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Personal Experiences
Why did I choose to go with the 3.4 DOHC? Well, I am 19, money is in short supply. Between the 3800 Series II Supercharged, 4.9 PFI, 3.4 DOHC, It was a tough decision. But in the end, I knew I wanted a motor that is unique. I took a look at the web site,, to price out a motor. I found several 3.4 DOHC�s in my area all priced at about $800, so I picked the best one out of those, the 1996 motor. Within a few days the motor was in front of my house, just as I was dreaming it would one day be. And so the decision was finalized. Immediately I bought a timing belt and put it on, I checked a few things out, opened the cam carrier covers to look at the cams. All looked good. Project 3.4 DOHC is a go. This motor swap was only my second one, and was destined to go into my 85 2m4.

The car is in somewhat rough shape, but that�s OK since it is the car to experiment with. Originally the 2m4�s old 2.5 was ready to throw a rod, and was so bad I had to tow it home. Total cost for the car $0, not a bad place to start. The first motor I put into it was the 1989 Pontiac Grand Prix LE 3.1L. The donor car was totaled, when a pickup truck T-boned right into the passenger door. Total cost for donor car $0, Now you know why I put that engine in. The final cost was about $400, which included the $200 I spent to replace all the brakes with Beretta calipers, pads and rotors, on all fours. When I did the swap I put in the whole driveline, including the 4T60 transmission. After about 5 months and 8,000 happy miles I decided to retire the tired engine and slipping transmission, which had 133,000 miles already.

The 3.4 DOHC is basically the same engine as the 3.1 was, just slightly bigger pistons, and a pair of heads that flow like no other. Just so you know, The 3.4 pushrod, and the 3.4 DOHC, share very little other then the basic layout of the block. The crank may be the same, as well as the rods, but not much past that point. The heads are definitely not interchangeable. Anyway, when my 3.4 DOHC was delivered, it came with a clipped wiring harness, no alternator, and no A/C compressor. I had to buy an alternator ($130.00) but the AC compressor on the 3.1 fit perfectly, and with a little shaving of the compressor manifold, the stock 85 Fiero hoses fit right on. As for the wiring, I decided that the best route to go was to take the 3.1 wiring harness, and adapt it to the DOHC, since the Fiero was already wired for that harness. Some lengthening and shortening of wires was necessary, but in the end it worked out ok. I added a map sensor, and eliminated the Mass air, as well as disconnected the crank position sensor, and the cam position sensor. For the computer I originally planned to learn how to re-program the computer and just tune it myself. When the time came, I had to have this car on the road, so I spent $100 and bought a 93 3.4 DOHC computer with the chip. I didn�t know this, but figured it out soon enough, but the computer plugged right in, not one wire had to be moved. The engine runs "OK". The final total cost so far is about $1500 not including the first motor swap, or the brakes.

The decision on what transmission to use was a tough one, I had the 125C all ready to go, and the cradle was already modified to fit the 4T60, so I could have used either. But, since the 4T60 was slipping and needed a rebuild, I figured I�d just throw it in with the 125C. Either way, I was going to have to make it so a TV cable would hook up to the throttle. The transmission didn�t last long at all. The first time on the throttle, it slipped going into third. I wasn�t near floored. Then a few days later, and just the second full throttle run, 2nd went. so all I had was first, and a slipping third. I rebuilt another 125C for it with racing clutches and the whole 9 yards, It still blew the transmission in short order. But it still drives; I just have to go real easy on it when it shifts from first to third.

About the performance, I was fairly impressed with the amount of power it has, Its not punchy or very responsive at low RPM, but if you were to floor it from a standstill, you�ll smoke the tires. The 3.1 performed about the same at the bottom, just the 3.1 seemed more punchy and responsive at low RPM. As the engine hits around 3K, it begins to come out of hibernation, by 4K its at about its peak, and holds it well past 5K, but begins to loose its thrill above 5,800 RPM up to the 6500 rev limiter. If you ask me, the engine needs a 2 stage intake system. Gas mileage so far for me has been Horrible, to pathetic. Something isn�t right for sure, 12-15 MPG. It also pings a lot even on 94, but I�m sure that has a lot to do with it being so far out of tune. I just drive it real easy.

Maintenance, you wouldn�t believe how easy this motor is to maintain in a Fiero compared to how hard it is in the car it comes from (96 Grand Prix GTP). You may have paid your mechanic $1,000 and lost the car for a week just to change the timing belt every 30-60,000 miles. In the Fiero, it can be done in about an hour, It isn�t that hard, and no, the engine does not have to come out. Best bet in any case, when you get the motor, CHANGE THE BELT while its still out of the car. Another thing you may have heard about, the spark plugs. Anyone who says its really hard, or thinks you may need to drop the cradle, hasn't really worked on this install. Its easier then the stock Fiero V6 by FAR. and if you have a pre 96 DOHC, its even easier. On the 91-95 engines, The spark plugs in the rear are accessible without removing the intake plenum. On the 96 you have to remove it, but still isn�t much of a challenge. The front 3 plugs, are a little more difficult. I suggest you remove your dogbone or engine supports so that the engine is free to move around. Using a bar clamp (preferably quick clamp) you can pull the engine away from the firewall, then you just remove the decklid, from the hinge bolts on the decklid. Be sure to scribe alignment marks before your remove the decklid though. With this done, you should be able to pull the wires out and change the plugs. See? not so bad.

Reliability: Personally I have had a few problems, nothing too serious though. Other then blowing up 3, yes 3, 125C Automatic transmissions, I have blown 2 heater cores. I guess the pressure surge in the water when the engine hits 6500 RPM is enough to blow it. I have yet to be left stranded, so reliability has been pretty good.

Author: Matt AKA Fierobsessed

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