The 1989 volvo 740 non-turbo has an LH2.4 Jetronic system (or any car with the LH 2.4 system) that actually retains a memory even when the car is off by using a little juice from the battery. The memory is kept on-board to optimize fuel ecomony based on a variety of driving parameters.

If your idle is lumpy then pull the codes using the FAQ to fix the underlying problem. But as you found out by pulling fuse 1, you effectively unplugged the power to the computer and reset it. You can do the same by disconnecting the battery or resetting the computer at the box (usually). This will fix the running issues for a short while but the problems will return (as you probably also found out).

Your computer # probably ends in 561 and it is a bit flaky (do a search) as it may not power up the fuel pumps and can have some issues with cold starting/running. The better computer replacement is a 951 if you can find it and have tested spare 561's on hand.

Hope this helps,

Tim

I doubt there is any connection between pulling the fuse and change in performance, after all when ignition is off all power to the ECU is off.

How weird. This deserves a bump.

Hope that someone who can answer this will see it . . .
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Mama always told me to eschew obfuscation . . .