2004 Discovery II
~80k mile P38 4.6 motor
Recently purchased. Many issues already addressed. One at a time... Thanks for the help so far!

Issue description:
Slight vibration from the motor, only noticeable at idle. Once rpm goes up over 1,000 the vibration is not noticeable. Engine revs free. At idle it is noticeable from the driver's seat, you can feel the truck shaking a little at idle. Also noticeable under the hood. Seems more pronounced once truck is at temp. Not noticeable when moving, but at a stop light it's easily noticeable. Prevalent in Park, Neutral, and Drive. Not noticeable worse or different in any gear.

• Less noticeable when engine is cool
• Not super consistent, seems to vary. Once the motor is at operating temp it seems most prevalent
• The motor is smooth. No ticking, tapping, no bearing whine or noise from serp belt (serp belt is new)
• No SES light or pending codes
• Oil change 2k miles ago, new plugs, new air filter, cleaned PCV, new MAF, new fan clutch, new auxillary/ac fan
• All plug wires are plugged in
• New motor mounts
• ** replaced both idler pulley's shortly after purchasing the vehicle***
• New front driveshaft
• The truck consumes power steering fluid. I add a few ounces of fluid every week
• Coolant temps are perfect - sub 200 all the time. 188-194 most of the time. 197 peak temp at idle. No coolant loss.
• I've added about 1/16ths of a quart of oil since I've owned the truck. It does not seem to consume much oil at all
• This issue has been present since I bought the truck, before any of the above maintenance items were addressed.
• I have not ruled out it being a deeper issue - but the motor seems to run really well, all vital signs are normal. have not had a compression test or oil pressure test, but there has been no reason to do those. The motor has been pretty much perfect. Power is good, revs smooth, again no coolant usage or leaks, no oil leaks.

I am leaning towards it being a worn bearing on the alternator, ac compressor, power steering pump or one of the other pulleys. Other than the idlers, the other accessory pulleys are a little rusty and look original. Another possibility could be the trans/transfer case mounts. The motor mounts are new, but I bet those are all original.

From what I have listed above a pulley going out makes the most sense to me. But, since there is no noise coming from the bearings, none of them are noticeable out of balance or shaking. I've felt the alternator, ac compressor, and power steering pump while the motor is running and none seems to be vibrating differently or more than the other pulleys.

Any trouble shooting recommendations? Where would you start?

 

How bad is the vibration? Mine moves a bit at idle but just enough to giggle the coffee in your mug if its in the cup holder. As soon as it comes off idle it's smooth, and rock solid at 70mph. Mine was like that before and after new HGs. Never really concerned me and not really different than any other 10+ year old truck I've ever owned.

Remove the belt and start the engine. That would rule out any of the accessories.

 

Good feedback @Red5 .

It's enough to jiggle the coffee in a cup holder. You can feel it very noticeably in the seat. I had a D1 and do not remember it vibrating like this, but that does not mean much. The truck is buttery smooth off idle, and also at 70mph now that it has a new front driveshaft.

I had a buddy of mine look at it with me today. He seems to think it's normal as well. I guess with new motor mounts I expected it to vibrate less. But if the motor mounts needed to be replaced (done by the PO) I am willing to bet the rest of the rubber mounts needed to as well. The vibration could very well be caused from the motor shaking the rest of the driveline, and the driveline transferring that vibration into the cabin. It

The truck is still new to me so I am pretty sensitive. My last vehicle was a 7.3 Ford Powerstroke, so I am used to some serious vibration.

 

harmonic dampener ?

 

@XRAD possible for sure. Visual inspection while running doesn't leave me to believe any of the pulley's are the issue. Harmonic balancer looks like it spins perfectly, no wobble. I need to pop the belt off and feel the pulley's for play.

Any experience trouble shooting the harmonic dampener?

 

body mount bushings? Could be just the frequency at idle is just right to cause the cab to vibrate? I've certainly had stranger things happen.

 

@whowa004 totally possible.

I was paying close attention wheel driving today. There is a vibration at speed as well, but not as pronounced as the one at idle. I have all terrain tires that vibrate a little so it is tough to distinguish between different vibrations.

I am less suspect of the pulleys/harmonic balance etc. after watch the engine run for a while when I got home. I still may pull the belt off and see if any of them have any play.

The engine was out to have the motor mounts done right before I bought it. I have had to tighten several bolts (bell housing - which was missing hardware, lots of hose clamps, SAI bolts, several other misc bolts) and am now wondering what could have been left loose that would cause this vibration. The bell housing is good now, the motor mounts are good (although I will put a wrench on them for good measure), so what else would have come a part when the motor was out that could be loose? Would the transmission have to have been loosened up to allow the motor to come out?

The motor does not seem to vibrate more than you would expect from an old V8. But the vibration is pronounced from the driver's seat. I wonder if it's the transmission or transfer case that is causing it?

 

Exact same thing happens when my truck has 02 sensor codes. When the sensors can't determine a fuel mix for you, it can start just dumping gas as a preset and it very noticeably shakes my truck at idle and goes away otherwise. Not sure why you wouldn't have codes, but something to consider. Check your front o2 voltages to make sure they are alternating up and down.

Can't imagine a pulley would be shaking it without making noise, and you already said it happens no matter what gear you are in, so that would rule out the transfer case and transmission for me. If it isn't the o2 sensors, I would probably be thinking mounts...2 motor mounts, one each side, and a transmission/transfer case mount a little further back. Can't remember if there are one or 2 or which part it is on but I know it's there

Hope this helps

 

Very helpful @Old Bertha . I have a video of the 02 readings actually. Idle O2 sensor readings:
I have befriended the well known local Rover mechanic. When he first looked at my truck I had him look at O2's and they passed his test.

Thanks for the thoughts on the pulley's. I agree that this is a little to significant to be pulleys. I can live with worn trans mounts if that is what it turns out to be. I don't know how long I can live with it...

I am leaning pretty heavily towards mounts - I will put a wrench on the motor and trans mounts in the morning. The PO replaced the motor mounts, so am I thinking if they were worn out, the trans and transfer case mounts are probably shot as well.

 

Quick update:

When I made this post I had no SES light. I’ve put a new MAF on the trucks s thought that fixed the SES light I have had for P1171 and P1174 for a while.

O2 readings appear fine

I have been unable to locate any vacuum leaks

The PO of the truck suggested replacing fuel injectors, but did not say why.

I have decent engine performance but very poor MPG. I have to try to get 11 mpg.

Should I replace the fuel injectors?

Are the lean codes possible a source of the vibration?