I have a 1980 450 SL that starts up fine when cold but, gives me a problem when the engine is warm. It's fuel injected. Any suggestions would be appreciated.

* Kcole, many of the 4.5 liter v-8s (M117), which had a production run from about '72 to '80, had that problem; a lot of cranking on hot start with an eventual uneven, faltering start. Like classic vapor lock. If you look under the hood of any of the later aluminum v-8s, which superceded the 4.5, you'll find a fuel cooler (part of the AC suction hose) to combat exactly that problem. Unfortunately, I don't know of any way to retrofit one of the later fuel coolers to the earlier 4.5 cars.
* I'm assuming here that you've checked out all the obvious (less expensive) maintenance items; plugs, plug wires, compression test, and fuel pressure test (line, control and rest pressures). If the line pressure is off, you can shim the pressure regulator in the fuel distributor to bring it back into spec. An incorrect control pressure curve would call for a new control pressure regulator (aka warm-up regulator). A deficient rest pressure will often point to a leaking injector or a leaking 1-way valve at the fuel pump. The fuel pump 1-way outlet valve isn't expensive but a little tricky to replace. The injectors, with new seals, are more expensive and time consuming. Depending on your resources, any/all of the above parts should make a noticeable performance difference on an older car. But it may still not solve the hot start problem.
* The most expensive item (save the expensive stuff for last if you have a choice) is the fuel distributor. Your fuel injection system is the Bosch K-Jetronic system, which came in in '73 and replaced the initial D-Jetronic system. I currently have a client with a very nice '78 450SLC with the same hot start problem. He's had all the other work and the car runs really well but is a pain to start hot. But he's living with it rather than buy a new fuel distributor.
* Partially blocked cats would give you a starting problem, hot or cold, but it wouldn't hurt to have a shop measure back pressure and verify that they're clear. Similarly, a clogged fuel filter won't differentiate between hot or cold, but is a routine maintenance item.
* If you're getting the sense that you could spend a lot of money chasing this problem and never really solve it completely, you're right. But good luck minimizing it.
* I look forward to reading anyone's account who has solved this problem completely.

* Kcole, I forgot one of the most obvious causes for the hot start problem and that's a bad fuel accumulator. The fuel accumulator's function is to prevent vapor locking. Problem is that I don't believe a bad accumulator is detectable from watching the movement in the rest (fuel) pressure reading during the minutes after shutting off the motor.
* If someone reading this post knows how to diagnose a bad K-Jetronic fuel accumulator from a proof-positive (rest) pressure reading, I'd be interested to know how. K-Jetronic is pretty old now, and KE-Jetronic, which has a similar fuel accumulator function, wasn't used beyond early 90s, but it would still be interesting to know.

I chased hard starting cold and warm for over a year. Just from reading the posts here, and there are many many many of them, my hard start warm issure disappeared completely after replacing tired injectors.

I began replacing everything from the tail forward. Process of elimination and a few,I mean alot of, dollars later my SEC starts up instantly hot or cold.

I first replaced fuel pump and filter. Then I got a new fuel pressure valve(at fuel pump). I thought the one that came with the new fuel pump may be bad. No improvement.

So I got a new accumulator and replaced all rubber fuel lines at the pump.
No improvement. I got another new accumulator on the off chance the first one was bad. Nothing.

I put in new injector seats and seals thinking I was loosing fuel pressure there. I also installed new injector breather hoses accross the top of the manifold. No improvement.

Finally I bit the bullet and sprung for a new set of injectors with another set of injector seals. Keep in mind, this baby has now 236,000+ miles on origonal pistons and rings.

Wow!!! I mean, wow!!! What a difference in performance too. I only drive 7 miles a day one way to work, but it's at 85 to 90mph. The torque is very impressive. Even with the mileage I have.

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Selling used Mercedes parts on Ebay...always changing...
cdplayer51 take a look!
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Wow
Glad you got some results
seems these cars had a hot start problem as manual even males reference to it

My 82/83 500 SEC is facing simlar problems
I just got it a month agan
trying to get the bugs out of literally
its crawling w baby spders that hatched as it seen sitting for a while..

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83 500 SEC Euro 198K

mine is a 1985 380sl. I had a warm start problem only. it was good at cold and it was good at hot. I started at the back with check valve by the fuel filter and worked my way forward. Finally gave up and let my mechanic solve it. They changed the distributor ($800) and warm up slide valve which made the problem disappear.

I've solved many old style electronic injection system "hot soak" problems by drilling a VERY small hole, at an angle, through the pressure regulator seat (inside) allowing the vapors to be purged and allowing fresh fuel to enter the lines. It creates a slightly longer crank time at every start but you never have the hard start and rough, choking running when started hot. I learned this at my first dealership about 1979. This takes technical knowledge of the workings of the regulator and a steady hand but it works great.
The newer system, the CIS, is another matter and the fuel distributor can be a major headache. If you take off the air cleaner and push the intake flap down and see fresh fuel on the throttle plate, you have a leaking fuel distributor.....an expensive problem. Putting a pressure guage on either system and watching the pressure drop will tell you if you have a problem or not. Clamping the lines between the engine and tank, ( one at a time if you detect a definate pressure drop) can tell you if it's a check valve on the pump...another possibility in all benz injection sysems or the pressure regulator....or at a worse case scenario, a fuel distributor...which again you look at the throttle plate looking for any liquid raw fuel.

my hard starting problem was resolved by replacing the fuel dist. the fuel accumulator, and the fuel check valve. very expensive, as the dist. alone cost $1080. the car starts fine now when hot. i had to bite the bullet and spend the money, but, at least i saved the starter motor which i'm sure is not cheap either.

I have 1978 450 SL that does not start. I changed the fuel pump, accumulator, filter and rebuilt the fuel distributor. Controlled pressure to the warp um regulator is good. I can hear the fuel injector's noise when the air sensor is depressed with the ignition key ON. Checked the voltages across the 4 & 6 ohms resistors including the coil and they are within the specs. There is a spark in the spark plug when pulled out and grounded. The engine turns but will not start.
I need help in getting the car started.