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Old 03-12-2017, 10:52 PM   #1
telly
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Transmission? Poor acceleration.

Ok, I guess I'm ready to hear what I don't want to hear. Here's the story:

Had someone helping me to change the oil in my 72 C20. Turns out they drained the transmission and then added 4 quarts of oil to the engine. I drove it about 10 miles then started to feel a lack of acceleration. Got it back home and figured out what happened, so I refilled the transmission and changed the oil. Problem is I'm still having acceleration problems. What's the chance the transmission isn't damaged and not in need of repair? If not a transmission problem, what do you guys think?

Edit: I used Dex/Merc transmission fluid to refill.
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Old 03-12-2017, 11:15 PM   #2
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Re: Transmission? Poor acceleration.

Draining the transmission pan then driving the truck around will smoke a transmission in a hurry!!!!

Guessing this is a TH350 as it's one of the few GM units to have a factory drain plug you can actually remove without a fight.
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Old 03-12-2017, 11:43 PM   #3
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Re: Transmission? Poor acceleration.

Yes. The tranny pan has the drain in its front/center.

Is there any hope for the tranny? It still moves the truck, and can accelerate, but it's not nearly as strong as it was.
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Old 03-13-2017, 10:07 AM   #4
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Re: Transmission? Poor acceleration.

Your trans is toast. I burned up a couple th350's when they were only a qt low. I lost a trans line once and th350 was toast in 1/2 mile. I have never had a th350 with a drain plug. That would be handy sometimes but not this time. Th400's are much more forgiving on being low on fluid but not the th350's. Sad story.
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Old 03-13-2017, 10:16 AM   #5
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Re: Transmission? Poor acceleration.

I'll crawl under it today to see if I can identify the transmission. I don't know that it is a th350 or not. I bought the truck from someone who bought it from someone who married an engine and transmission to a neglected shell. I was given no information about the engine or transmission, and I'm not much of a gearhead. I am able to work on my vehicles, but I'm definitely not real knowledgable.

Right now the truck accelerates decently from a stop (although not like it did), but the lack of acceleration is really noticeable in higher gears.
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Old 03-13-2017, 11:37 AM   #6
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Re: Transmission? Poor acceleration.

More than likely friction clutches burnt causing them to slip.

I've seen used TH350's -at least in this area -for $150-$300. I'd probably consider a rebuilt one before a used one these days but in the past I have installed used ones from running vehicles and had good luck.
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Old 03-13-2017, 07:49 PM   #7
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Re: Transmission? Poor acceleration.

If they drained a full 4 or 5 quarts out of the transmission, I'm surprised that the truck was even able to move at all, let alone drive it down the road for any distance. I've had them just a couple quarts low and they would take a good 10-15 seconds just to jump into gear.
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Old 03-13-2017, 10:26 PM   #8
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Re: Transmission? Poor acceleration.

Well, it is a TH350. Sucks, but I guess I have to give in and believe the truth.
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Old 03-15-2017, 06:52 PM   #9
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Re: Transmission? Poor acceleration.

How are you checking the trans fluid level on the dipstick? Engine running or not?
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Old 03-15-2017, 08:34 PM   #10
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Re: Transmission? Poor acceleration.

Yes, checking the fluid level with the engine running and at normal operating temperature.
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Old 03-16-2017, 07:23 PM   #11
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Re: Transmission? Poor acceleration.

Quote:
Originally Posted by telly View Post
Yes, checking the fluid level with the engine running and at normal operating temperature.
OK, just making sure. Sorry, doesn't sound like good news then. I'm still baffled at how you drove it at all with that much fluid lost. Should have slipped like crazy or as mentioned, never even went into gear. That's been my experience.
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Old 03-16-2017, 08:59 PM   #12
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Re: Transmission? Poor acceleration.

The truck still moves, just not much acceleration. I'll be driving the Camaro for the next month or so. Oh well, it could be worse.
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Old 03-16-2017, 09:27 PM   #13
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Re: Transmission? Poor acceleration.

Quote:
Originally Posted by telly View Post
The truck still moves, just not much acceleration. I'll be driving the Camaro for the next month or so. Oh well, it could be worse.
What do you mean by no acceleration? Do you mean the motor doesn't rev up or do you mean that the motor does rev up but the truck doesn't go?
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Old 03-17-2017, 10:05 PM   #14
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Re: Transmission? Poor acceleration.

One more time. Does the motor rev up when you give it the gas? If not you may have a motor problem and not a trans problem.
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Old 03-17-2017, 10:19 PM   #15
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Re: Transmission? Poor acceleration.

I'll go fire it up and see how high the rpms go.

I don't have a tach, so I don't know the rpms for certain. It doesn't seem to be hitting real high rpms though. If true, any ideas? What might have happened while there was waaaaaay too much oil in the block?
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Old 03-17-2017, 10:43 PM   #16
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Re: Transmission? Poor acceleration.

I see where having way too much can cause the rear main seal to blow, but I don't have any leaking oil on the driveway.
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Old 03-18-2017, 05:15 AM   #17
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Re: Transmission? Poor acceleration.

When is the last time the motor got a new set of plugs? If the trucks been sitting for any length of time before the mishap, maybe bad fuel and or a clogged fuel filter? Also, over filling the crankcase can lead to fouled plugs. Might take a look at the wires and caps on the distributor making sure they're all fully pressed on to the terminals. It's easy to inadvertently pull a wire loose when pulling the transmission fluid dip stick. I'd definitely install a new set of plugs before I gave up on the transmission. The fact that the 350 turbo is still pulling the truck is testament to how tough they actually are. If you did fry the clutches there should be some tell tale indicators such as a burnt smell in the fluid as well as suspended clutch material in the fluid causing it to be a bit darker than the bright red translucent fluid that was added. If the poor acceleration proves to be some fouled plugs, I'd buy the tranny a pint of this, http://www.lubegard.com/~/C-303/LUBE...uid+Protectant and myself a six pack of these, http://www.binnys.com/beer/Bud_Light...num_40500.html
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Old 03-18-2017, 03:28 PM   #18
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Re: Transmission? Poor acceleration.

This truck has been my daily driver...I only commute about 5 miles round-trip. I changed the plugs a week ago, changed the air filter, and reset the timing. I don't smell anything burnt when the truck runs, but it does have some white/gray smoke coming out of the exhaust pipes. I'm definitely down with the second link you provided, but I already have put Bar's Leak into the transmission fluid to help with any slipping.

When the weather dries out a bit I'm going to crawl under the truck and drain the tranny again, then drop the pan to see what's what. That way at least I'll know if there is metal shavings or not.

How would I go about "drying out" the block (removing the excess oil that may have been pushed into unwanted areas)? Where all do I look for issues?
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Old 03-18-2017, 05:46 PM   #19
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Re: Transmission? Poor acceleration.

I agree with others, if trans was slipping, you hear the engine rev up but the truck won't accelerate. From your description, engine isn't even revving. So... let's reset.

What have you changed since the incident? Oil, spark plugs, timing. OK. Anything else?

Excess oil in the engine probably didn't hurt anything as long as you drained it and refilled it to the proper level since then. If the cylinders were washed out, you may need to burn it out of the cylinders, which should have happened with a few drives already. Did you pop off your PCV hoses and make sure they aren't full of oil? Otherwise there really isn't anything else that would worry me.

The spark plug change and timing change does concern me. What gap did you use on the new plugs? Do you have HEI? Did you make sure you didn't accidentally swap any plug wires? Recheck the firing order.

How did you check timing? Did you make sure vacuum advance was unhooked and plugged? What timing values did you end up on?
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Old 03-19-2017, 12:48 AM   #20
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Re: Transmission? Poor acceleration.

Changed the PCV valve today. It was sludgy and didn't rattle when shaken. I then ran the truck for about 20 minutes and kept watch on the white/gray smoke coming from the exhaust (trying to burn off any excess oil that my have blown by while over-filled). After the white/gray smoke greatly diminished, I took the truck for a spin "around the block" (actually about a 3 mile loop). I don't seem to be having transmission troubles, at least no noise and/or clunky shifts. And, I still don't have any oil leaks that would indicate a main seal issue.

When I replaced the plugs, I did so one by one...remove plug wire, remove plug, insert plug, replace plug wire. I did not gap the plugs...I used them straight out of the box. It is HEI and when I reset the timing I did so with the vacuum advance unhooked and plugged and advanced to 12. I chose to reset the timing as I was getting some backfire upon deceleration...there is no backfire now after the reset.

The fluid in the tranny appears to be fine...not dark, although I haven't yet dropped the pan to see if there is any metal shavings in there.

*A bit of additional info...the truck will currently not spin its tires from a stop if traveling in a straight line, but it does have enough power to spin them when leaving a stop and turning onto another road.

**Another bit of additional info...I believe the engine to be a 350, but I can't be sure. I bought it from a guy who bought it from a guy...that guy supposedly built engines for racing and threw this one into this truck when the person he built it for didn't pay. The guy I got it from couldn't remember or didn't know what block was in it, and had no idea if it had been bored at all or what year. I haven't crawled back into it to take the numbers off the block. It may be a later 70's block, but I do believe it is a 350...just not certain that it's still a 350. With that information, what would you suggest the plugs be gapped to?
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Last edited by telly; 03-19-2017 at 12:54 AM.
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Old 03-26-2017, 12:04 AM   #21
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Re: Transmission? Poor acceleration.

The 6-7000 rpm part..espicially if its stock...unless you want to see the inside of your engine laying in your driveway

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Old 03-19-2017, 08:47 AM   #22
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Re: Transmission? Poor acceleration.

It sounds like you may have dodged a bullet here if you are still moving. Maybe the extra oil in the engine was doing something but I have no idea what. The plugs should be .045".
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Old 03-19-2017, 07:24 PM   #23
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Re: Transmission? Poor acceleration.

Ok. So gapped the plugs to .045, checked the fluid levels again and tranny was fine but the engine oil was dark. That surprised me since I'd not driven the truck more than 10 miles since changing it last. So I drained and replaced that. Then ran the engine for a while and now I'm hearing a "tingy" clunk when I change between Drive and Reverse and vice versa.
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Old 03-19-2017, 08:03 PM   #24
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Re: Transmission? Poor acceleration.

That could be a u-joint in your driveshaft.
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Old 03-19-2017, 08:04 PM   #25
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Re: Transmission? Poor acceleration.

Is that possibly due to the issues I've been dealing with or separate?
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