ATF additive suggestion for 85 Ford C6?

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I've had my 1985 Ford E350 class C motorhome with a C6 RWD (w/ diesel engine) for over 2 years and it's always had late upshifts (2nd at about 25, 3rd at about 35-40) but has never otherwise really been a problem. When manually shifting from 1st to 2nd while below 25mph, it instantly shifts smoothly. 2nd to 3rd is also smooth, if not a bit too smooth. When shifting into D or R, there is no noticeable delay, so the clutches seem ok. It's relatively low mileage (about 75k original), just 36 years old.

It also noticeably downshifts to second when braking/idling down to a stop. It's not harsh, but it's noticeable since the engine RPMs go up some. Sometimes if I roll through a yield at about 5mph, it's still in second gear for some reason, so it doesn't always downshift to 1st like it should. More recently, it has started having a delay when taking off from complete stops, which may be a "still in second gear when stopped" issue, where it has to downshift to 1st before it takes off. No bucking or banging, just a delay when taking off from the stop. It's like there is a delay, then it grabs and takes off. it does that maybe 35-40% of the time when stop and go city driving.

I'm suspecting it may be hardened seals, which I know means "rebuild time" but it's not a good time for me. If I could put it off for 6 months, that would be very helpful. I am a full time RVer so it's my home and my "daily" driver, though I'm often parked for 1-2 weeks off grid on BLM or FS land, so it doesn't get a ton of miles on it relative tp people who commute 5 or more days a week. If gets maybe 250 miles a month, some months. Others it gets far more. It got about 1,300 miles within a few weeks last November.

I replaced the fluid last summer by dropping the pan and also draining the torque converter, so I know the fluid itself is relatively fresh. I put in conventional Valvoline Dex/Merc, a new filter, and a cheap aftermarket pan with a drain bolt, so at least replacing the pan fluid will be as easy as an engine oil change.

I'm debating if I should try half a bottle of additive such as Lucas or Bars trans fix, run it for several hundred miles, then drain the pan and replace the fluid. That's what this trans tech says he would do:

 
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I would not use those products you listed, Lucas is thick like molasses.. What I had good success with is Sea Foam. It can help with the issues of a dirty valve body and help with the seals. Also, make sure your vacuum modulator is good. Just pull off the vacuum line attached to it, if fluids leaks out, it is bad. The fluid will suck up into the engine and lower the transmission fluid level over time.
 
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Cubey

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I would not use those products you listed. What I had good success with is Sea Foam. It can help with the issues of a dirty valve body and help with the seals. Also, make sure your vacuum modulator is good. Just pull off the vacuum line attached to it, if fluids leaks out, it is bad.

Well those were just two common brand names I tossed out as suggestions.

I replaced vacuum modulator with an adjustable one a couple years ago since I thought that may have been the problem with the late upshifts. I also tried adjusting the kickdown lever up at the engine end, and also the "vacuum regulator valve" (VRV) that controls vacuum to the vacuum modulator based on the throttle position. Nothing I did changed it. The needle was there and I put it back in with the new vac modulator. What's odd is no fluid came out when I took it off, despite the trans not being low on fluid, and it was parked on a pretty flat surface.

Maybe I should just go find a cheap RV park to stay in for the next 4 months, so I only have to drive it 2-3 times a month just a few miles to get groceries, then start heading back towards Arizona in October (about 500 miles) and consider having the trans looked at, if not flat just rebuilt at that time. At least it shouldn't be too expensive on a van labor wise, since the doghouse cover makes it so easy to access the upper bell housing bolts compared to a pickup where it's buried. And C6s are some of the cheapest to have rebuilt complete with a new torque converter, spec'd for higher performance.

Almost considering slowly heading back east this summer and having the guy from the video do the rebuild this fall there in Amarillo Texas once it's below 80 degrees there (i hate the heat). I've watched a good many of his videos and he seems to know his stuff. I can save more money towards the rebuild if I'm not paying RV park rent, even with 8mpg. It's about $43 per 100 miles, based on current diesel prices. I'm just concerned about how much longer it's going to last. I do have RV roadside assistance through my insurance, which also covers the cargo trailer (so it won't get left on the side of the road) so it's not like I'd be totally stuck, but I'd rather plan the rebuild time than be forced into it.

Since it's so old and it's due for a rebuilt, I'm less shy about the idea of trying an additive to give it at least an extra 6 months lifespan.

Also reconsidering my route plans to avoid steep mountain grades to help avoid blowing seals, if that's a thing to really consider. I'm in central Utah right now. Was considering going to Flaming Gorge but there's mountain passes to cross to get there. I could go north by way of Salt Lake City instead and avoid all that.
 

Cubey

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The Seafoam product I see specifically says it won't swell seals, which is probably what it needs: https://www.oreillyauto.com/detail/...5-ford-e-350-econoline?q=trans+additive&pos=4

Some people swear by LubeGuard products, and they make a "stop leak" product. Yeah it's not "leaking" technically, but seal swelling is what it would do basically. https://www.oreillyauto.com/detail/...-ford-e-350-econoline?q=trans+additive&pos=16

I don't suppose draining the pan and putting in some Valvoline Maxlife is worth the money for it's "seal conditioners"? The pan holds 5qts, so I can easily drain that from the pan and replace it with Maxlife.

Valvoline themselves make a "stop leak" product which I might trust more than other brands since it's probably mostly ATF. If I drain the pan, I'd have room to put in a gallon of MaxLife ATF and a quart of this: https://www.oreillyauto.com/detail/...-ford-e-350-econoline?q=trans+additive&pos=18

The C6 is 7qts including 2qts in the TC, so it might be enough to help it, without it being "too much" on the ancient seals.
 
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Nut sure it is going to help, but for Dexron III transmissions, I use a TES-295 fluid.

It is the same viscosity as Dexron III, but a full synthetic.

Only problem I have had is the cost...

 
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Does your trans take Type F? I was going to suggest Lubegard red, until I read this on the product page: "It can safely be added to any automatic transmission fluid (ATF) including full synthetic, semi synthetic, conventional, and low viscosity fluids except CVT, DCT, and Ford Type F [1986 and earlier models]"

OK--I just looked back and saw that you used Dex/Merc. If that's what the trans takes, LG red is compatible.
 

Cubey

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Does your trans take Type F? I was going to suggest Lubegard red, until I read this on the product page: "It can safely be added to any automatic transmission fluid (ATF) including full synthetic, semi synthetic, conventional, and low viscosity fluids except CVT, DCT, and Ford Type F [1986 and earlier models]"

OK--I just looked back and saw that you used Dex/Merc. If that's what the trans takes, LG red is compatible.

The dipstick says "Warranty requires Dexron II oil" and has the Ford logo, so I used Dex/Merc. I can't imagine the dipstick was replaced with from another vehicle, since it's crazy long, as they are on vans. And it's all metal and heavy duty like you'd expect from the mid 80s. I took a pic when I had the pan off and I could see the end of the dipstick poking down so it's proabbly correct. The refill amount with TC drain was about close to 7qts and that matched up with the dipstick reading.

The part number on the dipstick is D9UP-7A020-AA. When I looked that up, I found the following 1980-89 Ford truck document online that says Dexon II for C6. The general consensus is that they quit using Type F in the late 70s, but different people say different years. (76, 77, 78..)
 

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I would do a drain and refill (& filter change) with something like castrol hm dex/merc and add a few bottles of lubegard red; it's very good stuff and the ester might help your seals. I think the castrol green hm fluid is a bit thicker than other dex merc fluids, max life I believe is much thinner. I run the castrol in my '86 f-250 with the c6. A remote filter plumbed in the cooler lines can help extend life as well. I agree with the transynd reccomendation as well. It's what I run with my 4r100s.
 

Cubey

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I would do a drain and refill (& filter change) with something like castrol hm dex/merc and add a few bottles of lubegard red; it's very good stuff and the ester might help your seals. I think the castrol green hm fluid is a bit thicker than other dex merc fluids, max life I believe is much thinner. I run the castrol in my '86 f-250 with the c6. A remote filter plumbed in the cooler lines can help extend life as well. I agree with the transynd reccomendation as well. It's what I run with my 4r100s.

As mentioned, I did a full pan drop, TC drain, filter replacement, and fresh refill last year with Valvoline Dex/Merc. The old fluid was a bit dark but not awful, still red. It had been serviced at least once, since it had a 4WD style filter in the RWD transmission. (4WD can go in RWD but not RWD in 4WD due to the deeper 4WD pan). I put a RWD style filter.


"A few bottles"? That would be grossly overdosed. It calls for 1oz per 1qt in the system, so 7oz in my case. The typical retail bottle is 10oz. It also says it helps seals, so nothing more should be needed.
 
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Cubey

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Does your trans take Type F? I was going to suggest Lubegard red, until I read this on the product page: "It can safely be added to any automatic transmission fluid (ATF) including full synthetic, semi synthetic, conventional, and low viscosity fluids except CVT, DCT, and Ford Type F [1986 and earlier models]"

OK--I just looked back and saw that you used Dex/Merc. If that's what the trans takes, LG red is compatible.

Just spotted this too. Castrol says Dex/Merc for ALL 83-96 Fords.
 

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Does your trans take Type F? I was going to suggest Lubegard red, until I read this on the product page: "It can safely be added to any automatic transmission fluid (ATF) including full synthetic, semi synthetic, conventional, and low viscosity fluids except CVT, DCT, and Ford Type F [1986 and earlier models]"

OK--I just looked back and saw that you used Dex/Merc. If that's what the trans takes, LG red is compatible.
1980 was the last year that anything put out by Ford called for a Type F fluid in the transmission.

The classic failure mode for the C6 is the forward clutch seals get hard and allow too much hydraulic pressure to escape so shifts are delayed due to low line pressure. This is usually worse when cold. Additives like TransX can help to soften the seals, but any improvements will be termporary. A few ounces of brake fluid would probably work just as well. You should put off using seal softening additives as long as you can because eventually the seals can get too soft and turn to mush.
 
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Nut sure it is going to help, but for Dexron III transmissions, I use a TES-295 fluid.

It is the same viscosity as Dexron III, but a full synthetic.

Only problem I have had is the cost...

Not cheap, that's for sure. The new Allison fluid replacing TES 295 is TES 668.

www.allisontransmission.com
 

Cubey

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You should put off using seal softening additives as long as you can because eventually the seals can get too soft and turn to mush.
I think that's why the trans tech in the video recommends using a half dose and then draining the pan and replacing the fluid after a short run time, so they can swell but don't get ruined from being exposed long term. I guess I'll wait like you said. Manually shifting still works for the time being as needed like I have always done. I've put over 6k miles on it in the condition I described, with a pan drop about 2k miles ago. So it's holding up well, all things considered. Regular high mileage ATF might be all it needs to help it too. Easy enough to drain the pan and put in some of that in. $17 for a gallon of SuperTech high mileage Dex/Merc, might be worth trying.
 
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Another thing I remembered... since you say the transmission downshifts early, I suspect there could still be an issue with the vacuum modulation or the vacuum signal itself. The IDI diesels don't produce the right manifold vacuum to control the transmission shifts so the vacuum to power the transmission modulator valve gets produced in the injection pump. A control valve sends a variable vacuum level to the modulator valve based throttle position. It's called the vacuum regulator valve, or VRV. There is an adjustment procedure on those that you may be needing to have done which might help fix those early downshifts.

Edit:
Found a post on ford-trucks.com with good info on this topic.
 
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I had great success with Lubegard Platinum in my older Ford transmissions (2001 generation tough). Castrol Motorcraft V fluid drained every 30-50k miles, had also replaced their pans with the Dorman ones that had a drain bolt.
They never failed or had any issues, up to about 200k miles. Those cars died of rust and totaled by teenager.
 

Cubey

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Another thing I remembered... since you say the transmission downshifts early, I suspect there could still be an issue with the vacuum modulation or the vacuum signal itself. The IDI diesels don't produce the right manifold vacuum to control the transmission shifts so the vacuum to power the transmission modulator valve gets produced in the injection pump. A control valve sends a variable vacuum level to the modulator valve based throttle position. It's called the vacuum regulator valve, or VRV. There is an adjustment procedure on those that you may be needing to have done which might help fix those early downshifts.

Edit:
Found a post on ford-trucks.com with good info on this topic.

I have an 87 F250 with a 6.9 and C6 also, and it's last owner had the C6 rebuilt at a shop about 5 years ago. No problems with it at all, aside from "morning sickness", which the Dodge van I had with a rebuilt 727 (Chrysler's C6 equiv) also had. Every trans shop I have asked about that all just tell me that's "normal". Whatever. My E350's original C6 doesn't have "morning sickness" at all.

Anyway, I swapped out the VRVs between the 87 F250 and the 85 E350 and nothing changed on either vehicle with each other's VRV. The truck still up and downshifted normally, and the RV still upshifts late and downshifts early.

Oh..... and also worth mentioning, kickdown doesn't really work on the E350's C6 either. Can have the pedal to the floor and it stays in third. I can downshift to second manually and that works fine. I have tried adjusting the screw for the lever to both extremes and done short city driving tests and it doesn't make any difference.

I did vacuum system diagnostics with a gauge before and after replacing the vacuum pump shortly after I got it since it had a bad vacuum pump and brake booster, and the vacuum seems fine after replacing those two parts. No issues with vac or braking like you would expect from a vacuum leak somewhere on the VRV/vac modulator lines. Only time I ever lose vacuum was when the alternator V-belt went when I was driving on the highway and I only heard a small clunk. Had no idea what it was until 30 miles later when I hit the brakes. I brake early and no one was in front of me so I just two-footed it until I got to a nearby truck stop, and put on the spare belt the next morning once the engine was cooled off. The vac pump is powered from the alternator on a sub-belt, so if alt belt goes, so does vac.

The green one (left) one is from the E350 and the yellow (right) is from the F250. Only real difference is the hose barbs. One on the E350 is straight, while the F250's are both angled. That upper rubber piece can be removed and swapped out, which is what I did. I think I left the F250's VRV on the E350 since the E350's didn't hurt the F250's C6 behavior. I didn't see any point in swapping them back again.

I marked it on the F250 with a pencil before swapping them so I could put it back like it was. I tried adjusting it on the E350 and it made no difference at all.
 

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Sounds like you've got a pretty good handle on this stuff, Cubey. The only thing that comes to mind about the kickdown is to wonder if maybe you've got extra thick carpet or floor mats limiting throttle pedal?
 

Cubey

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Sounds like you've got a pretty good handle on this stuff, Cubey. The only thing that comes to mind about the kickdown is to wonder if maybe you've got extra thick carpet or floor mats limiting throttle pedal?

Nope, I checked the throttle pedal travel in relation to the injection pump's throttle lever after I put an electric lift pump (after a year of owning and driving it) because it was having timing issues until I put a regulator to further drop the fuel pressure below the already somewhat low pressure of 5-7. I could have the pedal to the floor and it wouldn't go over 55-60. Once I put the fuel pressure regulator, it fixed it back to normal. The cable might be SLIGHTLY stretched (a few mm max) but I don't think it's enough that it should disable kickdown.

Something is just screwy with this transmission in several ways is all. I think I might just go the route of draining the pan since it's just a matter of removing the drain plug, dumping in a gallon of SuperTech HM ATF and topping off if needed with the spare regular Valvoline Dex/Mec I keep on hand. Maybe the additives in that ATF will be enough to help it, and won't be as harsh as dumping in a bottle of pure additive. It costs about the same as a bottle of additive too. I have a couple of days to think about it before I head on to the next city with a Walmart and many auto parts stores.
 
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