AC pump engaging causes engine stumble

I own a 2003 Ford Ranger 4x4 Automatic with the 4.0l V6. I bought the truck a little over two years ago with 150k miles. The Engine had alway had a slightly rough idle and I never really noticed until a friend pointed it out at around 170k. Every time the pump engages after a short warm up period there is a distinct stumble in the engine, although it never stalls or trips a check engine light. I have replaced the spark plugs with four prong pre gapped Bosch platinum plugs and new plug wires as well. I have installed an Optima red top battery that tests at 1100cca and have tested the voltage draw when the AC engages and it only drops .2 volts from 14.48v to 14.24v. I thoroughly cleaned the MAF sensor and before changing plugs I ran a can of non chlorinated brake cleaner through the throttle body to clean out carbon deposits and dirty throttle body. The engine still stumbles. Yesterday at 182k I disconnected the low pressure switch on the AC pump and boom, smooth low idle with no stumble. What could be wrong with the AC pump that could cause this stumble in the engine?

It could be binding up due to worn shaft bearings.

Try removing the serp belt and checking for lateral and axial play, as well as trying to turn the pulley with your hand. Post the results.

How can I get the pump to engage with the belt off, the bearings in the idler are fine but the pump won’t engage, do I pull the whole belt off and run it without a belt?

A few things . . .

You should remove the throttle body and clean it on the bench, to make sure it’s as clean as possible

Clean idle air control valve

I’m not a fan of those 4 electrode Bosch plugs . . . unless that’s what it came with from the factory. And I know that was not the case on your engine. You probably spent more than on the factory plugs, and you gained nothing

In some cases, cleaning the idle air control valve is just not enough. In some cases, the stumble is due to an idle air control valve that is hopeless and needs to be replaced

As mentioned above, the ac compressor itself could be the problem

But I would absolutely rule out the other things first

I would replace, not just clean, the idle air valve. They are not expensive and cleaning it will not help if the electrical portion of the valve is weak.

I would install the correct Motorcraft or Autolite spark plugs and throw away the Bosch. They never help, they sometimes hurt.

Then see how it idles.

@asemaster Does Autolite make the plugs for Ford?

I’m asking because they both have those stripes on the porcelain

Oh, I’m not sure. I just prefer to use the proper spark plugs in any engine, which in this case is Motorcraft, with Autolite being an acceptable substitute. Chryslers get Champion, GM get Delco, Toyota Denso or NGK, etc.

I can’t tell you how many times I’ve cured a rough idle complaint by simply removing Bosch Platinum or Bosch+4 plugs. Bosch makes some fine spark plugs for OE applications, but using them across the board is just crazy. They create as many problems as they solve.

@asemaster Speaking of proper plugs

When I was still working at the Benz dealer, a colleague brought in his sister’s S500, which she had recently bought used. He was working on it during lunch hour

Anyways, it looked pretty clean, and she paid a nice price, but the idle was awful. It was overdue for plugs, so my colleague decided to replace them. The plugs he removed were non-factory plugs. In fact, they were cheapo champions . . . !

Naturally, he installed the correct Benz platinum plugs. That engine was idling perfect after that

Whoever installed those champions sure wasn’t doing anybody any favors


A kid brought me a Honda that would run rough when the ac was engaged. Plugs were totally shot. Replaced plugs, wires, rotor and cap and all was well. Did you replace the wires? I would replace the iac on yours and go from there.

The issue is that when the AC was disconnected the engine idles smooth and has no stutter. The plugs and airflow sensor can’t be the problem because the engine is fine without the AC engaged.

Clean the throttle body on the bench

Replace the idle air control valve

Report back after that, please

I’m not suggesting you try it with the engine operating. But you just might be able to feel bad shaft bearings without the clutch engaged.

Well the battery can’t be the problem either but you seem to have replaced that too for some reason. The MAF also won’t cause the symptoms you’re describing but you cleaned that.

Without some scan data or testing to see what’s going on we’re all just guessing blindly here. I want to know what the A/C pressures are with the compressor running, I want to know long term and short term fuel trim, I want to know TPS voltage and I want to know where the IAC is commanded to be. I want to know which/any cylinders are misfiring at the time of the rough idle.

Without that, the best advice I can give is to replace the IAC and get those plugs out of there. And yes, the plugs could be the problem.