Car Surges (RPM Surges or Lurches) - only when A/C on and outside temperature above 70F. Ford 500

Maintenance/Repairs
1

Vehicle:
2005 Five Hundred (220,000 miles)

Symptom:
Car surges (lurches or RPM shoots up to 1500-2000) when outside temperature is 75+ F and ONLY when A/C is on. In addition, when outside temp 80+F, A/C occasionally not as cool, and engine temp goes to HOT (nearly overheating. Shutting off A/C gets engine temp back to normal and surging issue goes away.
Less frequently, car’s RPM drops to nearly stalling and on a couple occasions the car actually stalled.
Symptoms began around 100,000 miles. Lastly, this happens ONLY when at idle (usually in gear) or when coming to a stop or driving slowly, like in a parking lot.

Corrective steps taken to date:

  • Throttle body cleaned by dealer mechanic (I’m not 100% sure it was a thorough job.)
  • Thermostat changed.
  • A/C recharged.
  • Belts & fans checked - OK.
  • Engine computer reset (?) - battery disconnected by mechanic for 30+ minutes to “reset” computer (suggested in another forum.)

This has been going on for at least 3 years now. I’ve been to my Ford dealer and an independent mechanic’s shop numerous times for this. The dealer was never able to reproduce the problem. The other mechanic finally experienced the surging. Oddly enough, after each visit to the shop, the car seems fine for the day and then the symptoms begin to return in full force!

2

My first thought would have been that your AC system is low on refrigerant and the compressor has to work extra hard to build pressure in the system. But since you already recharged the AC, I’d take a look at the intake air temperature sensor.

3

Thanks for the suggestion. I’ll put that on the list for my mechanic to check out.

Today was a 90+F day and my A/C occasionally lost some of its cooling power, to where it was almost warm. The surging issue was still prevalent although, surprisingly, not as frequent, but more pronounced (stronger surges). The 3rd symptom, engine temp near critical, was also present. It seems these three symptoms may share a common problem, or each one is feeding the other. I don’t know except that these symptoms only happen when 2 conditions are present: 1) It’s really hot out, 2) The A/C is on.

I’m researching this on and off line, and hope a solution will present itself so I rid myself of this and share the solution with anyone else dealing with this.

Leads thus far, which address the A/C effectiveness & surging:

  • Intake air temp sensor (thanks Nedley)
  • Idle Air Control Valve
  • Vacuum leak(s)
  • Failing compressor or A/C unit
4

I think you can probably take the IAC off that list. When those fail, you tend to have no idle and the engine just craps out.

If your engine is still overheating, even after installing a new thermostat, that is weird, but might be explained by the engine having to rev excessively to keep the compressor running. Recheck your refrigerant pressure just to rule out an AC leak. If that’s good, then I think a bad compressor is your next most likely suspect. Good luck.

5

If the A/C is low on refrigerant, the compressor actually has less of a load, but it will cycle on/off more frequently if it is very low. It cannot “work harder” to build pressure, as regardless of the state of charge, it is only turning as fast as the engine is. If it is OVER charged, it could be causing a heavy load though. Since you’ve had the A/C serviced, I’d pretty much rule it out.

Your mechanic disconnected the battery to reset the computer. This is kind of pathetic, and a short-term solution at best. Did he connect a scan tool to determine if there were any pending codes stored in the computer? Silly question, but has the car ever had a basic tune-up, with spark plugs being replaced? Does the car get the same gas mileage as before the problem occurred, and does it perform normally? (except for the surge)

Are you positive that both cooling fans are running? If they are not, it will cause the engine to overheat and cause reduced A/C output. Many cars have two electric fans–the low speed one will kick on when the temp rises slightly above normal, while the high speed one will come on as the engine heats up further, and may run continually if the A/C is on–depending on how the system is programmed. It may also be worth taking the car to a dealer and seeing if there’s updated ‘firmware’ available for your engine computer.

In addition to the other suggestions others have made, I’d consider the throttle position sensor as a possible reason for the surging (if your car has one), or the electronic throttle if it’s “drive by wire”

6

Oblivion is right. I was careless in my description. The compressor won’t actually “work harder”, but it will certainly “work more” if the refrigerant level is low. The way it will feel is this; whenever it’s hot, the compressor clutch will engage with a pronounced click, suddenly putting an extra load on the engine and dropping the RPMs. The IAC then does its job, forcing the engine to surge suddenly to maintain its idle speed. That could be the surge you’re feeling. If it’s hot out and you’re low on refrigerant, the compressor will have to engage more often, and you’ll notice it a lot more. the frequent surging could also explain a noticeable rise in running temp, but I don’t see how it could get to the “critical level” you describe, so the engine overheating is probably caused by something else. But I’d still have that ac system checked for leaks.

7

Thanks for all the comments. Here’s an update (in case anyone else is having this issue so you don’t suffer through all the steps):

The A/C was overcharged and it now performs better, but the engine coming close to overheating continues, albeit with less frequency.

Both A/C and radiator fans are working, so it could be that the compressor bearings are wearing out, maybe. It’s one theory on the table.

I replaced the Mass Airflow Sensor which seemed to help with the surging issue, temporarily. The surging continues.

I read in the owner’s manual that whenever the battery is disconnected you have to run through a procedure to have the computer recalibrate idle speed control. This is why the mechanic did this the one time. I’ve since done this procedure a couple times. Each time, the surging issue seems to subside, and then it slowly comes back. Utterly maddening.

I have read in multiple forums that the throttle body could be the culprit as well as a TCB having to do with the throttle body and firmware update to the PCM.

I contacted Ford Motor Co asking to just point me to the right component as this is something other owners are experiencing. In fact, the NHTSA has multiple complaints logged for the surging issue. Ford wants me to go to the dealer for analysis. I will go to my mechanic (not Ford) to check out the TCB. I’ll also go to the Ford dealer if my independent mechanic can’t help and ask them “off-line” what is the faulty component so they don’t officially admit to any design or manufacturing issue. (A couple forums stated that the throttle body was redesigned because of performance failures.)

Throwing more dollars into this without a 95%+ certainty of what the cause is, is not going to happen. If I can’t get a mechanic to say, “Oh, I know what this is! I’ve seen/heard of this issue and the cause is…”, then although I really like this car (sans the issue at hand) and would love to keep it for another 100k miles, I’m just going to drive it as is until it dies or breaks down or requires any substantial routine maintenance (like breaks or tires).

I will come back and post the solution if I obtain one.

Thanks again to all the suggestions, ideas and perspectives!