Engine revving but doesn't pull

[morphiend]

I was driving home from the city and my engine seemed to be revving a bit high. I got onto the freeway and it started revving quite high and wasn't pulling equal to the amount of gas I giving. I was unable to accelerate pass about 40mph. It also was very slow to change from reverse to drive. I'm thinking low Tranny fluid. Any other suggestions? It's a 72 GMcC 2500 auto.

[truckster]

Originally Posted by morphiend

I was driving home from the city and my engine seemed to be revving a bit high. I got onto the freeway and it started revving quite high and wasn't pulling equal to the amount of gas I giving. I was unable to accelerate pass about 40mph. It also was very slow to change from reverse to drive. I'm thinking low Tranny fluid. Any other suggestions? It's a 72 GMcC 2500 auto.

Transmission fluid is the first thing I would check. And I'd avoid driving it until you do.

Whatever you do, don't change the fluid. Unless the fluid's been changed regularly, that's often a death sentence for TH350s.

[mrolds88]

Good place to start. Check it in neutral, engine running, level ground, parking brake engaged, and operating temp. Regardless of level check condition of fluid. Is it pinkish? Smell funny? Pull the dipstick, wipe it off, then reinstall it. Pull it again and check the reading. If you have to add fluid, do it about a half quart at a time. Also, if it is low, here's a couple of things to check to see why it's low. Leaks. Obvious wet spots on the ground. Places to check. Oil pan, front yoke on driveshaft, front pump seal (right behind the engine), oil cooler lines, and oil cooler (in radiator).

[hugger6933]

If we are losing tranny oil lets not forget about it using oil. It could be pulling it up through the vac line and sucking it into the engine. It is odd and rare but it can happen if the diaphram on the modulator gets ruptured.

[Molberg]

Plugged tranny filter possibly..

[Hawghauler]

I think after determining if the level is correct, the next most important thing is the smell.
If it has a strong burnt smell, your probably cooking your clutches. It does sound like the level is low.
Another few places to check for leakage is the dip stick housing, the o-ring is a common wear item. Likewise the speedometer gear housing o-ring can fail with age. Then the most common, tail shaft seal around the driveline. Often the rear seal doesn't leak unless the vehicle is accelerating, or parked on a hill nose up. While it's driving you can't see it leaking, but it will leave a trail you can follow on the bottom of the truck, and you'll likely see a drop on the bottom of the seal housing left behind. All three are easy cheap fixes and should be done anyway if your trans is old and you haven't already replaced them in a long time or you don't know the history of the trans.
If your fluid is low, you absolutely must not drive truck until it is filled. If the clutches aren't already bad, low fluid will definitely cook them in a hurry.
You should always keep an eye on all your fluids. If it is a DD, you should check them at least monthly. If it sits awhile between drives, you should probably check them each time you drive it.
Hopefully you're just low, and haven't done any lasting damage so far. Check the smell...
Good luck,
Merry X-mas,
Curtis

[morphiend]

Thanks for the replies.

Truckster could you explain this for me? Its not a good idea to change the tranny fluid?

Whatever you do, don't change the fluid. Unless the fluid's been changed regularly, that's often a death sentence for TH350s.

[Sammy]

Changing the fluid can cause clearance issues that the old sludge is now present in the transmission. Can be caused from normal wear on the bands over long periods of time or from neglect.
I have seen good working transmissions after simply changing the fluid and filter began to slip immediately or in a short time afterwards. By no means am I an expert on transmissions. Sure someone with expertise will chime in on this subject.

[morphiend]

I did some research on this and a lot people say that if a tranny is on its way out, and the fluid hasn't been changed regularly, changing the fluid will finish it. There are different explanations why but a lot of people supporting this. So in my case I've only had the truck for a short time. Same with the owner before me so I don't know the history of the tranny. I haven't smelt any burning and until yesterday I didn't notice any symptoms. My question is should I top up the tranny and see what happens or should I drop the pan, replace the filter and do a full change or should just do a full rebuild. I've never rebuilt a tranny so this would a big process for me.

Thanks for all help.

[cool68]

bad torque converter?

[Molberg]

If its slipping badly already and the fluid level is up to operating range... what do you stand to lose?

[Molberg]

This happened to a truck belonging to a friend of mine. Slipping badly, would only catch if revved up. (fluid was fine) We agreed to change the filter. Before I got there he was in his shop revving the snot out of it... it caught so hard the timing chain skipped. So we ended up replacing a timing set, and the tranny oil/filter. Been fine ever since. Pump cant generate pressure if the filter is plugged.

[chewychevy67]

Originally Posted by hugger6933

If we are losing tranny oil lets not forget about it using oil. It could be pulling it up through the vac line and sucking it into the engine. It is odd and rare but it can happen if the diaphram on the modulator gets ruptured.

This happened to me last year. I thought I had a cracked head becacse the exhaust wes so white. that was how i figured out what it was.

[morphiend]

I topped up the fluid. It was a bit low but not terribly. The color looked ok. Didn't smell burnt. I took it for a short drive and it is still doing the exact same thing.

I don't think it's the torque converter as it has trouble shifting between drive and reverse. I think my next step will be to change the filter.