Overheating issues

[jetjock]

It appears to me it's functioning too. At least the refrigeration portion. How can the low side be 25 psi and the evap not be cold? And if it isn't cold how can the low side be 25 psi? Refrigerants have a pressure/temp relationship you know. It's what makes them refrigerants...

 

[CyFi6]

It appears to me it's functioning too. At least the refrigeration portion. How can the low side be 25 psi and the evap not be cold? And if it isn't cold how can the low side be 25 psi? Refrigerants have a pressure/temp relationship you know. It's what makes them refrigerants...

Tested again with the car nice and warm, high blower, recirc, windows open etc ambient around 100 and in the shade
High side: 275
Low side: 28
Vent: 58
No bubbles

I see what you are saying about the pressure temp relationship, but the evap is at around 28 psi and is not getting cold. When on the freeway for a while, or cruising steady speeds for a long period of time, once the inside of the car cools down a bit, vent temperatures do come down. The clutch never cycles, telling me the evap never is getting below the ~34 degree cycle temp. I can feel the suction line coming out of the evap, it is cool, but not cold. No moisture or condensation on the line anywhere, no drips from the drain tube.

I dont really understand the pressure temperature relationship of refrigerant. For instance, if I drained all but about 6 oz of refrigerant from the system, low side pressure would be extremely low right? So just because low side pressure is low means the evap is cold? In that case when I had the system under a vacuum, the evap should have been frozen. How do you know refrigerant is making it into the evap?

When the sensing bulb sees a warm outlet temp of the evp, it opens up more right? When it opens up, low side pressure would increase (according to my common sense, don't know if that is accurate), and the evap would cool off from the greater amount of refrigerant. If that is the case, the evap got colder when pressure increased? Doesn't make sense to me how evap temperature can be determined strictly by pressure. If the TXV is restricted, wouldn't that lower evap pressure and increase evap temperature?

Not attempting to argue because I know what you are saying must be correct, I am just pointing out the things I don't fully understand, maybe you could fill me in.

 

[jetjock]

It's not pressure alone, it's the change in it. When a liquid or gas expands it cools. The TXV "sprays" refrigerant into the evap. There's a large delta P across the valve and as the pressure of the refrigerant decreases across the orifice it cools. Same reason a spray can of something gets cold when used a lot. Pumping the system down without anything in it is not going to work

 

[CyFi6]

So the higher the delta p the lower the temperature? So my system should be working better at 275 head pressure/28 low pressure than it would at the specified 210 head pressure/25 low pressure? Only problem is it doesn't, and the evap is definitely not reaching 26 degrees.

 

[IBoughtASupra]

Unless, I missed something. You should try getting another shop to evacuate and recharge with fresh R134. Some machines allows you to recharge with recycled gas which is what you pull out of cars before performing any A/C work.

Stay there and watch them and ask to see to make sure they add fresh refrigerant and it is the correct amount, it is 1.65lbs the TSRM says 1.7 but the yellow sticker on my 88 says 1.65lbs.

There has to be something that we are missing, maybe adjustment on the expansion valve. If it is miss adjusted it won't open properly, that is, if it has an adjustment like some cars.

 

[jetjock]

He's R12.

The only thing I can think of for good low side pressure with high superheat is not enough liquid is making it into the evap. Have you rechecked subcooling?

 

[CyFi6]

It was at about 143 degrees at the receiver according to my IR gun IIRC at about 275 PSI high side, which is about 20 degrees of subcooling. Seems a little high if I am not mistaken.

IBAS I am pretty sure the TXV has an adjustment inside the inlet, but that requires discharging the system and removing the high side line, it should be preset from the factory. How would you ever know here to leave the adjustment?

 

[jetjock]

Well, low boiling = low cooling. If the TXV bulb isn't cold the valve is going to be open, which begs the question as to why more liquid isn't getting in. What's the suction line temp again? Do you ever see vent temps of less than 45 F?

 

[CyFi6]

I did see ~40 degree vent temps after a lot of driving on the highway with the blower on medium and the vent on recirc, if the blower is on high I never saw anything below 45 deg at the vents no matter how long I drove, and it was around 50 deg for a long time before coming down past that. My car cruises on the highway at 3k RPM btw. If i turn my blower speed down it will usually lower the vent temps, but doesn't help cool the car down obviously.

I don't know the temperature of the suction line, especially while driving, but I felt it after holding the engine at 1500 rpm for a couple minutes and it was cold, but not freezing by any means, and no sweating whatsoever. The only time I have had the compressor cycle is when it is night time/lower ambient, cruising at steady speed, and on low or medium fan speed.

When I rev the engine, low side pressure drops slightly below 20 psi, and as I hold it, low side pressure rises back up into the acceptable range. Does this indicate that the TXV is actuating?

 

[jetjock]

Sounds like it's working to me. The line may not sweat with the low RH you have out there but it should be cold. As for vent temp being coldest on low blower that's completely normal. Has to do with that residence time thing you don't believe in

 

[CyFi6]

LOL... If the evap is not functioning properly aka not getting cold, it can only absorb x BTU's, so if its on low blower or high blower, its only absorbing the same x amount of BTU's, so with more air volume the temp drop isn't as great, right (larger volume of air dropping the same BTU's = warmer air)? If the evap is getting cold enough (~34 deg) with the proper superheat, I don't see why it wouldn't blow sub 40 degree air even on high blower (it will do this with lower ambients, or used to at least). In my case, the evap is just not getting cold enough, so the higher the fan speed the higher the superheat, IMO. With the fan on low, the suction line WILL get colder than if the fan is on high, telling me its a refrigeration issue and not a residence time issue causing warm air output.

If it was a residence time issue, the suction line would be ~34 degrees regardless of fan speed, right? If I understand your residence time argument, the speed of the air is not allowing the evap to fully absorb the heat, but if this were the case, the evap would still be cold and the air just wouldn't be transferring the heat into the evap coils. In my case, the evap coils are receiving as much heat as possible (air out of the evap very close to evap temp), but the evap is not getting cold enough. Looking at the evap out of this car, I am impressed. The evap itself is very deep, and the fin density is very high. Not only this, but each fin is perforated.slotted to allow even more heat transfer. An aftermarket evap I looked at was a joke in comparison to OEM, the fin count was much less on the aftermarket, and the fin material itself was much thicker on the aftermarket.

In other words, you are stating there is an issue transferring the heat from the air into the evaporator coils, where as I am stating there is an issue with getting the evaporator coils cold enough.

 

[Nick M]

I did see ~40 degree vent temps after a lot of driving on the highway with the blower on medium and the vent on recirc, if the blower is on high I never saw anything below 45 deg at the vents

Technically, that is a good reading in Phoenix. 40 is a rule of thumb for A/C. Go much above and people notice. But for many in the summer the high is 85 or so. Not 100+. My car gets colder and colder by the minute, and I can make people ask to ease up on the A/C from the cars really good seal to atmosphere. I wonder if your recirc is really recirculating. You can always shut off the A/C at night when you won't roast and see if the windows fog up. You don't have the number 2 water temp switch? It comes on at 220 or something. I would have to look in my book.

 

[CyFi6]

Technically, that is a good reading in Phoenix. 40 is a rule of thumb for A/C. Go much above and people notice. But for many in the summer the high is 85 or so. Not 100+. My car gets colder and colder by the minute, and I can make people ask to ease up on the A/C from the cars really good seal to atmosphere. I wonder if your recirc is really recirculating. You can always shut off the A/C at night when you won't roast and see if the windows fog up. You don't have the number 2 water temp switch? It comes on at 220 or something. I would have to look in my book.

When it blows 40 degrees, I am in heaven.... the problem is that it takes about an hour to get there. And don't you dare make a stop at the gas station or those temps are going to take another 20 mins to achieve. If I jump in my car that has been outside, turn the ac on, and drive around town (most of my driving), the vent temps are 60+ for at least 10 mins, and after that, they hover around 60 (on med blower, high blower goes to around 65), in other words, I am sweating my bawls off.

I don't have the temp switch and the fans do not come on due to coolant temperature, I know this for sure. They only come on on my car when the AC is on and above a predetermined pressure on the high side. My car was originally NA 86.5, keep that in mind.

 

[3p141592654]

For any heat exchanger the heat-transfer rate (Q) is given by .

Q= [m* Cp (Tout – Tin)]cold
= – [m* Cp (Tout – Tin)]hot

where m* is the mass flow rate, Cp is the coolant specific heat, and Tout-Tin is the temp difference across the exchanger. Note that there is no "residence time" specified, because it is not a factor.

The equation above basically says that the heat transferred from the hotter fluid (cabin air for an A/C) must equal that transferred from the colder fluid (expanding R12).

So, when we reduce m* (eg. lower blower speed) then Tout-Tin must become more negative (colder vent temp Tout) in order to keep the equation balanced. This is why a lower fan speed results in lower vent temps.

This system still appears to be starved for refrigerant, high superheat and subcooling. The TXV is the usual culprit, but since it has been replaced twice that cannot be the issue. Possible culprits are a blockage (seems unlikely given all the work done), or the compressor is weak (?).

 

[CyFi6]

This system still appears to be starved for refrigerant, high superheat and subcooling. The TXV is the usual culprit, but since it has been replaced twice that cannot be the issue. Possible culprits are a blockage (seems unlikely given all the work done), or the compressor is weak (?).

Well I had a nice break from the heat, AC worked great in the sub 90 degree weather . I was hoping to spot your Supra when I rolled through Thousand Oaks
But, now that I am back in the hell hole, the AC is back to its same old self.
Can a weak compressor produce the pressures that I am seeing? I would think the high side would be low and the low side would be high in the case of a bad compressor? Not to mention it was replaced (reman) last year.
Thanks

 

[CyFi6]

Does it seem feasible to you guys that the Koyo aluminum radiator is causing a big enough restriction to reduce airflow through the condenser, leading to my problems? The guys on the AC forum don't have any other ideas as to why my high side pressures are so high, and it's about the only thing left that is a possibility. The higher fin count along with the increased thickness of the radiator may be a concern. Thinking about picking up a stock replacement, but it is going to suck if it doesn't yield any results. I would have installed the stock pusher fan long ago as well, but being pre 88, all the hardlines would need to be replaced with post 88 lines in order to allow it to fit, and honestly, I don't know how much air that little pusher fan can move.

 

[IBoughtASupra]

Doesn't your condenser have a pusher fan that comes on with the A/C is on.

You could fit a fan to pull air, working the same way the fan clutch does, through the condenser and put it in between the radiator and the condenser. Is that what you meant?

Higher PSI readings on the high side means the gas is not being cooled, which is the job of the condenser.

Your condenser is not cooling the refrigerant because it is running hot and not being cooled itself. It could be the radiator as it is more dense. I say try to put the fan in between the condenser and radiator first, before buying a stock replacement.

 

[3p141592654]

You're right, if the compressor was undersized your pressures would not be as they are. I don't buy the radiator argument either, because your subcooling is fine.

It may be that you are simply loading the system beyond its design limits. A superheat of 70 deg definitely supports that argument, and with subcooling fine I don't see any way you can get more performance with fans and airflow over the condenser.

I guess by now you have followed teh tubing and checked for kinks and dents that could block flow. I keep meaning to measure the diameter of my low side tubing to compare with yours. Maybe they beefed it up on the 87+ models.

Next time you come through TO let me know. A Supra is a pretty rare sight here these days, roads are clogged with BMWs! :icon_mad:

 

[Nick M]

Get a Tahoe/Yukon with its giant radiator and giant condensor. Or move to Flagstaff like I told you.

 

[CyFi6]

Doesn't your condenser have a pusher fan that comes on with the A/C is on.

You could fit a fan to pull air, working the same way the fan clutch does, through the condenser and put it in between the radiator and the condenser. Is that what you meant?

Higher PSI readings on the high side means the gas is not being cooled, which is the job of the condenser.

Your condenser is not cooling the refrigerant because it is running hot and not being cooled itself. It could be the radiator as it is more dense. I say try to put the fan in between the condenser and radiator first, before buying a stock replacement.

I don't have the pusher fan on my car, they were only installed on some 88+ Supras, and they changed all the lines in front of the condenser to accommodate it, so I wouldn't be able to bolt one on. I have installed a small aftermarket pusher fan last year and it didn't do crap, the clutch fan and aux fans pulled more air than that thing could push, so I got rid of it.

You're right, if the compressor was undersized your pressures would not be as they are. I don't buy the radiator argument either, because your subcooling is fine.

It may be that you are simply loading the system beyond its design limits. A superheat of 70 deg definitely supports that argument, and with subcooling fine I don't see any way you can get more performance with fans and airflow over the condenser.

I guess by now you have followed teh tubing and checked for kinks and dents that could block flow. I keep meaning to measure the diameter of my low side tubing to compare with yours. Maybe they beefed it up on the 87+ models.

Next time you come through TO let me know. A Supra is a pretty rare sight here these days, roads are clogged with BMWs! :icon_mad:

Only thing that makes me wonder is that when I get the car up to speed, it really does cool a lot better. Driving around town gives me warmer vent temps than on the freeway for sure. And yeah, I have triple checked all the lines for restrictions. Considering they didn't change the fittings etc on the receiver or evaporator etc (I installed an 88 evap in my car) I doubt they increased the line size on the later models, but could be wrong.

Get a Tahoe/Yukon with its giant radiator and giant condensor. Or move to Flagstaff like I told you.

You are definitely onto something here lol. Only problem is I don't have the means to support two cars right now and the Supra is only a pain in my ass 4 months out of the year :icon_razz