All,

I have a 2001 4.6 with 48,000. I've recently been experiencing a very rough idle. The tach appears to be stable (not moving up and down) but the truck kind of feels like it's shivering. This is only felt while in drive and depressing the brake. Also only felt when the vehicle is at operating temperature. It accelerates perfectly though. All plugs and coil packs recently changed. No CEL light on. Any thoughts?

Tony

 

Make sure connections are ok. No cracked spark plugs. Check for vacuum leaks. If things check out, the dealer can try to recalibrate the pcm. Ford is always coming out with new calibrations which can be downloaded at the dealer with the WDS scan tool. It will recalibrate several concerns at once for multiple customer complaints. As long as yours doesn't already have the latest update in it.

Robbie

 

Is there a simple way to check for vacuum leaks?

 

all the vaccum hoses run behind the throttle body. Check them for any cracks and sharp bends that could restrict air flow.

 

I have the same problem. Been trying to find what the problem is. Took it to the ford place and had it hooked up for $80 and they could not find anything wrong. Just recently had a body lift taken off, so I am checking the vaccum lines in the morning. Is this problem terminal, can it cause damage?

 

oh by the way post asap when you find out what it is. Ironically I have a 2002 Sport w/ 4.6 and 48,000 miles.

 

Will it vibrate at 1600-1700rpm in park?? Hold it steady at that rpm, let me know.

 

Mine was idling kind of rough and then I cleaned out the throttle body (didn't remove it, just sprayed into it and used a soft toothbrush to loosen the thick stuff). I also cleaned the mass airflow sensor with some electronic parts cleaner. The idle is a lot smoother now but still has some hint of roughness. I am going to try the 1600-1700 RPM deal and report back as well.

On a side note, I drove my boss's Toyota Land Cruiser the other day and my truck idles the same as it.

Happy Truckin!

 

jstang,

It does vibrate between 1600-1700 rpm. It kind of feels like the motor is unbalanced. Really weird.


Anthony

 

Well... i can safely say i have been suffering this EXACT same problem! lol been killin me for a while, but i finally got it figured out! thats just how it is!!!! lol ive talked with many people, including ford themselves, and some local hard tuner shops, even went the route of replacing the spark plugs and wires TWICE once with autolite wires and autolite copper plugs, and once with motorcraft wires and autolite APP104 double platinum plugs... no difference... ill bet every once in a while when you come to a stop, it gets REALLY noticable, you drive again for a little bit, come to a stop, and its almost not even there anymore... a slight shudder and the idle drops down to about 500RPMs... this is what i have been informed of, these cars have a computer system that is set up to bring the idle down as low as it can go for maximum performance/fuel economy. I watched the guy explain it all to me as it was happening, even on a warm startup, when you first start it, the idle jumps, and lowers itself to just under 1000RPMs, then slowly brings itself down to just above 500... where the computer says its time to stop just try not to pay attention to it, from what a million people have told me, its perfectly normal

 

1Bad97F150 is pretty much right about the Idle Air Control valve. On a carb, the butterfly valve was cracked open to allow idle air to pass. The idle speed adjustment screw just cracked it more or less. The butterfly valve in the throttle body on these and all new cars I know of completely closes, no air passes. Instead the idle air is routed in a finger sized idle air bypass. There is an IAC valve that partially blocks that bypass. When the valve extends, it blocks it more (lower idle), when it retracts, it blocks it less (faster idle).

The goal isn't lowest possible, it's the most stable. The computer determines the 'best' idle speed based on startup (flare on startup referred to earlier in this thread), and engine temperature. The computer always monitors RPM from the CKP. If the RPM goes below 'best', it opens the IAC and vice versa. If you turn the steering wheel at idle, the IAC opens up a little. If the AC kicks on, the IAC opens a little bit. If a cylinder starts misfiring, it opens the IAC a little. It doesn't enumerate the reasons it should open, just as the RPM falls due to any of these, it opens it some. If the RPM increases, it closes it some.

All of this happens when the throttle is closed. I knows the throttle is closed from the throttle position sensor (TPS). When the throttle is NOT closed, the IAC opens more with RPM. This is so if the throttle snaps closed, the engine will come down to idle RPM gracefully, not suddenly and perhaps stalling. This is similar to the dashpot function in a carb.

The closed throttle and open throttle modes of operation are why some idle problems are diagnosed IAC problems when really the TPS is incorrectly showing off throttle. Another false reason for false IAC diagnosis is the throttle body won't quite close all the way due to carbon buildup (from EGR and PVC recirculation).

Back to the original problem, since the idle is stable it doesn't sound like an IAC problem. You can verify this by watching for the flare on startup coming back down smoothly to 700 or 800 RPM. Flip the AC off and on and turn the wheel several turns and make sure the IAC corrects dips in the idle. If a manual trans, at idle in gear, hold the brake and use the clutch to drag down the idle to 600 for say 2 seconds. Push the clutch back in and the idle should flare up over 1000 and settle back down gently. If so, the IAC surely isn't the problem.

Most secondary ignition (coil, wire, boot, plug) problems are worse under load and this is at low load so that doesn't sound like it. Most fuel delivery issues are worse under load so I wouldn't suspect a pump or filter. EGR flow causes rough idle but no problems at speed or under load. Mechanical problems cause a rough idle but most do under load too.

If it's small and at a place not likely for most other common problems, I'd suspect a calibration. These are strategies and tables of parameters the PCM uses for engine control. You get a new set by having the PCM reflashed.

Good luck,
Mark Cantrell

 

Count me in to the 1600-1700 vibration club too. I agree, it seems like something is very unbalanced.

 

1600 - 1700? never had that issue.... ill check it tomarrow, but as far as im concerned, my truck is smooth after 700 RPMs... ill look into it though I saw a label near the throttle body stating that i shouldnt clean it, due to a special coating... is this a serious thing? if so, is there a way i can clean it without harming it? and should i only use electronic parts cleaner on my MAF sensor? i already sprayed a lil carb cleaner inside the hole where the MAF sits, put it all back together and everything seems to run fine, and this was about a week ago... should i spray some electro parts cleaner in there now just to make sure its done right?

 

Dont worry about that sticker, its perfectly fine to clean it, my instructor at my tech school worked for ford for 25 years and he told me to do it on my mustang when it had a rough idle and i did and it cleaned the gunk outa there and ran much better. When i cleaned my maf sensor on my mustang i took the actual sensor out and cleaned it with electrical cleaner and a q-tip. Righ now im trying to figure out why my 97's 4.6L truck's idle drops down to like 300-400 rpm then dies on me at a stop. i think its the iac valve. The truck has 323,000 miles on it