I did not do a detailed writeup with pictures, but here are the basics:
I retained the springs from the 2.5 with no issues. The GT I believe has higher rate springs as it has the sport suspension. Here are the items to address:
• You need the fuel pump and throttle cable from a 2.8L
• The left side for/aft coolant tube on the 2.5 is about 6” longer on the 2.5. You can cut it down, but remember the flare is no longer there, so double clamp it. I changed to the 2.8 coolant tube because I had one. It is a lot of work to snake this tube around the front suspension.
• The for/aft heater pipe that connects to the thermostat housing through a molded hose is different for the 2.8. I changed mine, but you don’t have to…just use more heater hose.
• There is a heater line that goes into the water pump on the 85-86 2.8 (hard line from the firewall, along the frame rails and to the back of the engine compartment). A hose connects this line to the water pump. You will have to replumb it from the 2.5 to the 2.8 (this line is eliminated on the 87-88 cars because the water return is plumbed into one of the for/aft coolant tubes).
• You need the dogbone from a 2.8…it is shorter
• You need an air filter housing, plastic elbow that is below it, and rubber boot that goes between the throttle body and air filter. It is a very tight fit around the shift cables if you use the Isuzu 5 speed. I made a new boot, but at least one person has used the stock 2.8 air intake boot.
• You need the engine harness from the 2.8+ECM+ECM mounting bracket (the 2.5 and 2.8 ECM have mounting screws in different places).
• I swapped in the engine bay cooling blower, and the rear wiring harness to power the blower. The blower cools the distributor and alternator. It was eliminated in the 1988 model and many people say it is not necessary.
• The 2.8 harness plugged in to the 2.5 harness and all worked with no issues. Harnesses are slightly different between AC/non AC, 4 sp/5 sp/auto. Depending on what configuration you have, there are wiring differences relating to clutch start lockout, auto/manual trans, AC etc. As far as I know, the locations for all wires are the same on the main connectors, but the wires may be omitted for some configurations. If you do not have access to wiring diagrams in a shop manual, try gaining access to the ALLDATA database, which seems to have the same information as a GM service manual. I have access to it at my local library. Sorry, you have to go to my library as it is not available on the Internet. I had to swap in the main harness from my parts car, as the fuse panel was badly corroded.
• The motor mount is the same, but I put in a new one, as well as new transmission mounts.
• I replaced all engine seals, the timing chain (the engine had 90K on it), the water pump (cast rather than plastic impeller) and had the injectors cleaned. There was a post somewhere on PFF on cheap but good places for injector cleaning. The car does not leak coolant or oil at all; a good investment while replacement was cheap and easy.
• I think the AC brackets are different; my coupe did not have AC, but I swapped it in from my parts car.
• I used the alternator from the 2.8 w/AC which is larger
• I also swapped in the AC, but the main wiring harness for AC controls is different. In the end, I had to swap the main harness only because the fuse panel was badly corroded on my otherwise rust free car. I was glad I had a parts car.
• I installed a new Fiero Store HD RAM clutch for the 2.8 with the Isuzu throw out bearing. You must match the throwout bearing to the transmission. This clutch works very smoothly and with low effort. A great clutch. The Fiero store told me to use a 2.5 clutch. While this would work, it is smaller in diameter and would wear out faster.
• I used the flywheel that came with the engine; 85-87 2.8 engines require balance weights on the flywheel. 88 engines are neutrally balanced. Putting the wrong flywheel on an engine would lead to early failure.
• I think the later year fuel lines are the same 2.5 and 2.8, but earlier are different (not sure)
• I installed the exhaust system from the 2.8. I would replace all of the exhaust manifold bolts with high strength (9.8) studs. My engine had three snapped off and I had to drill one out. It is much easier to do this with the engine out. Also check the exhaust manifolds for cracks. The one on my parts car (that had a blown engine) was cracked on the cylinder where the piston had a hole though it.
• I also swapped in cruise control and it works!
• The backup lights do not work and I have not yet had a chance to look at why.
Hope I did not forget anything.
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