4.0 Block Compatibility???

Can I swap the block from a '99 XJ into a '91???

Do you mean "block," or do you mean "engine"?

The block is fully compatible and can be swapped.

The head is different. The ports may be shaped or located slightly differently, which could cause some difficulty in mating up with your manifolds. If you keep the '99 manifolds on the '99 engine, you just get to deal with sensor issues. The '99 doesn't use a temp sender in the head for the gauges -- the gauges in the '99 get data from a central data bus (the PCM, in other words). Your '91 has a temp sender in the left rear corner of the head for the gauges.

Also, externals like power steering pump attach differently on the '99 engine than on the '91.

Eagle said:

Do you mean "block," or do you mean "engine"?

The block is fully compatible and can be swapped.

The head is different. The ports may be shaped or located slightly differently, which could cause some difficulty in mating up with your manifolds. If you keep the '99 manifolds on the '99 engine, you just get to deal with sensor issues. The '99 doesn't use a temp sender in the head for the gauges -- the gauges in the '99 get data from a central data bus (the PCM, in other words). Your '91 has a temp sender in the left rear corner of the head for the gauges.

Also, externals like power steering pump attach differently on the '99 engine than on the '91.

Click to expand...

Block...

I'm thinking of using a '99 block and intake manifold, and pretty much everything else out of my '91 (head, accessories, etc)...

Take a close look. I believe if you use the '99 intake you will have to buy a '99 power steering pump and mounting bracket.

Eagle said:

Take a close look. I believe if you use the '99 intake you will have to buy a '99 power steering pump and mounting bracket.

Click to expand...

Thanks for the heads up...

I have the chance to get the complete '99 engine, so I might do that just to be on the safe side. Were there any other accessories that were improved over the '91? I thought I had heard that the water pump and alternator were improved....but I could have been mistaken...

The water pump was improved, but I'm not sure at what year. The '03 block I used had the improved water pump. The '00 block I have doesn't. Not sure on the alternator. I put a '95 head on the '03 block. Not to many problems. I had to file on the head as the dowel pins on the block prevented the head from aligning with the bolt holes. I think I'm the only one who has had that problem. The 96+ blocks and heads have dowl pins to locate them and the head gasket. Also, the 96+ head bolts are 12 point. two of them have location dowels for the new valve cover gasket.
Tom

Question...
I was thinking of throwing a new. like 2003 or so style 4.0l into my '91
These new motors have distributorless ignition and it just looks like theres a sensor where the distributor goes....

can I take out that sensor and throw in my distributor?

creeperjeep said:

Question...
I was thinking of throwing a new. like 2003 or so style 4.0l into my '91
These new motors have distributorless ignition and it just looks like theres a sensor where the distributor goes....

can I take out that sensor and throw in my distributor?

Click to expand...

Yes.

creeperjeep said:

Question...
I was thinking of throwing a new. like 2003 or so style 4.0l into my '91
These new motors have distributorless ignition and it just looks like theres a sensor where the distributor goes....

can I take out that sensor and throw in my distributor?

Click to expand...

I haven't put the distributor in my '03 block yet. The sensor looks the same with a gear. One problem I ran into with my '03 block, the engine brackets didn't match up. The one of the bosses on the intake/exhaust(driver) side stuck out to far. One on the distributor side didn't stick out far enough. THe block was out of a Grand Cherokee. I had the driver's side one cut and rewelded. I though of machining/cutting down the boss, but the possiblitity of messing up entered my mind. I usually go with the rule of modify the part that cost the least or is easiest to replace. I can replace the engine bracket fairly easy. On the distributor side I made a spacer. I think it was .920 in length, but not sure on that. I grabbed a standard spacer at work and cut it down on a lathe. My '03 block also has the oil sensor towards the rear, while the other blocks I have ('88, '95, '00) have it on the oil filter boss. Not really concerned about it.
Tom