368 8-6-4 Woes.
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500, 472, 425, 368 Discussion, 368 8-6-4 Woes. in Cadillac Engine Technical Discussion; I have a beautiful 1981 Sedan de Ville with about 232,000 on the clock. I bought the car in 1989 ...
  1. #1
    tfd52 is offline Cadillac Owners Member
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    368 8-6-4 Woes.

    I have a beautiful 1981 Sedan de Ville with about 232,000 on the clock. I bought the car in 1989 with about 59K on it and with the 8-6-4 disabled.

    My problem is that the car has start to run roughly, with a very rich exhaust.

    I have a "check engine light on", but can't get any codes out of the Climate control unit, a "No Charge Light" on (but my battery/alt tester shows about 14.8 volts at idle) and an intermitant "Stop -- Engine Temp" (not coolant) light flickering off and on.

    I've changed the plugs, HT wires, cap, rotor, changed the filters and strayed the TBI down with cleaner, still no luck.

    The several mechanics I have taken the car to basically laugh me off.
    "too old,.....that V8-6-4 is a POS.....get rid of it, etc"

    Since I can't readily find anyone to work on the thing, I was wondering how difficult it would be to take the engine down to the heads, and rebuild it from the heads up, checking all the vacuum lines and sensors along the way?

    I did have a mechanic suggest doing a carburetted 350 swap on it, stating "At least, you won't have any problems finding parts for it".

    How difficult is it now to find the ECM and the various emission sensors for the car? Was the last guy giving me good advice?

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  3. #2
    CadiJeff's Avatar
    CadiJeff is offline Cadillac Owners Connoisseur
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    Re: 368 8-6-4 Woes.

    368 is a good motor, It sounds like your ecm may be shot but what do you want from a 232kmi car. Your best bet for a replacement is likely in salvage yard.

  4. #3
    AllCads is offline Cadillac Owners Member
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    Re: 368 8-6-4 Woes.

    Can you explain more specifically what the car is driving like?

    Seems like a whole lot of driving between your car and you to give up on now.

  5. #4
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 368 8-6-4 Woes.

    If you are mechanically inclined there is hope. If not it was a good ride. Assuming that you can troubleshoot, check the manifold air temperature sensor against the resistance values posted in the service manual. The connection to this sensor was a big problem on other models of the era.
    While you are at it, the coolant temp sensor tests the same way. Also look for a bad MAP and or BARO sensor. Easiest way is by substitution (you do have running spares don't you?) Vacuum line to MAP can become clogged with carbon and mess up the vacuum signal.
    Another route is to drop in a carbureted engine but there are a number of changes which add up to more work than repairing the DEFI IMHO.

  6. #5
    tfd52 is offline Cadillac Owners Member
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    Re: 368 8-6-4 Woes.

    Thanks for all the replies.

    The car runs fairly well, but the exhaust is very rich. Stumbles when cold, but moves out still fairly well when warmed up on the freeway.

    I don't have any back-up sensors though, Ape. Where do I go to find them?
    Will I have any luck in a junkyard? and who is to say those sensors are OK after 25 years?

    Thanks again. Looking forward to your replies.

  7. #6
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 368 8-6-4 Woes.

    To live with a car like this one needs spare parts. Start checking b-boards like this one. Post some want ads. Scour Ebay. I've sold several 8/6/4 ecms on ebay in the past. You have to turn over some rocks. There are several people on this board who have non-running 8/6/4s. Best of luck.

  8. #7
    tfd52 is offline Cadillac Owners Member
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    Re: 368 8-6-4 Woes.

    Thanks for your reply.

    I am still leaning towards taking everything off down to the cylinder heads, and re-installing everything from the head gaskets back up.

    But, before I do so, just a few quick questions. Does anyone repair-recalibrate the ECM, and any of the sensors that are no loner available from GM? BTW, how much of the emissions control devices are still in the inventory with GM? I've had pretty good luck with several 70s Chryslers that I still own, so I was hoping GM would be a similar in their parts location ability throughout the US.

    Basically, how tough would this job be, given the parts availibility, and if the availibility of parts is too tough, should I give consideration of installing a carb'd 350 into the Caddy?

  9. #8
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    jayoldschool is offline GM RWD V8 addict
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    Re: 368 8-6-4 Woes.

    rockauto.com has just about every part listed for your engine. I have an 81 FWB coupe, and I have checked out everything I can think of just to see availability. You could even just get the part numbers and take them into your local place. The kids behind the counter are much easier to deal with when you just give them a number and say, "Get this part."

  10. #9
    JTraik is offline Cadillac Owners Enthusiast
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    Re: 368 8-6-4 Woes.

    No offense, but did you even bother looking? Partsamerica.com has all of the sensors and the ECM that you need from several manufacturers nonetheless.... so does rockauto. Partsamerica.com has free shipping for over $50.

  11. #10
    tfd52 is offline Cadillac Owners Member
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    Re: 368 8-6-4 Woes.

    Thanks to both of you for providing me with this information.

    Thank you !!

  12. #11
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    Phantom&RedGoat2004 is offline Cadillac Owners Fanatic
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    Re: 368 8-6-4 Woes.

    The 368 is a VERY problematic engine. The previous owner had the variables disabled because they didn't work, and at only 59k miles, that says something. Because these engines are problematic, the 425 is actually pretty close to the same efficiency (someone correct me if I'm wrong here - I'm going on what a mechanic I ported for had said - I can't prove emissions compatability between the two), and a carburated 425 with a good carb might solve your problems all together and it would use the same cradle and accessories (not to include intake/exhaust manifolds or heads). Or if you wanted to stay fuel injected (what I would do), some of the '77-'79 425's had FI (195hp/L35) that would bolt up the same, although you would have to use the computor and the sensors from the car you got the 425 from. The 368 is Cadillac's bastard child, and the only way I know to fix them is to be rid of them, but please don't go to a Chevy motor. There are still too many options out there. PRG04

  13. #12
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: 368 8-6-4 Woes.

    Quote Originally Posted by Phantom&RedGoat2004 View Post
    (someone correct me if I'm wrong here
    OK you asked. You are waaay off. The '81 368 was a great engine for it's time. The 425 was close in economy on the highway. There you are right on.

  14. #13
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    Phantom&RedGoat2004 is offline Cadillac Owners Fanatic
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    Re: 368 8-6-4 Woes.

    I meant "efficiency" in reference to the ability to pass smog-laws, and fuel mileage of a 425 vs. a 368 that has a disabled variable system should be fairly close. If I'm wrong, I should make note.

  15. #14
    The Ape Man's Avatar
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    Re: 368 8-6-4 Woes.

    Federal and state emissions regulations would not allow the 425 to have been marketed in 1981 without a fuel and ignition system overhaul. The 8/6/4 only increased highway mileage by a few percent. It was a gimmick. They would have been far better off using an overdrive/lockup transmission but GM couldn't seem to make a good one back then. Every 8/6/4 disable job which I've seen only involved fooling the computer. No parts were removed. All that stuff stayed. The later HT-4100 engine used the same DEFI system and they all ran great right until they blew apart (which happened quite often with the RWD cars).

  16. #15
    tfd52 is offline Cadillac Owners Member
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    Re: 368 8-6-4 Woes.

    Thanks for your input.

    But to the point, how difficult is a carb'd 425 swap into a '81 Sedan de Ville?
    Straightforward?

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