I have an 88 GT with a 3.4 auto and an R134 conversion. It was my daily driver but the car sat for a couple of years until I finally pulled the cracked header and had it welded. I had the car smogged a couple of weeks ago and have been daily driving it back and forth to work. It's only a couple of residential miles each way. I had not had it on the freeway until today.

It's been very hot here. Over 100*almost every day for a couple of months. I recharged the AC around the time I had the smog check and it seems to be working fine on my short daily commute.

Today I took it out for a little joy ride and noticed an odd behavior when the A/C is on. I drove it about 20 miles on some 30 -50 mph twisties with no problem. It wasn't until I got on the freeway that I noticed a kind of pause and surge at about 60 mph and above. Not a stumble really, just a kind of repetitive push and pull sensation that went away when I turned off the A/C. I tried both the Hi and normal positions with the same result. When I turned it off the problem went away. Has anyone had a similar problem? Is it a problem with the A/C compressor, the clutch or...?

There is a HP drawdown any time the ac clutch engages. I think someone had estimated about 5 HP on the Fiero. There's also an extra load placed on the charging system with the ac clutch engaged and the radiator cooling fan running, and the alternator can add additional drag. Do you have a new battery or is it an old one? Watch your volt meter when this is happening.

Yes, AC eat some HP outright and the Power needed also eats some more HP as everything loads the alternator. More so w/ SI Alt use on most Fiero but 88 use CS130 > 90 amps peak power as OE.

If have wiring problems anywhere then can have this problem and Volt meter on dash likely anything but stable.
AC compressor w/ problems can cause more load then normal.
Alternator iffy or replace w/ smaller unit can cause this.
People w/ "Power Pulleys" that alone can cause this and other problems.

Before pulling/replacing parts... Start w/ clean/fix all grounds in engine bay, battery cables and + "box" under C500.
Coat ends bolted/screwed to engine etc w/ silicone or brake grease.
This often will help many problems not just this.
See my Cave, Wire Service and rest of section.

Also pull AC/Heat Blower Resistor and Motor and look at them and inside the box...
⚠️ Warning: These Resistors w/ crap buildup can burn a car down.
https://www.fiero.nl/forum/Forum2/HTML/141959.html
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

I can feel this a bit in my 4.9 car.
You might put some gauges on it and check the high side pressure. If it's excessive, it might be putting more than a normal load on the engine.

I bet its a bit low and the AC is cycling off then back on.... Or it could be too high and cycling off due to high pressure as Radar suggests. I have only had the high situation un heavy acceleration. Best to throw some gauges on it.

If the battery is weak then the current surge /load when the AC Clutch is energized may create the push pull situation that you describe . An overcharged A/C system can also do this by placing a large load on the engine when the compressor kicks in. .

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I noticed this issue on my Duke after the 134A conversion every time the compressor kicks in. Seems like the 134a was requiring more HP to run the compressor.

The problem is less noticeable now after my engine swap, but it is still there. I've had the conversion for at least 5 years now, nice cold air, and no problems other than the slight drag when the compressor kicks in.

Bear in mind that these are "Cycling Clutch" air conditioning systems, it's normal for the compressor clutch to cycle during operation. This is how GM prevents evaporator freeze up on these cars. Pretty typical for 80's era GM.

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Rod Schneider, Ball Ground, Ga.
"You can't have too many toys!"
1988 Fiero GT
1988 Porsche 928S4
1987 Corvette
2016 Mustang EcoBoost
Van's RV-6 airplane