Car loses very noticeable amount of power with a/c on

[billfrank85]

I know that's usually the case for most cars but I feel like this car ( 91 GM stock 5.0 with 3G alternator) gets sluggish acceleration a lot more than it should for what it is, at least compared to other a/c equipped cars I've owned. I've already done a complete tune up with Motorcraft plugs, wires and a cap and rotor with brass contacts. New air filter and fuel filter. New PCV valve, grommet, and filter underneath. Car accelerates and drives perfectly fine with the air off. I have plans to go through and test all the fuel injection related sensors as well as running a can of Seafoam in the near future. At some point I also want to remove the upper intake and replace the vacuum lines. Anything else to look for?

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[His Royal Ghostliness]

Nah, these do that a lot. Especially noticeable if you have highway gears (2.73s or 3.08s) in the rear axle... Kinda sucks if you ask me, some older (carbed) cars actually have an idle kick-up solenoid to deal with the issue and it worked quite well, the SEFI cars obviously don't have that however they're supposed to be able to sense the increased load on the engine and do something about it via the ECM. The worst offenders are the CFIs, these don't idle worth a damn with the A/C on while in gear (like when waiting on a traffic light), and once they're moving the power to get up to speed seems zapped in like half.

[DerekTheGreat]

Pretty much what HRG said, except for the CFI idle part. That depends how well the CFI goodness was tuned and how sensitive the dashpot for idle control is. My '85 CFI car was just fine, although the dashpot moved the idle up too high and required excessive brake pedal effort to keep the bastard stopped. Didn't know those were adjustable until after the car left my hands, so I probably could've tuned that out too. However, I just put a new compressor on my '88 and it does seem to idle and move easier than it did. Also doesn't make much noise either, old one kinda sounded like someone was shaking a couple of spray cans, I just figured that was normal up until it shit the bed.

I've pretty much owned nothing but low powered turds and they only got worse with the A/C on. Worst was my four banger ranger, damn that was a turd. Firebird doesn't seem to be affected by the compressor much any more but back in the day before any mods, oh yeah, turd. I remember once my dad went to do a burnout in front of my uncle and it was nothing short of underwhelming. It wasn't until he turned the A/C off that the one wheel started to peel.

[GM_Guy]

Get used to it. Just switch the a/c to vent if you need to hustle quickly.

Alex.

[sly]

Yup. My 88 with 3.55 (towing) gears does this too. Always has. Welcome to low output V8.

[Tiggie]

When you have 130 horses at the wheels, the 20 the compressor draws makes a big difference. Now try a 140ci inliner with one of the giant York or Tecumseh compressors.

[His Royal Ghostliness]

That depends how well the CFI goodness was tuned and how sensitive the dashpot for idle control is. My '85 CFI car was just fine, although the dashpot moved the idle up too high and required excessive brake pedal effort to keep the bastard stopped. Didn't know those were adjustable until after the car left my hands, so I probably could've tuned that out too.

That may be it, this one doesn't seem to do much at all sometimes. Gonna mess with it some more today, it's really pissing me off...

[Chuck Norris]

Some GM cars have a tie-in to the IAC and the AC. If the IAC is non-functional, the AC does too. I guess it is supposed to increase the idle to compensate for the power draw the compressor creates. Too bad the 5.0 doesnt do so, or at least not adequately. I dont remember that problem with my 4.6 cars - maybe a smidge less power (or a skosh, maybe not a smidge) but barely noticeable.

[DerekTheGreat]

That may be it, this one doesn't seem to do much at all sometimes. Gonna mess with it some more today, it's really pissing me off...

Let us know what you find! I wasn't sure how you adjust it and didn't find anything anywhere but now wish I still had mine just to tinker with that. First thing I had to do to get that dashpot to start working was to route the vacuum lines correctly as some other guy did me the favor of changing, plugging and removing stuff.

Some GM cars have a tie-in to the IAC and the AC. If the IAC is non-functional, the AC does too. I guess it is supposed to increase the idle to compensate for the power draw the compressor creates. Too bad the 5.0 doesnt do so, or at least not adequately. I dont remember that problem with my 4.6 cars - maybe a smidge less power (or a skosh, maybe not a smidge) but barely noticeable.

The 5.slow in my firebird has that and so does the 7.4 GMC, manual states it's supposed to bump up RPM by 200 IIRC, it does move it some on the Fireturd. I have noticed the idle increase on the Ford when A/C is engaged so it does work.

[87gtVIC]

On an unrelated yet kinda related note....when my car was still stockish (lopo and 3.08's) and dailyed back in college (2008/9ish...she was off the hook for junior and senior year) with the air on the car accelerated and down shifted into first like a BOSS around 40ish mph. Made me smiles! Never did that with the air off. Guessing the extra load of the a/c caused this to happen.

[gadget73]

Add more power and its not quite so bad. Mine unfortunately has a compressor that sounds like a dead air raid siren so I know the damn thing is running even if I don't feel it tremendously. Anyone that has ever ridden in my car has asked wtf that awful noise was. Motorshaft Reman FTW. It is cold though.

[His Royal Ghostliness]

Let us know what you find! I wasn't sure how you adjust it and didn't find anything anywhere but now wish I still had mine just to tinker with that. First thing I had to do to get that dashpot to start working was to route the vacuum lines correctly as some other guy did me the favor of changing, plugging and removing stuff.

The vacuum lines on this car are unfucked with as far as routing goes, and any and all questionable ones have already been replaced. So she should be doing what she's supposed to be doing, but she ain't... Honestly I'm right now questioning how that servo even works, it's got two chambers but only one vacuum line (hooked to one chamber only). And the whole thing can be slid in-out towards the throttle body which in turn turns the same throttle lever the pedal cable hooks up to, that part is easy. Guess I'll be digging into the shop manual to figure out exactly how that stupid thing is supposed to work...

Add more power and its not quite so bad. Mine unfortunately has a compressor that sounds like a dead air raid siren so I know the damn thing is running even if I don't feel it tremendously. Anyone that has ever ridden in my car has asked wtf that awful noise was. Motorshaft Reman FTW. It is cold though.

I'll take a dead air raid siren over the loose marbles inside sound this one is making! Does make cold tho, like yours, and there have been other more pressing expenses that already used up this summer's budget (like the damn heat-soaking starter), so not gonna replace it till it actually dies. Which hopefully will be next year at the earliest.

[gadget73]

If you're playing with CFI, definitely make sure the throttle kicker works. You'll have to dig (or ask Pete) but there is a way to jump the appropriate solenoid to ground to verify that it and the dashpot work. There are a couple of adjustments on that thing too. There should be one for the "sled" that the dashpot sits on, and the other one is the plunger tip itself. I want to say the sled is the base idle and the tip is how much it kicks up with AC, but that may be backwards.

[His Royal Ghostliness]

There is vacuum present at the line hooked up to the dashpot, so solenoid obviously works.

What I find interesting is with the engine turned off and no vacuum anywhere, if I disconnect the line from the servo and then step on the throttle the servo's tip will pop out and not retract all the way back in when the pedal is released. This suggest to me the servo may be working backwards, as in the vacuum from the solenoid actually makes it drop back down to low idle, and it's a spring inside that makes it push on the throttle lever when no vacuum is present. But again, shop manual consultation is needed for that. When did all this shit came into existence anyways, '81?

[gadget73]

think so, different eec but probably functions the same.