76 Eldorado Rough Idle - HELP
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500, 472, 425, 368 Discussion, 76 Eldorado Rough Idle - HELP in Cadillac Engine Technical Discussion; Hi all, I really need your help on a confusing problem. I'll try to give as much info as I ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    76 Eldorado Rough Idle - HELP

    Hi all,

    I really need your help on a confusing problem. I'll try to give as much info as I can. I have a 76 Eldorado that's my daily driver (yes, it's true) with 60k miles on the clock. I bought the car around 55k and though it ran/started/drove pretty well, it drove like the float level was set incorrectly. It would spark knock ever so slightly at highway speeds even with a fully operational EGR (yes, I checked it), and correct timing. Anyway, I finally got around to taking the top off the carb to reset the float level this weekend. It seemed like a perfect day (75 degrees/sunny) when all of a sudden a wind/rain storm came up out of nowhere. I had already taken the air horn off the Quadrajet and reset the float level (which indeed was set incorrectly), I just needed to put the air horn back on. I quickly covered the carb with a towel and put the hood down waiting for the storm to subside. When it did, I put the air horn back on, hooked the choke and accelerator pump back up, and started the car.

    I drove it around for about 5 miles and it ran much better and smoother. The hesitation when I floored the car was 100% gone. Needless to say I was pleased. So, anyway, I drove the car to work today and as I'm about to get on the freeway, I notice the idle is a bit rough. Everything above idle was still perfect, even with just a slight throttle tip in, but the idle seemed lopey. I wrote it off as a fluke, got on the freeway, then once I got off the freeway, it seemed like the car was only running on 6 cylinders at idle. Again, anything above idle it ran great, but the car was visibly shaking at idle speeds.

    After work, I went to start the car, and it fired right up and purred like a kitten. I thought all was well. I drove about 15 miles down the road and stopped in a gas station (one downside of driving this car). When I fired the car back up, the lopey idle was back and it stayed all the way home.

    After letting the car cool off for about 4 hours, I fired it up again, and the lopey idle remained. I thought I’d first check to see if it was an ignition issue before pulling the carb apart again. I put new plugs/wires/dist cap & rotor on the car about 10k miles ago, but thought I’d check anyway. I found pulling the wires on cylinders 4, 6, and 7 made no difference in idle speed. I then pulled the plugs on these cylinders and found they looked normal—no fouling at all. I then swapped a plug/wire from a good cylinder to a bad one and the problem remained on cylinders 4, 6, and 7. I then got out my timing light and checked the wires to see if the light flashed on these cylinders—it did. Next, I got out my compression tester. Long story short, cylinders 4, 6, and 7 have excellent compression.

    So, what is going on here? Is this a fuel problem? And if so, how can that be? My problem is on opposing cylinder banks. What should I do? I’m very confused and would appreciate any help you could offer before I cash in my manhood at the local mechanic shop.

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  3. #2
    BRUCE ROE is offline Cadillac Owners Fanatic
    Automobile(s): Eldorado
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    Re: 76 Eldorado Rough Idle - HELP

    The first thing to do is make sure your timing chain is good. Any original has plastic teeth on the cam sprocket, which can fall any time at this age. A sloppy chain will increase pinging, cancel any other improvements, and maybe strand you with a permanently damaged engine. My daily driver is a 77. Bruce Roe

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
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    Thread Starter

    Re: 76 Eldorado Rough Idle - HELP

    Hi Bruce,

    Thanks! I understand what you're saying about the cam chain, but I don't think that could be causing my problem with cylinders 4, 6, and 7. This one really has me stumped. There's no drop in idle speed when I remove a wire from any of these cylinders. I'm going to put in 3 new plugs right now and maybe switch distributor caps and see if that makes a difference. Very strange.

    I was thinking if dirt got in the idle circuit, my problem should be localized to a complete cylinder bank (e.g. cylinders 2, 4, 6, and 8). But, clearly it's not.

  5. #4
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: 76 Eldorado Rough Idle - HELP

    Pull your distributor cap and check the resistance at the contacts. Look for carbon tracking across the contacts inside the cap. Electric current, like water, will seek the path of least resistance.

  6. #5
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 76 Eldorado Rough Idle - HELP

    It might be the carb idle circuits. There may be caps on the idle adjustment screws, probably removable. Make sure both screws do affect idle. Ignition energy problems usually get worse at higher power. Bruce Roe

  7. #6
    awadecki is offline Cadillac Owners Enthusiast
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    Thread Starter

    Re: 76 Eldorado Rough Idle - HELP

    Thanks! Took the top of the carb off today and sprayed some compressed air in all of the orifices, put it back together, and purrs like a kitten again. Took it on a 100 miles trip and runs 100%.

  8. #7
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: 76 Eldorado Rough Idle - HELP

    Sound as if your carb picked up some dirt during that rain storm.

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