Hey y'all.

1997 Blazer LS, Automatic Transmission, 4.3L

Been having this issue off and on this winter. Kept trying to figure it out, but I'm honestly at a loss, and and I can't find anyone reporting a similair issue.

When the ambient Air temperature (NOT engine) is above 20*F, Blazer runs fine and dandy, happy as a clam. At highway speeds, when the Blazer goes into Overdrive, I'll usually reach 2K RPM at about 68MPH.

When the ambient Air temperature is BELOW 20*F, an issue arises at highway speeds. When the Blazer would normally drop into Overdrive gear, it suddenly bogs down as though having power issues, then kicks itself out of Overdrive, staying at higher RPMs in the 2-2.5K range @ 60-65 MPH. Additionally, the SES light will also begin to flash, and pulling over to check always nets the same code: P0300.

It does not matter for how long I've driven, what the engine coolant temp is, or what have you. If the temps outside are currently below 20*F, this will happen if I attempt to take her on the highway and go above 55MPH.

Plugs are all brand new AC Delco Copper.
Wires are AC Delco Wireset and also brand new.
Distributor cap and rotor are new as of a year ago. Inspected last week and found to be perfectly fine, no moisture buildup or corrosion.

I'm not sure what my issue may be, and I'm looking to see if anyone has perhaps a lead of where I might look? I use this truck to get me to and from work everyday, which is 15 Miles of highway travel back and forth, 30 miles in a day. I'd like to be able to comfortably drive her on those colder days, esspecially with this wacky weather the Midwest has been getting of this back and forth rollercoaster.

Thanks in advance,
Border

 

I'm wondering if it has something to do with the transmission, sounds like a fluid is getting thick and causing it the work harder. What type of fluids do you have all the way around? Engine and transmission, as well as maybe look at fluid in axle housings, check for water.

I honestly have no idea on any these issues personally, but to be it sounds like its fluid getting thickened by the cold.

 

Engine has 10w30 Oil, filled to proper capacity and haven't noticed a loss. Rear differential got a fluid change a few months back, filled with proper gear oil (no LSD additives cause mine isn't an LSD). I'm usually running in just rear wheel drive, only using 4wd in snow, of which we haven't gotten any worth using for months.

I don't know about the transfer case fluids, but as far as transmission, I gave a quasi-flush to the system by unhooking one of the cooler lines and having it drain into a bucket with the engine running, whilst filling it with new fluid. I did this back in early 2019 maybe? Reason being is that the fluid in the transmission had taken on a color of strawberry milkshake, indicating to me that coolant and transmission fluid had mixed previously. I replaced the radiator as my suspected culprit, and to be extra sure, I went ahead and bypassed the in-radiator cooler and instead, hooked up an auxillary transmission cooler as the main cooler, sitting in front of the AC condenser. So right now my transmission fluid flows from the lines directly to a seperate cooler with fins, bypassing the radiator completely. I don't know if maybe that could be a culrpit? I don't see how, unless it's somehow cooling the fluid TOO much.

But I suppose anything's possible. The fluid used was the recommended fluid for these transmissions. I forget the type off the top of my head, but I know I researched before doing. I also changed the filter at the time of flush.

 

Hmm... theres a transmission guy on here that knows everything about them, maybe he will see this.

I'm not too sure. Will follow tho