Surge when using A/C, defrost etc

[WPG SHO]

Hi everyone, I'm new to the forum and I have a '95 MTX. It has a slight surge whenever I use A/C, defrost, or basically anything other than off or vent. I watched the A/C compressor, and the surging doesn't match up with it engaging and disengaging. I'm assuming it is a vacuum leak somewhere. Any suggestions?

Thanks in advance.
Chris

 

[Mr Anonymous]

What kind of RPM change are you seeing when this happens?

 

[WPG SHO]

Maybe 100 RPM, barely noticable on the tach, but very noticable at slow speeds (in 1st or 2nd) and can even be felt at highway speeds

 

[Mr Anonymous]

Ok, just to double-check, when saying you have an MTX, you're saying you have a 5-speed and not an automatic, right? (Reason I ask is because that behavior is kind of typical with an ATX when the torque converter is locking/unlocking with the A/C on).

I realize you said you weren't seeing the A/C clutch cycling at the same time, but is it maybe close, like surging a second or so before the clutch engages?

You might also want to try the KOER test and see if you get an IAC code. Surging at steady throttle can often be traced to a failing IAC valve.

 

[WPG SHO]

Yes, it's a 5 spd, the A/C clutch cycling isn't close to the timing of the surging, you can hear the engine surge a a few times between cycles. I read the codes, as well as running the KOER test, and didn't come up with any codes for either.

 

[Mr Anonymous]

Since you have an MTX, you can try cleaning the IAC. Here is some info from SHOTimes:

My SHO Surges at Steady Throttle - Why?

Cleaning the Idle Air Bypass Valve (IAB)

 

[SASHO91]

i read the IAB article. Will this really help out with power? Or only when it is dirty, then it helps out restore it?

thread hijacked!

 

[WPG SHO]

Will do, thanks for the advice.

 

[WPG SHO]

Ok, cleaned the IAC, reset the computer, and it drove nice and smooth for the next few days, but today it started surging again. Any other thoughts?

 

[SHO Type U]

Happen to hear any "clicking" by chance?

 

[Tommy's SHO]

It surges because the engine is put under load by the compressor. I think it's normal that it does that.
When idling and the "AUTO" mode on, my ATX surges from 750 rpm's to 900 and back whenever the compressor clicks on/off.
The only time the compressor will not turn on is in the "OFF", "FLOOR" and "VENT" modes.

 

[WPG SHO]

Nope, no clicking. Why do you ask? A/C compressor isn't cycling when it surges.

 

[HopefulSHO]

Feel like it gives it a bit of a throttle for one second, and then backs off?
Does it do it consitantly (like, can you time it?)

 

[NoFlipping]

I have the exact same thing happening in my 95 MTX. It's not like it surges up AND down, it's just that when I cruising along and I turn on anything that engages the A/C compressor, it's like I just ever so slightly increased the gas I'm giving the engine. ALL of my seals/sensors, clutches, wires/plugs, etc. are less than 1000 miles old, so I'm no thinking it's a deficiency or maintainence issue.

I think it's a PCM compensation curve kinda thing for when the A/C's on. The most annoying part of it it seems to run BETTER when the A/C is on, and by that I mean just SET to be on, not merely when the A/C clutch engages (if that made sense). Another way - it's like the engine has been given a little "boost" just in case the A/C happens to engage. I have NO sensation when at steady throttle of the A/C cycling - just when the EAC button has just been activated/deactivated (because the reverse is noticable as well.)

 

[Xs SHO 1]

A/C Compressor?

 

[NoFlipping]

Xs SHO 1-

Are you asking what an A/C compressor is, or suggesting that the compressor might be bad?

In my case, the A/C is strong and cold - new clutch/field/plate about 2 months ago. The problem I'm describing has more or less existed for as long as I can remember.

 

[Xs SHO 1]

My guess was either the IAC, TPS or A/C compressor. Since you mentioned that the A/C compressor is new, maybe its not it, but then again, just because its new, it doesn't mean it didn't go bad already....

 

[Rob94]

Use a set of gauges and check your AC pressures. You may be low on refrigerant, which will cause rapid cycling and a slightly noticeable surge. If the pressures are fine, you may have a failing clutch cycling switch, which is located on top of the accumulator.

 

[WPG SHO]

Yeah, it basically feels like you have a shaky throttle foot, the revs pick up a little, and then drop back down. It happens on anything except vent, floor, and off.

 

[Xs SHO 1]

You may be low on refrigerant....

That's another possibility....