My 01 tundra can bog down under load - going up a long hill or towing a trailer - with the speed and RPMs steadily dropping until it finally downshifts, and the RPMS shoot up to 4k or so, but I don’t regain much speed. This is without cruise control. If I have the cruise control on the problem does not occur. The truck slows, but in a normal way expected under the circumstances.
The CEL is not on. No codes are being recorded. The Tranny fluid is full and smells fine. Ideas?
I suspect it has to do with being in overdrive (torque converter lockup).
Sounds like cruise control keeps it in overdrive vs manual control unlocks and downshifts more readily.
This might be intentional.
I have tried turning the overdrive on and off to see if it makes a
difference when I am not using cruise control, and it does not make a
difference. One thing that does make a difference is ambient air temp. On
hot days the problem is worse.
I was wondering if some vacuum circuit gets bypassed when using cruise
control. If a vacuum leak is bypassed. ??
The Tow/Haul mode being on should actually make the problem a bit worse when you’re in control of the throttle. It prevents excessive hunting and purposely slows down the shifting. The adaptive cruise control might be more responsive to engine load. Engine load problems will usually be worse on hotter days, the air intake is less dense. Less oxygen, less fuel injected. What happens if you romp down on the gas pedal when you feel it starting to lag? Does it downshift sooner and perk up?
If I stomp on the gas when I first notice it bogging down it does not
downshift and accelerate. It continues to bog. If I feather the pedal it
can sometimes perk up… but not by downshifting, It just briefly gets
power and accelerates “normally” before bogging down again. If I continue
to feather the pedal it will sporadically accelerate, but not for long.
If I stomp on the gas when I first notice it bogging down it does not
downshift and accelerate.
Hmmm, that’s a tough one. Normally, I would say you have some fundamental driveability problem and start troubleshooting from that perspective but the fact that when using CC it doesn’t occur, that is a strange wrinkle…
I’m beginning to think that it is Dutch Elm Disease or Poltergeists. Thank
you so much for your input. I appreciate your taking time to think about
this with me!
Yeah, it could be Poltergeists … lol … That’s the ticket!
hmm … let’s put on our thinking caps, how does the engine and transmission decide when to downshift? It seems like it must use from among these inputs to decide
Guessing the problem is something with all that. Since it goes away with the cruise engaged, and no codes come up, my first bet is the throttle position sensor is not reading accurately for some reason. Next up: the transmission gear selector position isn’t making it to the computer correctly. After that, the vehicle speed sensor is malfunctioning.
Are the conditions truly the same except for cruise control? This sounds like classic fuel starvation from a clogged fuel filter or failing fuel pump. If you are using the throttle more aggressively than the cruise control does, that could explain the phenomenon; the cruise control may be a red herring.
Wow! That’s some deductive reasoning. But if none of those sensors are
throwing a code, perhaps they are only malfunctioning intermittently. It
sounds like this will take some serious attention by someone who a) cares
and b) knows enough. Those two characteristics are hard to find in one
mechanic!
Thanks for the suggestions. It is great to have a list to start checking
things off of!
Good point. As a matter of fact, I think I’m less aggressive on the
accelerator than the cruise control, since I can see the hill ahead of me,
and I start to accelerate before the cruise control would. Still, what will
it hurt to slap on a new fuel filter? Perhaps a fuel pump is cheap enough
to replace diagnostically, too. I’ll put those on my list. Thanks!
On re-reading your post, I realize that I am also at about 6,000 feet
altitude. I wonder if the lower 02 density combined with the 220,000 miles
on the engine are factors.
You engine computer knows the altitude from its sensor readings, and adjusts the fuel injection amounts for the reduced O2 when at higher altitudes. At least it should do that. I wouldn’t assume that’s the problem unless the problem goes away when driving at lower altitudes.
[quote=“GeorgeSanJose, post:13, topic:94447, full:true”]
You engine computer knows the altitude from its sensor readings, and adjusts the fuel injection amounts for the reduced O2 when at higher altitudes. At least it should do that. I wouldn’t assume that’s the problem unless the problem goes away when driving at lower altitudes.
[/quote]Regardless, normally-aspirated engines do lose power as altitude goes up. However, given how many miles are on the truck, the engine may be worn out.
The problem might be with the throttle control actuator.
https://www.rockauto.com/en/moreinfo.php?pk=3769359&cc=1375365&jsn=358
Or the throttle pedal position sensor.
http://www.carproblemzoo.com/toyota/tundra/2001/accelerator-pedal-sensor-problems.php
Tester
Thanks, Tester! These are some new ideas which seem probable… esp the
throttle position sensor. The article you linked to on carproblemzoo says
that a malfunction will trigger a code, and there aren’t any codes coming
up, nor the check-engine light. That doesn’t mean there isn’t a
malfunction, though.
When the CEL comes on, I always check, and every time I’ve found an engine
there, so I don’t really trust the CEL. :-)
lol … that’s a good one OP!
The V-6 uses classic throttle operation, the V-8 has electronic throttle control. Which engine do you have?
If the engine holds at 2500 RPMs at full throttle and won’t downshift I would manually downshift.
It’s a V6. When this happens, I’m usually trying to maintain highway speed.
Even if it slows to 50mph, I don’t think I’d want to downshift. Would I?