V8-6-4 is no 425!
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500, 472, 425, 368 Discussion, V8-6-4 is no 425! in Cadillac Engine Technical Discussion; Hello! Have a 1981 Fleetwood Brougham Coupe with the V8-6-4, 235k miles and it is still nice cosmetically. Just had ...
  1. #1
    Bro-Ham's Avatar
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    V8-6-4 is no 425!

    Hello!

    Have a 1981 Fleetwood Brougham Coupe with the V8-6-4, 235k miles and it is still nice cosmetically. Just had a ton of work done to fix some problems and to attempt to make the car as smooth running as my previous 1977 and 1978 cars which had 425's. Mechanic put in a new computer, exhaust, tune up, belts, hoses, etc. Compression was checked and it is all good! The car runs much better now yet still does not have a velvet smooth idle and acceleration of the 425. We have not yet replaced fuel injectors and the plugs we put in are platinum and the mechanic feels they may make the car run hot. We have the shop manual. Want this car to run like a Cadillac should! Are we missing something? Ideas???

    Thanks in advance!

    Dave

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  3. #2
    The Ape Man's Avatar
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    Re: V8-6-4 is no 425!

    Your 8/6/4 should easily idle better than a carbureted 425. Pay close attention to proper ignition timing. Also I'd suspect either a vacuum leak somewhere around the intake manifold or throttlebody base gasket. If the car was driven by a 105 year old woman with one eye then you might just have some carbon buildup on the valves. This happens to the 425/472/500's even under normal driving. I like the old transmission fluid engine clean. Get the engine warmed up then rev it up to 2000 rpm and hols it there. Slowly dribble a quart of transmission fluid down the throttlebody alternating between the 2 throttle bores. Don't pour it too fast as all hell will break loose. The slowest stream that you can pour with the container hald sideways is fast enough. It will take a full 5 minutes to use the bottle up. Your car will also smoke from the tailpipe and you will worry that you just melted something important. This will clear up after a half hour or so. BTW this is a great way to ruin a neighborhood picnic as there will be lots of smoke. It may even smoke when you drive it at first. I have been doing this with my Cadillacs every 6 months for the last 15 years. I even had one of those old catalytic converters from the 70's with the hex plug that still worked and got the car through NJ emissions every time. That thing stayed clean and working for more than 25 years. It finally broke in half one day or I'd still have it! Also swore by this with my fleet of Ford 300 F.I. trucks. Those trucks were always being driven hard and they still built up carbon on the valves. The carbon would make the trucks stumble and misfire just off idle. The trans fluid cleaning and a couple of extra degrees initial timing advance fixed it every time.
    Assuming everything else under the sun checks out then you can cheat. The fuel pressure regulator is built into the throttlebody. Try increasing fuel pressure while watching exhaust emissions. If the hydrocarbons decrease slightly with a boost of fuel pressure then it will be OK to run it that way. I have cheated this way with this engine several times when I gave up looking for reasons. Don't increase it more than a couple pounds.

  4. #3
    Bro-Ham's Avatar
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    Re: V8-6-4 is no 425!

    W O W ! !

    Thanks for the ideas! Will see what happens! I just drove the car tonight about 50 miles and the check engine light came on from time to time and the trip computer turned on and off! I really hope this car can have its issues ironed out!

    I appreciate your thoughts!

    Dave

  5. #4
    The Ape Man's Avatar
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    Re: V8-6-4 is no 425!

    Grab the codes and see what they say. Codes tell a story like used spark plugs. That may help figure if something is out of range. Do you have the service manual yet?

  6. #5
    Bro-Ham's Avatar
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    Re: V8-6-4 is no 425!

    Hello!

    YES, have the service manual. The last time we checked the codes the computer was indicated to be defective. With the new computer, the car seems to have NO power, in fact, yesterday afternoon I took it on a 150 mile drive and it could not make it up any kind of big hills on the highway without a good run up to them otherwise the car would lose power significantly by the top! Plus, the car has always hesitated slightly when it is in 6 cylinder mode but now it is acting funny in 8 cylinder mode and 4 seems OK but all in all the car is really misbehaving! Prior to fitting the new computer the car ran fine with the exception of the bad performance in 6 cylinder mode and also, previously, the check engine light would come on after driving the car about 5 minutes - - every time! Oh, last night, and the night before, the check engine light would come on just a little bit, then fade out, then come on again - - but not stay on. Also, the trip computer display would turn on and off and after driving for awhile stayed on. I really wish i could turn off all this electronic garbage or find a way to be assured that it will work and not cause problems.

    Thanks for the help!

    Dave

  7. #6
    The Ape Man's Avatar
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    Re: V8-6-4 is no 425!

    No power and vibration in the 6 cyl mode is pretty much normal. 6 is just there as a bridge between 8 and 4. If you ask for more power in 6, the thing will switch back to 8. What are your codes right now?

  8. #7
    Bro-Ham's Avatar
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    Re: V8-6-4 is no 425!

    Hi!

    Can I check the codes myself? My mechanic did it previously so I don't know how. Is it checked through the climate control or external device?

    Thanks for your help!

    Dave

  9. #8
    The Ape Man's Avatar
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    Re: V8-6-4 is no 425!

    Yes. The proceedure is in the manual. The codes are there also.

  10. #9
    Bro-Ham's Avatar
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    Re: V8-6-4 is no 425!

    I will check it out! Thanks for the advice!!! Dave

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