My car starts perfectly when cold.
If it sits for about an hour it starts slightly rough.
When hot it starts, it starts very rough for a few seconds, then smooths out and idles fine.

It acts like its getting too much or not enough fuel at hot start, but I have not noticed and black smoke.

I am getting around 12 mpg in the city with the AC on and a semi lead foot, which seems a bit low.

I am thinking I should probably replace the O2 sensor since I dont think it has ever been replaced and I am sure there is another sensor that is bad, like air temp or water temp maybe.

Any opinions on where I should start ??

 

My 83 CIS car has done similar, although the rough starts seem to need more sitting time than that before they occur. Anyway, takes more cranking after sitting a couple of hours than when cold to fire up, and sometimes misfires for a few seconds. I have just replaced the pressure accumulator as it was wet on the supposedly dry side of the diaphragm, and waiting to see if the problem is fixed now. This unit is in front of RR wheel under the cover. Some had a vent tube going to breather case (hight towards the rear in there) which you could pull and check, mine just had a small screw - undo and see if wet in there. 928 Intl have them on the shelf.
jp 83 Euro S AT 49k

 

Just guessing -- dirty or old injectors. Try running some injector cleaner.

 

Tamons;

First place I always check on a hot start issue is the fuel pump check valve; at the outlet of the fuel pump.
You can try to isolate it by pinching off the return line after the car is shut off. This will in essence hold pressure in the system. Make sure you remove clamp just prior to starting.
Do oyu have a fuel pressure gauge on the rail? Makes for an easy DX.

p.s. I assume you car is L-jet...

 

You know what that is probably it. Occasionally, if I jump in and crank, it will not even fire. If I bump the key to accessory a couple of times it usually cranks right up.

As far as the pressure guage, I installed my VDO guage on my 951 and that was just sold, but i think its time for another one.

Thanks

 

Scott-
I'm not familiar with the check valve, or how pinching the return line would help diagnose this. Can you explain more about the problem and procedure to diagnose?
Just curious in case it ever happens to my '83...
Thanks!
Rick