car problem stumble @ very low rpm

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2005 ford focus 2.0 pzev
Symptom is present when engine is fully warmed up. Worse when a/c is on. Engine light is off and there are no codes saved. 10,000 miles on new Autolite AP104 spark plugs, and coil boots.

All online forums seem to end with no fix/solution for this topic. I am 99% sure a mechanic has seen this problem and it's a common problem. Not sure it's so easy to trace, though. Has anyone fixed this problem before?

Off topic. Could my serpentine belt have lost flexibility/pliability after 150,000 miles? If so is it adding unnecessary load to the engine?
 
Any issues with starting/cranking? Good power and mileage still? How is the gap on the plugs? OEM coils with just new boots? Did you use appropriate dielectric grease? Better, worse or same during wet weather? Was it doing this before the tune up?

Edit to add: I don't think the serpentine belt would have any effect on this. But low idle speed would. Also, if the ECM isn't bumping the idle during AC operation it could make a minor miss at idle more noticeable.
 
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Are those the proper spark plugs for it? I know some Hondas can have issues when not using NGK/Denso plugs. The owners manual even states to use those two brands.
 
there are pcm updates for hesitating and stumbling, do you know if you have the latest software installed? there's tsb's for it.

seems to be common issues with the engines running lean due to vacuum leaks (pcv hose, intake gaskets, vacuum hoses, and air intake tube leaks after the MAF).
 
Originally Posted By: gathermewool
Your drive belt has 150k miles on it?

Yes. It still looks good.

Originally Posted By: HerrStig
Are the injectors partially clogged?

Is this checked on a bench?

Originally Posted By: buck91
Any issues with starting/cranking? Good power and mileage still? How is the gap on the plugs? OEM coils with just new boots? Did you use appropriate dielectric grease? Better, worse or same during wet weather? Was it doing this before the tune up?

Edit to add: I don't think the serpentine belt would have any effect on this. But low idle speed would. Also, if the ECM isn't bumping the idle during AC operation it could make a minor miss at idle more noticeable.


This year we have had a lot of rain. I don't remember any stumble when driving in the rain. Since then we've returned to a normal pattern and the stumble has quickly gotten more frequent, and more intense. It even stumbles when backing up! Idle speed does not pick up or slow down when I turn the a/c on. I checked it in "D". Is it supposed to pick up?

Originally Posted By: satinsilver
Are those the proper spark plugs for it? I know some Hondas can have issues when not using NGK/Denso plugs. The owners manual even states to use those two brands.

No. Per the Autolite catalog AP104 is the same plug as OE.



Originally Posted By: t1snwrbrdr12
there are pcm updates for hesitating and stumbling, do you know if you have the latest software installed? there's tsb's for it.

seems to be common issues with the engines running lean due to vacuum leaks (pcv hose, intake gaskets, vacuum hoses, and air intake tube leaks after the MAF).


I have checked tsb's by vin and there was nothing for the pcm. Is there always a tsb issued for a software update?
 
Not sure. Most manufacturers don't have tsbs for every update. I worked for Chrysler, they didn't. If we had weird drive ability issues we would first check for updates and perform them then recheck regardless of what the description says it fixes. Chrysler flat out told us in training that they wouldn't list all the concerns that updates address. I have no clue why. And I haven't worked for another manufacturer so I'm unsure if they are all the same.
 
I dont know about modern cars. I am driving a 80s tech carb mistubishi sedan. For tbe a/ c it has some kind of throttle idle compensating device. What it does is it increases the engine rpm in conjuction with the a/c throttle setting on the carb.
i once had it fail last year ..my engine shudders and loses about 250rpm at idle.
 
Originally Posted By: HerrStig
Are the injectors partially clogged?


Not likely. You would also notice stumble with a cold engine(open loop) when fuel demands are higher. I would be suspect of the throttle position sensor possibly having a dead spot. You can verify smooth linear operation of the sensor with an analog ohm meter.You might want to try cleaning the throttle body with the correct cleaner. EGR is a possibility but the engine would likely stall out with an EGR which was starting to hang up in the open position.I would check for codes too. A vacuum leak likely would increase your idle speed.

A belt cannot cause this.
 
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Originally Posted By: Olas
Possibly EGR but I'd be looking for a vacuum leak and/or sticking IAC valve, then fuelling, timing, ignition, compression.


Located a vacuum leak using rubber glove and cigar smoke thru brake booster vacuum line method. I don't know if sealing the leak has resolved the stumble problem. I'm going to let the permatex set before I drive the car. My coworker drives this week.

I have modified the intake manifold. Swirl plates were removed 3 years ago. At that time I made epoxy plugs on the ends of the IM flange where the swirl plates shaft once was. Today when I tested for a vacuum leak I got a wisp of cigar smoke from one of the epoxy plugs.

I have never used the cigar smoke method before. The rubber glove was rubber banded onto the throttle body. I just blew until the rubber glove was inflated, put the cigar down on the driveway, and squeezed the glove until a wisp of smoke appeared from the epoxy plug.
 
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When you are confident that there are NO vacuum leaks, get Torque on your phone and watch live data relating to ignition and injection.

It's unlikely but possible you have a bad engine mount (or two)
 
1. dump a bottle of chevon techron in the gas and drive like crazy. This should clean up the injectors
2. I am not sure if you model has an idle screw to set the idle speed, just use it to raise the idle couple of hundred rpm keep it from dying while you figure out the problem.

I am idle dying issue which was due to faculty ground
 
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