Engine hesitates at idle when in drive or reverse

[ncorcoleotes]

Hi, my 1970 280sl will sometimes hesitate when idling while in drive or reverse. When the engine is cold it starts fine and idles well. Once it warms up, if I sit at a red light for a minute or two, the car will idle normally then the idle drops for second, like it hesitates or is not getting enough gas. The injector system has been completely rebuilt, the fuel pump is new, all the spark plugs and wires are new, we installed an electronic ignition, etc. The only thing I have not changed were the fuel injector nozzles. The entire motor was just rebuilt by Metric Motors in Los Angeles. The car runs perfect and has lots of power. Any suggestions?

Nick
 

[Cees Klumper]

Sounds like your idle air/fuel mixture is off. You can do the "split linkage test" to verify (do a search, lots of information here). That's where I would start. Report back so we can help more. Oh and welcome to the club!

[rwmastel]

Nick,

That's great that so much is new or rebuild, but you'll have to test things and work with facts.  Do you work on the car yourself?  Linkage test is a great start.  You can also search forums for fuel flow rate and press tests.  Cycling the fuel back to the tank effectively keeps it cool and prevents vapor issues.  Keep posting, you'll get lots of help.

[ncorcoleotes]

so just to clarify, the car will idle fine for about 10 to 15 seconds, then the tach drops down to 3 or 4 rpm, then idles back up to normal. This only happens after the engine is warm, and is in drive or reverse, normally happens at a stop light. Any insight would be appreciated.

[Pawel66]

I am not a mechanic, but this sounds like a need for a Linkage Tour - tune up of idle mixture both in Park or Neutral and with gear in.

It should be preceded by timing check, dwell check and valve clearances check.

As suggested - a quick check with split linkage may provide a clue, as suggested. Or looking at spark plugs.

Not sure what "3 or 4 rpm" means, but I understand there is some regularity with idle.

[Lori]

In an automatic, there is another component that could be the culprit.  The constant speed solenoid.  The CSS is operated by 2 pressure switches -1 on each side of the transmission.  One triggers on Reverse and one on Drive ( I forget which is which).  Check your transmission fluid when hot and make sure it is at the proper level. 

[Pawel66]

Indeed the CSS may play a role here. It is part of the Linkage Tour adjustments, to set the idle speed with gear in.

Depending on how your car is configured Air Conditioning may also play a role. Do you have Air Conditioner? Are the symptoms present when it is on?