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Thread: 3100 to 3800 Engine Swap

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    Senior Member Ray_McAvoy's Avatar
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    Question 3100 to 3800 Engine Swap

    My mom has a 94 Cutlass Cruiser with a tired (over 200,000 miles) 3100.

    I recently acquired a wrecked 95 Buick LeSabre with a nice low mileage 3800 that we're thinking of swapping into mom's Olds. I believe it's the L27 Series I version of the 3800.

    I think both cars have 4t60e transaxles and the plan is to use the entire drivetrain from the Buick due to the lower mileage. And that should allow the Buick's ECM to be used as-is without having to re-program it for a different final drive ratio. The LeSabre does have the passkey anti-theft system, but I've read that I can bypass that by building an oscillator circuit that'll feed a 50kHz square wave to the ECM's "fuel enable" line.

    My biggest concern is with the engine wiring harness. The LeSabre engine harness looks like everything goes through one hole on the passenger side of the firewall where it connects to the ECM and under-dash harness. I'm guessing the relays for the fuel pump, cooling fans, and such are all under the dash on the LeSabre too. In comparison, the A-body engine harness hooks to the bulkhead connector behind the driver side strut tower in addition to going through a hole in the passenger side of the firewall. And all the relays are out on a bracket along side the radiator.

    Anyway, it looks like it'll be a fair amount of work to modify the layout of the LeSabre's engine harness to make it fit the A-body. So I'm wondering if something like a 94/95 or so U-body van L27 engine harness might be more of a direct fit.



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    This can be done but you would have to study the schematics backward and forward and either splice or repin wires... Using the 94-95 U harness may make things easier... Then you will need a lot of the U brackets to make it all fit in the car... As well the 94-95 3800 U body did not use the Pass Key II system, so security is not turned on in the chip... This could easily solve your bypass issue...

    If you do this I will wish you the best of luck... It takes a bit of work to get to where you want to be... For a while I ran my 3800 Series III under Series II induction with the 95 U PCM that came in the van except I used a 1995 Bonneville chip with the Pass Key Flags turned off.... Later we updated to a 97 PCM...

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    Sounds like a fun project. I know someone with a '95 LeSabre, and that thing will GO!

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    Senior Member Ray_McAvoy's Avatar
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    Quote Originally Posted by Jr's3800 View Post
    This can be done but you would have to study the schematics backward and forward and either splice or repin wires... Using the 94-95 U harness may make things easier...
    Thanks Jr. That's what I figured ... that the LeSabre harness would involve lots of splicing/re-pinning. I do have full schematics for both cars as well as a spare 94 A-body 3100 engine harness that I can use for connectors/parts if I end up having to go that route.

    So am I correct in assuming that the U van engine harness uses the same general layout and bulkhead connector styles as the A-body? And will significantly reduce the amount of wire re-routing/splicing necessary as compared to the LeSabre harness? Unfortunately, the U vans don't seem to be all that common in the salvage yards around here so I'd probably have to to buy a van harness/PCM and have it shipped from wherever I can find one.

    Then you will need a lot of the U brackets to make it all fit in the car...
    Yeah, a complete parts van would deifnitely be a good source of brackets. But given their scarcity around here, I have already thought of a plan B. That plan is:

    -transaxle mounts - I figure I can re-use the existing ones in the Olds since both cars have the 4t60e.
    -lower engine mount/bracket - I've heard that they're the same for 3300 & 3800 and I happen to have a junked out 92 A-body with a 3300 that could donate that part.
    -upper engine bracket - I'm thinking I can cut/weld/modify the one off the 3300 to make it fit the 3800 and connect to the existing "dogbone" in the 94 Olds.

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    Quote Originally Posted by Century7667 View Post
    Sounds like a fun project. I know someone with a '95 LeSabre, and that thing will GO!
    My 88 Cutlass Ciera has the LG3 3.8 (basically the older version of the 3800) and it's a night and day difference compared to that worn out 3100 in mom's wagon!

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    Ray,

    I can't say for sure that they are the same as the A body, But the van was very much based on the A body, things did change a bit for the later 3800's... 94-95 were a bit better and last years a 3800 was used...

    Don't quote me, But I believe a lot of the brackets and the like from the 3300 should fit the 3800... Only one way to know for sure tho...

    I was blown away when the Series I brackets, The U bracket under the oil pan, the AC compressor( had to grind an inch off the back for the oil dip stick tube ), the trans to block brackets more or less fit the series III with a minor amount of work... The 3300 was more or less a short deck version of the 3800... A lot of those parts may just work for you...

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    Quote Originally Posted by Century7667 View Post
    Sounds like a fun project. I know someone with a '95 LeSabre, and that thing will GO!
    The 95 was really a special car.... GM had some funky things going on... In 93 GM went to a newer ring set up( Less friction, lower emissions ) as well they went to a roller pivot for the rocker arm to again reduce friction... as well for 93 the Compression was bumped to 9.0 : 1 .... 1995 saw a few more changes, the L27 had Powered Metal Forged Main Caps, the block was tapped for the series II oil pan and also used the Series II 2170 Light Weight Crank, these engines had slightly lighter rods as well... So not only could this engine throw the torque to the ground it could spin due to the lighter crank... Put a 3.06 gear ration behind this engine and you have a tire smoker of a Buick.... If you had a Buick Limited with the Touring suspension( F41 ) then you got a 3.06... Tire roaster that can handle....LOL

    Sometimes I love the little things GM slipped in there all secret like....LMAO

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    Senior Member Ray_McAvoy's Avatar
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    Quote Originally Posted by Jr's3800 View Post
    Ray,

    I can't say for sure that they are the same as the A body, But the van was very much based on the A body, things did change a bit for the later 3800's... 94-95 were a bit better and last years a 3800 was used...

    Don't quote me, But I believe a lot of the brackets and the like from the 3300 should fit the 3800... Only one way to know for sure tho...
    Thanks again Jr. I'll keep an eye out for a 94/95 van so I can at least look at the engine wiring layout and see how close it looks to the A-body. I'm thinking it might not be a bad idea to pick up a factory service manual for one either ... the U-van schematics will probably answer a few of my questions and should come in handy if I end up getting/using a van PCM/harness.

    I probably won't be starting work on this engine swap project until sometime this summer so that'll give me some time to pull those 3300 brackets, test fit them on the 3800, and work out a plan to deal with the wiring. I'm expecting to run into some pluming differences with things like the PS hoses, fuel lines, and radiator hoses, as well. But I think I can take care of those things with a combination of 3300 A-body parts and some home-made stuff. In any case I definitely plan to make a list of whatever parts I use off other year/make/model vehicles to keep for future reference when buying replacement parts.

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    Quote Originally Posted by Jr's3800 View Post
    The 95 was really a special car.... GM had some funky things going on... In 93 GM went to a newer ring set up( Less friction, lower emissions ) as well they went to a roller pivot for the rocker arm to again reduce friction... as well for 93 the Compression was bumped to 9.0 : 1 .... 1995 saw a few more changes, the L27 had Powered Metal Forged Main Caps, the block was tapped for the series II oil pan and also used the Series II 2170 Light Weight Crank, these engines had slightly lighter rods as well... So not only could this engine throw the torque to the ground it could spin due to the lighter crank... Put a 3.06 gear ration behind this engine and you have a tire smoker of a Buick.... If you had a Buick Limited with the Touring suspension( F41 ) then you got a 3.06... Tire roaster that can handle....LOL

    Sometimes I love the little things GM slipped in there all secret like....LMAO
    Despite the challenges, I would love the opportunity to drop a 3800 into my '95 Century. I wonder which engine is cleaner? Should be legal in Texas if is is same year or newer and if it will smog.

    Ken T.

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    Ken the 95 3800 should be cleaner... But the 3100 was not bad either... The 1995 3800 Both series I and 2 were meeting the lower emission standards before their time really... In 1995 all 3800's were very very low emission... Due to certain controls on the cat it could not and would not be called Low Emissions.... Once the cat was up to temp the emissions was just about nothing... I think the right number was .0002 grams per mile... My 03 Bonneville is actually certified as an LEV....

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    That's good to know. I'll keep that project on the someday/maybe list!

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    Update:

    I haven't been able to find a 94/95 U-van with a 3800. However, I have been studying the wiring diagrams for both the 94 A-body and the 95 LeSabre and taking notes on what circuits are the same/different.

    At this point, it looks as though modifying my spare 94 A-body 3100 engine harness to work with the 3800 will be considerably easier than trying to make the 95 LeSabre harness fit the A-body as I had mentioned in my original post.

    To make the A-body 3100 harness fit the 3800 it appears as though I'll have to do the following:

    (1) Re-pin the PCM connectors (using the connector bodies that fit the 3800 PCM). The majority of the circuits are the same (or very similar) so the 3100 harness has most of the necessary wires present ... just need to move them over to the appropriate slots in the 3800 PCM connectors.

    (2) Re-route wires in the engine harness to account for physical location differences of some sensors, ignition module, etc. on the 3800 vs 3100. Will also have to cut/splice some connectors off the 3800 harness since many of these parts have different type/shape connectors.

    (3) The EGR circuits are different (3-solenoid on the 3100 vs. linear actuator on the 3800) so I'll have to re-wire that a bit. I'll probably take these wires (4 of them) out of the LeSabre 3800 harness rather than re-using the EGR wires from the 3100 harness just so the color code will match the 3800 diagrams for future troubleshooting purposes.

    (4) The cruise control setup is different. The 3100 has a separate cruise control module with only 1 wire feeding a signal back to the PCM. While the 3800 has the cruise servo controlled directly by the PCM. Looks like I'll have to run some wires from the cruise switch to the 3800 PCM. And then some more wires from the PCM to the servo. I'll also have to change the cruise disengage switch at the brake pedal since the LeSabre cruise servo needs a vacuum vent connection. I think a switch from an older A-body with the vacuum type cruise servo should do the trick.

    (5) The AC compressor wiring is different too. The 3100 uses a continuous running variable displacement compressor and only has one pressure switch tied to the PCM. The 3800 has the compressor cycled on/off via the PCM and has 3 pressure switches (one of which is on the compressor itself). The AC in the Cutlass Cruiser doesn't work anyway (leaky compressor shaft seal) so it won't be a big deal if I can't get the AC working again. I'll at least run the necessary wires through the harness so they'll be there for future hookup.

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    I have to wonder if a 1992-1993 3300 harness would be easier?
    Alan Moore - TOAD Roadside - Worthington, OH

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    Senior Member Ray_McAvoy's Avatar
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    Quote Originally Posted by occupant View Post
    I have to wonder if a 1992-1993 3300 harness would be easier?
    I have actually considered that ... especially since I have a 92 Century parts car with a 3300/4T60. This car will be donating motor mount brackets and other miscellaneous parts to the swap.

    At first, it looks like the 3300 harness might make things easier due to the physical layout of the parts on that engine being similar to the 3800. I don't have any wiring diagrams for the 93 to know for sure, but I think I'd still run into a number of issues with the 3300 harness such as:

    (1) I'd likely have to re-pin the PCM connectors on this harness too.
    (2) I'd still have to add some wiring for the EGR ... 3300's didn't use an EGR if I remember correctly.
    (3) I know the 93 & older A-bodies use the vacuum style cruise servo like the 3800 engine donor LeSabre, but is it controlled by the PCM or a separate cruise module?
    (4) I haven't really looked at the 3300 AC setup to see what type of sensors/switches is has.
    (5) I believe the 3300 uses batch fire injection with only 2 wires (one for each bank of injectors) leading back to the PCM and no cam position sensor. While the 3800 has sequential injection with 6 wires (one for each injector) leading back to the PCM and wiring for a cam position sensor. The 94 3100 harness has the advantage here since it's is wired just like the 3800 in this aspect.
    (6) The 92 has the 4T60 transaxle so I 'd have to add some wires since I plan on using the 4T60E transaxle out of the LeSabre. The 94 A-body already has a 4T60E so the 3100 harness is all wired for that type of transaxle (although I will have to double check the connector pin-outs to make sure they're the same).

    Thanks for the suggestion though.

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    How soon are you taking on this swap? I still have a nearly complete '95 3800 parts van. Good harness, mounts, and a recent ECM.
    -Andy

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