[DML] Re: rough running engine

["Bob Brandys BobB@xxxxxxxxxxxxxx [dmcnews]" <dmcnews@xxxxxxxxxxxxxxx>]

Eric,

Sounds like your engine has a vacuum leak(s).  When the engine is cold, the mixture is richer and  small vacuum leaks are compensated for by the rich mixture.  Once the engine warms  up and the computer goes into “closed loop”  the mixture leans out - controlled by the O2 sensor. 

Also when the engine warms up, the various engine components change is size and vacuum leaks can become worse. 

On older cars, without all the plastic junk hiding the engine),  there were a couple of tricks used by us old mechanics to find vacuum leaks. 

The one I used most is  spraying carburetor cleaner (one that contains no water) all over the seams in the intake manifold.   The engine will smooth out where the leak(s) is now sucking in extra flammable liquid.   The challenge with the D engine is that with the ram tubes on the top, it can be very 
difficult to actually spray into all the knocks and cranees to find the leak(s).

Years, ago, car manufacturers recommended using a tank of propane and spraying that into the various seams.  Never really like that method or had any luck with it, because the fan would blow the propane away.  Might work on the D engine, but then you just don’t know where the gas is going. 

Unfortunately,  I have had engines that have vacuum leaks, and we knew where the general area was, but all  the metal mating surfaces looked fine and even after replacing the gasket and checking squareness, the leak(s) were still present.

All we could do in those cases, was to cover the leak area, with high temperature silicone.  Not very pretty but fixed the leak. 

I suspect that the leak is around the fuel injection valve.  I have found these to be the hardest to deal with. 

Bob

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