OT Question for the FI Engine Guys

[Greg Maier]

I have a 2000 Jeep GC with the 4.7 L V8. When the oil gets dirty (around 3000 miles), the engine will begin knocking. When I change the oil, the knock instantly goes away. I have no answer as to why this would happen. Does anybody have any ideas?

[mphatc]

3000 miles is a bit early for this . .I'd expect it to happen later . . 4500 miles or even more.

Leading me to ask numerous questions . . .
how is the vehicle driven? around town or open road?

Your oil is breaking down, and no longer lubricating as it is supposed to, but that may not be all!

How many miles on this engine? How has it been used, towing? and what weight and kind of oil have you been using?

FWIW, in 30+ years of engine building experience this is ussually an indicator of low oil pressure caused by numerous reasons, some are

Worn bearings
worn oil pump
poor quality oil

Either way, when this happens your engine is getting lots of wear somewhere.

I advise changing to a synthetic oil ( Mobil 1 or Redline) of the factory recommended weight, likely a 5w 30 and use a factory oil filter. WIX and Purolator are good if a dealer is not close by.

MPHATC

[Greg Maier]

I've been using Mobil1 5W-30 oil. The Jeep is driven both around town and on the highway. I don't alwys change the oil at 3000 miles. I have probably gone as many as 5000 without a change (That is what I thought one of the advantages of Mobil1 was).

[mphatc]

Definately not something I would expect to see with Mobil 1!!!!!
I use rthis stuff in racing engines, some of these run 3000 - 4500 miles on track and the street without any worries!

does this happen on every start or at idle all the time when the oil is old?

MPHATC

[Greg Maier]

No, it just happens under hard acceleration when the oil is old. When the oil is fresh, I can accelerate at full throttle with no knock. I use 89 octane gas.

[mphatc]

Is this a knock . as in lower end engine noise, or is this pre-ignition ping rattle ???

Mario

[Greg Maier]

This is a pre-ignition engine ping. That is what I always thought a knock was.

[mphatc]

This is caused by oil breaking down and getting into the combustion chamber and lowering the octane level of the fuel.

How does it get there . . . past worn rings, and or valve guide seals, or thru the crankcase ventialtion system into the intake tract then lowereing the octane of the fuel . . .

ahh the internet, your intrepretation and mine of a knock . . . :bonk: :banghead:

:beer:

Now to solve it . . .

better brand of gas . . no generic stuff!
Trust me here, I just rebuilt a BMW M3 engine because of this! cost the guy $7K!

run 93 octane . .yuch . . expensive

tune it up . . plugs, fuel filter,

Sell it buy a Ford powerstroke diesel . . and a bigger Donzi

Mario

[gcarter]

Mario, I've often wondered if automakers were going too far with lower viscosity oils in the search for lower friction, better fuel economy. Now I live in Florida and it's hot here most of the year. I use 20-50 in all my Toyota pick-ups, when I made the switch, oil consumption went down in all three trucks. Could this help?

George

[mphatc]

Absolutely . .

especially in your climate


mario

[MOP]

Greg does it have much blow by, pull the oil cap take a look when it well warmed up. If it does you could try one of the carbon buster products, I have done that with good results to quite a few motor. I end up doing it every year for the Reli's down in Costa Rica theyhave crappy gas.

Phil

[Greg Maier]

Actually, it did have a crusty black buildup on the inside of the oil cap. I'm not really sure of the significance of this.