Block heater season! '97 E300
 

I bought this block heater cord from NAPA auto parts.
https://www.napaonline.com/en/p/KAT28216

I used it for the first time last night, 22F when I started the car this morning. Seem to take about the same 5 seconds as always for the glow plug light to go out before starting. No appreciable increase in warmup time while driving.

Is the cord too small/weak to make a difference? Maybe it's not making good contact at the block heater element? (I hit it with a wire brush before installing, but the prongs looked pretty corroded and/or crusty.)

The heater in my '96 makes a pretty noticeable difference in glow time and starting ease. It works better in an enclosed or semi-enclosed environment, a place where the vehicle is shielded from winds. One thing you can do to verify if it's working or not would be to plug it in for at least a couple hours, then pop the hood and feel the engine. It'll feel warm to the touch if you've got a working heater.

I'm not sure about your model, but earlier versions used outside air temperature to determine glow cycle time. If that's the case, you might not see a difference with that.
A bit of silicone dielectric grease should help with the corrosion problem.

How long did you have it plugged in?

I remember by E300 would be plugged in for about 2 hours and the top of the engine was warm to the touch in 25* ambient

Plugged in 12 hours. I never felt the motor.

Parked outside.

Dielectric grease sounds like a top tip! Keep that darn water out and keep a good connection.

If you look at the product page link, it has the cable size. I think it means 16 gauge wire. Is that typical for this application?

I'm buying an outdoor timer tonight so I can run the heater for a more reasonable 3 hours tomorrow morning.

The new timer from the big orange store works fine. I have one of those extension cords with a clear plug with a light. Anyway, after almost 3 hours in 22F - 25F temps, nothing on the engine felt warm. If my NAPA part is faulty, can't I tell that with a multi-meter? I'll try to test that, and clean up the prongs on the connector, this weekend. I'm going to see some people at an MBCA meeting tomorrow who might have a thermal sensing gun for testing.

Measure the resistance of the block heater from the plug end of the cord. Should be somewhere 15 to 30 ohms if all is good.

"Measure the resistance of the block heater from the plug end of the cord. Should be somewhere 15 to 30 ohms if all is good."

From the plug end of the cord to ....... what? Not sure what you mean. I assume I'm measuring resistance within the cord, not the block heater itself. But, where exactly am I placing the probes of the multi-meter?