Problem with overdrive? Need your thoughts.

[craigp40]

Gents, I wanted to get your opinion on something I'm noticing with my truck. It's a '97 Chevy Z71, 350 engine, and approx 97k miles. The engine runs perfect. However, over the past year I've been noticing my gas mileage decrease quite a bit. I used to get about 18 on the highway, and now it's about 12. I know there are several things that can be contributing to this, but here is the funny thing I've been noticing.

When going down a hill after I've let off the gas, the truck used to roll like a demon. I was always on my brake to keep from slamming into the back of someone because it would actually speed up. Makes sense.
However, now when I do this, my truck actually slows down. It seems like the transmission is holding back the truck from drifting. Even on flat roads, I'm constantly on the gas to keep the truck moving at a reasonable speed. I've been counting the shifts of the transmission and I am getting four gears. It just doesn't seem like it's going into overdrive. If this is the case, I'm sure it is affecting my gas mileage, but what are your thoughts on it. Please let me know if I may have left out some info that you need to help.

Thanks.

[777stickman]

It may not be shifting into 4th. The fourth shift you are feeling could be the torque converter locking up in 3rd. Do you have a tach? You can tell the converter is locked with a tach by adding some throttle and the tach should increase only with a speed increase. If it's not locked you'll see about a 200 rpm increase. Try this, next time you think you're in 4th pull the selector to 3rd. See if you get a shift change. Go from there.

[MT-2500]

You might need to get it on a eng/trans/Bi directional scanner and check things out.
If you find any codes post back code no.
Also may be time for a filter and fluid service and check for anything in the trans pan that doe not belong there.
MT-2500

[craigp40]

Thanks guys. I haven't scanned it as MT suggested, but here is a bit more info:

- When driving on level road at 60mph, tach reads approx 1750 RPMs. Pulled the gears down into 3rd, RPMs jumped to approx 2400. Seemed normal to me.

- Did the same drill on level ground, but this time stepped down on the gas to make it come out of OD. RPMs went from 1750 up to approx 2100.

- Here's the weird part that I don't understand. Traveling down a hill at 60mph. Let off the gas to allow the truck to drift. Truck drifts fine for about 3 seconds at constant RPMs. Then, RPMs drop approx 300-400 RPMs and the truck starts slowing down. I can distinctly hear the engine change tone like it is downshifting, but the RPMs fall off. Once I slightly apply gas, RPMs jump back up and truck drifts fine. Let off the gas, and the same cycle repeats itself. This doesn't make sense to me.

[maxwedge]

When coasting you will not see a significant rpm drop untils till the tcc unlocks then the trans becomes more " freewheeling", normal, 55 up, say.

[777stickman]

What's happening with the rpm drop is called "engine braking". I live in the mountains and my '98 7.4 Sub does the same thing on long downhills. Sounds like your tranny is performing as advertised. I agree with MT about getting it scanned for codes though........Steve

[craigp40]

I just don't ever remember it doing this before. I understand what you are saying about the engine braking, but I didn't even think this truck was equiped with that feature.

Is the scanning you are talking about the same scan tool that they use at Auto Zone? Or is this something I have to get a garage to do?

[maxwedge]

No autozone here, it would have to be a bi-directional trans capable scanner in the hands of someone with experience in it's use and applications. The engine braking is not as obvious in o/d as 3rd, when coasting at a certain speed on decel the tcc un locks which drops the rpms.

[Mr. Smith]

Originally Posted by craigp40

Gents, I wanted to get your opinion on something I'm noticing with my truck. It's a '97 Chevy Z71, 350 engine, and approx 97k miles. The engine runs perfect. However, over the past year I've been noticing my gas mileage decrease quite a bit. I used to get about 18 on the highway, and now it's about 12. I know there are several things that can be contributing to this, but here is the funny thing I've been noticing.

When going down a hill after I've let off the gas, the truck used to roll like a demon. I was always on my brake to keep from slamming into the back of someone because it would actually speed up. Makes sense.
However, now when I do this, my truck actually slows down. It seems like the transmission is holding back the truck from drifting. Even on flat roads, I'm constantly on the gas to keep the truck moving at a reasonable speed. I've been counting the shifts of the transmission and I am getting four gears. It just doesn't seem like it's going into overdrive. If this is the case, I'm sure it is affecting my gas mileage, but what are your thoughts on it. Please let me know if I may have left out some info that you need to help.

Thanks.

When my 97 w/5.7 mileage went to the crapper I changed my fuel filter. Went right back to normal. It was that easy.

[craigp40]

Thanks for the reply Mr. Smith.

Actually, I just did that about 2 months ago. No big change. I'm also considering the pre-cat oxygen sensor. Any way to determine if this is bad? I'm not getting a CEL, but would it light up if this sensor was degraded?