1978 280SE DOHC inline 6cyl - stalls when warm

[BenzGmarket]

OK, I am new to MB cars.  This 78 280SE is my first. It has the Bosch K-jetronic system in it.  No electronics in the fuel system.  

This is the problem.

The car start right up when cold, so I take assume the cold start valve is working.  The car runs fine until it reaches operating temperature.  When warm it stalls when at a stop light.  And is hard to start when warm.  It also misses bad and runs very rough when warm.  Sounds perfect cold though. I am learning that these are way different from American cars in that you can't just drive the crap out of them and park them until they fall apart.  You have to constantly tinker with them to keep them in tune.

I have Bosch book on the K-jetronic system, and have purchased the hi pressure fuel pressure gauge to do the required diagnostic tests.  But I am going into this blind.  I figure I should start by taking the tank off and replacing the sock filter and pre-pump (if it has one).  Then fuel filter outside the tank and accumulator.  I guess I could just test the fuel pump to make sure it is pumping like it should.  

Again, the car runs fine cold but when it warms up the car stalls at stop lights and is even harder to keep running with the a/c compressor engaged. Just runs horribly when warm.  At the same time I am suspicious of the A/C compressor because it squeaks and squawks even after I replaced the clutch.  So maybe it is dragging down on the motor.  But non the less, with or without the a/c compressor on it runs horribly if at all only when warm.


After replacing the in tank filter and accumulator, what should I check then.  Someone told me that it is most likely my warm up regulator.  Can I adjust this or do I need to get a new one.  I could use any advice on where to start.

Thanks

[OzBenzHead]

[...] You have to constantly tinker with them to keep them in tune. [...]

I find quite the contrary to be true. I have MFIS, CIS (K-Jet), and ECIS (KE-Jet) Benzes, and none of those ever goes out of tune between services -- that is, correct, regular, by-the-book services. My only Benz that needs an occasional fuel-system tinker is a twin-Zenith-carby model.

[...] the car runs fine cold but when it warms up the car stalls at stop lights [...]

Something similar happened to me a few years ago with my '79 280SE. It turned out to be a worn distributor (a rare occurrence, apparently). Replacing the distributor fixed it. Not saying that is your problem, but I went right through the fuel system, finding nothing wrong with it, before I discovered the dodgy dizzy. When warm, the dizzy would not retain its advance/retard setting; the vacuum advance unit was okay, the contacts were okay, the condenser was okay, the ignition module was okay ... so I bit the bullet and bought a brand-new distributor. That was two or three years ago, and I've had no trouble since.

A new distributor from an Australian M-B stealership would have cost me $2370. I bought mine (genuine M-B / Bosch part, not aftermarket rubbish) from an independent supplier for $110.

Good luck with your problem solving.

[s class]

I suspect ignition on this one.  You need to make sure the distributor cap and rotor are good.  They need to be genuine Bosch.  Aftermarket ones may look the same, but will break down when hot, giving you these sypmtoms.  Similarly, your ignition lead supressors must be good, and the correct metal jacket ones. 

Coils can also give trouble when hot, but I suggest you start with the cap and rotor. 

[macatron]

As simple as it sounds...spark plugs.  My 280SE ran great when cold but when the temperature would begin to creep above 180F or so, it would start to sputter on acceleration and when it got really warm, it would barely accelerate past idle.  I was using platinum Bosch plugs my car just didn't like.  They fouled in 2,500 miles.

[Bandolero]

When the motor is warm, adjust the mixture screw down near the side of the distributor till it idles at about 600.
Then see what happens when you start from cold to make sure that is all still ok.

However, it could also be the warm-up regulator that needs an o/h..

[Tony T]

My '78 280E had a similar problem that I thought was fuel related in that as soon as it was at operating temerature, it would stall then only run at full throttle. I replaced pretty much every fuel system component without success.
It turned out to be the rotor in the distributor.
So, I tend to agree with others here that a look at the ignition system may be a good place to start.

[BenzGmarket]

Thanks for the replies.....but the first thing I did was replace the cap and rotor when I got the car, so thats not it.  I went ahead and purchased a new accumulator, fuel filter, and fuel damper.  So we will see if that helps.

How the hell do I get to the fuel tank?  I want to clean the sock filter in the tank but cannot find it.  I know it is above the rear axles, so I pulled everything out of the trunk (no luck), pull the back seat out (no luck).

Any suggestions?

[koan]

From underneath, the hose that feeds the fuel pump or filter screws directly into the tank strainer.

koan

[AAB]

OzBenzHead,

Where did you buy your $110 distributor ?

I am having similar problems as MenzGmarket,..ie cutting out when hot in my 1974, 450SEL.

Only started recently,.goes like a rocket when cold, but it cut out reversing down my driveway a few minutes ago !!

Regards,

Alex

[OzBenzHead]

OzBenzHead,

Where did you buy your $110 distributor ?

Alex

Alex: PM sent.

[flutes]

...and I'm starting to have similar problems in mine.  Not cutting out, but feels like half the power is gone when the engine is hot.  That could explain my crappy fuel consumption as well.

[AAB]

OBH,

Got your message thank you,..tried to thank you by reply e.mail but it did not go through.

Replaced the rotor with a spare ( from a $500, 450SEL wrecker that I bought a couple of years ago) & is a vast improvement,..still to see if it is cured entirely though.

It has an slightly uneven idle ( not all that bad, as is not trying to cut out as before ), but hopefully will be cured by adjusting the air intake.

My fuel economy has been lousy as well, so I hope for some improvement there as well.

Regards,

Alex

[BenzGmarket]

   Ok, today I finished replacing the accumulator, filter, fuel damper, and plugs...still have the exact same problem!!!  Just spent two days doing this and the SAME freaking problem.  Runs like shit when hot and stalls out when at a stop light.  I also put injectors in about two months ago.  So it must not be a fuel problem.

Also still has the same miss, at the motor you cannot hear the miss, but you can hear it clearly in the exhaust system.  The resonator has a huge hole in it.  I wonder if because the resonator is falling apart there is a blockage in the exhaust or maybe the catalytic converter is clogged....any ideas?

I am at a loss now, and I cannot find a mechanic that knows how to work on this car any better than I do.  Actually, I think I have done better than the several mechanics I brought it to.  I even brought it to a shop that specializes in foreign cars.

So frustrated, just want it to be dependable.

[TJ 450]

I would have a close look at the rest of the ignition system as a starting point, in keeping with what s class suggested. What sort of condition are the ignition leads in?

When you removed the previous set of plugs, was the any signs of fouling? It may be burning oil and subsequently fouling the plugs under certain conditions.

I would also have a leakdown test done, as you may have some basic loss of compression in one or more cylinders of which no amount of tweaking is going to fix without removing the engine.

These cars should not go "out of tune", unless there is a problem. The fuel system is also very reliable, more so than any modern car.

In any case, it's best to start with the basics and go from there, taking a systematic approach.

Tim

[oscar]

Do you reckon a cold start injector stuck open might cause the problem?

On compression, I was thinking about compression issues too before, but if it was low, you'd think warm running would be better than cold running due to compression improving when the engine's hotter.  That's what I've always assumed.  The world according to Google however seems divided.