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'Lumpy' at idle and light throttle pressure 200

posted by  gbraley on  Sat Jul 14 11:00 CST 2007
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last visit: Wed Feb 6 12:53 CST 2008
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I bought this '93 240 stick in June. First two tanks of gas gave 29.5 MPG and seemed to run well. Tuned it up with rotor, cap, flame trap air filter, oil change and Bosch Platinum plugs. Mileage immediately dropped to 25 - 26. Seemed to be harsh and acted like spark was "breaking down at higher revs. Drove it like this for 3 weeks and 1500 miles. Reading on the board last night and came across a post mentioning problems with platinum plugs. Researched and found lots more posts. This morning I changed them out to a standard NGK. Amazing the difference! Pulls strong right to red line. HOWEVER, although I don't remember the idle being perfectly smooth with the platinums, now it seems to miss a lot at idle. Also when cruising at light throttle (40 MPH in 4th for example) the misses are very noticeable. Standing on it immediately cures the problem. Runs great from 1/4 to wide open throttle.

To troubleshoot, I assumed that it is running on four cylinders with intermittent drops to three. If it was a particular cylinder (plug or injector) it should run on three, intermittently dropping to two until you pull off the plug wire on the offending cylinder, when it should run steadily on three. Pulled off the plug boots one at a time, with no differences between cylinders. Checked all the vacuum hoses and all appear tight.

Anybody have any ideas?

Thanks,

Graydon






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