From 368 EFI to Q-jet
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500, 472, 425, 368 Discussion, From 368 EFI to Q-jet in Cadillac Engine Technical Discussion; OK guys, here’s my question Will a Q-jet carb (M4ME) bolt on directly on my current EFI intake manifold? My ...
  1. #1
    Cerbera is offline Cadillac Owners Member
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    From 368 EFI to Q-jet

    OK guys, here’s my question
    Will a Q-jet carb (M4ME) bolt on directly on my current EFI intake manifold?
    My reason for his question? Allow me to explain

    My car (83 Seville with 80 368 EFI engine) runs primarily on LPG (Liquified Petroleum Gas)
    This is mainly done in The Netherlands for financial reasons. Because unleaded fuel runs about 9 dollars per gallon and with a fuel consumption of 15 mpg or way less, not funny.
    However our goverment needs money and decided to tax all classic cars running on diesel or LPG between the age of 39 and 25 years old.
    In turn this means that a lot (American) classic cars need to converted back to run on unleaded again to avoid this tax. Otherwise I’m looking at 3000 USD a year in road tax. Once again, that’s not funny.

    Now I know there’s more in converting the car from EFI to a carb than just that, so if there any tips on that too it would be highly appreciated.
    Already thought of fuel pump btw.....

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  3. #2
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: From 368 EFI to Q-jet

    You will need a manifold to go with that carb and a different engine wiring harness. I am sure the manifolds for the 368 engine are different and not a good interchange with the 425/472/500 engines. If the engine has been converted, perhaps these parts are available in Europe.

  4. #3
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: From 368 EFI to Q-jet

    Is your EFI set up the stock Cadillac TBI system? If yes, just run it with regular gas, that's what it's designed for. However, I suspect that the LPG conversion is not the GM TBI set up. Take the air cleaner off, and take us some good pics, we'll be able to help you. It either already has an intake that will accept a Q jet, or, it has the EFI intake that will take the two barrel TBI set up.

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    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    Hello Jay,

    Yes the stock TBI system is in place. The LPG conversion just bypasses it. I will run it on regular gas because I know it can take it.
    Unfortunately I forgot to mention that the previous owner (or the one before that) had the LPG installed but the installer cut some wires and sensors so thats why I have to convert to a carb.
    I will take some pictures as soon as possible.

  6. #5
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: From 368 EFI to Q-jet

    We've got all the wiring info you need if you want to put it back together. Cheaper than carbing it!
    70eldo likes this.

  7. #6
    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    Sure would be cheaper building it back together, but would it be possible? thats the big question for me for now anyway.
    Anyway here are some pictures
    general view

    connector idle motor cut through

    MAT sensor I'm not mistaken

    original TBI as you can see

    not quite sure what this is?

    EGR blocked off

    not quite sure what this one is but its ok

    whats this one by the way?

  8. #7
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: From 368 EFI to Q-jet

    That block - off plate at the back of the manifold Is where the original EGR valve was located.

  9. #8
    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    It seems your right on the manifold issue, I did some calling around and the 80 EFI manifold does not take the Q-jet carb. Unless you have the manifold of 80's Fleetwood or Deville.
    However I'm curious about the wiring harness. Why wouldnt the harness be ok for the Q-jet?
    MY car runs on LPG and all the functions on my (digital) dash are working

  10. #9
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: From 368 EFI to Q-jet

    You may be able to get by adapting your harness for use with the carbed engine. It would be easier (less confusing for future trouble shooting) to replace it with the original harness.

  11. #10
    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    Well, the car had its HT4100 engine replaced by the 368 along the line.
    So I can only guess which harness is in at the moment, the original one or did they replace the harness along with the 368 engine?
    I really dont have a clue......

  12. #11
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: From 368 EFI to Q-jet

    The wiring for the 4100 should be very similar to the TBI 368. I can see in your pics that the engine is not the 81 8-6-4, so I'm thinking that they just used the 4100 wiring. Might want to pull the computer out from under the dash to see if it is the 83 4100, or something else.

  13. #12
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    Re: From 368 EFI to Q-jet

    Welcome and good luck Cerbera!
    Are you in contact with the Cadillac Club Netherlands? They have some gear heads that might be able to help you out on site too.

  14. #13
    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    @jayoldschool

    I heard the same thing about the wiring for the 368 and the 4100 but wasnt sure this was true.
    Will check the computer as soon as possible so I can be sure which computer it is. And also check if it still has the MAP and Baro sensors as well.

    What do you think of the wiring itself? worth to give a try and order some parts at Rockauto?
    As far I can see I would need a MAT sensor, ISC connector, MAP sensor (if its not there) and possibly a new ECU (if its the wrong one)

    Concerning the Q-jet, I took a look in the service information manual and it also has an idle speed solenoid so it would need a signal from the ECU as well somehow.

    By the way, I did manage to find a Q-jet here amazingly, for a price of course

    ----------

    @70eldo
    Well not yet actually, will do as well.
    can use the luck thats for sure

  15. #14
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: From 368 EFI to Q-jet

    The idle speed solenoid on the Q-jet was principally installed to elevate the idle speed when the A/C compressor was in use. Not sure about the later models.

  16. #15
    Cerbera is offline Cadillac Owners Member
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    Re: From 368 EFI to Q-jet

    Ok thanks, sounds logical. That solenoid wouldnt be necessary at the moment as my A/C still has the old R12 refrigerant in it and I need to convert it anyway but thats for later....

    1:I found out that it has a remanufactered ECM still in place (1226930) unfortunately.
    2:I've checked if I could find the MAP and Baro sensor but it seems to me their not there? see pictures below
    Unless I'm not seeing what i'm supposed to see.


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