1999 Suburban AC Question

[L31MaxExpress]

For what it’s worth, here is GM’s spec for the a/c for a 98…

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That GM spec is the bare minimum and how they CYA'd their way out of loads of warranty repairs. Properly functioning, I have never had a GM AC that cools anywhere close to as poorly as that chart indicates. It is similar to how they deem 1 qt every 1,000 miles as normal oil consumption. I have had 350K mile worn out engines that did not burn that much.

 

[sneakingfart]

Yea, I definitely get that.

I just went outside, late evening, 82 degrees on the mirror temp display. I think it's a little cooler. Both dash and rear ac full blast, let it run for 5 mins and then increased RPM to 2k. Temp settled at 42 to 44 degrees. 65% humidity and 77 degrees per the weather report. Overall, seems reasonable actually. I wonder if the issue in traffic is honestly the lack of an electric fan for the condenser? But I will also power wash it just in case it's dirty from the back.

 

[L31MaxExpress]

Thank you. The accumulator is quite cold (if I am only running the dash AC on a low or medium setting, it would actually form frost on it, as well as the tubing going into the compressor low side. And I did replace it when I was fixing the AC system (wish I hadn't actually, as I did kink the tubing going to the evaporator while trying to remove it). It was quite seized. That alone does introduce a restriction into the system, as it's a decent kink, but removing the dash to replace the evaporator is something I am hoping not to do, lol. Do you think that could be a contributing factor?

Without removing the fan clutch to physically inspect it, there is no wobble or lateral motion of the fan, and it is tight to turn. At 1500 RPMs there seems to be quite a bit of air flow. I don't have a frame of reference unfortunately, as all my other cars have electric fans, but I did have a Tacoma 20 years ago, and this seems to blow harder (bigger fan certainly). I will try the paper towel roll trick when I run out of my roll of towels.

The condenser is clean, at least from the front. There are a handful of bent fins from road debris, but nothing significant. Can the condenser be dirty from the back, where it faces the radiator? If so, any good way to clean it without removing it? Should I just blast it with my power washer? I have a 3700 PSI washer. But yea, I wish it was as simple as a heater core valve that is no longer competent, but I didn't get any change from clamping the tubing that goes to it.

But I agree with you, there does seem to be some loss of efficiency. With the dash AC only, at highway speeds, and 91 degrees, I get 40-42 degrees. On a warm night (no sunlight to warm up the car), I can get 38 degrees idling with just the dash AC. The major loss of efficiency seems to be when I turn on the rear AC blower.

52 at idle is pretty good. I was about 56 to 58 today with all blowers on max at idle. Not terrible, still a good 30 degrees lower than ambient, but if I can find more efficiency out of the system, I would definitely take it.

Easiest way to get to the back of the condenser is remove the upper half of the fan shroud, remove the upper radiator mounts and tilt the radiator back. Then there is a nice gap to clean out whatever has accumulated there.

Another thing that really kills the efficiency. This was the OE evaporator core at ~70K miles on my 97 van. I had to replace it in 2011. It was leaking. You can remove the blower motor and gain fairly decent access to the core in your truck. When they get plugged up air bypasses the core and they do not cool as well. I have been meaning to pull the cover back off the van to install some sticky backed 1" wide x 1/2" thick dense foam tape to seal the core to the box. One thing GM should have done that would have helped vent temps. When the evaporator plugs up with trash, more and more air bypasses the core and flows around it thus less and less cooling even though the vents seem to blow about the same. I have had the intention of sealing the core to the box even before I built the engine, just never have taken the time to. Luckily GM van evaporator cores are super easy to get to. The 80s G-Vans and the Express all have them under the hood on the front of the firewall.

Rear A/C will kill the front performance a bit, especially at idle. The rear expansion valve opens and drives up the system pressures. Overall though you get more cooling even if the vents are not as cold. In practice I run all my rear units on low fan speed unless I have someone riding back there, my 2011 Pathfinder included. The interiors cool off more quickly with the added CFM and evaporator BTU capacity.

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[sneakingfart]

Wow, thank you for that. I'll definitely see what I can clean up as time allows. The truck has 180k miles and I'm almost positive the airbox and back of the condenser were never cleaned. Hopefully all the thing needs is just some housekeeping. Thanks again for all your help.

 

[sneakingfart]

I just wanted to thank you all again for your help. I cleaned out the airbox (wasn't too bad all things considered), and in the process, I found that the fresh/recirc door wasn't actually going all the way up. So it was always pulling fresh air. I did replace the motor for the door before, as it was broken (left that damn upper screw off), perhaps it wasn't calibrated properly, but it would stop halfway up. I removed and reset the motor, and it seems to be working fine. That, combined with cleaning the evaporator, on the last 90 degree day we had, I got about 46 degrees on the center vent while driving, with the dash vents on med-high, and the rear vents on low. Overall, I think it's right where it should be.