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HT4100 4.1, 4.5, 4.9 Discussion, Spark Plugs: What do YOU run? in Cadillac Engine Technical Discussion; Originally Posted by stickpony iridium autolites ive been reading this entire thread...i think your fueling problem could be a small ...
  1. #61
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: Spark Plugs: What do YOU run?

    Quote Originally Posted by stickpony View Post
    iridium autolites

    ive been reading this entire thread...i think your fueling problem could be a small vacuum leak in your throttlebody gasket...spray some carb cleaner at the base of the throttlebody all the way around it, see if the engine idle drops a little when you do that....
    Done. No dice and gasket is new.


    also, this constant business with changing out injectors, changing spark plugs, etc...the issue is elsewhere. have you checked to see if you are getting a constant 12v to your power steering input on the ecm? if not, it will cause a constant rich condition but not set any codes...the power steering switch cuts the 12v only when there is power steering cramp at idle and the ecm richens the fuel mixture to compensate so you can turn yoru steering weheel easier at idle... BUt if the switch is bad, and doesn't allow the 12volts through during normal driving conditions, the ecm will constantly think the power steering is cramped...the computer doesn't always throw a code either when this happens....
    I have not seen any evidence to support that on this vehicle. The PS switch just tells the computer to bump the idle up via the idle speed control motor.

    also, your reman computer might be for a 91 or later deville/seville, since they have the same part number, but different memcal(chip)... i would take the chip out of your orgiginal stock computer and put it into the new computer....
    I have always used my original PROM for the 4.5L.


    also, have you considered the fact that maybe you have bad compression on one or more of the cylinders? there is alot of ground you have not covered.. but you keep changing the variables by just changing out parts... you need to go back and start over with the basics...
    I havent been keeping up to date on my work in the boards so there are gaps, holes, and things I forgot to mention. I havent been following this thread because the engine is now in a whole different state of parts, testing, tweaking, etc etc... The only ground I havent covered is a compression test and possibly going through the o2 sensor grounds.

    I believe the stumbling issue that this thread was about was simply a heavily worn distributor cap... I cant remember. Perhaps I should be a little more diligent about noting my progress on these boards as they sort of serve as a diary/history of events. I did however have 2 bad sets of injectors one 'new' and another 'rebuilt' which caused the hard hot starts and seriously compromised the diagnostic journey.

    Anyways the car is running quite well on 19# injectors albeit with poor mileage. I havent had the green to spend on a set of the CORRECT 16# injectors. I had a set of BMW 16.6# injectors but for some reason it pinged like a sonofabitch on those. When I decide to dig into that issue I'll start a new thread.

  2. #62
    caddeville's Avatar
    caddeville is offline Cadillac Owners Enthusiast
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    Re: Spark Plugs: What do YOU run?

    stumble is MAT sensor, manifold air tempurature. Located beside the alternator in the intake manifold. Had same issue with my 91 deville and buddies 89 deville.

  3. #63
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: Spark Plugs: What do YOU run?

    Heh, looking back this thread makes me laugh. I was so frustrated, lol... Too many variables popping up during diags, and on top of that, using the car everyday for work. I think the answer was a combo of heavy 19# injectors, too much timing, and mostly a well worn distributor cap due to my MSD 6A eating the contacts. Now I regularly sand them clean.

    I have the correct injectors now and fuel data is accurate again. Current config is 12deg BTDC and copper core plugs @ one heat range colder than stock (Autolite 104). The MSD 6a provides spark through a brass cap instead of aluminum... I also relocated the MAT sensor to the filter box which seems to have corrected a 'flat spot' in the fuel delivery. Cause of that is unknown as everything is at spec in the fuel system... Having the sensor report a colder air temp results in more initial fuel when accelerating. Just enough to correct the problem without affecting gas mileage. Good enough for me. Motor is happy.

    I have no future plans for it now that the trans is really showing its age at just 95K miles. Seems the pump was damaged some time ago after switching to Dex 6. It got really noisy and stayed that way after switching back to regular fluid. An autopsy of the trans filter revealed aluminum shavings presumably from the inside of the pump housing.

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