450 SL in Denmark

Hi Peter,
I agree there are no signs of rich running to be seen on the plugs. More than 100 km should be more than sufficient to show abnormalities.

All your readings seem to be o.k. for me, too...I've read all through the thread now and thought about the symptoms.
I don't remember having read about the MPS having been confirmed to be correctly adjusted.
Has this been done?

Regards
Norbert

Hi Norbert,

The MPS could be the culprit. However, I acquired it from Volker and I am sure he was diligent in his testing before selling it. It holds vacuum.

Before I took the car for another run today I performed a smoke test to see (again) if there are any false air leaks. That was not the case. I then took it for a run and from cold and up to operating temperature it runs and pulls like a champ. When it reaches operating temperature and when accelerating moderately until ca. 2000 rpm it stumbles and jerks. Once the rpm is between 2-3000 or if I accelerate hard, it goes away. Not completely, but much better. At cruising speed (70-90) there is practically no issue. At highway speed (130) it drives and accelerates without problems.  

When I got home and with a really warm engine I performed yet a smoke test and suddenly I saw smoke coming from below and behind the throttle housing. At first I thought it came from the TPS sensor shaft, but after having carefully looked again I am pretty certain that it comes from the intake manifold behind the throttle housing. False air would certainly "confuse" the MAP sensor into believing that the engine is under load, thus sending a signal to the EDC to increase opening times, right? 

Could I perhaps be on to something?

Best, Peter  

Hi Peter,
if I had known you have a MPS from Volker, I wouldn't have considered it could be faulty.

Yes, you are correct: ECU injects more fuel when the absolute pressure in the manifold rises.
A vacuum-leak lets the pressure rise...

Regards
Norbert

Hi Peter,

sorry to hear that you are still in search of your problems and sorry for late reply due to holidays. Any false air leakage causes troubles. The lower engine load the more it will be. MPS will report wrong load to ECU and that will enrich fuel mixture.

Thanks, Volker. At least I get to clean the valley, which was full of old leaves, etc. You can clearly see that it has been hot down there and I think I'm lucky that it hasn't cause a fire.

From the picture cylinder 1 and 4 and maybe 7 do look as if something has been going on there. There are no apparent signs of leaks in the intake gaskets, but the rubber grommets between the lower and upper manifold are very hard. I have not inspected them in detail yet.

Best, Peter   

Hi Peter,

it lloks that you are on right track. Get some Dunkin Donuts and replace. 

Hi all,

I have now had the intake manifold and the valley cleaned which is nice in itself. In addition, all the lower rubber donuts have been renewed and so has the actual gaskets between the head and the manifold. 

Did it solve my running problem? Well, sort of. The car is indeed drivable and drives notably better. Smoother acceleration and a different shift pattern in the gearbox. Before, it would shift much sooner from 2nd to 3rd and now it goes naturally higher up the rev range before it shifts. It pulls evenly and steadily from 2000 upwards. From around 2000 rpm it runs beautifully. What is then the problem?

The problem is (still) that from 0 km/h and until the engine reaches 2000 rpm (no matter the gear), the engine stutters and there is intermediate "puffing" from the exhaust. It also smells of fuel. The idle is still rough and it smells of fuel. The idle screw on the ECU is turned all the way counter clockwise (i.e. lean). If I attempt to enrich the idle mixture, the engine reacts immediately and almost stalls.

So I have performed yet a new smoke test both with cold and warm engine and the conclusion is that it is tight (with a bellow on the throttle housing and opened flap). Only a slight trace of smoke from the throttle shaft and from two of the bolts for the cam covers. Hardly anything. I also measured engine vacuum which shows 680 mBar with hot engine. Prior to the renewal of intake gaskets and rubber, the vacuum was around 560 mBar. So clearly an improvement.

Any idea where to look? Can I still have a vacuum leak with 680 mBar idle vaccum?

Iginition timing is still on the mark and so is the fuel pressure (2 Bar). Dwell is smack on 30 degrees. 

Can the ECU be sending faulty signals to the injectors? Can something have happened with the MAP sensor? 

 





Best, Peter 

Hi Peter,

you are getting closer to solving your engine issues! Youcan be proudof yourself.

Hi all,

Yesterday I checked all the valves and it turned out that 5 valves needed adjustment. One exhaust valve could not be tightened any longer, which means that it has a 0,05mm larger gap than it should. The first impression is that the extent of puffing out of the exhaust has notably gone down. Now there is only an occasional "puff", maybe every 10th second, whereas it was almost always there prior to the valves being adjusted. I guess it means that I need to change the thrust washer (bigger, I presume?) which should allow me to tighten the valve a bit more and hopefully get rid of the remaining puffing. I wonder whether it is complex to change this washer. Any experience from you guys?

Anyway, since this is the first time I've adjusted the valves myself, I'm pretty satisfied with the result.

The issue with stuttering at lower rpm seems to be the same, i.e. unaffected by the valve adjustment. Now I need to mobilise the courage to pull the distributor, dismantle it, clean and lubricate and go through the hassle of setting the dwell.  

Best, Peter

Hi Peter,

check both: Valve clearance on hot engine and distributpr timing on cold and hot engine.

Thanks, Volker! Considering that the engine behaves totally normal with no puffing during warm-up and only starts to puff when it reaches operating temp., does that provide a pointer to where I should begin? Valves or distributor?

Best, Peter

Hi Peter,

what I have seen with old ignition distributors is that they tend to hang as grease and dirt is sticky. In such cases I take them apart, clean and test everything and adjust on test stand after new grease and reassembly.

Puffing in exhaust is typically wrong ignition timing or wrong valve timing.

Hi Volker,I have changed the vacuum hose and now I get 5 degrees ATDC at idle. Went for a ride and things are slightly better. Still judders at low rpm, but not as much as before. Still puffing randomly from the exhaust in idle. Mixture screw all the way counter-clockwise. I adjusted the timing to TDC at idle and around 15-16 degrees BTDC at 1500 rpm and around 22-23 degrees BTDC at 3000 rpm. (should be OK compared to spec, although in the high end) and went for a ride. That improved the low rpm driving (below ca. 1200-1500 rpm) even more, but there is still some juddering when pulling away from a red light, etc. and it still puffs from the exhaust randomly. I'm of course now over-sensitive and maybe expecting too much from an engine that regardless of good compression, etc. still has around 450.000 km on the clock.  

Best, Peter

Hi Peter,

it is correct that your timing at idle is wrong. It should be 5° AFTER TDC. You measured before TDC. It seems that your vacuum box does either not work or distibutor is badly moving. That should be corrected.

Hi there,

I have been looking at ignition timing and have arrived at these values after some adjusting.

Timing at idle: 5-6 degrees BTDC (a long way from spec of 5 degrees ATDC
1500 rpm: 12.5 degrees BTDC (within spec)
3000 rpm: 20 degrees BTDC (within spec)

That seems OK, although the static timing seems a bit too advanced. I later discovered that most likely my vacuum box on the distributor is not impacted by vaccum. The box itself holds vacuum so either there could be a fault in the change-over valve or a leak in the vacuum lines. Will check this tomorrow.

Anything to be worried about with the static timing? It has been raining heavily in DK so I haven’t had the chance to drive the car with the adjusted timing.

Best, Peter

Natürlich

Daimler-Benz

Hi both,

Thanks to both of you for your patience and advice. I'll measure and adjust.
"DB"? - apologies if I'm being a bit slow here 

/Peter

Hi Peter,

I fully agree with Norbert. Ignition timing and its change over rpm and vacuum is something you should check well. I once had a 350 SLC of a friend and his igntion distributor timing would create the problems you describe now.

More than 600 mBar vacuum in idle is a good value.

Hi Peter,
did you check the ignition timing following the instruction given by DB and noticed by Volker before?
It is really important especially in the lower engine rev's range.

I don't remember you reported having checked this in full.


Regards
Norbert