weird overheating prob

[gundamx_105]

stock 93' 240 radiator and cap

non turbo sr20de(rwd)

stock fan- (I am using the stock elec fan from a 93, probly the ac fan, but i have been running it like this with no probs.)

replaced original thermo 4-5 mos ago with 91 se-r thermo

I first noticed the over heating on a hour and a half trip, all highway. when i reached my destination the stock gauge read at the last notch, and it was just starting to boil barely. i let it cool and drove it home that night it got hot but not that hot. after driving it awhile the gauge goes up real slow, i can't see it go up it, i look down every so often to find it has went up a notch. it stays there, it never goes all the way to "H", but starts to 'bubble'. it seems to cool down some when off the highway! also the last time i drove it on the highway for 25 min. the gauge went down slowly from the last notch to normal then back up slowly twice. the heat blows very hot and will bring the temp down alittle after several minutes.

i have red, sandlike, soot in the overflow. in the past i had put that orange coolant for newer chevys in it right after i swaped it. i changed it soon after and noticed a red color in the new coolant. I figured it was a little rust from the pipe that i used for the top rad hose(i am using the stock 240 rad). maybe this is somthing to worry about?

i found alot of posts on overheating but no definate answers. my engine has run fine the way it is, don't think its the fan or radiator. there are no leaks. and it doesn't smoke but, i did have slight coolant loss after driving it like this several times.

how do i properly flush the system. where is the bleeder? could it be the radiator cap, if so do i buy a 90' 300zx tt radiator cap for a replacement?

[s13sr20chris]

just upgrade to the nismo cap which any dealer can get you. its $25. it has a higher pressure than the TT. i am thinking you may want to pull the water pump and check the impeller. it sounds like a typical case of rotten water pump. a flush is just drain coolant, fill with water and flush chemical, run for 20min with heat on, drain, water, run, repeat many times with water and then the last time do it with 50/50 coolant. this will give you a slightly leaner mix of coolant/water which is good for heat transfer. running distilled water is a very good idea for longevity of parts. bleeding is simply jacking up the front, fill, loosen bleeder screw in intake manifold, take out screw untill you have water gushing out heavily, reinstall, run motor with heat on and let it burp. during the burp it may help to periodically raise engine rpms for a minute or so. 240's are hard to get the air out of.