W108 3.5v8 hot running and starting issues

[Hemal83]

Hello

I have a 1972 280SE 3.5. It starts well when cold but does surge up and down. This settles but the car will stall and not start when warm after about ten min.

It will later start if left for an hour.

Any ideas on how I can diagnose this?

Could the auxiliary air valve cause this?

Thanks

Hemal

[Todd Miller]

The air valve, and the fuel pressure regulator are probably where you want to focus your attention. When the regulators fail, the system runs too rich, so you'll get good cold starting, but it'll flood during attempts to hot start.

[cth350]

This thread is for a different set of symptoms, but it has some basic debugging techniques...

http://www.peachparts.com/shopforum/vintage-mercedes-forum/168614-idle-hunt-2mins-only-when-cold-280se-71-3-5-v8.html

My old 300sel 3.5 would hunt when the electric sensor on the air cleaner was connected. Not that disconnecting it was a good cure, but it was another diagnostic data point.
-CTH

[aluminum]

I would first concentrate on the ignition >
As ignition components warm up if a weak area is evident it will brake down often .
Coil / Plug and Spark Wires / Resistors / Points and " Condensers as example
First thing I would check is how much spark it coming from the Coil wire to Ground as well as the Plug wires > Check it cold < Then same check when its Hot or warmed up
( can do this just cranking with the key on ) < Trigger Points / Fuel Pump Relay >

[Graham]

If this is a djet, what you describe is vey common.

More than likely the AAV (aux air valve) is stuck and not opening or closing smoothly. You can remove and test in hot water or with a hot air gun. It should be fully open at room temperature, then as coolant heats up, it should be fully closed at 150F. That surging is often a symptom of a stuck AAV. You can usually stop surging by reducing idle rpms, but then you may have more of a problem when hot. Happens a lot. AAVs are expensive if you can find one. Try to fix the one you have.

You should check your fuel pressure - put any simple pressure gauge at the end of the line to the startup up valve. Turn key on - you should see about 29 psig. If you set it using the fuel pressure regulator, about 32psig might help with the hot starts. Car will run a little rich, but more likely to start.

[berfinroy]

Graham is pretty much on the money, but doesn't mention that part of the reason is modern ethanol laced/winter blend fuels. Modern fuel injection systems operate at much higher pressures, and therefore don't have vapor lock problems that the old D-jets had @ 30+_ lb presure.
If you get all the other parts in the fuel delivery and managment system working to spec (including trigger points-often overlooked for both cleanliness and point accuracy), plus ignition, proper plugs/wires andtrigger points, and kickup the fuel pressure regulater a few point to 32 psi or so, your car will be OK with modern fuels.
as the saying goes, ask me how I know...