: Engine year compatibility

07-16-09, 02:03 PM
I've got a 1999 Seville STS with the typical head gasket problem (91000 miles). I found a mechanic who has access to a 2003 Northstar (33000 miles) engine that he says will work in my car. I thought I read on this forum that the 2003 engine won't work in a 1999. I that true? If there is some modification needed, what's involved?

07-16-09, 02:26 PM
It's possible, but not easy. Some others will chime in. I think the consensus is that it's not simple enough to really be feasible. You don't say if there's anything wrong with the '99, but if it's just head gaskets, just repair your '99. the hard and tedious part if the R and R of the engine/trans/steering rack/cradle. The actual repair is pretty simple and not strenuous. Here is my HG thread - it will give you some idea of what you are in for:


07-16-09, 03:09 PM
Can you tell me exactly what doesn't fit? I know there were some internal changes to the later engines, but what won't connect or bolt up? The mechanic says no problem but naturally I won't just take his word for it.


Mark C
07-16-09, 04:47 PM
Cutouts on the crank timing wheels have different spacings for the hall effect crank position sensors on the 2000 and up vs the 99 and down, you will never get your old PCM to accept the signals coming from the sensors as being valid. You would have to change your old crank out and fit it to the new block. Other than that the heads are different, the ignition system is different, the bolt holes for the head bolts are different. Don't even know if you could use 2000+ headbolts and your old heads if you could get around the hall effect sensor issues with the crank.

Your stuck using a 98/99 block as the 97 and earlier blocks have different motor mounts and the earlier blocks are not tapped for the later motot mount bolt holes.

07-16-09, 05:03 PM

.............and those two posts ^^^ are only the tip of the iceberg. 2000 saw redesign in almost every aspect - ignition, mechanicals, PCM programming, wiring and connectors, drivetrain controls (transmission and suspension), and so on.

The later engine will fit, but to make it run is an entirely different problem.

07-16-09, 06:22 PM
What was he going to charge you for the '03 including labor? Your probably better off studing the one you have. http://www.northstarperformance.com

07-16-09, 07:52 PM
Many of us have repaired engines with 100K+ miles on them and gotten many additional miles of life from their car. No machiining of engine parts or replacement hard parts are usually needed - just gaskets and seals.

Also, if you read my thread, you noticed that i replaced my first engine with a used one, and that one experienced the same problem 23 months later. I would have repaired the first engine had I known what I have learned here. You have a huge advantage of the knowledge base here.

07-16-09, 10:00 PM
A Northstar is good for at least 250,000 miles with reasonable care. So doing the HG's at 100K, if the rest of the engine is good, is the better option.

I do not think the mechanic realizes how many changes were made to the block and electronics in 2000.

07-17-09, 01:11 PM
Listen to Zonie - a lot of what I learned was from his threads and posts. He has done this procedure more than once, and he does know what he is talking about!