Odd downhill engine behavior

[JB.]

Haven't heard any mention of this behavior, but it's pretty subtle in high range so it's possible someone might read this and find out that they have it as well.

It's very noticeable off road in low range so that's how I'll describe it.

On a downhill slope using the engine for braking but using some throttle to hold a steady speed, the engine will throttle back periodically for a couple of seconds and then resume the previous speed. It occurs in a somewhat regular pattern. If I'm holding 3000 rpms it cuts back to 2600 rpms before resuming 3000. Doesn't happen under 2000 rpm. I've experienced this many, many times over a long period of time and there's no chance it's a touchy pedal and I'm doing it myself. It will do it with the cruise control on in high range under the proper downhill conditions. It happens with the A/C off and I cleaned the MAF, so it would seem to be neither of those. If I didn't know about it from off roading I might not even notice it on the road. It becomes very evident if I watch the tach, however. I can live with it since it doesn't affect acceleration and is only an engine suppression instead of, say, a surge of some kind. Kind of annoying, though. It changes the way I use the gears for braking off road.

If I have to take it to the dealer I don't know how I'm going to get them to duplicate it.

Any ideas?

 

[Koz]

Interesting, maybe it has something to do with the intelligent trans.

Koz

 

[JB.]

Interesting, maybe it has something to do with the intelligent trans.

Koz

It'll do it when I'm locked into 1st so I don't think so. With the
hundreds of thousands of 4th gens out there mine can't be the only one doing it. It's hard to notice in street driving, though, so I'm not holding my breath for solutions. It only occurs when holding a speed with some throttle against gravity while above 2000 rpms. If anyone wants to try to duplicate it, I use 3000 rpms as a point which will reliably produce the effect.

Thanks for the bump.

 

[vumcrab]

JB,

Does it feel like a downshift at first followed by a hard upshift once you get to the bottom of the hill?

My 2005 V6 4runner does this a lot on the street. Even on level road with pressure on the gas pedal, it will sometimes downshift when I just take my foot off the gas and start braking. It is annoying.....

 

[JB.]

JB,

Does it feel like a downshift at first followed by a hard upshift once you get to the bottom of the hill?

No, it's definitely an engine output issue and not a transmission one. I first noticed it while holding speed in 1st-low on a long, steep downhill stretch. With the low gearing, the effect is very noticeable. It's just like taking your foot off the gas and then putting it back down. If you do that in 4-high where you're using the gear to hold speed on a long downhill, the effect is pretty subtle. Now imagine the engine does that by itself with no pedal movement. Hard to notice on the highway.

 

[hossmaster]

Maybe the transmission is checking to see if it still needs to apply that engine breaking to slow the vehicles decent. Temporary letting up on the braking power to test the amount of decent and determine how to handle the task at hand. Don't know just a thought.