Driving car on D3 in the highway bad for engine?

I have a 2000 Honda Accord ex. Recently I have a vacumm issue while driving in d3 but not d4( not so far ) but I have driven the car on the highway in d3 several times because one of my friends who works on cars told me that it saves gas if I had done that and i was online and read that it's better for my car if I have start with d3 then shift it to d4. Stupid and I know not to believe everything I've read online. Anyway those trips are 30-40 miles and I driven it on d3 on the highway about 8 times. About an hour long trip each time. Did I do major damage to my car and is the vacuum problem because of my driving?

Driving in D3, which is simply "drive" and not "overdrive" will not do any harm to the engine even at highway speeds. However the more important concern is what is causing the problem and will it leave you stranded as it worsens. So your real issue is the cause of the transmission not working in overdrive (which saves fuel) and your safety.

I do not see a connection between driving in D3 and a vacuum problem. You have burned a little extra gasoline. And maybe your engine will only last 350,000 miles instead of 375,000 miles. If your problem is only when you drive in D3, then just use D4 from now on.

The vacuum problem is not a problem it is your engine preventing you from ruining your transmission by going too fast with the wrong gear selected. The automotive engineers did their best to design an Idiot-Proof car, and in your friend who works on cars is their test case of the month.

Leave it in D4. There's no need to run in D3, friggin' PERIOD.

No you wouldn't do any damage to the engine or trans driving in D3 the engine will just rev harder, and use more fuel, (steep hills it would be better to use D3)
around town D3 would be better than D4 because it gives you a better torque available.

Who ever fed you this info Is so full of ****. I would sue them for the damages, driving in d3 on the free way your driving at a higher rpm level. Causing lower miliage, Its an automatic transmission never meant to be shifted up and down.manuely. To save brake wear and tear you shift the tranny down going down hill.

Well yeah--it will wear it out a little faster. If you have a vacuum gauge on it it will show better vacuum in 4th because it is less efficient when over revving. That's just the nature of the beast and the gears are made to match it.

Cars in the 70's and early 80's did not have D4… They only went to D3. They got really bad gas mileage.
That is what you did,
Just put the car in drive, That is what it was made for.

If you have a vacuum leak you'll be running lean and the ECU will richen the mixture by pulsing the injectors longer. That ruins gas mileage. Get the vacuum leak repaired if you really do have a problem. I drive in the drive position but downshift to D3 when going up large hills or long grades on the expressway. The motor has more power for the hill and runs better because you're not bogging it down under load.

You know your friend? The one that works on cars. Ignore everything he ever says. You think he knows more than the designers and engineers at Honda?

Only to your wallet really. OD is for better fuel economy. A honda auto is pretty much a computer controlled manual trans. What vacuum issue do you have?