I have a 3.0 in my car and use the stock gtv6 radiator. I have not had a radiator-related overheating problem and am not aware of a general issue with the gtv6 having a radiator capacity issue. Coolant tank cracking seems to be the biggest issue.
I live in NJ and it is reasonably warm here in the summer. @Gepetto
's car is in NY. I do see my temps on a wonderfully inaccurate gauge system going up and down in a narrow range but nothing concerning. I have to give the dash a little tap to get the temp gauge off zero when it is cold, oil pressure always reads high, fuel gauge response not linear, tach and speedo needles hard to tell since the tips were clipped after twisting in hot weather. LOL
Milano 2.5 stock radiator does seem insufficient for hot weather and a common replacement is to use the Milano 3.0 radiator with higher capacity. I am pretty sure Milano radiator is not a direct fit for the gtv6.
I was not thinking that the radiator here is an issue although a "new" radiator might have a blockage in it or not flow as well as the original one, resulting in the system not cooling as well with fans off, and faster cycling with them coming on and off. Perhaps the t-stat is not opening fully now. I even thought that perhaps the fully shrouded radiator does not cool as well with the fans off since the airflow is now pushed through a smaller area and between the fan blades, versus the stock configuration without a full shroud. But the description of the gauge behavior seemed pretty normal to me for a warm location and normal driving.
, would be interested to know why you replaced the radiator and chose a new radiator instead of having the original one repaired (if it was leaking or choked up)? I presume you had an original fan die and did the SPAL replacement?
Thanks for chiming in (as I have what used to be your car...) LOL. The original radiator from 1985 was re-cored as it was just about corroded through and about to fail. Though, it still worked very well but it was a preventative measure. The re-cored radiator was tested for blockage and it's fine, but I didn't think about the fan shrouds. The Spal fans do have a shroud whereas the originals didn't...that might be the culprit. Either way, I may put in a brand new thermostat just to be safe. Or, put one that opens a little sooner (instead of full open at 190F, maybe full open at 185?). BTW, the Spal fans are fantastic! But, I find they do draw more current. So much so that the alternator ended up failing (probably from having a max draw on it so much of the time in the summer with the A/C on...). Switched to 100amp alternator, and it's still a little finicky as the lights will dim when fans come on, especially at idle (this didn't happen with the original). So if anyone knows where I can buy a brand spanking new alternator (instead of rebuilt) that will fit a GTV6, do let me know. I suspect the rebuilder of the alternator didn't do that good a job.
Incredibly, all my secondary gauges are accurate (yes, we tested them with calibrated instruments), while the speedo reads about 10% slow (but only above 50mph) and the tach reads about 150rpm higher than actual rpm.
The beguiling part is that the other night it was quite cool out (upper 50's). Driving from NYC to NJ, lots of stop and go and once I got on the open road, the temperature still rose and fell (with the fans cycling on) even though I was doing about 65-75mph. Particularly on the Verazzano bridge - temp went 3/4ths the way to the 212 line before going back. On the way back, without any stop and go and quickly getting on the highway (and, hence, temperature never got that hot), temperature gauge was right at 175 the whole until I got to local streets - just like the good old days...