C6 Hesitates to go into Drive

[MadJoe]

Hey guys, my C6 is starting to hesitate to shift into D when cold. It seems to get quicker about it the warmer it is (never instant, though), but if it's cold it will not go into Drive until I goose the throttle a little (which reveals some slop in my U-joints with a little "clink" each time), and then if I stop shortly after it will disengage sometimes needing another goose (and clink). I did let it idle in D for a short while yesterday, and it did eventually start to move forward, but it took about 15-20 seconds (I'm not absolutely sure about that because it wasn't actually intentional, I had shifted into D and thought about something I needed to do before driving, but knowing it wouldn't go into D I just left it there while I took care of whatever it was I needed to do and then it started rolling forward) for it to do so. It shifts perfectly fine as long as I'm moving, and goes right into Reverse no problems. Any chance this is a simple fix like low fluid* or a filter change?
Thanks in advance!

-Joe

*I know I should have checked this first, but I just haven't had time because the few times I've driven it long enough to get it up to temp lately It slipped my mind, or I haven't had the chance because things are hectic right now (we're building an addition for our handicapped daughter and doing a lot of the work ourselves because of cost). And now that I'm thinking of it and have some time, I'm at work.

[colnago]

I had problem a few years ago where it wouldn't shift into Drive in cold temps until the engine was warm. I freaked out about low fluid, clogged filter, worn friction discs, etc. Long story short, I have at least one bad intake valve on my 352, and the engine wasn't creating enough vacuum until warmed up. So, in my case, it was engine, not tranny. You might check the line going up to your intake, and make sure the modulator itself is good.

Joseph

[1972hiboy]

Always verify fluid level first.

[Busboy]

I had problem a few years ago where it wouldn't shift into Drive in cold temps until the engine was warm. I freaked out about low fluid, clogged filter, worn friction discs, etc. Long story short, I have at least one bad intake valve on my 352, and the engine wasn't creating enough vacuum until warmed up. So, in my case, it was engine, not tranny. You might check the line going up to your intake, and make sure the modulator itself is good.

Joseph

I challenge that diagnosis. The vacuum modulator circuit has zero to do with engagement. If the vacuum leak was causing a low engine rpm then maybe that would have caused the issue.

[MadJoe]

Thanks guys, I'm going to check the fluid level as soon as possible since the vacuum in the truck is pretty strong (according to the after market vacuum gauge the PO installed, anyway).

-Joe

[Jacksdad]

Mine would drop in and out of drive. The fix? Adding enough trans fluid...

[colnago]

I challenge that diagnosis. The vacuum modulator circuit has zero to do with engagement. If the vacuum leak was causing a low engine rpm then maybe that would have caused the issue.

I'm just trying to point out that it might not be the transmission itself.

Joseph

[MadJoe]

Well I checked the fluid level today and, if I'm reading it right, it's a little over full. Not by a lot, the stick says if it's between the arrows don't add fluid, but it's just slightly (maybe a 1/2" at most) over the arrows. The truck was up to op temp idling in Park when I checked it. Occasionally there were bubbles in the fluid (I checked multiple times), but it certainly wasn't frothy. Just a couple bubbles on the stick. The fluid is still reddish pink, and smells like transmission fluid, no hint of oil or gas in it that I could tell. Also no saw dust, haha.

I checked the vacuum advance and it's working fine, so if there's a vacuum issue it's definitely the line to the transmission (is there one?).

Any more ideas? I'd change fluid and filter, but from the color of it the PO did that very recently so it isn't the problem (but might have been done to hide the problem).

Thanks again,
-Joe

[colnago]

I checked the vacuum advance and it's working fine, so if there's a vacuum issue it's definitely the line to the transmission (is there one?).

Yes, there is. You should have a small gold or silver can on the passenger side of the C6. This is your vacuum modulator. There should be a short length of rubber line connecting it to a hard line, and this hard line should run up and tap into the intake manifold. If you disconnect the modulator from the rubber line, you should not have any fluid in the line. If you do, the modulator needs replacing. Depending on the year of the C6, some modulators screw in to the C6, and some push in and are held in with a clamp.

Some modulators are adjustable by inserting a small flat-blade screwdriver in the rubber-hose end. My current one isn't adjustable, so there's nothing to mess with.

Joseph

[cep62]

Add a bottle of Lucas.

[Busboy]

Don't add anything to it. If it hesitates excessively then it's days are numbered. If it isn't too bad then it might run for years more.