Low Hwy Speed Power - Worried about my torque converter

I have an '00 Olds Silhouette. 3.4L engine, 4 speed (4T65E) auto trans. Vehicle & motor have about 107K. The trans has about 50K. It was replaced by GM under warranty back at 56K or so. I bought it at 104K from the orig owner so I don’t know if it was new or reman, or if that included a new converter (and GMs service folks couldn’t tell me when I called).

The issue - it has great - to me even surprisingly good torque at the low end. This van has absolutely no trouble off the line and in anything like back road and rural highway driving.

But, take it onto the highway, get up to cruising speeds in 4th gear (OD), and it cannot get out of its own way even on the mildest of grades. It has absolutely nothing to give. On anything other than flat road or downhill, you have to kick it hard enough to downshift in order to maintain speed.

It can’t be as simple as gear ratios. I have had a Dodge Caravan for a long time. It has a 3.8 - a little larger - but essentially the same gear ratios, and it effortlessly maintains speed in 4th gear even on reasonably steep grades - and I am driving exactly the same roads. (4th gear is about .7:1 on each vehicle).

I have only the most basic knowledge of how the auto trans really works. One vague bit of “knowledge” is that if the stator clutch in the TC is not spinning right you can get fluid turbulence at hwy speeds, and thus no power. Which is why I’m worried about the TC.

Extra Info: The van runs really well. The transmission behaves itself perfectly well - smooth engagement and shifts, no signs of problems. It was flushed at 93K, so the current fluid has only about 14K on it. The fluid is clean and at proper level, but does have one weird thing about its color. It isn’t brown or black - but it also isn’t red. It is orange. I did a blotter test on it, and it looks perfect - its just not red. I asked a guy at my shop who had no real answer, but just said different fluid brands can vary in color.

I know that there are at least a couple of people out there with these vans (Roadrunner?), and maybe this is “normal” and just something about the design that I don’t get.

Or maybe there is some trick I can try to learn about the converter (but you can’t do a stall test on this van).

Or maybe just some other thoughts or words of wisdom.

If you’ve slogged through all of that - Thanks.

I’m just going to bump this once in case any takers come along.

The story is long b/c I’m just trying to put out as much info as possible.

Short story: I have a van with good low speed pick-up and no high speed/high gear pick-up. I have another van w/ similar motor size & gear ratios that drives very differently.

The fact that both engines are about the same size means little. Some engines have more low-end torque compared to others that rev high to make more HP. I doubt that your torque converter hypothesis holds up. If it is not functioning correctly at highway speeds in the top gear, it would be doing the same things at lower speeds in the lower gears, essentially repeating itself every time you run through the gears. The other thing is that I think your transmission would be overheating. The power lost in turbulence would have to be going somewhere and that somewhere would be heat.

Two prior repairs come to mind that had similar symptoms. They started out like you’re describing and got progressively worse. The first turned out to be a partially clogged catalytic converter. It was choking the motor. Easy to test with a pressure gauge in the manifold.

The second was a failing fuel pump. It ran perfectly but had diminishing power. Eventually (like within a week or two) it couldn’t climb a moderate hill. Another easy check to do.

Good luck!

Thank you both for your ideas. On the engines, I should have said similar size / HP / torque. Going by specs on paper they are not much different from each other.

I did wonder about the overheating as a symptom. I don’t know how much heat or how long it takes for it to show up in the fluid, but the fluid shows no signs of any overheating. I also recently had the opposite problem with the engine - coolant temp too low. A new thermostat fixed that.

As for the TC, my understanding was that if the stator is allowed to turn the wrong way the problem would only show up at high speed - not at lower - but I also don’t know since my knowledge of how this goes is fairly rudimentary and largely conceptual.

I doubt that there is a problem with exhaust restriction as I have gotten in the habit of putting a vacuum gauge on it everytime I change the oil including a long rev looking for any early signs of exhaust problems. I just do that to keep an eye out for problems and to make sure I am familiar w/ how the engine behaves.

I should have thought of the fuel pressure and will hook up a gauge & drive around with it the next chance I get.

Thanks again.

I agree with the others. If your tranny were the cause you’d be experiencing problems throughout the range of its operation, and at highway speeds you’d smell the fluid burn, which would show up when you check it. I don’t think this is a tranny problem.

I agree that it’s more likely an inability of a tired fuel pump to deliver enough oomph to maintain flow rate on the highway or the inability of the engine to be able to breath out…a clogged cat converter perhaps, or a rusted and hanging baffle in the muffler.

It’s also a possible bad fuel pressure regulator. Or even a clogged filter.

Have you had any fault codes?

Hmm, we were apparently typing simultaneously…

I would not write off a plugged exhaust. It could easily not show at rest, even revving the engine, and be unable to freely pass enough exhaust gas to enable sustained highway speeds.

Thanks mb - I’ll hook up the fuel pressure AND vac gauge & drive around for a while.