Keep in mind, the ramiles lives in Texas! Summer ambients can get to 110f and we shouldn't compare results from the northeast. These engines were designed to run with coolant temps up to 230 and running constantly at 230-240 isn't going to hurt anything.

One of the problems with upgrading the fans for early C4's (84-89) is the single puller fan is an integral part of the radiator plastic shrouding and if you want to install an aftermarket dual fan set up, the whole plastic housing has to be cut away. This makes the mounting of the radiator and condensor really wimpy.

I agree with the idea of running 75/25 for the summer but maybe switch back to 50/50 during the cooler months. I know a lot of guys that have two mixtures, with one sitting in bottles at all times. Just make sure you are using distilled water in both solutions.

 

Factory design, which includes volume and fixed surface exposed to air flow, is for average operating temps between the thermostat and the threshold for fan operation - non a/c. A/c operation is to maintain the head pressures I posted and the a/c guys don't really care what the coolant temp is until head pressures get into the danger zone (400 psi), at which point it's not only going to boil over, there's a good probability of a hole in the hood. My cars, including the Vette don't exceed 212 to 215 at 100 to 105 degree ambients with the a/c on. I get the same at 50 degrees - though it takes a lot longer for the fan to come on at that temperature. If you're getting into the 230's with the a/c cranking, I'd look into it as you don't get real good vent temps if you're throwing off that kind of heat. 15 to 20 year old a/c systems usually have a problem or two - generally metal - moisture from improper topping off or charging - lack of oil and a host of other things that can be verified by simply checking pressures.

 

SunCr - Thanks for your detailed considerations on the A/C – it’s a technical area I don’t have enough knowledge or experience in. The car in question here was converted to R134 this last April when I had to replace the original condensor, and was told it was charged with 1.65 lbs (which I think is right) to the SAE J1661 standard. Only anomaly I see is that occasionally, a few minutes after a warm start, and only while stopped, the system goes hot with no cooling, as if it is automatically cycling off like it was reaching some limit. In that case it will clear itself and go cold again after 20 – 40 seconds, or go cold if I can pull away from the traffic and drive it. Any chance that would be the “plugged” A/C condition you talked about?

The voltage stays fine both on the instrument gauge and in the data log. Will take your advice to check the pressures as soon as I can get a manifold gauge set.

Not sure I am following you on the ECM fan control info. My car is using a $6E BIN and the only applicable data log variables I see are coolant temp, startup coolant temp, and fan duty cycle. Are there values for the condensor pressure/temp in $6E and how do these factor in? All the previous posts for the 165 ECM seem to say that fan program logic is based only on coolant temp and whether the AC is on or off. Any additional info here is appreciated.

Tom, thanks too for your input on different fluid, and the excellent workmanship in the radiator you sent me.

 

Rule of thumb for a conversion is 80% of the R12 charge (2.25 lbs is the factory fill). Another rule of thumb is that high side should never exceed 2.2 to 2.5 times the temp of the air across the condensor. The R134 molecule is smaller and it's easy to pack too much in. Depending on what else was done (at the very least the Accumulator should have been swapped out), there's no way to know if it's good or bad. Checking operating pressures is the place to start (and while you're at it, stick a thermometer in the center vent). If the Accumulator wasn't swapped out, there's some probability that's it's falling apart and plugging things up.

The fan signal is provided by a switch on the high pressure line. The ECM grounds the main fan relay when there's voltage which occurs when the switch opens. The pressure that it opens and closes at is built into the switch, not the ECM. It will open at 230 psi and close at 190. That means the ECM sees voltage when it's open - no volts when it's closed - fairly simple really.

For R134, it would be better for it to open at 200 to 210 psi with a reclose at around 170; but a different switch doesn't exist. A lot of tunes seemingly ignore the a/c function - I can only speculate as to why - but if it's been eliminated you should try to have it reinstalled. You're not going to outguess that switch even though by today's standards, it's old school (by '92, GM switched to a thermistor which relays voltage relative to actual pressure; by '96, they were using a variable speed secondary fan relative to this signal). R134, at temps above 100, can get out of control if not controlled properly at startup.

 

SunCR thanks for the follow up. I did have the accumulator and the orifice tube replaced at the conversion.

There is an ECM constant labelled "Fan 1 Duty Cycle if MPH < Minimum and A/C Pressure High" and it is set to a value of 99.61% so I assume that is the fan programming for the switch on the high pressure line you describe. It does appear to still be an operable part of the programming, but I'll need to research it some more to be sure.

I did data log several parked sessions today. The ECM data logged coolant temp was always within 2 to 5 degrees F of the instrument reading and was usually higher. It was almost a pleasant day here with the temperature only 90 degrees and relatively dry. Parked with the A/C on full the car reached only 208F after 20 minutes. I also tried it paked with the A/C off and temperature dropped to 195F in five minutes - I wasn't able to wait to see the steady state A/C off.

With the weather beginning to change it looks like the steady state temp with mac A/C whil parked stays at a reasonable number now.

Thanks to all for their great inputs and advice. They provided a much better understanding on the many considerations involved, and plenty to work on during weekend car time.