'80 305/TH350 losing power in 3rd

[deadllama]

Hey y'all. My truck is my first old project vehicle, and I could use some guidance.

I've got an '80 C10 Silverado with a 305 and TH350. I bought the truck knowing both were in rough shape, but I've been driving it every so often without incident until the last month or so.

The truck idles fine, revs fine at idle, and runs fine in first and 2nd, but it seems like it's shifting from 2nd to 3rd too early. When it shifts from 2nd to 3rd the revs drop way down, and the truck seems to lose quite a bit of power, like the motor is bogging down. If I really stay on the gas, it will shift normally, but I can't do that in traffic, and even then it feels like it's missing some power in 3rd. It never wants to get over 45-50mph.

Today when that happened, I was going up a slight hill and I had to get on the gas a little bit to keep from slowing down too much in traffic, and the revs picked up slightly, but the motor started cutting out. I thought it was going to die completely, but it never did--just stopped running for a half second and came right back to life, maybe three or four times. Once I topped the hill, I made the block and went back home instead of driving it to work like I'd planned, and it ran fine the rest of the way (though I was babying it at that point).

I initially suspected the transmission. For one, it's got 150k miles on it. Also, it's always leaked a little around the front, like the input shaft seal is worn out. I have only had to add fluid once, though--it's not a big leak.

Today made me wonder, though. The truck has always run fine until today. Could a dying transmission cause what I'm describing, or should I check something else first? The fact that it started cutting out makes me wonder if I should check the fuel pump instead?

[Captain Swampy]

Yes, it sounds more like an engine problem. The fuel pump is a possibility. If there's 150,000 on it, it needs a timing chain. The throttle shafts could be worn and leaking air. Start with a tune up and put some Seafoam in your gas. Regular copper plugs, I like NGK or AC Delco.

[VileZambonie]

Sounds like you have a fuel starvation issue. As far as the trans goes, do you have the detent cable hooked up and properly adjusted? How about the vacuum modulator? Have you ever replaced it or adjusted it?

[deadllama]

Sounds like you have a fuel starvation issue. As far as the trans goes, do you have the detent cable hooked up and properly adjusted? How about the vacuum modulator? Have you ever replaced it or adjusted it?

The cable is hooked up and, back in the summer when I was driving it more, seemed to work fine. I haven't adjusted it. I have not ever touched the vacuum modulator.

[JohnnyPopper]

Have you tried to manually shift into second under those conditions?

It does sound like the cable or modulator is not downshifting.

You can test it by getting on a flat stretch and at 35-40 mph.

I was also thinking that a tune up might be in order.

[deadllama]

I haven't tried it, mostly out of fear of exploding my transmission and getting stranded on a morning when I needed to get to work. I will try that next weekend when somebody can come get me and see what happens. I'm guessing it's also possible there's a vacuum leak somewhere; it looks like the vacuum hoses are all factory and/or pretty old. Would the cable keep it from shifting into 3rd too early, or just make it kick back down into 2nd if it did shift too early?

I have no idea what I'm doing, so I'll ask the people here who know better: is a tune up something I should attempt with my total beginner skills (I have a Haynes manual and some tools and I've fixed bikes before, but this is my first project vehicle) or should I find a mechanic who can do it?

[JohnnyPopper]

2nd gear will take you from 25 mph to 50 easily, so if you're in that range, you won't hurt he trans by manually shifting. Just don't drop into 1st...she won't be happy. 

On that motor tune ups are easy:

Buy plugs, gap them, new spark plug wires, new cap and rotor.

You can use the existing coil in the cap, but for a few more bucks, go with a new one. Use the dielectric goo on both sides of the rubber pad between the coil and the cap.

Now here is the really important part: take pics... from as many angles as necessary and at each point of disassembly.

The firing order is usually what gets messed up, 5 & 7 get switched. Double check that you have the order clear in mind-write it down, make a graph, whatever it takes to keep it right. It's usually cast on the intake manifold behind the carb/tbi. I think it's 18436572

Your right bank of cylinders are ODD,  from the front it's 1,3,5,7. On the left they're EVEN, from the front it's 2,4,6,8.

Before you take anything apart, identify the #1 plug wire on your existing cap. Take a sharpy and write that on the new cap. It will be your road map going forward. The rotor inside turns clockwise.

Before removal, number the top of each old cable in it's firing order, after removal, match lengths with the new cables and mark them so you know what spot they take up. They make silver sharpy's now!

You're ready to disassemble and reassemble.

I recommend using some type of anti-seize on the plug thread, just a dab.

Finally, each cable has a correct position at both ends, at the cap and at each plug. Make sure they are seated all the way in the cap, and make sure you feel the snap at the plug. Loose or poorly seated cables will haunt your life after a short time...

You'll do fine man! Just like riding a bike!

[deadllama]

Finally got all the parts in and found the time to do this--replaced the coil, cap, rotor, plugs, and wires. (Special thanks to JohnnyPopper for the encouragement and the walk-through.)

The cap and rotor were pretty old but the plugs didn't look bad, just normal wear as far as I can tell (based on the Haynes manual and lots of Googling). The old ignition coil definitely had what looked like burn or track marks on the outside of it. The whole thing was an Accel HEI Super Coil and cap (and probably distributor too) that look like they've been on there forever. Truck runs a lot better--a lot more smoothly. Due to the ancient glasspack mufflers on it I'm sure my neighbors appreciated how much better it sounded 

Took it out for a test drive to see what problems still remained. It still shifted into 3rd way too early and really bogged down... it never cut out like last time, so I have to think the tune up helped there. This time I manually shifted into 2nd when it happened, and the downshift was pretty hard, but it worked. I was able to basically leave it in 2nd and only manually shift to 3rd when I needed to. Not ideal, but it seemed to work fine that way. If 2nd is supposed to start around 25, it may be shifting into 2nd too early too. I'm thinking of ordering a new (adjustable) vacuum modulator for the trans. Does this seem like the logical next step?

[JohnnyPopper]

Good job man!

Sound like your coil/cap were the main culprit, straying electrons are no joke.

I'm not that hep on modulators except when they leak and you start sucking ATF down the intake and the resultant blowing of white smoke.

Do a search on modulators here, I think I saw recently a post about when and how they come into play.

Manual shifting won't hurt your transmission as long as you go easy. I shift between 2 and 3 all the time when it's appropriate.

Have a great weekend! Oh, and when did your Llama die?

[deadllama]

I'm not that hep on modulators except when they leak and you start sucking ATF down the intake and the resultant blowing of white smoke.

Do a search on modulators here, I think I saw recently a post about when and how they come into play.

I found some other old threads about the same problem that suggest checking the vacuum to the modulator and making sure the detent cable is adjusted... I'll be checking that at some point this week, and adjusting the cable if it needs it. Hoping it's not something more serious than that, but if it is, my father in law is a mechanic who started his career in a transmission shop and I bet we can figure it out.

Have a great weekend! Oh, and when did your Llama die?

That's the thing about just using the same username all over the internet from high school on... eventually you forget the inside joke that was behind the username in the first place.

[deadllama]

(accidentally double-posted instead of editing...)

[VileZambonie]

Adjust the detent cable, replace the modulator and see how it behaves. If it still upshifts too soon, buy a governor service kit and install it.