1999 Chevy K2500 Suburban 6.5 Diesel (Automatic)

[101Volts]

Update: I got the first fuel economy reading: 12.09. However, that's with a leaky fuel injector on part of the tank and with cold weather and usually a bit of idling to prevent engine damage.

I'm still replacing the transmission fluid with some Valvoline Full Synthetic. ($16.97 per gallon at a nearby Walmart.) The fluid has been getting cleaner with every time I replaced it in the pan, though even after three changes it was still brown. Is it a 12.5 quart system? I read somewhere that it is. I used to have a difficult time just moving the transmission lever which got easier after changing the fluid in the pan just once.

I also have the ScanGaugeII plugged in to it and the pre-heater (if it's working; it seems to work) is used before I start up the vehicle. I'm not sure what the heater goes to though - if it goes to the engine coolant it doesn't work (no difference in temperature.) But maybe it goes to the oil pan?

[oil pan 4]

Sub 15mpg is pretty normal for 90s 2wd 6.5L suburban.
I believe they come with 4L80E transmission, which isn't super efficient and they tend to have 3.73 or 4.10 gears.
Check out dieselplace dot com.

I say if you are using trying to flush the transmission get a pan with a drain plug and use the cheap fluid that is only about $10 a gallon.

The coolant heater is in the water jacket, if its working, after you plug it in you should start to hear a crackling, sizzling or hissing sound from the water in contact with the element boiling. If its quiet and not sparking when you unplug it its likely dead.

[101Volts]

I was looking up Differential Fluids and Autozone's computer suggests 80w90. But is there any problem with running 75w90? I saw some nice Mobil-1 Full Synthetic in there which I was thinking of using.

[oil pan 4]

I put mobil1 synthetic in the diff of my half ton suburban back in 2008 or 2009 and have not had any issues.
I will most likely put that same oil in the 10.5 inch 14 bolt diff on the C2500 I just picked up.

When changing the diff fluid I recommend putting the cheap stuff in there running around on it for a day or 2 then keep changing it every few days until it becomes clear. On that first change take something like WD40 (not brake cleaner) and spray out the bottom of the diff, that is where all the metal shavings and gunk are going to collect. If you clean the bottom of the diff it should only take one flush to get it cleaned out.
Also wear gloves and use safety glasses.

If the transmission looked like it was filled with used motor oil then the rear diff will likely look about the same or worse.

[101Volts]

I didn't mention this earlier, but we had the reverse go out. (Edit - That's why I posted about the ATF being black.) So, $1,050 later the thing is back on the road working with not just the band fixed but the mechanic also worked on another band which needed work and flushed the transmission, transmission cooler and torque converter. The vehicle coasts longer now.

Now I'm working on the power steering system, changing the fluid. The old stuff was junk and smelled burnt.

[aardvarcus]

I hope that resolved your transmission issues. You may want to check that your ac condenser, radiator, intercooler, (anything in the front air is supposed to flow through) is not clogged or stopped up causing the transmission to overheat due to inadequate flow through the cooler. I now know that was the reason I had many many transmission issues in my 91 Chevy K2500.

[deejaaa]

Originally Posted by aardvarcus

I hope that resolved your transmission issues...........:

X2.
money well spent but a nightmare nonetheless.

[101Volts]

Originally Posted by aardvarcus

I hope that resolved your transmission issues. You may want to check that your ac condenser, radiator, intercooler, (anything in the front air is supposed to flow through) is not clogged or stopped up causing the transmission to overheat due to inadequate flow through the cooler. I now know that was the reason I had many many transmission issues in my 91 Chevy K2500.

Thanks, though he did flush the transmission cooler. And the transmission is working a lot better than it was, unless I'm missing something.

In other news, I flushed the power steering. I changed the fluid in the reservoir several times yesterday, and after that I put almost three full cans of Seafoam Trans-Tune through it. The old P.S. fluid was black and not very effective; The new fluid is not quite 100% clear but I can see through it a whole lot better than the old stuff.

[aardvarcus]

I wasn't referring to the tubes (liquid passages) being open, but the fins (air passages) through the front. If they are crushed or constricted, they can prevent airflow, which will make your transmission overheat, fluids turn dark, and ultimate failure. Since you had transmission issues that included dark fluid and since your power steering fluid is also dark, I would check this.

Backstory:
Some genius pressure washed the front of my old 1991 Chevy k2500 before I owned it, which folded over all the fins of the AC condenser, seriously restricting the airflow through the radiator and transmission cooler. Me, not knowing much about cars at the time, repeatedly took the truck to the shop for various engine and transmission issues, which I now know were symptoms of the AC condenser problem. Many thousands of dollars later, I finally figured out the actual problem on my own, no thanks to any of the many mechanics who had seen the truck, removed the AC condenser and all the issues went away. If I had replaced the AC condenser with a new one the day I bought the truck, I would not have had any of the issues I did. That is why I suggest checking the fins of your AC condenser, radiator, etcetera and make sure air can flow through and that they are not clogged.

[101Volts]

Originally Posted by aardvarcus

I wasn't referring to the tubes (liquid passages) being open, but the fins (air passages) through the front. If they are crushed or constricted, they can prevent airflow, which will make your transmission overheat, fluids turn dark, and ultimate failure. Since you had transmission issues that included dark fluid and since your power steering fluid is also dark, I would check this.

Backstory:
Some genius pressure washed the front of my old 1991 Chevy k2500 before I owned it, which folded over all the fins of the AC condenser, seriously restricting the airflow through the radiator and transmission cooler. Me, not knowing much about cars at the time, repeatedly took the truck to the shop for various engine and transmission issues, which I now know were symptoms of the AC condenser problem. Many thousands of dollars later, I finally figured out the actual problem on my own, no thanks to any of the many mechanics who had seen the truck, removed the AC condenser and all the issues went away. If I had replaced the AC condenser with a new one the day I bought the truck, I would not have had any of the issues I did. That is why I suggest checking the fins of your AC condenser, radiator, etcetera and make sure air can flow through and that they are not clogged.

Oh. I'll do that, thanks. I'm not sure if it's that or if one or more previous owners just neglected to change fluids.