4.3 Mercruiser 94 vs 98 interchangeability

[Ozzzy]

Hey all,

So my 94 4.3 is getting tired. It runs closed cooling system. I have scored a 98 engine that I’m looking to rebuild while still being able to use the motor in the boat. The 98 is an 8 bolt intake vs the 12 bolt in the 94, and from what I can tell doesn’t run the closed loop cooling system. What can I do/ what do I need to do to make the 98 engine work?

Thanks in advance.

 

[achris]

Hi Ozzy, and welcome aboard.

... and from what I can tell doesn’t run the closed loop cooling system.

Says who? (ie, that's incorrect)

What can I do/ what do I need to do to make the 98 engine work?

You're going to need to change a LOT of stuff for the 98 to utilize all the 94 gear. Mid 96 was when a lot of changes occurred to the V6, and most of the bolt-ons on the 94 will not fit the 98 without some 'creativity'. As you have already seen, the 94 is non-vortec, and the 98 is vortec, so anything heads, manifold and fuel system isn't going to travel across.

If you're willing to convert the 98 back to 94 type spec, it can be done, like using Vee-belts instead of a serpentine belt. That solves a few problems straight up. And using the 94 thermostat housing will allow you to use the 94 closed cooling system... Your biggest issue is going to be ignition. Specifically the timing curve. The 94 is running Thunderbolt IV, and Merc never made a TB-IV module for the vortec V6. You CAN run the TB-IV system in the 98, but you'll not be getting the full benefit from the engine engine. I would suggest you update the module to the TB-V (uses the same distributor and sensor. It's just a new module that you'd need)...

Most of the other stuff should bolt on. If the 98 is out of a car then I would suggest you need to change the camshaft too (the one in the 94 is the same as Merc used in the Vortec blocks, so you could use that one), and the CPS (Crankshaft position sensor) will need to come out and a blanking plug be fitted (or leave the sensor in and just not connect it).

Chris.......

 

[Ozzzy]

Thanks Chris.

So can I use the 8 bolt manifold and put my 94 carb set up on the 98 manifold? Would the heads off the 94 bolt onto the 98 engine? That’s assuming my 94 heads are ok..no issues to convert back to making the 98 into 94 spec, it’s been plenty fine for the last 5+ years I’ve owned the boat, just that it’s well tired now, and has done a lot of hours. The 98 motor is originally a boat motor so cam should be all ok, did read about that somewhere.

Of interest, the 94 motor in the boat now does have V6 Vortec on the rocker covers, but I have looked up the serial numbers and confirmed it’s definitely a 94 as thought. How do you think this would have come about?

Thanks heaps for the above.

Cheers

 

[achris]

Thanks Chris.

So can I use the 8 bolt manifold and put my 94 carb set up on the 98 manifold?

That should work...

Would the heads off the 94 bolt onto the 98 engine? That’s assuming my 94 heads are ok..no issues to convert back to making the 98 into 94 spec,

You can, but Why???? Those vortec heads are much much better than the non-vortec.

Of interest, the 94 motor in the boat now does have V6 Vortec on the rocker covers, but I have looked up the serial numbers and confirmed it’s definitely a 94 as thought. How do you think this would have come about?

Marketing. (I also had a 1994 4.3LX, and it also had 'Vortec' on the rocker covers, and the heads weren't vortec. )

Chris......

 

[Ozzzy]

Perfect! The carb was something playing on my mind as the linkage setups are on the opposite side so wasn’t sure how to attack that, that was my main concern. I knew I could swap the alt, power steering etc over with no issues. The harmonic balancer on the 98 is buggered, so was swapping that over anyway. The 98 is stripped, and will be spinning the crank up as previous owner noted the balancer was spinning out of round, so before I replace bearings etc I’m just going to check that’s running true, and if all good will get new bearings etc and heads cleaned up and will fire it back together.

Didn’t know the vortec heads were any different. I haven’t really looked into these motors much until now as mine has always gone really strong.

Do you know if the TB 4 and 5 are unplug/plug in, or will I have to rewire to suit the TB5 module?

Side note, got the 98 for a couple box’s to fill ol mates fridge, it has near new risers and manifolds on it, so am onto a win straight away!

Thanks for your help Chris, bloody awesome.

 

[achris]

Do you know if the TB 4 and 5 are unplug/plug in, or will I have to rewire to suit the TB5 module?

If you use the part number of the old TB-IV system, it will supersede to a kit that give you all the wiring and connectors. (The kit you're looking for is Merc # -115247T02)

Side note, got the 98 for a couple box’s to fill ol mates fridge, it has near new risers and manifolds on it, so am onto a win straight away!

Got any more mates like that?

Chris......

 

[Scott06]

I knew I could swap the alt, power steering etc over with no issues. The harmonic balancer on the 98 is buggered, so was swapping that over anyway.

you may want to verify the balancer can swap. I belive they have different part numbers between the 94 and 98 balancer

 

[achris]

I belive they have different part numbers between the 94 and 98 balancer.

They do, but only because of the belt system. 98 and 94 use the same crank, rods, pistons and flywheel... Balancer will be interchangeable.. but you may need to remove the front cover to take the reluctor wheel off. The snout on the 98 will be shorter than the 94, and that's why.

Chris...

 

[Lou C]

The Vortec heads are supposed to give you approx 25 more hp. When I replaced the heads on mine (first time for me taking apart an engine) I would have liked to go with Vortec heads but wasn’t sure there wouldn’t be complications with the valve train etc; since there are also other differences (pre Vortec has adjustable Vortec is non adjustable) so I went back with the pre Vortec heads & my original 12 bolt intake. Would have been nice to get 25 extra HP!

 

[nola mike]

Your biggest issue is going to be ignition. Specifically the timing curve. The 94 is running Thunderbolt IV, and Merc never made a TB-IV module for the vortec V6. You CAN run the TB-IV system in the 98, but you'll not be getting the full benefit from the engine engine. I would suggest you update the module to the TB-V (uses the same distributor and sensor. It's just a new module that you'd need)...

Chris.......

Hmm. Never gave this much thought when I swapped mine. I was under the impression that the ignition curves were similar between the TB IV and V (with the exception of some pre-programmed "routines" in the V). I thought that most of the special sauce in the V was due to advancing the ignition based on knock sensor input. Without the KS, I didn't think the V offered a huge benefit.

 

[achris]

Hmm. Never gave this much thought when I swapped mine. I was under the impression that the ignition curves were similar between the TB IV and V (with the exception of some pre-programmed "routines" in the V). I thought that most of the special sauce in the V was due to advancing the ignition based on knock sensor input. Without the KS, I didn't think the V offered a huge benefit.

Even without the KS, V offers plenty over IV...

 

[Scott Danforth]

easy method: unbolt complete 1994 bobtail. bolt in complete 1998 bobtail

where did the 1998 motor come from?

some light reading: https://forums.iboats.com/threads/complete-history-of-the-4-3-liter-with-casting-numbers.693935/

 

[achris]

some light reading: https://forums.iboats.com/threads/complete-history-of-the-4-3-liter-with-casting-numbers.693935/

Ah, the ultimate cure for insomnia....

 

[Ozzzy]

Hi Chris,

As I have more thoughts about this, do you know if there is a Diesel engine that will drop in with similar size to the 4.3 and that will work with the alpha one leg that I can look at by any chance?

Thanks

 

[Scott Danforth]

Hi Chris,

As I have more thoughts about this, do you know if there is a Diesel engine that will drop in with similar size to the 4.3 and that will work with the alpha one leg that I can look at by any chance?

Thanks

Mercruiser never paired a diesel with the alpha drive. Even the 1.7 diesel was paired with a bravo