What is the best way to tell if your engine is the original one? I got my jeep about a year and a half ago with around 235,000 on it and the guy said he thought the engine had never been replaced. It has about 237,500 miles on it now and the only thing I have done to the engine is change the oil, flush the cooling system and replace the valve cover gasket. What I find amazing is that it shows virtually no signs of internal wear, it has good oil pressure, doesn't burn any oil, doesn't overheat, has no blow by, has good power, and starts fine even on a 20 degree day after sitting for a week. It also doesn't have any unusual noises other than the normal 4.0 valve train noise that goes away when it is warmed up. As far as I can tell it looks like it has never been replaced and if this engine has over 237,000 miles on it that is very impressive considering how well it runs. I think the transmission is original also and it works great.

 

For the love of God, do not let your Jeep hear you talking like that.

First rule of Jeep club, is we don't talk about how well the Jeep is running, unless privately, and even then, only in hushed tones.

 

Look for a machined surface on the side of the block, next to the ignition coil. That is the engines build date code. The 1st digit is the year. The 2nd and 3rd digits are the month (01-12). The 4th and 5th digits are the engine size. The 6th and 7th digits are the day the engine was built (01-31). It may not be exact, but if it is close to the build date of the vehicle itself, chances are the engine is original. And never let a vehicle hear how well it runs. Unless you stroke it softly as you read it poetry by candle light.

 

4.0 Engine date codes

The date code is on the passenger side of the engine, on the block, just forward and up from the distributor on a machined flat surface.

The digits of the code identify:
 1st Digit-The year (8 = 1998).
 2nd & 3rd Digits-The month (01 - 12).
 4th & 5th Digits-The engine type/fuel system/compression ratio (MX = A 4.0 Liter (242 CID) 8.7:1 compression ratio engine with a multi-point fuel injection system).
 6th & 7th Digits-The day of engine build (01 - 31).
1. FOR EXAMPLE: Code * 801MX12 * identifies a 4.0 Liter (242 CID) engine with a multi-point fuel injection system, 8.7:1 compression ratio and built on January 12, 1998.

 

My code is 705MX06 so March 6 1997 and the build date is 5-97. So I believe it is original.

 

I don't even whisper it to myself in the shower...