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Old 08-08-2003, 11:52 AM
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450 SLC idle problem

I have a 1978 450 SLC with 118,000 miles. I am fighting with an idle problem. It runs fine and idles fine when cold but the idle gets rough and eratic when hot. I have changed plugs, wires, cap, and rotor. I have pulled the injuectors and cleaned but the holders aren't in the best shape. before I buy new injectors I would like some advice on this. Should I be looking at something else as the culprit? Any advice will be appreciated.
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Old 08-08-2003, 12:31 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Replace all the rubber connectors on the all vacuum lines.

Check the manifold for leaks by spraying carb cleaner around the gaskets and listen for change in idle. Spray some cleaner around the injector seals to see if any of them are leaking.

Read the posts about "cracking" the lines on the fuel distributor to see if you can isolate the cylinder(s) causing the problem.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 08-08-2003, 12:47 PM
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Join Date: Jun 2001
Location: CA
Posts: 531
While the carb spray trick is a good one, it is sometimes hard to tell where the leaks are as the stuff gets everywhere for the engine to ingest.

A more specific test is to take your car to your indy and have him pump inert white gas into the engine and see where it leaks. The $ spent for one hour of labor can save you lots of $ on replacing the wrong parts.

Mine leaked at the injector holders, plenum and intake manifold gaskets, and the fix, if somewhat laborious, is here, and resulted in the smoothest idling 380 ever. Your engine is similar.
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Old 08-08-2003, 05:17 PM
Registered User
Join Date: Jul 2003
Posts: 78
Is the idle also too slow when hot? If so, the auxiliary air valve may not be closing at the right temperature.
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