straight forward questions

Want to know how to get more out of your Beretta? Or have a mod you would like to share?
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3X00-Modified
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Re: straight forward questions

Postby 3X00-Modified » Mon May 20, 2013 11:58 am

woody90gtz wrote:I think that's an electrical problem anyway, so swapping a 3400 would not help.


x2 sounds like another member who's 3.1 wouldn't run and he tried a 3400 and that wouldn't work either.

The mechanical operation of the engine is quite simple... It only controls the cam so if you do a compression and leak down test and find no issues there and don't see any intake vac leaks then swapping motors is just a lot of work to find out you have an electrical gremlin.



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Re: straight forward questions

Postby Rettax3 » Sun May 26, 2013 1:08 pm

Tylerlp wrote:What do i need for my 91 3.1 5 speed to swap to a 3400? couldnt find a definite answer for how to actually make the custom front mount so how is it done? what are the measurements? i also have a bar that goes from one k frame to the other so is that going to be in the way? And last but not least can i use a 96 obd2 harness and just not plug in the abs stuff and the trans stuff?


Dang, I wish I had just made two or three front engine-mounts when I built the one for my '90 LG5 turbo 'Retta upon upgrading the block to a 3400... The one from the link is really nice, if you can actually build it (correctly), but what a beast! How many pieces go together on that jigsaw puzzle? Mine has two...
Plus the two nuts I welded in place for ease of installation.
'Retta Ft Eng Mt.jpg
Front Engine Mount in my '90 'Retta, using a 3400 block.

Anyway, there are really a lot of ways you can do this. If you are trying to avoid a serious electrical gremlin in your 3.1 MPFI system, then consider swapping-out to a 3400 PCM piggy-backed in your car to run the engine. I've done that for both my '89 GTU running the 3800 SC, and the '90 running a 3400/3.1 LG5 hybrid turbo, and an '86 Fiero GT now running a 3100. All three cars operate fine (except the Fiero is having a tranny issue, and needs a five-speed swap anyway) -I do have slight idle issues after heavy engine-compression-braking during deceleration on them because none of my piggy-backed PCMs receive a VSS signal -that runs straight into the car's original ECM under the dash for the GTU, directly to the speedometer in the case of the Fiero, and my turbo 'Retta has the wrong VSS because the Muncie 282 five-speed in it came from a Z-24 Cavalier and needs to be run like the Fiero. You wouldn't necessarily have to do it that way yourself, but the issues are very minor and barely noticeable anyway. Aside from the connections to the engine, the 3400 PCM only requires battery voltage, ignition voltage, ground, and a few odds-and-ends, depending on things like which wiring harness you want to control the fuel-pump, the 3400's or the car's.

On my '90 turbo, I decided to have the 3400 PCM run the A/C (untested, I don't use it anyway, but it should work if I ever charge up the system and plug it in), which required a swap to a three-pin A/C pressure-sensor instead of the Beretta's two two-pin sensors. I let the car's original harness run the fuel-pump, so I used the OPSU (Oil Pressure Sending Unit) plug from the car's harness and left the 3400's plug unused. IIRC, I did the same on the GTU. I did the opposite on the Fiero, and used the 3100's pump relay in the fuse-block to run the pump by splicing that in.

On all three cars, I hooked up an OBD2 connector just so I could interface with the PCM, it has a hot, a ground, and two connections running to the PCM.

I also ran a tach wire from the ICM into the '90 'Retta's ICM harness, but otherwise the 'Retta's ICM harness is not used. The tach in the Fiero is not compatible with the 3100's ICM output (I have a Beretta digital instruments swap planned for that car anyway, so I don't really care).

Also on the '90, I am using the 3400 PCM to run the cooling fans, but using the car's original fan relay spliced in -I might be changing that this summer just because the wiring in my '90 is in very rough shape.

You might have to play around a little with the alternator's wiring, but it shouldn't be very difficult.

Because all three of my engines were originally sourced from automatic cars, there are a few connections that have to be grounded from the PCM so it thinks it is in gear, otherwise you will have a much lower rev-limiter (roughly 4k RPM, depending on the engine), rather than 6k+.

I originally had a '99 Grand Am GT PCM running the 3.4 hybrid in my '90 turbo, but had growing problems with the anti-theft system and the secondary computer necessary to disengage it so the car was becoming unreliable, so I switched it out for a Venture PCM without anti-theft, I highly recommend going that route.

These computers are designed to reside under the hood, so they can take the elements if you find it easier to fit it in with the engine, that gives you easier placement options.

Lastly, if the bar going across your subframes wasn't in the way on a 3.1, it likely won't be in the way on a 3x00 (the oil-pans are a little larger on a 3x00) -if it is, remove it.

As a side note, the list of parts you swapped doesn't mention the fuel pressure regulator. Honestly, it sounds more like a fuel issue than ECM or spark (although I don't have enough info to be certain), so consider buying/borrowing a fuel-pressure tester and making sure you have the right pressure before going any further. I don't mean to sound overly critical, but throwing parts at an engine isn't a very efficient diagnostic procedure, though it does get your engine a lot of new parts, I guess. :pardon:


1989 Super Charged 3800 Series II (First)Six-Speed Conversion GTU
1990 Turbo 3.4 Five-Speed T-Type
1990 4.0L 4-Cam 32-Valve V-8 Five-Speed Indy GTi (Rebuild Project)
1990 Stock(!) 3.1 MPFI Auto Indy
1995 3400 Top-End Auto OD Z-26


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