1972 350SL Poor Running Until You Floor It.

[mikesmercs]

Hello all, I havent driven my SL in two years but it had developed a very poor running condition when cruising at any speed until you floored the accelerator then it pulled like a train until you left off the gas and began to cruise again, replace plugs, points, condenser and HT leads.
Sorry if my explanation is a bit vague but its been a while since I drove it. Thanks in advance
Regards
Mike
Ireland

[mbdoc]

Most likely a fuel issue, sounds like the mixture is too rich.
Possibly a vacuum leak at the vacuum line on the mass air flow sensor.

[mikesmercs]

Thanks MBDOC, reading back on some of my posts on a UK forum, I can see that the car surges at idle when warm and increases until it sometimes cuts out, I had also disconnected the idle air valve to eliminate this item. I had checked the hose to the Air Pressure Sensor, squeezed it and it rose the revs, disconnected the four pin connector and it cut out.
Just a bit more info if it may help.
Thanks again
Mike
Ireland.

[Graham]

Originally Posted by mbdoc

Most likely a fuel issue, sounds like the mixture is too rich.
Possibly a vacuum leak at the vacuum line on the mass air flow sensor.

No mass air flow sensor on 72 350SL.

But certainly look for vacuum leaks.

Manual has a good FI system check list - not hard to go through and check all components of FI system.

Check that all vacuum lines are actually connected - small one from throttle body to switchover valve to distributor sometimes gets disconnected. Also check the one from the back of the engine to the MPS (Manifold Pressure Sensor) - it sends info to ECU to control mixture. There is also another connection on passenger side back of engine to transmission and climate controls - make sure they are connected and not leaking.

Another common problem is stuck AAV (aux air valve) which results in an internal vacuum leak.

Also check throttle switch - details are in manual which can be found on-line if you don't have one.

[Graham]

Originally Posted by mikesmercs

Thanks MBDOC, reading back on some of my posts on a UK forum, I can see that the car surges at idle when warm and increases until it sometimes cuts out, I had also disconnected the idle air valve to eliminate this item. I had checked the hose to the Air Pressure Sensor, squeezed it and it rose the revs, disconnected the four pin connector and it cut out.
Just a bit more info if it may help.
Thanks again
Mike
Ireland.

That surging is quite common. The ECU automatically cuts off fuel at about 1500rpm and then it starts all over again once rpm drop below a certain level. Once you have eliminated all vacuum leaks and are sure all components are sound, adjust hot idle speed to about 700rpm. Cold idle will be higher - maybe about 1100rpm. Don't set it too low because then you may have too low an idle speed when engine is hot and hot-start problems.