I've purchased a cheap LR3 with 170k miles on the clock, it has the infamous ford 4.0 SOHC V6, I've been trying to figure out a rough at idle that is occuring, Its not stalling nor is it having difficulty running but when I'm sitting in the vehicle you can definitely feel it vibrating the interior.

Here what i've got/done so far.
*Compression test on all 6 cylinders engine warm (engine temp 70-80*F) and DRY: PSI 155-170 with deviation <10% from highest and lowest
*Spark plug wires replaced (with denso), All 6 wires original, ends were deformed by heat, 2 were arcing on to the head.
*New set of spark plugs (BOSCH platinum tip), one of the plugs electrode separated from the body, plugs otherwise nominal, old plugs were Motorcraft AGSF 24PM
*Codes: P0420 (Bank 1 Catalytic converter efficiency below threshold) Data on Torque app verified inconsistent voltage readings on O2 sensor Bank 1 Side 2 (downstream), also assessed ie knocking the catalytic converter something is rattling in there, no malodorous exhaust fumes otherwise, the separated electrode spark plug is on the bank where the catalytic converter failed perhase it may have caused it to fail?
*Possible "death rattle"? Faint but noticeable valve train rattle/grinding at idle onlly, slightly more apparent when having the oil cap opened with ear close to it, no rattles at startup both hot and cold.
*Slight fluctuation at idle from 720-800, idle varies between these values incoherently
*Cleaned MAF, throttle body valve and replaced air filter, no changes with that.

Suspecting culprit for rough idle:
1. Vacuum leak, judging by the number of PVC lines with age and mileage, quite possibly may have a small vacuum leak somewhere, perhaps intake manifold gasket? Smoke test would rule out quickly but i dont have one.
2.Timing Relate, given its reputation of these engine spontaneously destroying its valve train, and a slight grinding sound that can be heard, maybe I have an issue with timing that is related to skipped or from stretched chain?
3. Catalytic converter? Probably from clogged or damaged catalytic converter may be restricting exhaust flow?
4.Motor mounts, Unlikely since it appears to be mechanically related,

I'm a little lost right now, perhase anyone could chime in give some advice and direction?

 

What fuel are you running. I have the same engine and run 91 octane (unleaded). Every 5000 kms I add a fuel system cleaner (not additive) to a tank full of fuel to keep the injectors etc clean. Your problem may be a dirty injector or two. My idle is around 750-800. Check the PVC valve at front of engine on rh side if you are standing in front of engine. I had a problem with my rear O2 sensors so removed and cleaned them and all okay - it occasionally gave a MIL light error. Need to check both idler pulleys on front of engine, one is fixed the other tensions the belt. Make sure you use a good quality engine oil - I use a 10w40

 

I use 91 octane fuel top tier (costco), getting close to finishing up the 2nd run of fuel cleaner, I use techron fuel treatment, I've replaced my idler and fixed (smooth) pulleys they were badly worn however the grinding sound still persist, im using Mobil 1 High mileage full synthetic 5w-30 and mahle oil filter. I'll look into the PVC valve, i;ve tried to remove the o2 sensors to replace them but they seem to be stuck solid, looks like i'll have to get some penetrating fluid soaking tonight.

 

If grinding noise still persists, have you checked the alternator bearing as it is known to fail. Depending where you live, I would up the oil to 10w40

 

Globetrotter448 I live in US we have some occasional cold weather that would roll during the winter, so im sticking with 5w-30 for winters and 5w-40 for summers, grinding noise is definitely timing chain and guides are making some noise, I ran the car without its drive belt for a minute to see if the noise persist and unfortunately it still dose but more subtle (a little rhythmatic chatter like rattle), the alternator was replaced by dealer in 2013 about 60k miles ago (now 168k mi).


I did some digging and did some Live Data Reading related to the emissions systems and the A/F sensors, I've concluded that the Bank 1 (aka U.S. passenger side) catalytic converts are done for and Bank 2 cats are on the way soon, given I have a P0420 codes and verified that all the sensors both down and upstream are reading well (all O2 and A/F sensors will be replaced when getting new cats in), the O2 post catalytic converter (aka B1S2) reading voltage is always on the low side and fluctuating all the time vs Bank 2 where its stable (90% or the time) with gradual adjustments to throttle inputs , and also fuel trims both long and short are not very consistent on Bank 1, however I find it strange that Bank 2 is running lean (not lean enough to trigger lean condition usually >25%) but with very little adjustments.

(Note that B1S1 is reading lower voltage 0.215 and frequently drops, normally O2 sensors reads between 0.600-1.200 depending on mfg specs, and gradually increase and decrease voltage dependent on load mine clearly runs low on bank 1 and fluctuates which isn't suppose to happen.


Same thing as above but graphed over 20 seconds, the highlighted in blue is the culprit catalytic converter, note that Bank 1 (B1 S2) was stable then it trends down, compared to Bank 2 Side 2 (B2S2 right upper conner) where is stable with occasional fluctuation. The bottom Left and Right box B1S1 and B2S1 is the Air/Fuel (A/F) sensor or Pre catalytic converter it appears to be in working order with some fluctuation but no trending up or down or drops in voltage (mA).


Fuel trims both Long and Short not sure what to make of it, but Bank 2 is running a little on the lean side (-9.4%) not high enough to trigger a lean condition (>25% usually), however Bank 1 fuel trims are constantly adjusting at idle.



I suspect that this may be the culprit of damaging bank 1 catalytic converter, a broken spark plug core on the passenger side (ie Right side or Bank 1) and also the fact that the previous owner was probally using regular instead of premium killing/poisoning their catalytic converter on bank 1, We'll see when we cut remove the cats and run the engine without them to see if it idles straight.

 

Its a fair chance that the broken plug would misfire

 

Still rough/vibration at idle even after replacing the broken plug, but i suspect since the plug was broken for quite some time thus long term of misfires has already incurred damages the catalytic converters.

 

A longish run where it gets the exhaust hot (especially if you have hilly areas nearby) may clean the cat for you