450 SL in Denmark

Well, here is an update on the almost ever lasting (same) problem. As a recap, the engine starts up fine and idles fine. There is an occasional miss here and there at idle. Once in gear it misses quite significantly and is difficult to get moving from 0-60 km/h. Once up to speed, it pulls nicely and evenly. Ocassionally, I experience that the engine is behaving normally and pulls as it should from 0 km/h, but then a day or two later, the same symptoms appear again. I feel I am chasing a ghost.

Here is what has been checked/replaced over the past several weeks:

• Fuel filter replaced
• Fuel strainer in fuel tank replaced
• New fuel hoses at fuel pump assembly
• Fuel venting system checked
• Fuel pressure OK
• Capacity check OK (1 L in 30 sec)
• New ignition points
• Dwell OK
• MAP sensor holding vacuum 
• Smoke test to check for vaccum leaks - my mechanic says there are no leaks
• Timing OK and the centrifugal mechanism is working as mechanical timing advance is working at 1,500 and 3,000 rpm
• When pulling the vacuum hose from the vacuum can at ignition distributor, the timing advances 5-7 degrees - so seems to work
• Tried to connect another working ECU - no difference
• All injectors are ticking steadily and evenly (although two injectors have "softer" pulses than others (have tried to switch them with other injectors to verify if it was the trigger points, but the "soft" pulsation followed along, so likely not trigger points)
• When I connect a T-piece to the MAP sensor and measure vacuum at warm idle I get steady-ish (perhaps sligtly fluctuating) 13-14 inHg or 340-350 mBar - that seems low. 
• When stepping on the throttle, the vacuum goes to 0 and then jumps to 20 inHg or 500 mBar - after that it settles again around 13-14 inHg. - I have inserted photo 

I am beginning to suspect that I might have a vacuum leak after all, even though my mechanic says that he has checked it and found nothing. What else can explain the low vaccum in the manifold?

Any thoughts?

Best, Peter

Hi Peter,

have you ever done a compression or better a pressure loss test on your engine. If piston rings or valves are not tight, you might also have low level of vacuum in manifold. That of course makes MPS to signal wrong load to ECU and enrich mixture. That would fit to low vacuum and problems in low rpm.

Also check all hoses connected to manifold whether they sit tight or loose.

Hi Volker,

Thank you. I will start by checking valve clearance. I have bought the correct wrench Hazet 329-4 on Kleinanzeige which will hopefully reach me when I'm in Hamburg next week.

The strange thing is that the behaviour of the engine is totally arbitrary. The other day it ran flawlessly across the speed range and during acceleration only to be totally unpredictable the next day where it did not want to accelerate but ran fine when up to speed. This afternoon, I just took it for a short trip to see how it behaved today and here it also missed and lacked power even when acceleration from 70 km/h onwards. This erratic behavoir makes it extremely difficult to diagnose the real underlying problem. I would suspect that if the rings are worn or the valves are out of adjustment or there is false air, then the symptoms would be the same all the time and dependent on load, speed, etc. and not erratic.

Best, Peter

Could the fact that I have two injectors whose pulses are softer, yet still regular, suggest that they are not providing enough fuel into the combustion chamber, hence leading to too low a manufold vaccum? I have not yet performed a volume and spray pattern test.

Hi Peter,

wrong injector pulses would affect mixture - meaning A/F ratio - but not vacuum level. Pistons moving down create vacuumwhen exhaust valves are closed and intake valves are open. If air can comes from another source, vacuum level will be low too.

So sealings under injector are a source, but injectors are not.

Well, I finally got around to pull the valve covers off - initially to adjust the valves. Before I got that far I went to check the timing chain. Here I discovered that the chain has either stretched or maybe even jumped a tooth as the timing marks on the cam do not line up with the TDC mark on the crank pulley. I think that I may have found the culprit of the low engine vacuum. It seems indeed that the valves and the pistons are not playing the same accord at the same time.

The first picture is of the RH cam with the crank pulley at TDC and where the timing marks do not line up. The second is the same place but with timing marks lining up at the cam but now the crank pulley is several degrees retarded.

Best, Peter

Hi Peter,

well that is of course a reason for wrong valve timing and vacuum. Did you not tell that you have been to a garage to look after your car? They should have checked that.

Now check chaing guides whether they are broken. You should still have steel guides. Those you can continue to use. If they are plastic, replace them. Then insert a new chain. If a tooth jumped over, it means 18° difference. You should also replace chain tightener.

BTW I received my ECU back and also another nice package. I say thank a lot you and will test soon. I am a bit tight in time as I had a lot to do and will now go for a motorbike tour in Alsace.

I’m glad to hear that both items arrived safely. The original steel guides are unfortunately no longer there, so it must have had a chain job done in the past. I wish you a good trip to Alsace. I myself are going to Munich tomorrow morning for my personal inauguration to the Bavarian Oktoberfest.

Best, Peter

Hi Peter,

have fub on Oktoberfest and do not forget your leather trousers (Krachlederne) ! I hope that you have a reserved table.

Good morning everyone,

The weather here in DK is nice so I took the Merc for an early run for the first time this season and I am still experiencing that the engine jerks/sputters under acceleration. When giving it some more throttle it evens out (for the better but not 100%) and begins to run more evenly. Under full throttle there is no problem and when cruising at 80-90 km/h it also runs OK. At idle at operating temperature it sputters/phuffs from the exhaust and it smells of gasoline.

When it is cold and during warm-up (while driving) there are no symptoms whatsoever - it seems to only show when the engine has reached full operating temperature.

Since it apparently is temperature sensitive I have checked the air and coolant temp sensors to see what ohm they read when warm:

Air = 180 ohm at 28 degrees and 340 ohm at 17 degrees
Coolant = 230 ohm at 90 degrees (operating temp)

Ignition has been checked and verified. Dwell at 30 degrees and timing at idle is 5 degree ATDC and absolute timing also OK. New NGK BP5ES spark plugs and BERU ZEF406 wires.

Fuel pressure is exactly 2 bar. I've also done a leak test to verify if the/some of the injectors are stuck open. It shows a fuel pressure of 1.8 bar after 20 minutes i.e. a loss of 0.2 bar. That should be OK.

The trigger points have been adjusted using Norberts tool. 

The potentiometer on the ECU is turned completely to the lean side (counter clockwise seen from the mounting position) and remember that it is still running rich also at idle. It have no ability to make a CO reading, but I am quite sure it is running rich.  

Any ideas?

Best, Peter

Hi Peter,

how is ignition timing and dwell at 1500 and 3000 rpm? And what vacuum does your MPS see at idle?

Hi Volker,

Here are the readings
      
                         Idle           1500 rpm            3000 rpm
Dwell             30 dgr.          30 dgr.                 30 dgr.
Timing 1        5 ATDC
Timing 2                            15 dgr. BTDC       20 dgr. BTDC 
Vacuum 1       12,5 inHg (315 mmHg)
Vacuum 2       16 inHg (410 mmHg)

Timing advance is right on spec and so is dwell at idle, 1500 rpm and 300 rpm.

I have used two different vacuum gauges, and they show different results - see also pictures. I tend to rely mostly on the hand pump version, which is also the one showing the lowest reading. That reading is too low.

With a warm engine I also performed a smoke test to see if there are any obvious vacuum leaks. That did not show anything. No leaks. These tests tell me that my problem is probably not injection or ignition related, but rather mechanically related. Could be many things, such as piston rings, wrong valve clearence, etc. But it points towards a compression test and even better a leak-down test, correct? 

 

Best, Peter

Hi Peter,

that vacuum level is too low in idle - even if both pressure gauges are scrap. I use a class 1.0 100mm pressure gauge -1 to 0 Bar.

Either you have a vacuum leakage or piston rings or valves are the issue. You should have >500 mBar at idle. Your vacuum level also makes the MPS signal load to ECU where there is no load.

And yes you should do a pressure-loss test and buy a proper vacuum gauge.

Thanks, Volker. I agree. I have arranged that the car will go to the mechanic next Sunday where he will perform a leak-down test. I'll report back.

Best, Peter

Hi Volker et al,

Here are the test results of a compression test. I've enclosed a photo as well, but will table it out here:

Cyl. 1 ==> 10 bar and spark plug looks fine
Cyl. 2 ==> 10 bar but spark plug looks a bit fouled (rich)
Cyl. 3 ==> 11 bar and spark plugs looks fine
Cyl. 4 ==> 10 bar and spark plug looks fine
Cyl. 5 ==> 10,5 bar but spark plug looks a bit fouled (rich)
Cyl. 6 ==> 11 bar and spark plug looks fine
Cyl. 7 ==> 11 bar and spark plug looks fine
Cyl. 8 ==> 10,5 bar but spark plug looks a bit fouled (rich)

It cannot be faulty trigger points as the fouled spark plugs do not sit in the correct groups.

I'm surprised about the compression levels as this is supposed to be a low-compression US-spec engine. Is this normal?

Is the next logic place to look the injectors themselves? I've already made a pressure test and the fuel pressure only drops 0.2 bar after 20 minutes and that is OK. Could it still be that these injectors are stuck open?

What now?

Best, Peter

 

Hi Peter,

looking at spark plugs, your injectors in 2, 5, 8 are from 3 different groups. So it cannot be ECU. Pressure loss test was done. That should exclude valve shafts and oil rings as well. Have you done a cleaning test of injectors and have you verified ground connections of injectors?

Thanks, Volker.

The wiring harness is new, but I will check the grounding connections to be on the safe side. I will also try to find someone in DK who can clean the injectors professionally.

Have you any experience with the solonoid in the injectors going bad, so that the injector does not close fast enough? - meaning that it continues to spray even if the signal is no longer there?

Best, Peter

Hi Peter,

well injectors can clog and normally that blocks them. But sometimes needle is fixed in a way that it cannot close and leaks permanently.

and if it is the injector, the problem should wander to next cylinder if you swap injectors.