Late Shifting in C6 After Road Trip

[colnago]

I took an ill-fated road trip this weekend (long story), and basically drove at 60MPH for a little over four hours (250 miles total). After the trip, my C6 is shifting later (and harder) than usual when first warming up. Once the engine temp gauge gets 1/4 up, she shifts fine. I had this experience during the winter, but figured it was a cold-weather thing. Right now, the ambient temps are ~100*F, so it's far from cold. Also, it wasn't shifting late before the road trip, and the temps were just as high.

I haven't checked the tranny fluid yet, but is it possible I cooked it on the drive? This is only the third time I've had the old girl on the highway. I do not know the history of this tranny, so it could have been flogged by the PO. I'm just looking for reasons why it might be shifting late. Am I due for a tranny flush? New filter? Rebuild time?

Thanks,

Joseph

[ToughOldFord]

A bad modulator can cause the late shifting. Only I've never heard of a bad one working once it was warmed up. Still might be a good place to start though, they are only about $10-$15 and are easy to change. But first check that the vacuum line is in good shape and connected at the intake manifold and is in good shape and connected down at the modulator. Leaking vacuum will cause issues. If you pull the vacuum line off the modulator and there is transmission fluid in the line then the modulator is bad.

Also, I'm pretty sure I recall correctly that at least one of the bands on the C6 is externally adjustable, you might want to do a search here for the procedure.

[Donnie]

Tough is correct...go with the vacuum circuit first.....the only thing that I can add is: you do not need a filter change, a C6 filter is just a screen, like a tea strainer, if there is enough material in the filter to stop the flow, you would be walking. as it would take 90% of your friction material to clog this useless screen.
And he is partially correct, that is the band..........there is only 1 band in a C6 & it is for 2nd gear & is adjustable, but that is not your problem either. MOST OF THE TIME--late hard shifts are vacuum related...leak anywhere in the engine vacuum lines or the modulator itself.........the hot/ cold issue has me stumped as well,,,,,check ALL vacuum circuits including a leaking power brake servo..........Donnie

[colnago]

Well, I just checked the modulator tonight for leaks in the diaphragm. Not a drop of tranny fluid in the line. Dang! I just knew that was going to be my problem!

This weekend will be a new carb base gasket, and chasing down any remaining vacuum leaks. I know I have low vacuum; maybe this has been my tranny problem all along. Still don't understand why it would act differently once warmed up, though. Does an engine generate more vacuum as it heats up?!?

Joseph

[Donnie]

Looks like you have a grip on your problem "LOW ENGINE VACUUM " It is a fine line in physics 101, if an engine develops more vacuum as it warms up? Some materials, be it aluminum, metal, or some gasket materials have the ability to expand with a temp rise...For example, aluminum expands at a rate greater than cast iron...So in theory, it is possible for engine vacuum to increase when your engine is warmed up, however the instruments needed to measure this SMALL increment are beyond the scope of the line tech. Myself , I would be investigating an intake / carb gasket leak.....
In short to answer you question, an engine does not generate more vacuum as it heats up, YOU have to find the source of low vacuum
HTH............good luck....Donnie

[1972hiboy]

What is your vacuum reading? Ive had the modulator screw back itself out before throwing the adjustment off. just putting that out there.

[colnago]

I'll check my vacuum readings this weekend, when I put a new carb base gasket in. Going by memory, my vacuum is ~12.5", but I'm also at 2000' altitude (SoCal high desert). But even with that, it's on the low side.

However, I think the light bulb just went off. The engine runs rougher when it's cold (so it's probably not generating much vacuum). Once it warms up, the engine runs fine (and is probably generating enough vacuum for the tranny), and the tranny shifts fine. I don't have a tranny problem, I have an engine problem!

In fact, I can do one better! After my trip, the engine wouldn't idle. I could hardly get her started. I ended up putting back in my old spark plug wires, and now it idles pretty well. Hard to start, though. So, what else could be the problem? I know! I put in a Pertronix system, and it's done me well for the past couple of years. But I bet my out-of-town trips have cooked it! I know the coil got pretty toasty after my drive. So, along with my gasket replacement this weekend, I think I'm going back to the basics: swap the Pertronix out for points! Then I'll check my vacuum, my tranny, my sanity, ... and go from a known baseline.

I will definitely keep everyone posted. Thanks for the input!

Joseph

[ToughOldFord]

I'd start with a compression check to make sure the engine isn't getting tired.

And when it's running sloppy spray around the seams of the intake to see if the gasket's leaking.

If you're lucky maybe it's just a carb problem.

[ToughOldFord]

And the running better with the old wires is weird/wrong. Are you sure you put the wires in the correct positions? How old are your plugs?

[Donnie]

12.5 is LOW, even at your altitude, are you cammed up? remember too, with your plug wires, the wires on the drivers side are not to be lined up in the loom.....# 7 & 8 fire sequentially, so they must be separated in the loom....I'm still betting on a vac cap off, power brake booster or base gasket.........LEAK SOMEWHERE????????????????????????????????????????? .................Donnie

[amycyclenut]

with your plug wires, the wires on the drivers side are not to be lined up in the loom.....# 7 & 8 fire sequentially, so they must be separated in the loom....

Wow, did not know this... makes sense thinking about it. I don't notice any issues, but I'll be making this modification asap!

[Donnie]

Amy, interesting that you picked up on that note....The reason being is that in the firing order 15426378..........7 & 8 are the only 2 cyls. that fire in sequence, so it is possible that the spark from 1 wire can influence the spark going to the next wire since their travel time is distanced by mili-seconds....put # 7 in the front of your loom & 8 in the rear..............Donnie

[colnago]

Donnie,

I wonder if that's why I got zapped when I tried turning the distro. I think it was #7 that got me (I think my getting zapped is on another thread, though, but to summarize, I was adjusting the timing and got a small zap when I put my hands on the distro).

I've checked the brake booster before, and so far as I can tell, no vacuum leak there. After the new carb base gaskets, I'll check the base of the intake manifold. Man, I hope it's not needing a new gasket; from what I hear, that stinkin' intake comes in at just under 100 lbs! Ugh!

Donnie, you said "12.5 is LOW, even at your altitude, are you cammed up?" I know I'm definitely at the low end of vacuum (although I've convinced myself that I'm within the acceptable range because of my altitude), but how do I check the cam? Do I need to take off the timing cover, or do I have to check it from the valves?

Thanks,

Joseph

[Donnie]

Joe, have you tried setting your timing with a vacuum gauge? Or by ear,........push it up until it pings or is hard to start.. What percentage of ethanol are you running in your area?...It is difficult to set the timing on these older engines going by the specs that were issued when the truck was new & gas was real gasoline. With the drop in octane & the addition of alcohol, today the pump fuel is a crap shoot...
Late timing does have some effect on your vacuum, as does a worn timing chain. Spray testing your intake for leaks is going to be a futile effort as most of your intake in under your valve cover ....if you didn't build your engine, it is a rather complex process to determine what cam that you have........For all sake's & purposes'.......let's assume that it is a stock factory cam, unless you suspect or know otherwise....
I can walk you thru most torque converter & automatic transmission problems, but there are a few guys on here that are more qualified than me to speak engine diagnostics with you...I'm not backing out, just giving you some engine straight talk....
I'll hang in there with you on this, I just thought that you may have a hot cam in that engine ..........HTH..Donnie

[colnago]

Donnie,

Excellent! I'll start with my carb base gasket, and go from there. After that, it's ignition. I have no idea what the ethanol content is where I live (SoCal, so it's probably the worst that it can be in the lower 48), but I do know that I have a small vacuum leak, and am now suspecting that I have a possible ignition issue. Thanks a zillion for hanging on.

On edit, I don't _think_ I have a worn timing chain, but if I replace it, that does let me check on the timing of the cam, and timing chain/gears are fairly inexpensive ... Any recommendations on chain/gear sets?

Also, now that I think it's an engine issue, and not a tranny issue, it's time for me to start posting in the Engine forum. Thanks for putting up with me for this long, everyone!

Joseph