I am wondering if there is a way to find out if my engine has been replaced. I have a 91 Cherokee Laredo that I just picked up about 5 months ago. I am having some running issues, hesitation when cold. So I decided to start with simple things, plugs, wires, cap, and rotor. I got everything for a 91 4.0. When I got into it, the rotor was different. So I did a bit more research at work and picked up a cap and rotor for a 95 and they seem to be correct. Plugs and wires were the same.

I was looking at the head and block and I found a couple numbers cast into them. But I don't have a clue what they mean, if anything. On the top of the head above the intake mani is; 0630. On the driver's side of the block, towards the front is; NVH and just below that; 53020569. That last number may be an 8 too, hard to tell.

Am I looking in the wrong area or is there some meaning behind these numbers?

Thanks

 

looks like 96-98

http://www.google.com/search?q=years...hrome&ie=UTF-8

 

The engine Build Date Code is located on a machined surface on the right side of the cylinder block between the No.2 and No.3 cylinders (Build Date Code Location).
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.

 

Great info guys, I will check the passenger side of the block when I get home from work today. If I can figure this out, it may help me figure out my hesitation problem too.

 

Looks like mine says 604MX12...

So I have a 4.0 Liter with multi-point blah blah blah that was built April 12, 1996?

From the angle I was looking at it looked like it ended with I2, not 12 but after looking at this, I would assume it has to be 12?

 

The part number on my distributor is 56027028. Which from a quick google search shows, that was used on 91-97. I wonder why the cap and rotor listed for a 91 from Napa don't work on it then...

 

I haven't checked the part number on my ECM yet but do you guys think there would be an issue if it is still running the 91 ECM with the 96 engine?

 

No, if all the sensors were swapped over from the original engine to the one that's in there now it wont hurt anything, the ecu just thinks its running a 91. The mechanics of these engines don't change a whole lot and as long as the sensors are correct the run just fine.

 

Well that is good news, thanks. Now I just have to figure out why it runs like crap when cold...

 

The most important sensor in the whole system is the coolant temp sensor, it tells the computer how much fuel to dump in based on the temp of the engine. If the sensor is faulty, the computer might read it as being up to temp, causing it to run like crap when it's cold.

 

That was the first thing I checked. The PO had an aftermarket temp gauge and the sensor for the aftermarket one was installed where the stock sensor goes, on the t stat housing. I found the stock sensor still plugged in to the wiring harness but just laying between the fuel rail and the valve cover. I disconnected the aftermarket sensor and re installed the factory one and checked resistance when cold and as it warmed up. All seemed pretty damn close to the specifications for it. And made no difference in how it ran.

I also tried a good known MAP sensor, replaced the O2 sensor (crushed wires), and replaced the intake air temp sensor (resistance seemed off). Fuel pressure is right at specification with key on, and at idle, and while driving. Fuel pressure doesn't change when it is hesitating either...

 

So, it turns out the hesitation has something to do with the outside temperature. I had mentioned that once the jeep warms up the hesitation goes away, but that was when it was fairly warm out. I drove it yesterday when it was cold outside, and the hesitation didn't go away, even when at operating temperature.

I guess my next step is to borrow the DRB 3 from work and check to see what the computer is seeing.