CHANCELLOR Helmut Kohl's visit to Washington was so pleasant that the general tone became slightly misleading. Listening to all the formal pronouncements of agreement, you might easily have lost sight of the reality that, one inch under that smooth surface, there remain important differences of view. Some involve tactics in dealing with the Soviet Union, and some involve economic policy.

French politicians win votes by pursuing their disputes with Washington publicly. German politicians win votes by pursuing them privately, and Mr. Kohl's visit here was a crucial part of his preparation for the West German election next March. He accuses his predecessors, the Social Democrats under Helmut Schmidt, of allowing doubts to rise regarding German allegiance to the Western alliance as the neutralist and anti-American rhetoric of their left wing grew more audible. There Mr. Kohl addresses the German fear of isolation -- one of the great central concerns, not to say obsessions, of postwar Germany's politics. The impression that Mr. Kohl wished to leave is one of total reliability, and total political stability.

That last point, stability, is currently a matter of some sensitivity in Germany. Mr. Kohl and his conservative coalition came to power seven weeks ago, not through an election, but because one small party previously attached to the Social Democrats chose to change sides. That kind of parliamentary adventuring is not popular in Germany, where it stirs memories of chaos and catastrophe. That's why Mr. Kohl and his government have been emphasizing, over and over, that they are going to be predictable, and they are going to deliver what they promise.

That means going ahead with the installation of the intermediate-range nuclear weapons, unless things happen in the arms control negotiations at Geneva. But things probably won't happen before March, since the Russians presumably will want to see how the German elections turn out before they begin talking seriously. The real test of the Kohl government's competence is more likely to be economic policy. The West German unemployment rate is still low by American standards, but it has risen from 3.4 percent in 1980 to 7.9 percent last month. It's hardly surprising that, despite his admiration for President Reagan, Mr. Kohl declined to interfere with the contracts for the Siberian gas pipeline.

The Kohl visit has been such a success, in terms of polite remarks and assurances of warm regards, that a careless listener might see only perfect unity ahead. That would be a serious mistake.