Severe spark knock under partial throttle
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Severe spark knock under partial throttle in Past Cadillac Vehicle Discussion; Here's another problem that my car is having and I'm hoping it is an easy fix. When driving at a ...
  1. #1
    TopherS is offline Cadillac Owners Fanatic
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    Severe spark knock under partial throttle

    Here's another problem that my car is having and I'm hoping it is an easy fix.

    When driving at a steady pace, if I push down partially on the gas to increase speed or to maintain speed when going up a slight incline, I get a very bad spark knock...the car sounds like a diesel. Here's what I've done, so far...

    1) Timing: I bought my car in September 2011 and the timing was way off. I had the timing set to factory specs. I got better power, but I also got spark knock. So last summer, I took it back to my mechanic...his response was to alter the timing. It got rid of the spark knock, but I had so little power that I could not pull into traffic safely. I've had the timing set back to factory specs. Power's good...spark knock's back. It's never gone away.

    2) Last summer, after having my timing reset to factory specs, I changed to using either mid-grade (89 octane) or premium (91 octane) gasoline. It's helped, but the spark knock is still present. I haven't noticed a difference between using mid-grade and premium, so I've stuck with mid-grade most of the time.

    The only other thing that I can think to do is when adjusting speed, to adjust it more abruptly rather than gradually...as heavy acceleration (especially to the point that the car downshifts) does not create the spark knock issue.

    I've noticed that the problem is worse when the outside temps are warmer. There are also times that coming off the freeway and being on surface streets, that the problem is so bad that the spark knock is accompanied by reduced power.

    Any ideas??? I appreciate your help in this.

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    cadillac kevin's Avatar
    cadillac kevin is online now Cadillac Owners Master
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    My broughams 307 had a similar problem, but only when making a hard right turn without stopping or going WOT then letting off. I never was able to figure it out (tried plugs and wires but didn't help). I ended up dumping B+G 44K in it, which helped noticably with the frequency of the problem, and running premium, which helped more, but the problem persisted.

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    SafariOne is offline Cadillac Owners Fanatic
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    Re: Severe spark knock under partial throttle

    You have to help us out a little too there dude. I have to assume it is your 87 that your talking about. Do you have the 4.1 or the 5.0??? Have you done any adjustments to your throttle position sensor?? Any codes coming up on the DIC?? We need as much info from you as you get from us...... stat.

  5. #4
    csbuckn is offline Cadillac Owners Master
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    Are you sure its spark knock? Have you tried treating it as a bearing knock and running heavier oils and such?

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    amunderdog is online now Cadillac Owners Connoisseur
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    csbuckn is offline Cadillac Owners Master
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    Maybe get an inspection camera in the spark plug hole to see how much build up is actually in the cylinders. Detonation for over a year would make me think the motor has had a rough life. Eventually it should burn off if its not running rich anymore, right?

  8. #7
    TopherS is offline Cadillac Owners Fanatic
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    Re: Severe spark knock under partial throttle

    Thanks for the quick replies. Sorry about the lack of info in my original post...I was pressured for time...but here's more info:

    * Yes, it's the '87 Brougham I'm talking about (5.0L V8).

    * I don't believe that the Throttle Position Sensor has ever been adjusted.

    * Check Engine Light never has come on, except on startup.

    * I jumped the ALDL and get "12" flashing over and over.

    * For the past year, I've been running 30W oil at the recommendation of my mechanic (I've had a couple of minor leaks, which have since nearly gone).

    * I don't believe that it's running rich...I don't get heavy gas fumes/smell ever.

    * My mechanic believes that there's likely a lot of carbon based on the age of the car and the number of miles on it (26 yr old car with 78K miles). He says it was likely never really driven to a point to burn off carbon buildup...and that there's not much that can be done now. He's never looked within the cylinders to see how much buildup there is, though.


    I've found a YouTube video that has a similar sound...Cheers again!!!
    http://www.youtube.com/watch?v=XiBzxxvy3FQ

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Severe spark knock under partial throttle

    You are screwed. Aren't these distributors computer controlled? Normally a distributor is infinitely adjustable unless it is controlled by the computer. My old 307 Olds was one of the few cars I ever had to custom tailor the advance curve. It was a pinger but an adjustable vacuum advance kit allowed me to take out some of that vacuum advance. Half of the problem was it was always locked up in OD at 40 mph.

    You say when you step heavily on your throttle that makes the ping go away. That loss of vacuum with the heavy throttle is causing the computer to get the signal to back off the timing; it's just not sensitive enough at light throttle. If you don't want to change the dist you may be able to help it with some stiffer advance springs. Maybe someone has some tips on how to raise the lock up speed to about 50. That would take some of the strain off these heavy cars going up hills at slower speeds.

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    outsider is offline Cadillac Owners Connoisseur
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    Re: Severe spark knock under partial throttle

    if it's carbon build up then you should be able to clean it out with some seafoam poured directly down the carb throat while running. Use about 1/2 bottle and start slow, gradually increasing your rate of pour until it chokes out the car and it shuts off. Let it sit for a few minutes and then start it back up. YOu'll see a lot of black crap come out of the tail pipe if it's carbon build up.

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    csbuckn is offline Cadillac Owners Master
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    After listening to the video, I'm with Al, sounds related to the vacuum advance. Something inside the dizzy seems wrong. Maybe the weights are sticking or broken. Maybe whatever is supposed to control the timing isnt working or getting the proper signal from wherever.

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    Re: Severe spark knock under partial throttle

    You've been running straight 30W? Bad advice.

    Check inside the dist for freely moving weights, springs working. Test vacuum advance mech. Check and set timing, properly according to the FSM procedure. Don't trust a garage. Do a Seafoam or GM top engine treatment.

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    TopherS is offline Cadillac Owners Fanatic
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    Re: Severe spark knock under partial throttle

    Quote Originally Posted by jayoldschool View Post
    You've been running straight 30W? Bad advice.

    Check inside the dist for freely moving weights, springs working. Test vacuum advance mech. Check and set timing, properly according to the FSM procedure. Don't trust a garage. Do a Seafoam or GM top engine treatment.
    Why bad advice on the oil? I don't know anything about what oils are best, so I'm asking out of curiosity. My mechanic said that the 30W would help slow/stop the minor leaks that I've got...and the oil drip pan seems to be catching less oil since switching...but if there's a better oil, I'm open to suggestion.

    I took of the distributor cap this morning and found 1/2 of the ballast resistor (AKA: distributor brush kit) rolling around the bottom...the other half still attached to the small spring by the coil. I don't know how that happened, or how long it's been that way, but it's been replaced now. I have no idea if this has anything to do with my car's issues, but thought it noteworthy.

    After replacing the broken part, I took a look at the distributor...I took off the rotor, but I don't know what I'm looking at/for (in respect to the weights/springs). I don't know how to test the vacuum advance mechanism???

    Yesterday, I went to buy Seafoam and was told to use Marvel Mystery Oil...that it was better. I did and I now must apologize for any damage to the ozone layer...I smoked out the neighborhood...white smoke was coming out somewhat as I was pouring it down the carb...but after letting the last bit sit for an hour, I drove off and...holy cow!!! White smoke poured out the back. James Bond's got nothin' on my smoke screen. I want to say that the treatment made a slight difference, but I'd bet that's more my imagination. But how long should I see smoke from the car? It appeared to go away after about 10 minutes of driving...but today, I pulled out into traffic and I could see a faint bit of smoke behind me...nothing like yesterday's smoke screen, but a little bit of smoke, nevertheless. Is this normal? By the way...no black smoke during/after the treatment. Not sure if this means anything...

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    outsider is offline Cadillac Owners Connoisseur
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    Re: Severe spark knock under partial throttle

    woah i never would have put marvel mystery oil down my carb! Not sure if it's OK to do that, but the seafoam it is ok to do that. With the seafoam i stopped seeing smoke after driving for a few minutes.

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    Re: Severe spark knock under partial throttle

    30W too viscous (thick) at cold temperatures. You are wearing out your engine every time you start it. Additionally, many 30W oils don't have the correct additive package for modern engines. Change your oil, go with a good 5W30.

    You should have used Seafoam. Stop taking advice from old mechanics and know-it-alls at the parts counters, and start listening to people that use and work on these cars every day (and have done so for a couple of decades).
    BigCadMan_1 and BigCadMan_1 like this.

  16. #15
    brougham is offline Cadillac Owners Master
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    Re: Severe spark knock under partial throttle

    It could be something wrong with it that the timing is advancing too fast. It can also happen with older engines that the ring on the harmonic balancer has moved a bit so the timing isn't really set where you think it is. Two more things to keep in mind if the seafoam doesn't do anything.
    I've never had an engine that would ping because of carbon build up but some types of engines are more picky to things like that then others.

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