123 backfire - detective needed

[Mr H]

Suggestions needed. My 123 230 started to sound odd - fine on starting but after 1 or 2 mins when warm the engine note changed quite noticeably - real grumble .... needed a service so new plugs, timing fine, mixture ok (and had to replace exhaust at same time) and now she spits badly (nearly a backfire) on overrun - particularly when coasting with foot off the throttle -and particularly downhill. Any suggestions to avoid local MB charging like a wounded bull for a diagnosis which usually involves replacing everything!

 

[star]

Suggestions needed. My 123 230 started to sound odd - fine on starting but after 1 or 2 mins when warm the engine note changed quite noticeably - real grumble .... needed a service so new plugs, timing fine, mixture ok (and had to replace exhaust at same time) and now she spits badly (nearly a backfire) on overrun - particularly when coasting with foot off the throttle -and particularly downhill. Any suggestions to avoid local MB charging like a wounded bull for a diagnosis which usually involves replacing everything!

did you fit an after market "cheap" exhaust, cos they sometimes pop on the over run.

 

[Mr H]

No, backfire was noticed after service but before exhaust fitted - decent quality exhaust anyway. The spitting is much louder/more "explosive" than the tinny "pop" of cheap exhaust.

 

[Amphiscian]

Carb or injection? With injection there shouldn't be any fuel injected on overrun above 1200/1300 rpm so no popping and banging. Can be quite easily put out of adjustment (and readjusted). Not sure about carbs but suspect similar set up.

 

[panason1c]

No, backfire was noticed after service but before exhaust fitted - decent quality exhaust anyway. The spitting is much louder/more "explosive" than the tinny "pop" of cheap exhaust.

'Popping/backfiring' on the overun is usually caused by a leak on the exhaust system..........while the engine is at tickover have someone cover the end of the exhaust pipe with a rag (or whatever) and listen for any 'blow' from joints along the length of the system.

 

[Mr H]

Still no cure to backfire!

Hi guys - still haven't solved this problem.
Backfires when under engine braking - most frequently occurs when going downhill with foot off throttle. Pops back rather noisily on overrun when revved in neutral.
'Tis an injection system.
Have eliminated exhaust leak as a cause.
Checked vacuum pipes and 1 (retard?) had a leak - fixed but still got the problem.
Gonna try some super-redex additive in case it's just carbon build up but I'm starting to clutch at straws now...
any suggestions would be gratefully received!

 

[Legoman]

your fuel/air mixture could be too lean , that will cause backfiring too.

 

[panason1c]

No, backfire was noticed after service but before exhaust fitted - decent quality exhaust anyway. The spitting is much louder/more "explosive" than the tinny "pop" of cheap exhaust.

As i understand the above, are you saying that all was well untill the exhaust was fitted?...........if so, the exhaust sounds like the obvious culprit.....check again that all the joints are perfectly sealed....also check that the seams of the silencer boxes are not leaking.......a leaking exhaust will suck in fresh air on the overun, this air mixes with any unburnt gases in the exhaust system and the result is mini explosions as the mixture ignites.

 

[Mr H]

No, I'm afraid I didn't explain the sequence very well. Strange noise/different engine note noticed one day - grumble under load when warmed up - so I serviced it which included mixture check (ok) -> immediately afterwards the changed-engine-note-when-warm was still there and I also noticed spitting/backfiring on overrun -but there was some blowing from rear box so replaced that to eliminate that possible cause (and, of course, the middle section fell apart so replaced that too!) - and the-changed-engine-note now couldn't be heard (!) but the backfiring was still there. On longer drive, I noticed that the backfire was (and is) particularly noticeable and frequent under engine-braking.
I and chums have checked mixture again; exhaust leaks (no hint of a one!); advance/retard vacuum pipes (one had a pin-hole -> replaced it -> very advanced ignition -> adjusted -> still backfires); and sprayed carb cleaner on pipe joints to see if there're any obvious leaks; currently got redex-type additive in the fuel in case there's carbon on exhaust valves causing after-ignition.
I've got this niggling thought that it could be related to the various warm-up regulator / ignition advance/retard boxes/senders etc etc which I don't fully understand and not sure where to start with. Especially as there was the "only after warm-up" change in the engine note noticed at the start of this story!
But she's very pretty!

 

[panason1c]

Are the new spark plugs the correct ones?...gaps correct?.........(heat range could be wrong)...try putting back the old ones and see if it still backfires.

 

[Mr H]

Will try that tomorrow - gaps are ok but will see if plug swap helps.
Also gonna try replacing the vacuum pipes one at a time and investigate advance/retard with a timing light to see what occurs.
Any timing degree data - tickover/advance/retard - available?

 

[Jim Lambert]

Anyone given any thought to the exhaust valves themselves? Sometimes a misfire can be caused by a heat source such as a small leak on the seat or carbon deposits becoming hot. If the valve springs weaken, or one spring of a multiple pair fails, the stems can also stick a little, or the valve can `bounce` and remain partially open, effectively being sucked off it`s seat by the gas flow on overrun. The springs can be checked by applying measured pressure to the top of the stem and friction by the difference in pressure in each direction of travel. A compression tester may show a faulty valve, but deciding whether it`s inlet or exhaust is a little more difficult! A leak-down type test where air is applied under pressure through the spark plug opening can be used and a stethoscope will indicate where the leaks are. These are aero engineering techniques - my first meeting with the `tired spring` syndrome was while flying a light aircraft, Continental 0-200 power, when I was `strongly advised` not to completely close the throttle on final approach, for fear of waking the occupants of the cemetery below.... I only did it once, which goes to show that experience teaches more than instruction.

 

[Mr H]

Many thanks for the various responses.... talk of aero engineering techniques makes me think I've definitely got to the end of my abilities to investigate this - perhaps I arrived there long ago! So it's booked into the nearest indie garage....
If it's an interesting diagnosis/resolution then I will share the tale for the greater good... if, as I suspect, it's just pure ignorance on my part, then let's hear no more about it - surely we've all got better things to do - it could happen to anyone etc etc

 

[johnny230e]

No, I'm afraid I didn't explain the sequence very well. Strange noise/different engine note noticed one day - grumble under load when warmed up - so I serviced it which included mixture check (ok) -> immediately afterwards the changed-engine-note-when-warm was still there and I also noticed spitting/backfiring on overrun -but there was some blowing from rear box so replaced that to eliminate that possible cause (and, of course, the middle section fell apart so replaced that too!) - and the-changed-engine-note now couldn't be heard (!) but the backfiring was still there. On longer drive, I noticed that the backfire was (and is) particularly noticeable and frequent under engine-braking.
I and chums have checked mixture again; exhaust leaks (no hint of a one!); advance/retard vacuum pipes (one had a pin-hole -> replaced it -> very advanced ignition -> adjusted -> still backfires); and sprayed carb cleaner on pipe joints to see if there're any obvious leaks; currently got redex-type additive in the fuel in case there's carbon on exhaust valves causing after-ignition.
I've got this niggling thought that it could be related to the various warm-up regulator / ignition advance/retard boxes/senders etc etc which I don't fully understand and not sure where to start with. Especially as there was the "only after warm-up" change in the engine note noticed at the start of this story!
But she's very pretty!

hi just wondered if you have resolved your backfiring problem on your car. I have developed a problem with similar symptoms and am looking for more suggestions ...john