Warm engine dies and wont start up

[Travis71280]

Hey everybody, I have been fighting the same issue for a while and am out of ideas. The car will fire up fine wiith the aid of the cold start, run for about 10-15 minutes until it starts getting warm and then dies and wont turn over until the engine is completely cool. Usually i can limp her back by running it off the cold start manual switch but the other day even that didnt work and ended up having to tow her with the truck. Its not the fuel injection pump overheating (fixed that) and its not because of vaccum build up in the tank (i popped the gas cap as soon as it happened to make sure). I'm thinking the problem lies somewhere after the injection pump. Any ideas? Something with the injector lines, check valves, or the injector itself causing vapor lock? The lines are cool on the injector pump side, but on the injector/intake manifold side it is scorching hot. Could there be a leak where the injector line comes into the injector causing this?  Sorry for all the questions, just stumped right now. Thanks for all the help in advance. By the way its a 1971 280SL and this car had been sitting a while before i started working on it.

[reggie]

Travis. dont suppose your barking up the wrong tree and its electrical.condenser or coil breaking down when it gets warm ?
Just a thought 
Reg

[al_lieffring]

Travis try connecting a pressre guage up to fuel line that goes to the cold start injector. Drive around with the gage connected and see if the fuel pressure is dropping off as you are driving.

If so, it sounds like you are about to join the "I'm looking for a new fuel tank club"

[Benz Dr.]

It could be the fuel pump, electrical, or a plugged return line. You'll have to do some basic testing before you start buying parts to fix things.

[Travis71280]

Well I don't think its an electrical problem because the engine is sputtering a little when trying to restart it (believe me I wish it was, a lot easier to fix) but ill check onh it if it happens again. The tank maybe at fault, l but it only happens when the engine is warm regardless of driving or not (had it happen just idling in the shop). Assuming its not the gas tank, what else could cause this? I'm thinking it has to be a vapor lock issue but could be wrong. Its not the electic fuel pump, just had it rebuilt and the return line is flowing now (thought that was it when i fixed it, but on the next drive it stalled again).

[Naj ✝︎]

 Assuming its not the gas tank, what else could cause this?

Could well be the tank.
Remove the return line from the tank and check if the line inside the tank is clear.
A choked line will stop fuel circulation and cause vaporisation in the electric pump rotor chamber.

Been there etc....

naj

[ja17]

Hello,

You can lift the rubber mat in the trunk and remove the round plug. You can then remove the fuel gauge sending unit and view the inside of the fuel tank. Use a flash light , be cautious of fuel vapors!  

If you see a lot of dirt and rust in the tank, it is most likely your problem.  Often times keeping the fuel level above 1/2 tank will temporarily cure the problem.  This allows fuel to spill over the top of the "flower pot"*  (*Dan's terminology, thanks! ) instead of trickling in the small opening at the  bottom.

See the information in the tech manual on the fuel tank for more details.

[julianmaba]

Travis - when you say the "engine won't turn over," I assume you mean to say that it does turn over, but won't start.

I agree with Reggie that it might be electrical (ignition). Just because it does sputter a bit on cranking does not mean it's getting a good spark (have you checked for spark on your spark plug leads)? I had a car that would not start (would splutter and try to start like yours - and it had spark at each plug wire), and it turned out to be the condenser (and it was a new one at that). Replaced it, and it ran like a top. Since the points and condenser are easy/inexpensive fixes — try replacing these before you dig into the FI - and see if this fixes the problem. Hope this helps!

[Travis71280]

Thanks everybody for all the help. The fuel tank shouldn't have any debris in it since I relined it last year using POR15 (but ill still check though) and the fuel injection pump is running cool when she dies and the fuel is definitely circulating. About the ignition part, what is the best way of testing it? Just run it until it starts dying, pull a sparkplug ground it on the engine head and see if she sparks? Or is there a better way? One thing I noticed is the coil gets hot after running it a while but I dont know if this is normal or not or how to test that too.

[Travis71280]

Something to add about the situation, usually I can nurse her back with the cold start valve engaged (except when I got stranded the other day) so it has to be something with the fuel system, right? Just throwing this out there, can there be something wrong with the injector line connections or the injectors itself that could cause this to happen? Still going to check the ignition system though.

[Iconic]

Travis71280,
You state you have flow at the return line, to the fuel tank, I assume.
I had flow too, but not enough.
Once I measured the flow at the return at the fuel tank, I saw I only had 1/5 of a liter in 15 seconds.
I cleaned out the return line.
Then I had 1/2 to 3/4 liter in 15 seconds. A huge improvement over 1/5 of a liter.
From that moment on, I've been driving the car, long distances.
So, I suggest measuring the flow at that point. If you have enough move on, if not, clean it out and enjoy the ride.
What ever it is, keep at it. These systems worked when new and you'll figure it out.
Keep us posted.

[Travis71280]

Well, I think she is definitely flowing but here is the thing, when I was having flowing issues before, the check valve where the return line comes into the injection pump was froze up completely blocking fuel flow. I had to end up drilling out the check valve after numerous attempts to clean it up and ended up putting an aftermarket 5lb fuel pressure regulator on the return line where it attaches to the fuel tank. After I did that the fuel injector pump ran so much cooler. I'm hoping to get to run it early this week and check the flow again to make sure the fuel is still flowing good just in case the fuel filter got clogged or the line itself like you said. Also a couple days ago I had the car idleing in the garage. She had a hard time idling at first (no cold running device, it completely froze up) because I just readjusted all the linkages again so everything would line up. She started running real strong and idling well once she reached 185F but once it got to above I'd say around 210F she started missing and died. I could restart her, but she would only idle for a little bit then die. As I said before all she did was just idle with an occassional rev. I'll definitely keep yall posted and thanks everybody for all the help. By the way, is there an easier way to test the ignition system than pulling the sparkplug to chech the coil? I do have a multimeter.

[reggie]

Travis, Try doing it in the dark like your garage or wait until it gets dark,Amazing what you can see better. Like the spark or tracking, Because you have had trouble before with the fuel side you may be blinded or fixed on that again, When it may be something simple, Hope you get it sorted soon.
Reg

[Travis71280]

Recken it would be a good idea just to replace the coil to see what happens (its only 40 years old ). There a lot cheaper than I thought and should solve any ignition related problems, right? I already had replaced all the ignition wires (dang squirrels chewed the originals up) and replaced the distributor cap previously.

[julianmaba]

Travis, I agree with Reggie - make sure the ignition system is working correctly BEFORE tearing into the fuel system (I can't tell you how many times I blamed fuel systems, for running problems only to find out it was the ignition!). To test for spark, unhook the high tension (spark plug) wire from the coil to the distributor. Place the distributor end of the wire near a ground (about 4 mm or 1/4 inch) — note that the rubber boot may prevent a spark, so use a screwdriver jammed into the connection if necessary and have the shaft of the screwdriver 4 mm from a ground. Have a friend crank the engine while you look for spark. Do this test both when it starts well/cold AND when it dies when hot — compare the differences.

Even if you do see a spark, replace both the points (gap them correctly, and be sure no oil or grease gets on your point contacts) and condenser anyway — very important — it's an inexpensive way of isolating the ignition system in your overall diagnosis (NOTE: To illustrate how, even when you have spark how sensitive ignition issues can be, I had an engine which would NOT idle smoothly (of course, I blamed the fuel system, vacuum leaks, you name it)!. I had spark at each plug (so assumed the ignition was in good shape), but in desperation (after almost replacing the carburettor) I finally replaced the spark plugs.  It turned out that the shop who last tuned the engine (this was a Range Rover V8) had installed the wrong spark plugs. Installed the correct plugs, and voila - I had a beautiful, dead smooth idle. So, just having spark is not enough — you need the "right" spark! Recommend you isolate any chance of this being an ignition problem before you do anything further with the fuel system.

[Travis71280]

Guess i'll be going through the ignition then . I'm going to try to get a new coil first and see what happens. I want to say I already checked the points and they look good, but ill change them out next and the condensor if the coil doesnt solve the problem.

[rogerh113]

If you want to eliminate any possible issues with points / condenser, and you are not a purist, you may want to shift over to the Pertronix 'points'.  There are a number of posts here about them, so lots of background available.  I have a driver, and drive it - this is one of the two moves from original that I have made, which have resulted in significant improvements in drivability (the other being alloy bundt wheels). 

Good luck with resolving your problem.  These can be frustrating and time consuming to hunt down, but you feel GREAT when you finally resolve it.  Follow the methodical approach these guys suggest and you WILL get there!!

Regards -- Roger

[Travis71280]

Just out of curiosity, how hard is it to change the points if you dont have a Dwell meter? I heard you can set it using a 4mm feeler but I could be wrong. I'm probably just going to replace the points, condensor, and coil (and possibly the resistor) just to completely isolate the ignition system (everything would be new except the spark plugs). Once again, thanks for all the help and advice.

[Benz Dr.]

Points are not that hard to replace but it can be hard to get them set right if thiongs are worn inside which is very often the case. I think maybe you mean .4mm and not 4mm gap?

You said something that caught my attention. You said the engine started to die and run roughly at 210 degrees F. That's pretty warm you know. I'd call that overheating which will make the engine start to pop and go lean. A lot of these problems can't be easily found or corrected by ' do it yourself remedies ' and in some cases could end up causing further problems. The injection pump is one of those things you can damage and not even know you did it. I encourage people to learn and all but some things are best left to the pros. I send a lot out and never touch it myself......
 I think 5 lbs fuel pressure is far too low. You need more like 12 - 15 before the regulator on the pump.  Could be why it won't run.

Don't let your engine get that hot if you can help it. Shut it off and let it cool off a bit and then try it again. Sounds like you could have an air lock in your cooling system.

[Iconic]

Travis,
What in the ignigion system will work for 15 minutes and then stop working? It is very simple to see if you have spark once your car dies. How about putting a timing light on a spark plug wire and see if it is still "strobing" wnen the engine dies?
If your car is like mine was when it died, it was like someone turned off a switch. So, I chased ignition issues for quite a while. Then, as I have already stated found my problem with not enough flow at the return to the tank. I did this because I went back to this site and re-read many posts on this issue.
Do you have an electronic ignition? What year is your car?
I believe some here feel strongly about setting your points with a dwell meter, and I am sure that is the proper way to do it, but it can be done by setting point gap (certainly not 4mm, but depends on the car (I also believe)).
I set mine by gap and it runs great. Maybe I'm just lucky. Ask me in a few years if it is still running great.
Also, many thousands of automobiles have run properly with a points ignition.
It is going to feel great once you figure this one out.

[Travis71280]

Thanks everybody, I think it was .4mm from the tech manual, my bad  . Anyways the ignition is the factory ignition and has the red top coil with a .9 ohm ballast resistor (I thought red tops had a 1.8 ohm resistor but thats what the sticker on the coil says ) and its a 1971 US 280SL/8. The fuel pressure regulator is set at 5lbs (thats the max setting). Hopefully tomorrow or the day after tomorrow ill be able to tinker with it some more, do the coil test and check fuel pressure. Also do you think that Overheating can cause it to lean out that bad and die? If thats the case I'll go get an electric helper  fan (I'm assuming that 210 is the last white mark before the red).

[ja17]

Yes, definately start with the simple and cheap fixes first.  Points can get fried from having the wrong ballast resistor. I believe youre coil should have the 1.8 resistor. Try to use original Bosch brand contact points they seem to last.  "Icor" ventilated points are also good.

Setting the contacts with a feeler gauge was a failry simple and common procedure in by gone years. A dwell meter is not a must but it lets you know that you have done it correctly. Be sure to use some lube on cam on the distributor shaft or the point gap will not last. Other brands I have tried just do not hold up.

***The engine should be in good tune before other expensive, complicated items like the fuel injection, transmission, and fuel supply are tampered with

[Naj ✝︎]

 Anyways the ignition is the factory ignition and has the red top coil with a .9 ohm ballast resistor (I thought red tops had a 1.8 ohm resistor but thats what the sticker on the coil says ) and its a 1971 US 280SL/8.

AFAIK, A'71 US 280 SL/8 should have transistorised ignition with a blue coil and two ballast resistors (0.4 and 0.6 ohm).

An easy test to check if your fuel return line is blocked:
Run engine till it stalls.
With Ign 'on' and electric fuel pump running, loosen the bleed screw on the fuel filter head in the engine bay.
Let the fuel run out for about 30 sec. Hold one finger in the fuel stream to see if the cold fuel  changes to warm and back to cold.
Tighten up the bleed screw and see if the engine will now run.

naj

[Travis71280]

Awesome, that saves me from crawling under the car again, also feel better about changing the points. Where is the bleed screw exactly?

[Benz Dr.]

Naj, how does that test work? It's something I haven't thought of or used.  

I simply use compressed air to blow through the line going back to the tank, and I remove the cap on the tank. If  the line is not plugged, you will hear air bubbling up through the fuel in the tank. I always use a rag around the end of my air wand because if it IS plugged, fuel will shoot back at you without something there to stop it!

Travis,  I think there's a number of miss matched parts on your car. If you have the red coil you need the 1.8 ohm ballast resistor. I've seen cases where the CD ignition system would work right with that set up and other times where it wouldn't. I never really determined why that happened, but it did.
Your point gap is determined by the required dwell angle, which in this case is 30 degrees. All aluminium, 6 cylinder, single point distributors use 30 degrees, regardless of the year.

I think you must be using a regular fuel pressure regulator for a carb type set up. That probably won't work. That's usually more than enough pressure for carb engines but not enough for MFI engines.
 There's a small fuel pressure regulator valve on the side of the injection pump. It should have a small hole in the center so that small amounts of air will purge through the system. Anything less than 10 lbs fuel pressure and the engine will run poorly and may stall. At 5 PSI, it will die. This fuel pressure regulator threads into the side of the IP. If it's damaged you should replace it with one that works and discard the other one you put on the system because it's not designed for what you're asking it to do.

In general, I find this injection system to be well designed and it will work well as a complete package. If you change anything on the system, it won't always work out. Everything has to be balanced to work in harmony if you want to get everything out of it that it can give you.