jtl1567

Hey everyone i am new to corvettes, i picked up a 1978 25th anniversary about a week ago and she ran great. yesterday i drove around quite a bit and it worked awesome but today i got in started it up and put it in drive and when i would apply gas it would bog down and then go but it dosnt want to change gears unless i take my foot off the gas to let it shift. i can floor it and the rpms will go high but it wont change until i take my foot off the gas then it will go up a gear. any ideas?
Thanks!

The13Bats

Start simple...check fluid and vacuum line...

jtl1567

i am a novice when it comes to engine work. i replaced the transmission fluid, it was a little dirty. and recently had the vacuum hoses replaced. so im not sure what to do.

jtl1567

could this problem also be caused by a exhaust problem?

tejasak

Sounds like a vacuum issue, you need to check the vacuum modulator. Check to make sure the vacuum line is not cracked and that it is dry. If the vacuum line is wet the modulator needs to be replaced.

jtl1567

Awesome thanks for the photo i had no idea that was there. i will put it up on a lift and take a look. the reason i asked about the exhaust is because i want to replace the current fake dual with side mounted exhaust.

The13Bats

Exhaust doesnt normally have effects on an automatic shifting that I know of,

On my 69 it has a th400 tranny my vacuum line the metal part got mashed almost flat and caused a problem much like yours....

jtl1567

if it is the modulator where can i purchase one? the only places near me are car quest and o'reilly auto parts

jtl1567

the modulator will be the first thing i check.

tejasak

If you need an excuse to change the exhaust, then yes, that's your problem.

But really I am pretty sure you have a modulator issue.

If you chase the metal line up to the engine you will find it also has a small rubber hose connected the the intake manifold, make sure to check it for cracks also.

jtl1567

haha that just may be my problem :P i was looking at some of the rubber hoses today and they are not in good shape. this corvette sat for 9 years then the guy i bought it from had it running for about 6 months.

jtl1567

now that i think about it the "leak" i was trying to fix on the transmission pan was on the from passenger side of the pan or appeared to be. so it must be the modulator or a hose near there.

13611

with The13Bats and Tejasak. Welcome to the world of C3's. You may not be handy with a wrench at the moment, but if you plan on driving your 38 year old Vette for an extended period of time you're going to get pretty familiar with the mechanics of it Keep us posted and good luck.

Paul L

They should have, or can get, that part for you. My local NAPA got me one in 24 hours for my TH400. One for your TH350 should be just as easy to find.

jtl1567

Awesome! you have all been a ton of help. i thought the repair was gonna cost an arm and a leg but its only like $12! the soonest i can get it up on a lift will be friday or saturday but i will keep you all posted. just looking at the modulator it just pushes into the transmission? Any ideas of a good place to get the side mounted exhaust?

Paul L

Yes, it just pushes in. But there is a forked bracket to hold it in place. Just a one-bolt job.

When I removed mine it appeared that the internal diaphragm had developed a pinhole leak and some transmission fluid had found its way in the vacuum tube. This caused some erratic shifting.

The13Bats

I couldnt afford a vette if I didnt do most of the work on it,
Get a good generic auto repair manual, not too new, something that covers the 70-80's not modern computer controlled everything,
Then if you really get into it go for a 78 assembly manual, they are nice to have especially if you are a bit new to vettes wrenching or both,

Paul L

He should actually go for the GM version. Can't beat it.

jtl1567

sweet thanks. im going to try and fine one!

Paul L

This on EBay right now. I am not permitted by Forum rules to link but just search on 1978 GM Chevrolet Shop Manual.