What conditions cause the RPM needle to fluctuate?

My wife said that it was moving all over. The 1991 740 wgn jerked, hesitated, missed... I don't know how to describe it... just twice when I drove it home. The RPM needle seemed okay. I didn't hear a backfire or anything like like. For an instant I thought maybe it was the transmission failing. Engine starts okay. Idles okay. Jerked twice when I first started to acceleerate into traffic onto the hwy . then everything seemed okay again. Then it hiccuped twice again after 10 miles of smooth driving. That's too "intermittent" for me to get a grip on. Spark plugs look okay but I changed them anyway. I need some troubleshooting advice. Thanks.

I agree with Chris about the power stage being the prime suspect here.

If your tach is jumping around like crazy (not the same as your actual engine speed) it means that there is a problem in the ignition primary circuit. Common faults are wiring connections and grounds, power stage, and the hall swith or flywheel pickup depending on which one your car has.

I had an intermittent misfire at high speeds on the highway with my '86 740 and the problem turned out the be the power stage. Replaced it last weekend and she runs like new.

Sounds like the ignition amplifier or power stage, which is located on the inside of the l/f fender. It's bolted on one end andf has a wiring harness on the other with a black boot on it. Usually when tachs develop a mind of their own with hesitation, that's the culprit. If it was a crank sensor, it wouldn't start at all. If the engine wiring harness is baked, I'd replace that too. I've seen quite a few older 700s with both problems, but the power stage would be the first thing to replace.

It the tach's jumping, it's an ignition problem, see the FAQ.

I think your fuel pump relay or crank position sensor (rpm sensor) could cause this if they had an accasional burp. Check the FAQ.

Could also be a vacuum leak. I find the best way to check for those is with propane, but a lot of people spray carb cleaner around for that. If you had a vacuum leak that only opened when the engine was under load, or moving, it might be hard to find in the driveway.
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Andy in St. Paul, '91 745 Regina 198K mi, '89 244 142K, '87 245 RIP

Hi, Andy When is the last time your coil and plug wires were changed, if never that could be the problem . A bad regina coil wire will eventualy ruin the coil causing a intermitent no start that will drive you crazy trying to find out why.You can get spray to reseal the wires for a temporary fix .Keep us posted .WILLIE