P048 code - detonation at load 95 deville 4.9
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HT4100 4.1, 4.5, 4.9 Discussion, P048 code - detonation at load 95 deville 4.9 in Cadillac Engine Technical Discussion; Have the infamous P048 code. Car runs great while cold, but when it warms up i get detonation/knock. So far ...
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    warnereg is offline Cadillac Owners Member
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    P048 code - detonation at load 95 deville 4.9

    Have the infamous P048 code. Car runs great while cold, but when it warms up i get detonation/knock.

    So far I have rodded out the egr tubes under the throttle body, the smaller one was completly clogged and the other was about 3/4 of the way there. Didnt help... Ive lifted the egr valve by hand at idle and the engine does stumble as it should, so I know from egr to intake manifold is kosher.

    All of my vacuum lines looked pretty rotted, on spare time I have replaced half of them, I have sprayed the other half down with carb cleaner and was unable to find leaks. The parking break return seems to work well, it has quite a bit of backpressure when pressed back down while in drive with breaks applied. The only thing that seems off as far as vaccum goes is one the vacuum reservoir, if I move the fitting side to side it leaks but seems to hold vacuum otherwise. Its a hard plastic fitting that clips on and looked like it has an oring on the inside. The line that is supposed to go from the charcoal canister to the fuel tank has been plugged, could this cause any problems?

    Could the vaccuum port on the throttle body to the egr solenoid be clogged? The throttle body had a bit of carbon built up on the bores. Didnt want to take it off and clean it if I didnt have to since its my daily driver.

    Also the pcv valve hisses but I I think its from around the sides of the valve, could this cause problems?

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    Re: P048 code - detonation at load 95 deville 4.9

    The only part of the EGR system that is monitored and could set a DTC is the solenoid.

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    warnereg is offline Cadillac Owners Member
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    Re: P048 code - detonation at load 95 deville 4.9

    Quote Originally Posted by Ranger View Post
    The only part of the EGR system that is monitored and could set a DTC is the solenoid.
    Any ways to test the solonoid other than with a vacuum guage? Ive read something about hooking up a test light to the leads, just dont know the exact procedure. I guess ill have to take a closer look at the solonoid itself and report back.

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    Re: P048 code - detonation at load 95 deville 4.9

    I'm sure there is a test procedure, but without a FSM I don't know how. You can probably test it through the on board diagnostics by going into the override mode, but again without a FSM I don't remember how. Maybe someone with a FSM or a better memory will chime in.

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    warnereg is offline Cadillac Owners Member
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    Re: P048 code - detonation at load 95 deville 4.9

    Quote Originally Posted by Ranger View Post
    I'm sure there is a test procedure, but without a FSM I don't know how. You can probably test it through the on board diagnostics by going into the override mode, but again without a FSM I don't remember how. Maybe someone with a FSM or a better memory will chime in.
    If someone could that would be absolutely amazing!

    Reporting back about the solonoid. First of all when removing the the line coming from the throttle body it was as hard as plastic and shattered into many pieces, luckily I had a spare. The line held vacuum and all was well. I could pull air through the solenoid in each direction with the car off, but seemed restricted.

    I also check out the fuel pressure regulator, its vac line was old and dry as well, so it got replaced. I hooped a longer line to it and used lung power to test vacuum, it held but I could taste fuel. This was also with the car off. I read that a bad fuel pressure regulator can also cause detonation. Im going to stop by autozone on my way home from work to have my fuel pressure checked.

    Does anyone know where to get the vacuum boots and smaller more rigid lines? I have a couple lines that are just temporary until I can find a better alternative. Mainly just the one for the egr, and the one going to the fuel pressure refulator.

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    Re: P048 code - detonation at load 95 deville 4.9

    I could pull air through the solenoid in each direction with the car off
    Hmm, that doesn't seem right. There should be no EGR flow at idle. That might be the cause of your detonation.

    I seem to remember replacing one of those on my brothers CTS several years ago. I forget the DTC that it set, but I do recall being able to suck air through it with no current applied. Could not do that with the replacement.

    Does anyone know where to get the vacuum boots and smaller more rigid lines?
    Just replace them with regular rubber vacuum lines.

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    warnereg is offline Cadillac Owners Member
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    Re: P048 code - detonation at load 95 deville 4.9

    Quote Originally Posted by Ranger View Post
    Hmm, that doesn't seem right. There should be no EGR flow at idle. That might be the cause of your detonation.

    I seem to remember replacing one of those on my brothers CTS several years ago. I forget the DTC that it set, but I do recall being able to suck air through it with no current applied. Could not do that with the replacement.
    Ive done some research on testing an egr solonoid, I cant imagine they vary too much from car to car. According to the guide, air should not pass with no power, and should given power (not sure if this is correct but would make sense it the solenoid were bad). If my understanding of the system is correct its acting more like the egr never opens when rpms increase above a certain number (2k?)

    I guess ill just have to remove the egr solenoid from the car and see what it does when given power.

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    warnereg is offline Cadillac Owners Member
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    Re: P048 code - detonation at load 95 deville 4.9

    Update: I loaned out a hand vacuum pump to test for vacuum leaks as well as testing the egr solenoid.

    The solenoid tested out good. Car running at idle there was no vacuum to the egr. With the soleniod unplugged from power vacuum rose to -18 hg almost instantly. Then with the car in drive with my foot on the brake I raised the rpms to around 2k and the solonoid opened as expected, when I let off the gas it closed showing no vacuum. So I suppose that the egr is not opening at all, so either the diaphragm is bad, the egr is dirty, or im not getting the backpressure from the exhaust. What would be the most likely culprit?

    As far as vacuum leaks go I found that the elbow connecting to the vacuum reservoir was in fact leaking, I could not pull past -8 hg. I cut off the elbow and put a new vacuum line in its place and it held -12 hg (which is the most I could get that crappy hand pump to pull). Seems to run a little more stable, but still have detonation, which I now believe could be mostly vacuum related.

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    Re: P048 code - detonation at load 95 deville 4.9

    When I had detonation with my 94 Deville it was the EGR valve itself. It tested properly when you put vacuum to it but we couldn't find anything else to cause the problem and changing the EGR fixed it.

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    Re: P048 code - detonation at load 95 deville 4.9

    I went ahead and ordered a new OEM EGR valve and recieved it from FedEx today. I went with OEM because of what ive read about the aftermarket ones...

    I will report back once I install it and give her a test drive.

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    Re: P048 code - detonation at load 95 deville 4.9

    Ok everything was pretty strait forward and went pretty much as expected. I did have to go out and buy a set of combination wrenches to get to the EGR bolts after I removed the throttle body since I dont have a set of crows feet. I also was not able to torque down the EGR to spec. Seems like even with crows feet it would still be dificult to torque the bolt under the fuel rail. Either way its only two bolts so id immagine the torque spec is pretty much just so you dont strip the threads on the intake manifold. I tightened them down as evenly as possible.

    Also on the torque subject, what is torque spec on the throttle body bolts?

    The only problem I am now having is that now that the trottle body blades and throats are carbon free (and I know this is common) is that it the idle is high. After I start it up the idle is way higher than it was previously, after about 10 seconds it comes back down until I hit the gas and it goes way up again and doesnt come down much, took it for a drive around the block and came to a stop several times and its like the blades are hanging open, and the idle eventually goes back down to a little higher than normal. I have read both that there is an idle learn procedure and that it will correct itself after driving. I need a definitive answer to go off of since its my daily driver. Any help would be greatly appreciated.

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    warnereg is offline Cadillac Owners Member
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    Re: P048 code - detonation at load 95 deville 4.9

    All is well!

    First of all I torqued the Throttle Body bolts to 14.5 ft lbs, which Ive read that spec is 10-12 ft lbs, nothing stripped so i think im all good on that one.

    Second cleaning the throttle body has improved throttle response, much quicker than before.

    Third, no more knock, i think that what I am hearing now is either the main bearing thump, or maybe I have a small vacuum leak somewhere, ive replaced over half of my vacuum lines due to dry rot.

    Fourth, gas mileage has improved greatly, before I was averaging 19 mpg. Took it for a drive anx by the time I reached my destination I averaged 31.5 mpg.

    Also, holding the EGR diaphragm all the way up and plugging the vac port to test is a myth, the diaphragm may stay up for certain EGRs that work that way. This is a positive backpressure EGR, you have to put the rectangular port to your mouth and blow while holding the diaphragm all the way up while plugging the vac port and it will stay up as long as backpressure is applied or vac port is plugged. My old one would not stay up, the new one will.

    Lastly, the idle relearned itself after about 15 miles of driving.
    Next im onto the radiator, head water outlet, and thermostat, all of which are leaking. Then the water pump cuz it leaks too. Lol.

    Other than all that shes running better than ever. Thanks for everyones input!

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