dprantl

Yep, PV=nRT (kinda). What is your coolant temp at? Do you have an IR gun you can use to measure the temp of the radiator and condenser? It does sound like the fans that replaced your viscous fan are not up to the task. There is a replacement for the front pusher fan that pushes much more air than the stock one.

Dan
'91 928GT S/C

Tampa 928s

Do you have a link for the replacment fan? I am using the Murph kit fans that should work as designed but just got the kit in and never drove it in the Florida heat.

dr bob

Tampa--

What do you see in the sight glass now, engine at 1500 and ALL fans running?

The low side pressure is tied to how much heat you are taking out of the car. What's the center vent temp with fan on sevond speed? With 32 PSI it should be barely above freezing, maybe 40 degrees. Assuming of course that you do have a liquid flow to the expansion valve.

High side pressure is generally a function of ambient temp and how much cooling fan you have to pull the heat out of the condenser. Since the compressor performance (suction pressure) is tied to condenser performance (condenser capacity), your car will be coolest inside with the lowest possible condensing temperature. But to get real cooling inside you need to have a fairly constant stream of liquid to the expansion valve; if you see lots bubbles or a lot of foamy stuff in the sight glass, you may have bled out too much refrigerant.

Post your temp results and sight glass observations please.

Tampa 928s

Dr. Bob here are the readings I got this afternoon:

I have a S/C and 4 Electric fans and the front mounted fan also.

I bleed off a little R-134a while monitoring my Temp. Got it down to about 27/250 with the first warm up went to 33.5 Degrees at idle. As it heated up and its hot 36/280 at idle and 25/325 at 2k Rpm. outlet temp 44.5.
It seems I may have an air flow issue?

Fan was on two, I never checked the site glass. Do you have a number for the amount of R-134 that is accurate?

Tampa 928s

Just took a reading:
Ran the car for a few minuites 30/185 at 35 Degrees. There is foam in the window.

dr bob

The 'foam in the window' is generally a telltale that you are now undercharged. Add refrigerant slowly, pausing for a minute after each puff of refrigerant, until you get to the point where there are few bubbles going past the sight glass. Again, fan on 2 and engine at 1500 RPM's. This will be close to your average motoring condition so it's generally a good place to start. There's alsways a temptation to 'just add a bit more to be sure' but you need to resist that. Err on the side of undercharging a little if you are going to err at all. The main reason for undercharging by weight is to avoid overpressuring the system during a long idle soak with reduced airflow through the condenser. R-134a has a steeper pressure vs temperature slope when ambients creep past 95º, and the pre-'93 928's don't have the dual pressure safety switch that would protect them. You can buy the better switch and retrofit to your car pretty easily, BTW. Anyway, the compromise between performance and system safety is with the condensed stream right at the bubble point.

Tampa 928s

Thanks

How do you feel about the high pressure do I need to address tha fan?

Tampa 928s

I have a question since the system is tight would it be possible to put R-12 back in it? I have placed ester oil in it, new R/D and expansion valve; any thoughts?
From what I have read R-134a is not worse that R-12 but absorbs the heat faster than R-12. This causes the condenser to not be able to transfer the heat fast enough.
I am still waiting on the fan or larger condenser option if any one has done this.

Also you said to install a new safety switch, where does it go how, do you hook it up and where can I get one. My switch that is off of the Receiver Dryer is bypass to provide fan on at all times.
Waiting for any replies!

dr bob

R-134a is not 'worse' than R12. it's just different. In that it has slightly different characteristics. It has a very slight difference it heat of evaporation, which equates to not absorbing quite as much heat. It also has the different temp vs pressure rise above 95º that can test the condenser and fan capacity. In the over-engineered 928 AC system neither of these will be a problem so long as the condenser is clean and the fans are working. I see no reason to even think of swapping R12 back in. If you have a surplus of R12, sell it and use the money for a vacation or a quarter tank of gas for your car.

The high pressure and fan question-- What's the high side pressure like with the correct charge in it, the one that has just some bubbles in the sight glass? If it's under 325 worst case, I wouldn't get excited about it. Do make sure all the leaves and crud between the radiator and condenser are gone. You probably did that as part of the SC install but had to remind you. Then keep an eye on your engine coolant temps. If they get excessive you may look at higher-capacity fans.

The pressure safety switch is different from the fan switch. Safety switch is in the circuit with the compressor clutch coil. On my S4 it sits facing up into the tubing manifold between the condenser and the drier. The tubing's really part of the drier plumbing. Anyway, the later cars that came with R-134a from the factory came with the better switch, and it screws right on where my old one attached. You may need to adapt the wiring connections a bit, depending on which switch connections you have now. Switches are under $30 most places. They are common not-Porsche-specific parts. Might cost less if you tell the sales clerk that they fit a VW, for instance.

Tampa 928s

Dr. Bob

If I'm in traffic it goes above the two bars way high than before the S/C install.

I would not mind adding a replacement fan in the front, the back is full with 4 fans. Since it seems like the air flow is bouncing off of the S/C it cannot get enough cooling. I thought maybe adding 4 smaller fans, two on each side to allow for a flow through, "THOUGHTS" ?

I will address the foam issue if I get a break this week it's lightning season and I'm in the telecommunications business, not that I am complaining.

dr bob

There are some fan and cooling experts out there with more experience on what to add or change. ON another thread, someone reported that one of their electric fans ended up pushing air the wrong way, so the fans were fighting. It's stupid stuff but worth a look. Somtimes it's that stupid stuff that causes the biggest problems. Easy to check/fix.

Also, if your car is running a lot hotter even while driving at normal loads (no boost) you need to figure out why. Hotter there means REALLY HOT going into the engine, and that's a recipe for engine failure under boost.


Don't hang on top of the towers when there's lightning...

Tampa 928s

Going down the road it's fine just sitting in traffic for any length of time.
It starts cooling down after I get going for a while, so I assume its an air flow issue.
The fan is pulling not pushing but the thought of reversing it to blow the opposite way has crossed my mind.
Hope someone chimes in on the fan options I would like to be able to actually drive this car and not keep it in the garage this summer.

Tampa 928s

Do you think by adding a little foam around the radiator might help?

dr bob

Original radiator?

Tampa 928s

Yes original !
I looked at it tonight and the fan is square in front of the super charger.