3.4 DOHC crate engine buildup for 87 Fiero.

[Aaron]

I had this engine sitting around for the last 5-10 years, and finally got around to selling it. A local guy bought the engine, and is paying me to dress it and prep it for the swap too.

The engine is a brand new, never run, 91-93 GM crate engine. I've installed 2-3 of these before, and couldn't be happier. The best running 3.4s I've ever played with.




Spent 3 hours at the junkyard today. If you haven't had the pleasure of working on a 3.4 DOHC W-body before, they are a royal pain in the ass. You don't even want to know the procedure for getting the rear exhaust manifold off, and out. Here is the "new" intale manifold, exhaust manifolds/crossover, water pump pulley, and the water pump inlet. These are from a 1994 Lumina. Unfortunately it was the only 3.4 car in the junkyard, as I still need a bunch of parts (Including a wiring harness). I'm going to try a couple Denver junkyards later this week. The good part is the parts are in very good shape, nothing broken and not much rust at all. With the help of generous amounts of WD40, every exhaust stud came off relatively easily (A shock, as they usually break 50%).





To order: Thermostat, oil pressure sender, intake manifold gaskets, plugs/wires, A/C bypass pulley, wiring harness, starter, coil pack bracket, IAT sensor, and some other stuff I can't think of I'm sure.

[The Dark Side of Will]

I have a set of the AIR exhaust manifolds, if he needs those for emissions purposes.

[Aaron]

Thanks for the offer, but we don't have emissions here. I was actually happy that the ones I found were non-AIR, means easier less cluttered install.

[crzyone]

Should be a minty fresh install. I love crate engines.

[Aaron]

I cannot say enough good things about these crate engines. It's so nice to work with a perfectly clean engine, and one you know doesn't have any stripped or cross-threaded bores. Anyone who has ever worked on a 3.4 knows how easily some of the bores get cross-threaded, specifically the lower intake manifold bolts. Love not having to worry about these things. And, the crate engines have been the best-running 3.4s I've ever been around. Every one has idled better than any 3.4 I've worked on, they've run smoother, and made great power numbers. I dynoed a bone stock Lumina Z34 with a crate engine at 180whp. Although this is right at 210bhp (Rated output), I was still surprised and quite happy considering 170 was not an uncommon dyno on the forums, and most guys only made 180 with intake/exhaust/chips. I spent $2300 rebuilding my first ever 3.4 DOHC, and although it ran perfectly, the crate motors cost less than half, and run perfectly as well. I don't think my buyer understands how good of an engine he's getting, and how good of a price he's getting it at. I wish I had had the sense to look for a crate motor when I blew up my first 3.4.

Today I drove up to Denver, and visited two junkyards, and left with a bloody anus and not much to show for it. I was hoping to get an ECU, wiring harness, and a few small parts. Only got the small parts. I still need a coil pack bracket, 91-93 ECU/PROM, oil pressure sender, and a 91-93 un-molested wiring harness. I did get some work done to the engine, but I'm overall disappointed with part availability. Today I put on the fuel rail/injectors, lower intake manifold, exhaust manifolds (Rear one with the EGR port welded closed), and the gear-redution starter (Love these things, half the weight and size of the factory DOHC starters, and they spin faster).

Interesting note about the 3.4s. There is a bore directly above the oil filter. I figured this was for the oil pressure sender, but after looking at it fairly closely, it only appears to be a bolt recess, not an oiled port. After thinking about it, all of my 3.4s have had the water/oil cooler previously, and that sandwich cooler has its own port for the pressure sender. I'm not sure if I need the sandwich cooler in order to get the oil pressure reading, or if my eyes couldn't see well enough and that port is oiled. Anybody know?

[Aaron]

Another helpful thing about this 3.4 in a Fiero, the cooling system. Most 3.4s have two coolant outlets from near the throttle body, one heater core feed, the other a heater core bypass. Well some have another coolant line exit by the oil filter to run to the sandwich oil/water cooler, after the cooler the hose runs up to the inlet I installed in the last picture of my first post. The inlet I picked up from the junkyard didn't have an oil cooler, and neither will this engine (Unless by some miracle I find one, not likely). Anyway, in the W-body the heater core returns to the nipple right above the inlet, again reference last pic of my first post. Well my crate motor came with this nipple capped off instead. Which works perfectly since the Fiero heater core return is already taken care of.

[The Dark Side of Will]

You mean that the heater core return T's into the right hand coolant pipe? That's only the case in '87 and '88 cars.

My Northstar '87 didn't give me heat in the winter until the main thermostat opened. I switched to the '85-'86 heater pipes and right hand coolant pipe so that the heater circuit was completely independent of the main coolant circuit. Now I have heat in 1/4 the time it used to take.

[crzyone]

I could see how that would help the engine in cold climates. Let's it warm up faster instead of the heater core taking a bunch of the heat away. Sucks if you need to go anywhere in a hurry but the engine will be happier.

[The Dark Side of Will]

The Northstar has a recirculating thermostat, so there's essentially no circulation through the main coolant circuit and the entire flow of the waterpump goes back into the engine, EXCEPT what goes through the heater core circuit, which bypasses the thermostat.

It really doesn't affect how long it takes the engine to warm up, and the occupant(s) are much happier with heat sooner.

[Aaron]

You mean that the heater core return T's into the right hand coolant pipe? That's only the case in '87 and '88 cars.

My Northstar '87 didn't give me heat in the winter until the main thermostat opened. I switched to the '85-'86 heater pipes and right hand coolant pipe so that the heater circuit was completely independent of the main coolant circuit. Now I have heat in 1/4 the time it used to take.

Ya that's what I meant, had no idea that was 87-88 specific, my customer's car is an 87 so that works.

It makes sense as the coolant from the heater core is still warm even after the core, but then gets mixed with a large portion of cold coolant before going to the engine again. I'm not going to do that on this build, it'd add additional work for the customer as he'd have to switch to the early pipes, and it's one more hose he'll have to run. If he wants me to I could, I'd just add a nipple for coolant return where the crate motor has it blocked off.

The 3.4 has two routes for coolant to travel if the T-stat is closed. First is the heater core, and second is just a bypass. The bypass is there in case the T-state is closed, but the engine's RPM goes high, it prevents the system from over-pressurizing the heater core.

I have guy over on the W-body forums who says he has everything I need, the coil pack bracket, ECU/harness, and the oil cooler. I'm not sure if I'll add the oil cooler, it'll end up costing my customer an extra $60-70, but that's his decision.

[The Dark Side of Will]

The 3.4 has two routes for coolant to travel if the T-stat is closed. First is the heater core, and second is just a bypass. The bypass is there in case the T-state is closed, but the engine's RPM goes high, it prevents the system from over-pressurizing the heater core.

If the heater return circuit is capped, then you only have the bypass.

The 3.4 TDC has a SBC style thermostat, right? That is, the waterpump simply pushes against a closed thermostat (with a small bypass hole through it to get warm water to the thermostat more quickly) until the thermostat opens.

In a modern recirculating thermostat, the thermostat is a diverter valve. When "closed" it sends the water that the pump moves back into the block. This assures that the block warms up more evenly than with the older style thermostats. As it "opens" the fraction of coolant that is recirculated drops from 100% to about 75%, with up to 25% going to the radiator (at least I think those are the numbers I read... it's been a while). However, the heater core bypasses this thermostat entirely, circulating all the time.

I thought that the 60 degree V6 didn't get a recirculating system until the VVT 3500 and 3900 were built.

[Aaron]

If the heater return circuit is capped, then you only have the bypass.

The 3.4 TDC has a SBC style thermostat, right? That is, the waterpump simply pushes against a closed thermostat (with a small bypass hole through it to get warm water to the thermostat more quickly) until the thermostat opens.

In a modern recirculating thermostat, the thermostat is a diverter valve. When "closed" it sends the water that the pump moves back into the block. This assures that the block warms up more evenly than with the older style thermostats. As it "opens" the fraction of coolant that is recirculated drops from 100% to about 75%, with up to 25% going to the radiator (at least I think those are the numbers I read... it's been a while). However, the heater core bypasses this thermostat entirely, circulating all the time.

I thought that the 60 degree V6 didn't get a recirculating system until the VVT 3500 and 3900 were built.

The engine's port for the heater core return is capped, but the Fiero still has its heater core return underneath the body, so coolant will still flow through both routes.

The 3.4 does have a SBC style T-stat. But when the T-stat is closed, water can and still does flow through the bypass and the heater core.

[Series8217]

AFAIK, the 91-95 LQ1 has a modern recirculating system. When the thermostat opens it closes the bypass passage. Aaron, can you take pics through the thermostat hole in the LIM before you put it together?

[Aaron]

Huge progress today. I went to the store and bought an idler pulley to bypass the power steering pump. I also put it on. I know, I'm seriously God's gift to efficiency. Still waiting on parts, which should come in tomorrow.

AFAIK, the 91-95 LQ1 has a modern recirculating system. When the thermostat opens it closes the bypass passage. Aaron, can you take pics through the thermostat hole in the LIM before you put it together?

I completely forgot, but you've corrected me before on this. I'll get pics tomorrow, but I examined it closely and here is how it works.

The heater core is fed coolant from the engine 100% of the time, from the port directly beside the T-stat.

The top port is the bypass. Coolant goes from the coolant "plenum" up around the throttle body, and then around the engine back to the water pump inlet. This port is fed exclusively from a passage directly behind the T-stat, and the T-stat does block it. So as the T-stat opens and closes it controls whether coolant flows up through the throttle body and the bypass. I'm not sure why this is though, and when it's open vs closed, I can't think of a reason, can someone explain that?

After thinking about it, I think I've answered my own questions. If the coolant is hot enough that the T-stat is open, bypassing the coolant could be extremely dangerous as it bypasses the radiator and goes right back into the engine. Serious overheating potential there. So when T-stat is closed, coolant bypasses to both keep throttle body from freezing, and protect the heater core for high RPM operation. When T-stat is open, then there's no need to bypass as the main coolant lines provide the heater core protection, and bypassing could overheat the motor, and it's very unlikely the throttle body can freeze when the coolant is 200+*. Makes perfect sense now.

Except for the engines with the oil cooler. Water comes from the engine block (NOT the heads), through the oil cooler, then right back to the water pump inlet. AFAIK, this is done 100% of the time, not controlled. Now the majority of engine temperature does come from the cylinder heads, but that's not to say none comes from the block, especially when the oil is heated past the coolant temperate. My concern would be coolant circulating the block, heating up, gong through the oil cooler where it heats up even more, and then is delivered right to the water pump and sent into the engine again? Seems dangerous, however it is a relatively small amount of coolant when compared with the entire system.

[Series8217]

Interestingly, the 96-97 DOHC cooling system doesn't close the bypass when the thermostat opens. There's just a restrictor in the bypass line.

[The Dark Side of Will]

I don't think the bypass limits pressure to the heater core... the waterpump just doesn't make much pressure. Also, the heater core is good for at least whatever PSI pressure cap GM put on the system originally.

SBC's have a small hole in the thermostat so that water flows through the thermostat at all times. If no water flowed through the thermostat when the engine was cold, the engine would heat up and overheat long before simple conduction warmed the thermostat enough to open. It sounds from the description like that's the function of the TDC/DOHC thermostat bypass, especially if it closes when the thermostat opens.

Still doesn't sound like a recirculating cooling system.

[Series8217]

I don't think the bypass limits pressure to the heater core... the waterpump just doesn't make much pressure.

It makes enough to crush 2.8 thermostats, and I've had more than one new heater core fail like Aaron, with the bypass blocked off.

[Aaron]

Ya I've been through 2 heater cores, but finally got my 3.4 turbo setup to where it won't blow them again. It's got the oil cooler, a coolant line to the turbo, and the heater core, all on separate pipes, so I don't see it blowing another.

A little bit of an update today, but still waiting on parts. Got the spark plugs in, as well as the T-stat and bypass line on.

Looking into coolant plenum, down to the rear cylinder head (Heater core is fed from the top right of the photo, directly with coolant from the rear head):



Looking into coolant plenum directly. The machined hole we can see is the only feed for the bypass line, it goes up into the intake manifold, across the throttle body, then down, around the engine, back to in front of the water pump:




Looking down into the bypass port, you can see the raised parts of the plenum where coolant goes to the left, then down, then to the picture above:



Today's progress:

[Aaron]

I finally sruck a deal and got the rest of the parts on the way. Wiring harness, ECU, oil pressure sending unit, and the ICM bracket.

[The Dark Side of Will]

It makes enough to crush 2.8 thermostats, and I've had more than one new heater core fail like Aaron, with the bypass blocked off.

Ya I've been through 2 heater cores,

Weird. I've never seen either of those things happen.