Difficulty Index: Experienced - Special Tools Needed
The Tuned Port intake was a vast change from the previous induction systems offered for Corvette. It relied on a simple premise: Increasing the runner length would increase torque.
The L98 Tuned Port took this theory to the limit. With long runners on each side of the plenum that curve around to direct airflow into the main intake housing, the Tuned Port is one of the most recognizable intakes ever made. It worked quite well, too.
Unfortunately, GM was still sending coolant through the intake manifold, so the biggest problem is that the gaskets are showing their age with coolant leaks. Usually, leaks can be seen in the pockets between the head and intake on the rear passenger side of the engine. There are a few more wires or lines running to the intake than there are with a typical carbureted setup in the earlier generations. But with just a bit more information, swapping out the gaskets can be done the same way as any of the sans-electronic small-block predecessors.
With the car timed and tuned, it was ready for another decade and a half of trusty service. Now we can drive it without any coolant leaks, smells, or stains on the driveway. Right. Like it will spend that much time in the driveway!
The first step is to remove...
The first step is to remove the plenum from the runners. This includes the throttle body as well. At this point, it can be removed as one unit. You can also drain the coolant from the engine and unhook the battery cables.
Next, remove the distributor....
Next, remove the distributor. We suggest marking the plug wires before removing them, and mark the rotor's location inside the distributor so you can get the proper timing when you reinstall the distributor.
You won't need to remove them...
You won't need to remove them at this time, but disconnect the fuel lines to the injector rails (don't forget to relieve the fuel pressure first) and remove the bolts holding the rails in position. You'll have to wiggle the injector rails to remove the tubes from the intake.
Remove the runners.
The intake runners have 40...
The intake runners have 40 Torx-head bolts holding them from the outside and inside. If the runners don't budge, make sure all of the bolts are removed or you'll be shopping for another set of runners and/or intake.
Up to 1988, the L98 used a...
Up to 1988, the L98 used a cold-start injector to richen the fuel ratio, much like a choke on a carbureted engine would. This will have to be removed as well.
Now the fuel injector rails...
Now the fuel injector rails can come out.
Since the EGR valve is hidden...
Since the EGR valve is hidden under the plenum, this would be a great time to check it for proper operation with a vacuum gauge. If it will not hold vacuum or doesn't operate when vacuum is applied, replace the valve.
Clean up any coolant, crud,...
Clean up any coolant, crud, or foreign matter before you start loosening the bolts for the intake.
The intake bolts are T45s....
The intake bolts are T45s. Remove them and carefully pry the intake off the engine.
Put some clean shop rags in...
Put some clean shop rags in each of the intake runners in the heads, and lay down some rags in the valley area to prevent any debris from getting into the engine. Now you can safely clean the head and block mating surfaces in preparation for the new gasket. Once again, keeping foreign matter from entering the engine is essential. Also, this is a great time to clean up the intake and runners.
Tap all the holes and clean...
Tap all the holes and clean the threads on the bolts because all of the intake bolts will need to be coated with Teflon sealant; any old sealant left could result in an improper torque reading.
Make sure the sealing surfaces...
Make sure the sealing surfaces are clean. That includes the bottom side of the intake as well. Use a parts cleaner to remove any grease.
We used Fel-Pro gaskets (PN...
We used Fel-Pro gaskets (PN MS93318) for the intake. The kit does not come with a new EGR-valve gasket so be sure to get one. We used a silicone gasket from a tube for the intake/block mating surfaces. Be sure that hot oil does not affect the silicone. There is only a five-minute maximum window before the silicone skins over and will need to be redone, so don't put on the silicone until you have the intake ready to go back on.
Reinstall the intake bolts...
Reinstall the intake bolts with Teflon sealer. Be sure to torque down the bolts before the silicone cures, using the proper torque sequence.
Now the EGR valve and fuel-injector...
Now the EGR valve and fuel-injector rails can go back in place. Tighten the EGR valve but don't put in the bolts for the injector rails since you'll need to move them around a little to get the runners back into place. Also, using a little bit of lubricant such as oil or transmission assembly lube will help the O-rings slide into place without damage.
The runners were slid into...
The runners were slid into place, which was made easier by being able to move the injector rails.
There are different sizes...
There are different sizes of bolts for the intake, and each one has a specific use. There are six small bolts (two for the fuel-line mount and four for the EGR tube). The next size up are the seven bolts for the plenum to the runners; after that, there are two bolts for the passenger side in front of the runners and the driver side back of the runners. The two second-longest bolts bolt in from the inside for the runners (front passenger side and rear driver side), while the four longest bolts go in the lower center spot on each side of the runner.
This is the time to connect...
This is the time to connect the vacuum lines to the fuel-pressure regulator and the EGR valve. You can do it later if you forget, but it's much easier now.
Next, the distributor goes...
Next, the distributor goes back into place. This is where marking the position of the rotor and body of the distributor really pays off. Once installed, the wires and cap can be installed.
The plenum was reinstalled...
The plenum was reinstalled on the runners. Notice that the throttle body is not on the plenum. It will be inspected and cleaned before it goes back on.
We continued buckling up the...
We continued buckling up the front of the engine by installing the A/C compressor and belt.
Before installing the throttle...
Before installing the throttle body, we used some Permatex Aviation Form-A-Gasket sealer on the coolant lines to prevent any leaking. Then we bolted the throttle body in place using the proper gasket, and hooked up the required electrical connectors.
Once all was completed, we...
Once all was completed, we fired the engine and set the timing. Since we marked the distributor, it was close to right-on, but we used a timing light to make it perfect. GM recommends 6 degrees if initial timing for '85-'88 cars, but Chris sets them to 0-2 degrees. He says they run smoother and have better low-end torque. The '89 and newer L98s were designed to run with more timing. We also drained the oil and installed a new filter and oil.
Between the top and bottom...
Between the top and bottom bores of the throttle body, you can see the difference that cleaning the throttle body can make. Pay particular attention to the area around the idle circuit and EGR orifices where the buildup will be the most severe.
Coolant was next. When adding...
Coolant was next. When adding coolant, always check for leaks and be sure to close the drain at the bottom of the radiator.