96 Deville won't idle down, hard start and backfires when cold
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Northstar Engines and System Technical Discussion Discussion, 96 Deville won't idle down, hard start and backfires when cold in Cadillac Engine Technical Discussion; I just bought this Deville. It has 83k miles. Recent tuneup. When the engine is really cold, it hesitates to ...
  1. #1
    magistrate_apeman is offline Cadillac Owners Member
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    96 Deville won't idle down, hard start and backfires when cold

    I just bought this Deville. It has 83k miles. Recent tuneup. When the engine is really cold, it hesitates to start and backfires thru the intake pressure relief valve like a Chevrolet that's out of time a little. Once it starts, it idles rather erratic until it warms up a little, and then it's smooth as butter. Then sometimes the engine wants to idle incredibly high, 1500 to 2000 rpms, no matter whether in Park or driving. I've cleaned the tbi opening and butterfly and cleaned the idle air control. I can't find any vacuum leaks. I think I hear a hiss from the backfire valve behind the power steering pump, but a shot of Gunk doesn't effect the revs at all, and then I'm convinced that it's just the belt and pulley whirring. Sometimes it idles fine, sometimes not. It's not throwing any codes, not misfiring. No horsepower shortage, didn't overheat over the 120 mile drive getting it home. What's going on here? It's a nice old car, but I don't want to start dumping a bunch of money changing parts in hopes of stumbling onto a fix. I've searched this issue extensively via Google and tried all the cleaning and pulling codes and so forth, but to no avail. Any advice would be much appreciated. Thanks...

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    I also pulled the line off the fuel pressure regulator and no fuel leaked out.

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    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Black magic: Find a 20 oz. jug of Chevron TECHRON and use that in the gas with a fill of 91/93 octane TopTier gasoline. (Google it). If that doesn't clear up the poor cold starts, then you're probably in the fuel injector cleaning business.

    When was the last spark plug and plug wire replacement ?? The correct plugs are AC Delco #41-950 Platinum plugs and an AC Delco wireset. Remove the 4 coils and the ICM and clean the contact bases as well as the coil towers. Make very sure the ICM ground wire is securely fastened to the ICM and engine. make sure the plug wires are connected to the proper (GM recommended) coil towers.

    For parts, talk to Chris in parts at Rippy Cadillac over there >>> and for parts info and pictures/diagrams use www.rockauto.com and/or www.gmpartsgiant.com .

  4. #3
    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Complete tuneup was performed, including new plugs and wires, by a Cadillac dealer little over a year ago. That's actually the only service record I have for it. Going to start back in on the car this weekend, beginning with a combustion leak test, fuel pressure test, and an injector leak test that I read about here last night. Also read that a bad tps would cause my high idle issue, though I was convinced that it should have thrown a code were it faulty. Then again, I haven't had opportunity to put any miles on the car aside from the 100+ mile ride home, during which it didn't act up. I'll use the bottle of Techron, too.

    How sensitive are these N*'s to fuel grade? I'm quite certain the previous operator (girl in college) wouldn't have paid for premium.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    The Northstars use a knock sensor to control maximum ignition advance if ping/knock/detonation is detected. While your 1996 engine is spec'd for premium, it will run perfectly well on 87 and 99.9% of the time you'll never feel a performance loss; However, if you intend to pull a trailer, do any mountain driving, or want to operate the car to its maximum potential, use premium fuels. Also, Google "top tier gasoline". Study.

    The 2000 and later Northstar is spec'd for 87 octane - combustion chamber shape change, compression ratio dropped from 10.3:1 to 10.0:1 with no performance loss. The manual does the flip of the above - it spec's 87 but says you might want to use premium under the above hard service conditions.

    These engines do NOT need a lot of ignition advance to keep the flame travel/combustion pressure in the most efficient part of the power downstroke.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Quote Originally Posted by magistrate_apeman View Post
    When the engine is really cold, it hesitates to start and backfires

    Once it starts, it idles rather erratic until it warms up a little

    Then sometimes the engine wants to idle incredibly high, 1500 to 2000 rpms

    I think I hear a hiss from the backfire valve behind the power steering pump

    Sometimes it idles fine, sometimes not.
    Sounds like a vacuum leak to me.

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    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    I was thinking vacuum leak, too, but I haven't been able to find one. Would that cause the backfiring and hard start?

    Also, can a tps go bad without throwing a code?

    What I'm thinking is there's a fuel leak somewheres causing the hard start and backfiring, as well as a bad tps or vacuum leak causing the high idle. I'd planned on working on it this week, but some extra paying work popped up and haven't had time. Going to rent a fuel pressure test kit and a vacuum test kit this weekend. Also going to purchase a combustion leak tester to Check the head gasket fiasco before spending another nickel. This Northstar is a weird rascal.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Yes, a vacuum will cause a lean condition and can cause backfiring and hard starts. Low fuel pressure could also cause it.

    Never heard of a bad TPS not setting a DTC.



    I also pulled the line off the fuel pressure regulator and no fuel leaked out.
    You might want to try it again and add vacuum this time either with a hand held vacuum pump or buy sucking on the vacuum line. On rare occasions the diaphragm rupture is so small that it takes vacuum added to the fuel pressure to get the fuel to leak.

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    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Had a few minutes to tinker today. 40 degrees out, car was a little slow to fire but no backfires or anything. Ran combustion leak test, test fluid never changed colors. In the process, however, if found a few rotten vacuum lines leaking. Replaced the easy one, the other runs down into the right front wheel well. Put a pressure gauge on the fuel system; I'm carrying 40+ pounds of pressure running. However, the pump doesn't kick on when I turn the key on, only when I'm cranking/running the engine. What's up with that? Isn't it supposed to kick on for 3 seconds or so to prime? Drove a few miles to the gas station and car wash, car runs and rides beautifully. Not so concerned about the head gaskets now. And that's as far as I made it. Time to hit the road again.

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    Oh, and I changed the fuel pressure regulator, too. Mechanic friend advised it, said that's why my fuel pump isn't priming when I turn the key on. He was wrong.

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    And the car has been idling normally for two hours. Warm weather? Yes, I even before the fpr swap.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Yes, the fuel pump should run for 2-3 seconds at key on. Not sure why yours doesn't.

  11. #10
    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Bad relay? Thanks for all you folks' input, btw.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    I would think if the relay was bad it would not run at all.

  13. #12
    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    I read somewheres around here that, in regards to the fuel pump, the key on-engine stopped priming function was completely separate from the engine running function, controlled by two separate components of the computer system. Ring any bells? Convoluted sentence that was, for sure.

  14. #13
    magistrate_apeman is offline Cadillac Owners Member
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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Found this snippet in the archives, from a fella name of Warren J: "The reason for the long crank time is that there is a secondary power path to the fuel pump. It is provided by the oil pressure switch after the engine has developed sufficient oil pressure. Your engine should start and run even if the fuel pump relay is in your pocket. A code would be set, of course."

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    Fiddled with the a/c a little the other day, too, just to see if it would take a charge. Never got the compressor to kick in. Previous owner said it was ice cold. Turns out, the a/c relay is right next to the fuel pump relay. Related? Perhaps. Previous owner also said he had a new wiring harness installed at some point. Don't know which one; no documentation.

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    Quote Originally Posted by magistrate_apeman View Post
    Found this snippet in the archives, from a fella name of Warren J: "The reason for the long crank time is that there is a secondary power path to the fuel pump. It is provided by the oil pressure switch after the engine has developed sufficient oil pressure. Your engine should start and run even if the fuel pump relay is in your pocket. A code would be set, of course.
    Not positive, but I think that may have been correct on the earlier cars (prior to yours).

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    Re: 96 Deville won't idle down, hard start and backfires when cold

    His engine/fuel pump does not depend on oil pressure for a 'permission to start' signal from the PCM. As oil pressure in these Northstars builds to over 5 psi the oil pressure sending switch (on the oil filter adapter) OPENS, sending an OFF signal to the PCM which tells it not to turn ON the oil pressure warning light/icon in the DIC. Fuel pump RUN is controlled by engine rpm, ignition signals (CKP, CMP), ignition switch position, and gosh knows what else - but it's not dependent on oil pressure.

    At key:ON the ignition switch sends a signal to the PCM which then does the one second fuel pump run/prime sequence. During START the fuel pump relay is held ON and the pump runs - the engine catches, run signals are sent from the PCM, you release the key and the fuel pump runs as long as the engine has rpm. The engine stalls for any reason or you turn OFF the key, the fuel pump relay drops out and the pump stops.

    You can completely disconnect the oil pressure sending switch in these cars and it makes no difference in fuel pump operation. Witness the several threads over the past 3 years where members have either broken the switch connector/wires or the switch itself and the car runs just fine with no messages. You just won't see the low oil pressure icon on the DIC - and that only stays on during the DIC prove-out at key:ON - or if the switch goes bad and you get a flickering pressure light.

    There are no "rollover" fuel pump safety inertia switches in these cars.

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