Stumble only when A/C is on

I have done a lot of work to tune up my 98 CSVT since I purchased it but have one lingering issue. Runs fantastic except when I have the A/C or Defrost on. At those times it idles fine but has an intermittent stumble under load or constant throttle. I have installed new Motorcraft plugs (properly gapped), new wires, upper 02 sensors, fuel filter, cleaned the TB and Mass Air and cleaned and lightly oiled the K&N. I also did the throttle hang fix involving the drilled brass caps. The car only has 84K miles and in all other respects runs perfect. Without the A/C on it pulls perfectly and so I don't feel it is a fuel issue It isn't a huge issue but I am pretty particular about how my cars run.

Anybody have any thoughts? Possibly dirty UIM or LIM? No codes by the way.

Thanks for your time.

I have the same issue. I just replaced my engine wiring harness that I had no idea had a lot of cracking insulation even a lot of corrosion under the cracks. I'm thinking the added resistance might have been the reason for the extra load on the electrical system when turning on the A/C.

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Seems to be a common thing with all Contours, my last one did it, and this one may as well, but I've been driving Contours for so long I probably don't even notice it anymore. Would only be under steady throttle at a cruise, almost felt like I had stopped giving it throttle for a second, like engine braking while driving. There was a thread about it a while ago and the consensus was the system just robs a lot of power.

Totally normal with the 2.5 Duratec. They just don't like air conditioning.

B3NN3TT said:

Totally normal with the 2.5 Duratec. They just don't like air conditioning.

Click to expand...

Agreed.

Something interesting is that when I had the throttle hang fix on the car, the car hated me with the A/C on. I put a new Motorcraft IAC on and took the throttle hang fix off the car. After that the car ran much better with the A/C on. I no longer needed the TH fix after the new IAC.

I'm having slightly different issues. Since I bought mine last August, I noticed that with the A/C on the car surges a little as the compressor clutch kicks on and off. What's more weird is when I'm slowing down with my foot on the brake, while still in gear, the car kind of "pushes" forward flow a split second while the clutch kicks off and back on. I have a 3L with under 100k on it (might be closer to 80k... but have no way of really knowing except from an assumption from a previous owner). The clutch is definitely kicking off and back on more frequently than it should so I need to resolve that short-cycling issue, but the slight surge I feel when it kicks off and then back on again seems a bit too "pronounced" to me.

And now, over the past couple of weeks, if I sit at idle with the A/C or defrost running for 20+ seconds the RPM will start to drop and bounce... and if I don't give the engine a slight rev, the engine will stall out. This only happens at idle andwith the A/C on. If A/C is off, or if I'm not idling, I have no issues holding a steady RPM nor any issues with the engine stalling. Any ideas???

Thanks everybody. I will likely start by replacing the IAC and see if I can cure the A/C as well as the throttle hang issue. I would really hate to think what a wiring harness would cost for this thing, let alone the pain in the A$$ that it would be to install. Lets all have a moment of prayer...

This reminds me of the SHO idle relearn procedure. It is all the same components (MAF, IAC) so it should be done this way in my opinion:

NOTE 1: The Ford owner's manual directs that the idle program be relearned following any service when the battery has been disconnected. To do this, it states that the engine should be started and allowed to idle for the (approximately) 70 seconds without touching the gas pedal. Adding the accessory loads ensures that the EEC and IAC valve can accommodate the additional air flow requirements and still maintain the programmed idle speed.

NOTE 2: It is preferable that the car be driven immediately after setting the idle programming. While driving, allow the coolant temperature to rise to the normal range and run the car through its entire performance range.

Idle Speed Program Reset Procedure
-----------------------------------------
1) Disconnect the negative battery terminal.
....Optionally, disconnect the keep alive memory (KAM) connector in the small
....wire also landed to the negative battery terminal.
2) Wait at least 30 seconds.
4) Reconnect the KAM connector or negative battery terminal.
5) Start engine normally (never press the gas pedal).
6) Allow the idle speed to settle for at least 30 seconds.
7) Within 70 seconds of starting the engine, concurrently apply all accessory loads.
....a) Climate control to MAX A/C.
....b) Headlight switch ON, brights ON.
....c) Turn steering wheel slowly ~1/4 turn side to side.
....d) Press brake pedal.
....e) ATX SHOs only: shifter to DRIVE.
8) Do not turn off engine for at least 5 minutes.

A lot more prior SHO owners on here that I thought! I built a beast N/A 3.3L '92 SHO several years ago. Had a shoe box with just over $11,000 of receipts invested in that car lol. Then bought a '96 SHO when someone hit me head-on and totaled the '92 (was a very sad day). Really fun cars. I definitely wouldn't mind another Gen III again at some point... much nicer ride quality than the 1st and 2nd gen.

Anyhow, thanks for posting that for us! I was thinking of pulling the negative terminal, for 60 seconds, put it back on, and start it up with the headlights and A/C on. The process you posted might yield better results. I'll let you guys know after this weekend if it fixes my problem with the stalling at idle.

Finally had a moment to try the reset procedure. The initial impression from my test drive is that there was a marked improvement with the stumbling issue. It does not appear that it cleared up the problem entirely but did make it much better. Under full throttle acceleration with the A/C and headlights on there was no "misfire" or hesitation but on steady, light throttle there is a very rare intermittent stumble. I think I can live with it now.
Thanks everybody for the input. I appreciate the help and I hope this helps somebody in the future.

A bit late to the party, but I'll chime in on 15 years with a '99 CSVT.

Never experienced a stumble while A/C is on, but when on, it takes a bit more throttle to get smoothly off the line than with A/C off. I feel the same thing with extra people in the car as well, so the drag of having the A/C on is like having some passengers.

Never had any stumbling behavior once moving along in higher gears, but I can feel the A/C clutch engaging and disengaging while moving along on the highway.

Some Ford calibrations used to richen up idle and off-idle transitional fuel when the A/C was engaged, and this can cover up the additional load of the A/C compressor, but the EPA started frowning on this behavior in the late 90's, so I don't think anyone does it anymore, and as fewer and fewer American cars are sold with manual gearbags, the need to do this is negated anyway.