normal; or if some tuning adjustments might take care of it.  It's not a big deal.  Just sort of annoying, irritating.  I'd like to make it go away if practical.

Any thoughts / suggestions appreciated.

Tom in Boise
'71 280SL 4-spd signal red w/lt. tan interior

[Ricardo]

Tom
 Mine does the same and I'm pretty sure many if not all standards are prone to this. I think I've read that it is possible to tune this out or that it may indicate some wear condition, maybe in the injection pump, though mine is rebuilt and still makes this sound, which some have stated they like.
Does anyone with an auto tranny experience this "harley" like burbbling?
Ricardo

[J. Huber]

Interesting timing. Last week, I noticed my automatic made this "burbbling" on deceleration -- I actually had to try it again to make sure I was not hearing things... Not anything very prevalent and had trouble getting it to do it again. It was not a backfire. Just a repetative gurgle.

My first thought was it was my rear-most muffler starting to show wear. 16 or 17 years since I changed it.

James
63 230SL

[Douglas]

My former SL was a standard and it did this same thing on decel. Hans at H&R tweaked the fuel mixture quite a bit, but the thing that cleared it up once and for all was a Pertronix ignition system. It turns out it was an ignition issue and not fuel-related. It went away completely after years of light backfiring on decel (and though it sounded like "gurgling," it is backfiring.)

Douglas Kim
New York, NY
280 SL #016220

[J. Huber]

Well, I guess I'll concede they were backfires! But around these parts, when a car really backfires, everybody thinks Ol' Uncle Jed found himself dinner...

I am due for a tune-up later this month -- so maybe mine was just a hint.

James
63 230SL

[Mike Hughes]

My 230SL automatic has politely backfired on decel when descending from about 2500 rpm to 1900 rpm ever since I've had it.  One hears it as much from under the floorboards as from the rear.  Some 25 years ago the original owner of my 230SL had the center muffler and pipes to the rear muffler replaced at a Midas Muffler shop.  The front pipes and rear muffler are original to the car!  This was a fairly common exhaust system repair in those days in this area.  Anyway, after checking the fuel and ingition systems over (and dealing with a leaky CSV!) my mechanic recommended the next step should be replacing the ancient bitsa exhaust system with an original M-B system.  He has discovered over the years that the internal configuration of the Midas muffler used for these repairs seems to encourage this kind of backfiring in these cars for some reason.

- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havanna Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)

[J. Huber]

Hmmm. I too replaced my original with a Midas back in the mid-80s, I think. They seem to last okay. The center muffler (resonator) did finally give in in 2001, so maybe the tail portion is going too. I replaced the resonator with an Eberspaecher from Miller's -- I'll probably do a similar move with the rear.

James
63 230SL

[Cees Klumper]

My automatic does this light burble on decel. It did not do this when it had an incorrect ignition fitted, so I tend to think it is ignition-related indeed.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[Douglas]

Since the subject of exhausts has been raised as one possible source, I should add that my former SL had a factory exhaust.

Douglas Kim
New York, NY
280 SL #016220

[TR]

Interesting info.  I'm running a new long-block engine (from Metric), with electronic ignition (installed several years ago).  Funny thing is both the old engine, and the new engine do the mini-backfire thing.  I recently changed from a M-B exhaust system to a performance system ... no change when it comes to the little backfires.

I haven't done an injection pump rebuild (ever).  That's about the only thing that hasn't been changed.

We'll soon be going go over the timing, fuel mixture, double-checking the exhaust, etc. etc.  But I'm beginning to wonder if these "pops", "burbles", "gurgles", "mini-backfires" are just something I'm going to have to live with.  I hope not.  That irritating little sound has always bugged me.

Please keep the ideas coming folks...

Thanks.

Tom in Boise

[athadani]

Common problem and I have had 2 280 SLs that backfired and I kind of enjoyed the throaty thunder. This is to contribute my experience of having the engine head gasket replaced (joint de culasse en francais)

No more backfiring
No more over heating
Super smooth automatic gear shifts up and down
Runs like new

Ashok

[hands_aus]

All these cars seem to back fire on deceleration.
On some of the 280 SLs a second solenoid was added to the inj pump with the aim being to cut off the fuel supply when decelerating. Hence no unburnt fuel would enter the exhaust system.
Later 280SLs had both solenoids removed.
You could try changing your air fuel mixture.
I have my 250SL running just right and I can almost at will cause the backfire noise.
According to the BBB the 250SLs are known to run rich.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[TR]

Interesting that a new head gasket might fix the backfiring.  Sadly, it hasn't worked in my car though.

[Eryck]

My car is a manual and it does the same thing. Usually when I let go of the accelerator when coming downhill...

1965 230 SL White Manual
Hong Kong

[TR]

Eryck -- Yup, that's symptom and the usual cause alright.  At least it's good to know I'm not alone with this mini-backfire thing.

Within the next couple of weeks I'll have everything checked over.  Based on all this input, though, I'm not optimistic the decel popping will go away.

Tom in Boise

[Tom]

Tom,

That model came with one solenoid on the FI pump that is called a deceleration solenoid. Intended to reduce fuel on deceleration. Many owners/mechanics have the wire to these removed as they sometimes lead to unexpected stalling. Backfire is more prominent if the wiring to this solenoid is removed. Check this out.

Backfiring is also attributed to the ignition system as Doug mentions as well as incorrect timing.

Best,

Tom

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

[TR]

Tom -- Thanks much for that input.  I'll definitely have it checked.

Tom in Boise

[Randy Woodward]

Tom, how are things coming with your back-firing problem? I also have a 1970 280sl 4-speed, which I got a couple months ago. I've had some work done to the engine, and the Pertronix ignition was the best change I made. However, I believe my timing is still off. I get those little annoying back-fires as well, just you describe. However, when I sustain 3000rpm or higher, I get some horrific back-fires with a ton of black smoke, but again, totally cool at 2500rpm or lower. Pretty weird.

Let me know where you are at with this problem. Given we have the same vehicle with the same problem, we need to keep each other updated.

Thanks, Randy

quote:Originally posted by TR

My 4-speed 280SL produces small intermittant popping sounds from the exaust when I take my foot off the accelerator and the car slows under its own compression.  These noises are occassional, and sound sort of like small backfires although are muffled way down.  I'm not sure if this popping is normal; or if some tuning adjustments might take care of it.  It's not a big deal.  Just sort of annoying, irritating.  I'd like to make it go away if practical.

Any thoughts / suggestions appreciated.

Tom in Boise
'71 280SL 4-spd signal red w/lt. tan interior

[Dick M]

Tom..
Could it be "blow back" from the after burners you installed?
My 280 also makes little popping noises when going down hill with foot off the gas... seems normal.

Dick M
1970 280SL

[nick350]

I have had this too and It was in the main related to what the Elecronic ignition did to upset the established way the engine was running. It's just a matter of tweeking I think. Also I think that we perhaps spend too much time doing stuff "by the book" rather than listening to what the engine needs and responding appropriately. I dont do any of this myself so in a way dont know what Im talking about, but I did have my car in and out for constant tweeks, with one thing affecting the other until finally it all came together and runs like a dream. Makes sense to me.

Nick350
r107 SL red
w113 SL white

[jameshoward]

quote: I dont do any of this myself so in a way dont know what Im talking about, but I did have my car in and out for constant tweeks

Classic. Love it!! (And couldn't agree more).

My car makes exactly this sound, especially when going down hill, foot off the accelerator. I don't have an electronic ignition and it runs too rich. Still trying to sort that out.

JH

[Randy Woodward]

Well fella's, it apears from this thread and another I have that I'm going to just have to keep tweeking it. There doesn't seem to be any magic formula but that. So help me out here, I guess you set the timing at different rpms? Is that right? I have no idea how that works, but given that my car is excellenet 2500rpm and below, and awful at 3000+, I'm going to need to make adjustemnts at those higher rpms....

[tuultyme]

Randy
If you are having difficulty over 3000 RPM you may be having issues with your speed relay not functioning.  I had mine rebuilt a couple years ago.  Search the site for speed relay for further discussion about this.  The speed relay is installed in 70 & 71  cars.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[TR]

Hello Randy and Dick,

Looks like it's been awhile since I posted this message.  But not too long afterwards, maybe about this time 2 years ago, we found an aged and retired M-B mechanic who was willing to do a tune up on my car.  Others, also seemingly well qualified, had tried and failed; but that kindly & wonderful old gentleman was able to get the 280SL running more smoothly than anyone else had, plus the popping sound that occurs during deceleration all but disappeared after he was finished.  Unfortunately I do not know what he did to make that happen, and I am sad to report that I'm now unable to ask him.

I know I am not providing any information that is the least bit helpful, and apologize for that. But whatever the man did, well, without any major effort or expenditures on parts, he was capable of making that irritating little “pop-pop” sound go away.  I hope someone knowledgeable about this symptom might be able to provide some useful insight for those who are living with this same vexing problem.

Tom in Boise
'71 280SL 4-spd, signal red w/lt. tan interior, restored/enhanced

[dendrinos]

Well i would like to find such a kind old man. Mine makes this same sound. It was totally rebuilt perhaps 10 years ago, but has been sitting since then.

It does seem to run well other than that. Other than the kick down not working . Need to look into that.

Pete

280 Sl
Michigan, USA