Pretty odd... almost died.

[ADVFJ80]

I was out with my 80 series on Friday on a moderate off-road trail for a shake-down run after some suspension work. All of that was fantastic, no issues.

What wasn't fantastic was my 80 and its decision to uncontrollably accelerate while in 4 low, when shifted into reverse. I freaked, mashed on the brake, and shifted to neutral immediately. This happened again, and again. In fact, the only instance it did not accelerate while shifting into reverse was in 4 high. It doesn't do it at all in 4 high. It is repeatable, however, in 4 low.

Thoughts on this oddity? I nearly filled my drawers the first time it happened and we were backing rapidly towards the edge of a shelf road. I feel like it's something simple, but I need a push in the right direction.

Thanks!
Randy

 

[ChaseTruck]

I was out with my 80 series on Friday on a moderate off-road trail for a shake-down run after some suspension work. All of that was fantastic, no issues.

What wasn't fantastic was my 80 and its decision to uncontrollably accelerate while in 4 low, when shifted into reverse. I freaked, mashed on the brake, and shifted to neutral immediately. This happened again, and again. In fact, the only instance it did not accelerate while shifting into reverse was in 4 high. It doesn't do it at all in 4 high. It is repeatable, however, in 4 low.

Thoughts on this oddity? I nearly filled my drawers the first time it happened and we were backing rapidly towards the edge of a shelf road. I feel like it's something simple, but I need a push in the right direction.

Thanks!
Randy

Motor mounts - and the higher torque in Low makes the engine shift and puts the harness against something where it shouldn’t be?

Similar thing once happened with Kevin / Tools R Us’ truck - in his case, putting the truck in gear while in Low nixed the locker circuit.

I know how this feels: in my K5, on a respect-commanding descent, a wonky connection at the TPS sensor caused a signal drop, and the ECU defaulted to 2200rpm. Not fun, and I imagine it’s even more scary when going backwards.

 

[ADVFJ80]

Motor mounts - and the higher torque in Low makes the engine shift and puts the harness against something where it shouldn’t be?

Similar thing once happened once with Kevin / Tools R Us’ truck - in his case, putting the truck in gear while in Low nixed the locker circuit.

I know how this feels: in my K5, on a respect-commanding descent, a wonky connection at the TPS sensor caused a signal drop, and the ECU defaulted to 2200rpm. Not fun, and I imagine it’s even more scary when going backwards.

Motor mounts look to be in great shape, but that's in park. Guess that's a place to start. Thanks!

 

[lumbee1]

Motor mounts look to be in great shape, but that's in park. Guess that's a place to start. Thanks!

Motor mounts are incredibly difficult to see if they are broken. The plate on the top and the bottom skews the view of the rubber sandwich in between. Shifting into drive and reverse and watching the engine is the best way to quickly tell if they are bad.

 

[ADVFJ80]

Motor mounts are incredibly difficult to see if they are broken. The plate on the top and the bottom skews the view of the rubber sandwich in between. Shifting into drive and reverse and watching the engine is the best way to quickly tell if they are bad.

Will 100% take a look. Thanks!

 

[Trollhole]

Is your vehicle lifted? Was it lifted prior. Is the LSPV correctly adjusted? When you say uncontrollably how many rpm are we talking about?

 

[60 toy ota]

This might not be very easy to answer, but when you say accelerate did it feel like a throttle input beyond what the idle control solenoid is capable of, as if the actual butterfly valve was being turned? Pretty crazy instance. I assume this is a 1fz we're talking about?

 

[leonard_nemoy]

Here is an easy way to check your motor mounts. Put your front tires up against a curb than stand on the brakes and give it throttle in 1st gear while having someone look at the mounts. Just make sure there are no people or things you don't want smashed in front of the vehicle....

 

[ADVFJ80]

Is your vehicle lifted? Was it lifted prior. Is the LSPV correctly adjusted? When you say uncontrollably how many rpm are we talking about?

It is, and was prior. Just went from roughly 2.5 to 4". Other than the springs sway bar drop brackets and front radius arms were changed.

I'd have to check the LSPV. I'm guessing the service manual has specs on that?

From idle to 2200 or so.

 

[ADVFJ80]

This might not be very easy to answer, but when you say accelerate did it feel like a throttle input beyond what the idle control solenoid is capable of, as if the actual butterfly valve was being turned? Pretty crazy instance. I assume this is a 1fz we're talking about?

Felt like throttle input. Smooth and steady from idle, up to over 2200 by the time I mashed the brakes and shifted to neutral. Yep, 1FZ.

 

[ADVFJ80]

Here is an easy way to check your motor mounts. Put your front tires up against a curb than stand on the brakes and give it throttle in 1st gear while having someone look at the mounts. Just make sure there are no people or things you don't want smashed in front of the vehicle....

I might be able to do that with a camera set up. Will check tomorrow.

 

[ADVFJ80]

What would other bad motor mount symtoms be? I've got no weird noises, clunks, bangs, vibration or anything else. Nothing like my '70 Chevelle when the motor mounts gave up the ghost all those years ago.

Randy

 

[leonard_nemoy]

What would other bad motor mount symtoms be? I've got no weird noises, clunks, bangs, vibration or anything else. Nothing like my '70 Chevelle when the motor mounts gave up the ghost all those years ago.

Randy

Fan hitting the fan shroud was my symptom.

 

[LongDuck]

Poltergeist? That would scare the out of me. Still, look into a young priest and an old priest...

 

[lumbee1]

I took a look at your original post.
In 4 low, it starts accelerating without your input but doesn't do it in 4 high?
When shifting to neutral, does the engine rpms immediately go down?
When it is accelerating in 4 low, can you hit the brakes and stop the Cruiser or does it try to push through your braking effort?