70MPH downhill in neutral truck rapidly slows down

Trip from the hills of KY to the mountains of WV. Total about 360 miles.

First half of the trip typical performance/MPG no worries. Stopped for gas and snack then back on I79. At this point it seemed as if I was to pulling a load and way more gear shifting than normal. Then noticed MPG went from my typical 18 to 15.

At 70+ and on a long steep downhill grade I put the truck in neutral. I understand physics but it was like I had my dam foot on the breaks. Now I am worried.

Off I79 and now state roads truck still fuched. Going to rough roads I put in 4H. What the hell truck is now lite as feather, fast responsive like normal!! Go back to 2 wheel drive and all is good.

What the hell just happened? Any thoughts greatly appreciated!!

 

Brake caliper stuck?

 

I should have mentioned that when we got off interstate, I did look at breaks and did see anything.

Not sure if it had anything to do with 4H but perhaps weird timing.

 

Is it below freezing currently? Perhaps frozen drums ? Did you your ebrake when parked? Doesn't sound like you when parked long. That is also a symptom of a faulty brake booster.

 

It was about 28 degrees when on the road. I didn't use ebrake when stopped but I do use it sometimes. But when I exited 79 all breaks were cold to the touch. Wouldn't they be hot

 

What's a break booster

 

Headwinds? Low tire?

 

Was it a straightaway or curved roads

 

Altitude, grade, wind. Likely one or a combination. I always feel like I'm towing a boat when I drive in the mountains.

 

Tires at 36. No real headwind. As mentioned I understand basic physics and at that speed and grade the loss of velocity should not have been that quick. I was as if the engine was breaking but I was in neutral.

 

It is a black round vacuum booster that mounts behind the master cylinder .. It's easy to tell when it goes bad . When you separate the master cylinder there is still pressure and you can hear it pop like a can of biscuits. Buuuut that' a new truck are you under warranty still?


So how long did you sit? Was Your ebrake engaged? Below freezing? If a brake booster goes the brakes get very hot and the rims may become discolored and too hot to touch. Something to look out for. Good luck . I just figured if you where in 4hi and in the east right now (snowing here in Ga) the rear drums may have froze when your ebrake was engaged and then it sat. Then as you drove and they where engaged there was heat from the engaged brake and it freed it up.

 

Oh nevermind not brake related..

 

Didn't use ebrake during our pit stop.

I was able to touch all four breaks disc & drums right after exiting interstate.

Still under warranty and will have it looked at.

 

Glad you made it. Keep us posted if they find anything.

G.

 

Any particular reason you were putting the truck in Neutral to go down the hills?

 

When a saw MPG took a dive I went into troubleshooting mode. I troubleshoot for a living and if I could I would have rebooted the dam thing.

 

When I had my 05 Xterra 6MT, I'd always run in neutral on these same hills. So have an idea how trucks should cost.

I guess I'm a bit nuts too.

 

Thanks everyone for your input.

 

Just a guess but probably to troubleshoot engine braking as a potential reason for the brake feeling.

Good job describing the event. I’ve felt drives similar to yours, although never as “on & off”.

 

In that case you were wasting fuel, you where better off leaving it in gear.