Well, here we go again. My much-used and much loved wagon, 187,000 miles, was running fine. I have routinely replaced the RPM sensor, radio interference relay and fuel pump relay after the engine mysteriously died at speed on the highway. I would restart immediately. Replacing those items soilved the sudden death syndrome. Now, after several long trips, the engine will idle fine, but stumble on acceleration... intermittently. Sometimes it runs for weeks. Sometimes it stalls and stumbles every day.
I have checked all vacuum hoses, especially the MAP hose. When I add injector cleaner the problem disappears for a while, then comes back.
I took the plugs out and they show an even white surface at electrodes. Plugs are 10 months old.
By the way, full loaded with my audio/visual gear, the Volvo gets 28 mpg highway at turnpike speeds and hauls as much as an SUV. Which is why I keep it.
Any ideas about the stalling? O2 sensor perhaps?
If the NKG plugs are white at the firing end electrodes but are not burned or pitted, do they need replacement?
Wires have been replaced. Distributor cap is almost new.
Fuel pump replaced 40,000 miles ago.
By the way, this car has the Rex/Regina management system.