Car shuts off

[MikeSimon]

New member here (see introduction) and don't know if this is the right place for this question. Need some help!

I have a driveability issue with my 1970(71) 280SL.

It will start when cold and run for a while until warmed up and then shut off. Will not restart until I wait for a while for it to cool down.
Then it will start and run for a while and shut off again.
The time the car runs is getting shorter and the time I have to wait is getting longer.
When this problem first occured, I asked a lot of "experts" and tried to do some research but could not get a definite answer for a solution to the problem.
A minor thing I did was change the capacitor on the distributor.
At one time, I heard that in cars that aren't driven on a regular basis, where rust and crud deposits on the bottom of the tank, these deposits will clog up the fuel return and the fuel pump will overheat and shut off.
After the car now sat for 20 years, I have finally decided to tackle the problem.
I plan to take the fuel tank out and clean/replace it. Also will replace the fuel pump.
Before I get there, I took the cylinder head off to see, if everything inside the cylinders is still O.K. It is! (lucky me)
next is cleaning everything up and putting the head back on before I address the tank.
Anybody any thoughts?

[Benz Dr.]

Could you remove the head gasket so we can see the top of your block?

[MikeSimon]

Sure can. What would the top of the block show?

[MikeSimon]

Here you go..

[Benz Dr.]

Clean of the top of any piston and write down the piston size. I think it may have been rebuilt once since it looks like it has low compression stepped pistons. Looks fairly normal but it's hard to tell by looking at a couple of pictures. For sure it's a late 280 engine.

[MikeSimon]

How does this relate to the running problem? The motor at one time threw a rod bearing, but the cylinders were not touched as far as I know.
The pistons are stepped.

[Shvegel]

Dan,
Is that a factory crown design?  I Have not seen that before but have seen it on some English pistons.

Mike,
I would strongly suggest replacing the valve guides while you have the head off.  The original guides have a tendency to slip down in the head and begin travelling up and down with the guides which causes a big oil leak and smoke off idle.  The updated guides have a step in them to keep them from slipping down.

Have you checked for spark when the car won't start?  It sounds to me like it might be the screen in the tank or the screen on the fuel pump intake.  Fuel pumps to the best of my knowledge don't shut down when they get hot but if a screen gets clogged it will clear marginally when the car shuts off.  I would test everything before condemning the fuel pump.  New pumps are expensive.

[MikeSimon]

Thanks for the advice. I will do all the usual jobs that should be done while everything is apart.
Reading through some of the threads here, potential problem areas causing hot engine shut off are fuel filter and the WRD on the FI pump.
Don't know whether the symptoms are exactly the same, though.
What puzzles me is that the engine shuts off and requires a cool-off period to restart.
I have a new fuel pump already. Also new fuel filters and injectors. Even have a used FI pump. I had 20 years to collect parts..not knowing what I would need.

[Shvegel]

When it dies does it die gradu
ally or is it like shutting off a light switch?

[Pawel66]

MIke, this cool-off time you say the engine needs to start again often is the time gasoline needs to soak through clogged filters and fill the last filter reservoir. I think these shut-offs may be symptoms of fuel starvation.

[MikeSimon]

It dies like I flipped an a off-switch. I agree, I think it is a fuel issue. That's why I believed the theory of the fuel pump overheating and shutting off.

[Cees Klumper]

As the engine dies suddenly rather than slowly, I would check the spark as I suspect an electrical/ignition culprit, more particularly the coil. If you have a spare coil I would try that. Or perhaps you have the original electronic ignition from 1970/71 that's malfunctioning.

[MikeSimon]

. Or perhaps you have the original electronic ignition from 1970/71 that's malfunctioning.

What is the "original electronic ignition"?  The car has points and a distributor.

[Iconic]

Mike,
The search tool is your friend.
Many, if not most, issues have been written about.
1. If you car has not been modified it has a distrubutor, points, and an electronic box under the battery which basically reduced the current going through your points which increased their life.
2. Your shut down failure is described exactly as a lack of fuel flow issue. Go ahead and simply check to make sure you have spark, then you can forget about the ignition. Measure the fuel flow at various parts of the system. When I did this I found a clogged fuel return pipe going back to the fuel tank. I drove the car for years (~8) after clearing out that clog (then sold it). I know it "feels" like an electrical issue the way it shuts off, but it was a fuel issue for me.
I went through eactly what you are going through in 2009 (except I didn't take the head off   ).
Good Luck !!
Mark

[Benz Dr.]

Dan,
Is that a factory crown design?  I Have not seen that before but have seen it on some English pistons.

Mike,
I would strongly suggest replacing the valve guides while you have the head off.  The original guides have a tendency to slip down in the head and begin travelling up and down with the guides which causes a big oil leak and smoke off idle.  The updated guides have a step in them to keep them from slipping down.

Have you checked for spark when the car won't start?  It sounds to me like it might be the screen in the tank or the screen on the fuel pump intake.  Fuel pumps to the best of my knowledge don't shut down when they get hot but if a screen gets clogged it will clear marginally when the car shuts off.  I would test everything before condemning the fuel pump.  New pumps are expensive.

Late 230SL and some 280SL pistons only come in the stepped version. With the late 230SL you can change the connecting rods then go with the earlier version pistons which I would highly advise as the late stepped pistons will only produce about 150 PSI compression and and a 230SL needs every bit it can get. 280 stepped pistons also lower compression.

[Benz Dr.]

How does this relate to the running problem? The motor at one time threw a rod bearing, but the cylinders were not touched as far as I know.
The pistons are stepped.

You removed the head so I'm commenting on that. Knowing the size of the pistons will tell you if the block has been bored.

[MikeSimon]

Mike,
The search tool is your friend.
Many, if not most, issues have been written about.
 2. Your shut down failure is described exactly as a lack of fuel flow issue.    When I did this I found a clogged fuel return pipe going back to the fuel tank.  Mark

Thanks, Mark! That's exactly what I had heard a while ago, and it referred to a blocked return in the tank due to debris.

[MikeSimon]

You removed the head so I'm commenting on that. Knowing the size of the pistons will tell you if the block has been bored.

I removed the head to make sure the inside of the motor was OK and not rusted from sitting for 20 years.

When a motor is bored, the compression increases. You are pushing a larger swept volume into the same size compression chamber. In order to retain the same compression (ratio), it makes sense to use a piston with a lower compression crown.
I tried to measure the bore and it is difficult to do the way the motor sits installed, but it is definitely larger than stock. It looks like it is 1mm over.
Again, nothing really of consequence as far as my running issue is concerned.

[kampala]

Mike,

I am not experienced like many on this site.  So take this for it might or might not be worth.

not commenting on head removal.

Assuming the car starts fine when COLD and runs fine for few minutes until warm and then shuts off.

Check Cold Start Valve (CSV).  If it starts cold nicely, your CSV is probably dumping fuel as it should when cold. 
However, might not be shutting off so it could be flooding the engine when warm.   You can test by physically blocking the fuel line at CSV.  Disconnecting Elec at CSV may not stop a faulty valve so you must disconnect and plug the line to confirm you are not providing fuel.  It may be harder to start cold with the line blocked but should start after longer cranking and see if it continues to run.   

WRD - this is also a common culprit.  You will read a lot about it, but need to make sure it shuts off when car warms.

Good luck.

[MikeSimon]

You removed the head so I'm commenting on that. Knowing the size of the pistons will tell you if the block has been bored.

Good eye, Dr Benz!

After cleaning a piston, the stamping relieved a "86.97", which should make this a 0.5 over???

What are the other marks? especially the 50 M/M ?

[Jordan]

Mike, the WRD is probably the easiest and quickest to check.  Once you have confirmed the CSV and WRD are functioning properly you can start looking for fuel blockage.  There are 3 filters to check, one in the tank, one in the fuel pump and one in the engine bay.  And check the flow rate on the return to the tank with the pump running and engine off.  There is lots written up about this.

[MikeSimon]

I cannot find any reference to the CSV in my manual. Where is it? What does it look like?

[Benz Dr.]

Good eye, Dr Benz!

After cleaning a piston, the stamping relieved a "86.97", which should make this a 0.5 over???

What are the other marks? especially the 50 M/M ?

I'm not sure about the 50 m/m but Sp is the running clearance which looks like .03 mm. Piston sizes are .5 mm in size increases and I think standard is 86.50 mm so maybe it's indicating .50 mm over size from standard.

.50 mm oversize isn't enough to increase compression ratio on any of these engines. If anything, they never seem to have enough compression after a rebuild because replacement pistons are slightly shorter from the pin to the crown. This is so that you can mill the top of your block to clean it up. You want your new pistons to actually stick out slightly from the parting surface so trust me, you don't need to decompress the piston crown. 

I milled about .040'' off the top of my block which gave me around 180 PSI compression. It's why we call it the Red Rocket. 

[kampala]

CSV


https://www.sl113.org/wiki/Engine/ColdStartValve

Right side of engine bay at intake manifold. 

[Shvegel]

Cold start valve only comes into play at startup.  Not the culprit.