My 1998 xj 4,0 has had a slight knock that is coming from the bottom end. It seems to be louder in the drivers seat. It does go away after 2k rpm or so.
It sounds like a hammer hitting the floorboard every rotation of the crank.
Runs perfect, and plenty of low end power, just knocks.
It has 205k miles and zero repairs ever. (not even cap and rotor, or water pump)
It does not come up on camera so a video is worthless.
So what do ya think?

 

1. Spring for a tuneup. At 205k, your ignition equipment owes you NOTHING. It is shot, regardless of how the engine runs. New Champion copper plugs gapped to .035, new wires, cap, rotor. Never underestimate the importance of secondary ignition components. Replacing them will pay you back quickly in increased gas mileage.

2. You could have bit of piston slap; not unusual. I hope that the oil has been changed every 30k or so??? LOL

 

I do check and replace the plugs and wires, it just never seemed to need a new cap and rotor, or anything else.
And yes I change the oil every 3K or so with a napa gold filter and rotella hd oil. It did have a wierd antifreeze issue a few thousand back, but it was just from rusty water in the overflow tank.
It doews leak oil quite badly (rear main and oil filter mount) but I would not think that would be a problem.
Also it it gets 26mpg highway and 20 mixed.

 

At 206K, I have been running Amsoil XL and there EA filters and have had no issues. I have a 10 month interval on earl and filter and well has treated the XJ pretty good. I did a full tune up back in Jan and inspected plugs last week and the NGK's at .035 have been really good to me. Maintenance Maintenance and good ole Maintenance have kept this 4 liter good and strong.

Oh and if you can, premium would be best but try to at LEAST run mid grade, well worth the extra few bucks.

 

I will stick with what I have. As for the fuel, that is a load of bull****. I get 26mpg with regular and I got about 15 with premium, and it added a bad spark knock. The 4.0 is made to take 87 octane as 87 is more cumbustable than 93 octane.

 

Well good or bad, I seem to get much better MPG on premium. I took a 900 mile drive NM to TX on 87, on the way back on 93 and had much better results. If you get good MPG on 87 go for it. I am just stating my facts, not trying to sell you on anything.

 

I did a cap and rotot today... wow... that may have been the reason it idled funny.

 

Cap, rotor, plugs and wires were required maintenance items to be replaced every 30,000 miles......

 

Didn't you just state it run's perfect up above,now it idle's funny? Anyways with that many miles things wear out abit and it doesn't sound to serious, but you might think along the lines of bearing wear, piston slap (already stated) wrist pin, etc.I think you'd be fine for a while longer good luck.

 

The way I'd approach this noise is this.

Monitor it.

If it continues to get worse, then start developing a plan to resolve. If it stays the same, it then becomes more of a "characteristic" of a high mileage engine, rather than some pressing concern.

You have to choose your battles with your engine. Don't over-react. With a situation like this, it "usually" (standard disclaimer) will let you know if complete failure is near.

 

just thaught id throw in that the timing might be off? thats the reason my xj knocks a bit, need to redo the timing before winter....

 

Well all these comments got me thinking, could it be that the flexplate is loose?
As for saying it runs good, That was generalized statement and I was just meaning that the engine is not missfiring and has no drivability problems. It has always had an issue of idleing a little high (1200rpm) for no reason. The noise has been there for 50k+ miles and has never really bothered me till I started to go to a automotive school.

 

What kind of oil pressure do you have at idle? Also I had a water pump knock on my 98 that i swore was a rod knock. I got one of cheap stethoscopes at harbor frieght, thats how I found it.

 

Can't say for sure. The sensor is bad at idle it reads between 5 and 30 psi, but at 2k rpm it reads 45-50. So it really turned out to be a water pump?

 

Sounds to me like this would be a good automotive school project: Check the torque converter to flexplate bolts for looseness.