Engine stalls after start up

[wwheeler]

I have a chronic problem that I haven't been able to correct. My '68 M130 280 SE engine will fire up and then die unless I open the throttle to catch it. It dies either hot or cold. On a rare occasion the engine may bring itself back to life on its own, but not often. It only dies after a start up and never after revving or when coming to a stop in drive.

I have done the following to the engine: full tune up, valve adjustment, new spark plug cables, throttle linkage rebuild, checked all starting aids for operation, have a new 123 distributor, compression test (150 psi and even). The original fuel pump is starting to make a variable whining noise so I know it's on borrowed time. With all of the above work, the engine runs well and strong with only a twitch at idle.

Anybody have an idea what might be the culprit?   

[awolff280sl]

I had the same symptoms as you. Turns out it was related to the linkages in my case. I found out through many trials and errors that after replacing the ball joints and bearings, the linkage became alot more precise than before. After setting the FIP linkage rod to 233mm and using the alignment hole too, it was critical to be sure that no other linkages were pulling the FIP off of its rest position. Sounds easy, but the renovated system is so precise that I hadn't noticed that the throttle rod was just a hair too short and was pulling the FIP open just a hair as well. I came to realize this only after trying to balance my fuel/air/rpm/& CO, and running into problems, one of which was that dying after starting. The difficulties were all caused by this linkage setup that I wrongly assumed was accurate. Make sure that the FIP is able to rest on its stop screw before adjusting the mixtures to try to fix your problem.
 

[wwheeler]

Thanks Andy.

I went back and checked the linkage. I remember when I first rebuilt it, I set the FIP rod exactly to 233mm and checked it three times. I then tested it with the alignment pin and found it was a tiny bit too long (2mm). I checked it again and decided to leave it and then set the throttle rod. At that time, I checked the mixtures for the entire RPM range. I found all except idle to be lean. I added a .003 shim to the WRD.

Since your post, I went back and reset the FIP rod to whatever the alignment pin dictated. It turned out to be 231mm. I went back and rest the throttle rod as well. Both the throttle and the pump levers are definitely resting against their stops as they were before. I don't understand why the alignment pin doesn't match with 233mm dimension? I am not sure if 2mm makes that much difference. The throttle screw adjustment is correct since I had rebuilt a few months ago and then double checked it myself. Unfortunately, the stalling persists.

I think I may know what the cause of the stalling is. When resetting the linkages, I also split the linkage and rechecked the mixture at different engine speeds. At idle, it was OK. From idle to 1500RPM is definitely lean and the engine will buck if held in that speed range. Anything higher than that is perfect. I now just learned the the low range mixture screw is for idle all the way to 1700 RPM. I previously thought it was just for idle speed. So maybe I need to turn the screw in a few clicks to richen the idle below 1700 RPM, and then go back and adjust the air screw for a good idle. Seems like a very lean mixture at that speed could cause the engine to stall.   

What's the verdict?

[awolff280sl]

I think you are on to something with the idle screw, sounds logical.
Also, I think I understand that you were running rich above idle so you say that you added a shim to the WRD.
Would you do us both a favor and double check that adding a shim to the WRD will lean out the mixture. That would then be the opposite of what I believe you do with the barometric compensator to make it lean things out. Thanks.

[ja17]

Hello Wallace,

The injection pump check valves or check valve seals can also cause these issues. If they are faulty, they can allow the fuel in the injection lines to run back down into the injection pump, partially empying out the injector lines. The next cold start, the engine starts and begins to run on the inatke starting solenoid. After the initial start the engine dies when the starting solenoid cuts out and there is not yet fuel in the injector lines.

An improved check valve is available if your pump has the earlier ones.

[wwheeler]

Andy,

The following thread link talks about wrd shims. That is where I got my info at the bottom reply from JA17. When I added my homemade shims to the wrd, it did indeed richen the mixture across all ranges. The upper two ranges were then perfect.

http://www.sl113.org/forums/index.php?topic=8276.msg53291#msg53291

I also did adjust the low range idle screw this weekend. I turned the screw 5 clicks clockwise to richen the below 1700 rpm range. It did as advertised. To check the mixture, I manually pulled the throttle solenoid to hold the linkage at a steady rpm. With the solenoid manually engaged, the rpm is about 1300. Then with the linkage split, I moved the FIP rod back and forth to determine my setting was neither too rich or lean. I then reset the air screw for the correct mixture/idle speed. 

One thing that occurred to me from all of this is that the function of the air screw is irrelevant once the throttle is slightly open. Strange thing is that the low range FIP mixture screw is effective all the way until 1700 rpm. Yet you use both of these to set the idle mixture and idle speed.

Result: Car now comes out of the gates like a quarterhorse and is very strong on the highway. There still is a little twitch at idle and engine still stalls either hot or cold when starting(ARRRR)! If you don't catch the engine before it stalls the first time, it gets harder with each successive try. When it does start after stalling, it stumbles for second before clearing its throat and then runs OK.

Great thoughts Joe, but unfortunately it also stalls when hot just after running. I would have never thought about the check valves.

If nothing else, I am learning a lot about this system!

[wwheeler]

I tried an experiment today when starting the engine. I hoped to determine if the engine was rich or lean when starting.

I split the linkage at the FIP and started the engine with the throttle slightly depressed. This would allow more air since the injection pump rod is still and lean the mixture. The engine seemed to start better and didn't stall after it started.

I tried it once again with the linkage split at the throttle housing. When depressing the throttle and starting, this would add more fuel and richen the mixture. The engine stumbled horribly and quickly died. I repeated the first test and again the engine started better.

This leads me to believe that I am too rich when starting. I rechecked the mixture at 1200, 2000 and 3000 RPM and all seem to be right on. I recently added a .003 shim the WRD housing to achieve this. I know the heat feeler on the WRD has worn too rich and I am going to correct that. But the stalling happens both when warm or cold. Could the heat feeler effect the mixture when starting eventhough the engine is hot? I have also checked the cold start valve and the leakage is very small. The starting solenoid seems to be operating OK.   

My mixture at idle is slightly rich. However, if I adjust the air screw (lean) to the correct mixture, the idle speed is too high (1000 rpm). I have to richen the mixture and bring the idle speed down to 850. The engine runs smoother when at 1000 than it does at 850 rpm. If I lean the mixture at the pump screw to correct the above, my mixture from idle to 1700 rpm is too lean. I seem to be stuck and not sure what to do. I must be missing something. 

Thanks for the help.

[hands_aus]

I thought these engines were only adjusted for idle mixture.
The injection pump does lean out the mixture as the revs increase.

The injection pump thermostat tends to not work correctly as it ages.
They can be tested. I think there is a module in the BBB about it.
It would be fairly inexpensive to buy a new one.
The expensive ones are for the 230sl.

Also the WRD plunger must move freely.

The BBB says that if the COLD START VALVE leaks more than 3 drops a minute the CSV needs to be replaced or serviced.

Check the colour of your exhaust pipe.
If it is sooty black ... too rich
silver gray is ideal

look at the colour of the plugs
similar colours similar indications

tighten any loose linkages.

[awolff280sl]

What I have discovered, at least with my car, is that adjustments to the FIP idle screw after a reasonable point in the tuning process, must be made 1 click at a time. Trying to fine tune the idle without taking the car for extended drives at higher speeds often resulted in a different idle speed after these drives. I was only able to acheive a good and stable idle mixture & rpm only after a combination of "Italian" and "German" tune-ups.
If it were me, I would adjust the FIP and air screws so that your car does not stumble even if the idle is as high as 1200, and then take the car for a good work out. Upon return, start the process of adjusting your idle in a small increment at a time, and then take it for another good run. I repeated this process several times over several days until I could finally zero in on a happy mixture & rpm. It has been very stable at about 800rpm since, although a recent 600 mile trip seems to have raised the idle to about 900, at least for now. I think the idle is quite affected by driving times, speeds, and styles.

[wwheeler]

Andy and Bob,

Thanks for the wise advice. Funny Andy should mention the unstable idle before the "International Tuneups". My engine's idle speed does fluctuate and has been driving me crazy. I have heard from many who's advice is a good work out cures a lot of problems. The weather is still nice here, so I think that should be my next task.

My exhaust pipes are a bit sooty, so I will give it a good work out and see if they improve. I think I will rebuild the CSV as it was inoperative for many years by the PO because of a bad relay. I will measure the leakage first.

[ja17]

Hello,

Using the split linkage method, I first determine if the mid and upper range of the IP is rich or lean and  shim or un-shim the WRD to compensate.  Next I use the split linkage menthod to determine if the low range is rich or lean and then use the idle mixture thumb screw on the back of the IP and the idle air screw on the intake to dial in the idle mixture and rpms. Then re-check the low range by splitting the linkage and adjust the thumbscrew again if needed. Control the idle speed with the idle air screw on the intake. Remember splitting the linkage should produce not more than 100 rpms when operating the intake linkage when the engine is warm and at idle. When the idle mixture is dialed in correctly the intake  idle air screw should be able to control final idle rpms and you should be able to increase or decrease the idle with this screw.

In the real world, most idle time  happens  at traffic stops with the transmission in gear. So I do the final adjustment with the car in gear at idle. If everything turns out right the idle should be silky smooth and not change more that 50 rpms in gear or out of gear. Idle RPMs should be around 700-750 rpms.

[wwheeler]

Great explanation Joe.

Can you tell me what the approximate engine test speeds are for the low, mid and upper ranges?

Just as I am about to zero in on my idling and stalling problem, my original long fuel pump goes out. The motor has had a variable whine for awhile now. While I'm at it, I'm going to rebuild the CSV to make sure there are no leaks. I am hoping these two items might have had something to do with the problems I have been having.

As soon as I get this stuff fixed, I'll be at it again!

[ja17]

Hello Wallace,

Sorry to hear about the difficulty with the fuel pump. Yes, this indeed can cause running problems. Fuel pressure and volume variations can cause issues.

Low range is up to 1,200 rpms,  mid range is approx 1,200 to 2,100 and toop speed range approx 2,100 to 6,000. These figures are not exact but overlap slightly. 

If the adjustments take too long  you should take the car out for an exercise session to clean everything up so fresh adjustments will be acurate.

These "mechanical marvels" were pre-computer age. Good rational thinking and careful observation will guide you to success!

[wwheeler]

Thanks!

[wwheeler]

When you spilt the linkage on the pump side, do you check the the mixture with the throttle linkage (adding air) or do you use the pump rod (adding fuel) or does it matter?

[ja17]

Hello Wallace,

I usually use both. They should both verify the same diagnosis.  For instance, if the engine is running rich , moving the linkage going to the intake (more air), the rpms will increase greatly.  Next move the linkage going to the IP (more fuel), if the engine immediately faulters this also confirms the mixture is too rich.   

If the engine is running lean, moving the linkage to the intake (more air) will cause the engine to faulter.  Next moving the linkage to the IP (more fuel) will cause engine rpms to increase!  These reactions confirm a lean mixture.

[wwheeler]

The low range on the FIP (0-1200rpm), includes idle speed (with the venturi closed) and up to 1200 rpm (with the venturi open). Is it true that the air screw is only effective when the venturi is closed at idle?

If I understand this then, the mixture adjustment above idle (venturi open) and below 1200 rpm can only be done by the fuel mixture screw alone. In other words, the amount of fuel is what can be changed not both the air and the fuel. I am I on the right track?     

[glenn]

WW, You want the least amount of fuel it takes to idle at 800 rpm- then cut back on air to 750.  With a warm engine, adjust idle air screw to get max rpm.  If it is over 800 rpm, stop engine and lean out idle fuel with the knob on the back of the FIP.  Repeat(max rpm with air adjust and fixed fuel) until you have 800 rpm.  Then pinch air back to 750-this gives you 50 rpm of 'rich'.

[wwheeler]

Great news to report.

After installing my rebuilt original long type fuel pump and a rebuild cold start valve, the engine no longer stalls after starting! The engine starts up quickly now and exhibits no signs of wanting to stall. My pump had gone out a month ago and had to rebuild (I did the CSV as well). The report from the re-builder was that the pump was very tired and the CSV was gunked up.

I went and rechecked the small bolt on the rebuilt CSV after installation and unfortunately, it leaks worse now that it did before. I will have to have it redone before I worry about setting the mixtures since I know a leaky CSV will throw it off.

While this is great news, I would like to know which component was the likely culprit. Any thoughts which one did it?