Engine knock - 4.9L w/ 90k
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HT4100 4.1, 4.5, 4.9 Discussion, Engine knock - 4.9L w/ 90k in Cadillac Engine Technical Discussion; Hello all, Purchased a used 91 about 6-7 months ago around 75k on it. Have replaced oil every 4-5k and ...
  1. #1
    Cream_Devil_91's Avatar
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    Engine knock - 4.9L w/ 90k

    Hello all,

    Purchased a used 91 about 6-7 months ago around 75k on it. Have replaced oil every 4-5k and put all new Monroe Sensa-tracs on all 4 corners. Car now has 90K and the only issues I've had is an intermittent E48 EGR code. Code will appear every few hundred miles and when it does I will press the trip button. It always goes away in less than 50 miles and will stay gone for another few hundred or so and then come back for a few miles. Don't understand, the tubes have been rodded out and I've cleaned the TB real good so I know that's not the issue.

    Now onto my knocking issue...

    A few days ago I was feelin froggy and decided to take it in to get the timing bumped up a degree or two after reading a lot of people getting a little performance by doing that. Well I honestly couldn't tell any difference and then the other night I started it up and it was making this knock sound at idle. I only hear it at idle and low speeds/slight load. (Under 20). When I'm driving anything above 20 or so I don't hear or feel anything out of the ordinary, only when I slow down to a stop you can definitely hear a knocking. I took it back and got them to put the timing back where it was at 10 BTDC. Knock is still occuring. Have taken it to a few places and more than one place has told me it sounds like the water pump failing. Maybe it's not timing related and the knock is just coincidence after getting my timing messed with (which I deeply regret now). Could a failing water pump make a loud knocking sound? It definitely is coming from that area. I'm about to go record a sound clip of it to help diagnose...

    Thanks everyone, this site has been a great resource and wealth of information.

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    AMENCADDY's Avatar
    AMENCADDY is offline Cadillac Owners Enthusiast
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    Re: Engine knock - 4.9L w/ 90k

    release the tensioner pulley on your belt drive and then you can manually spin the water pump and see if it is noisy or binding at all. In fact try all the pulleys (alternator, etc.) since the sound is coming from that area.

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    Cream_Devil_91's Avatar
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    Re: Engine knock - 4.9L w/ 90k

    Thanks for the response, in fact I just removed the belt and ran it for a minute and the noise is gone. I'm 100% positive it's the water pump goin out on me. The pulley spins fine by hand, no noises or binding, but when it's running with the belt on and I held a screwdriver to it, you feel some roughness in relation to the knocking. I just ordered a new delco water pump w/ gasket from Rockauto, the 137 dollar one that says OEM design. Not gonna chance it with any of the others. Talked to a guy today that was very familiar with the 4.9 and has done the water pump before on one, but he was very reluctant to say he'd be willing to do it again. He said it was quite a bitch to do and sounded somewhat intimidated. He never could tell me how much he'd be willing to charge for it, although he did say he'd need it about a day or two. I know the old man will do good work though and take his time on it...we'll see

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    Ranger's Avatar
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    Re: Engine knock - 4.9L w/ 90k

    Remember, you also have an idler and tensioner pulley along with an A/C clutch and an alternator, all of which have bearings that can and will knock when they go bad.

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    Cream_Devil_91's Avatar
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    Re: Engine knock - 4.9L w/ 90k

    Bit of an update...

    Went ahead and purchased the water pump and pulley along with Fel-Pro gaskets. Also got an O2 Sensor, rad cap, cap & rotor, plugs and wires, and a new thermostat since she'll be gettin work done. The stuff in her all appears to be original still. Purchased all AC-Delco OEM except the gaskets and pulley from rockauto with a 5% discount code I googled. It all shipped from a town about 45 minutes away and I got it next day! Couldn't believe it... I'm gonna wait till after all the holidays cool down though to get her in the shop...she'll be nice and ready to go for the new year then haha

    Anyways I have another question about timing and finding BTDC...

    As of now my timing is all screwed, she is just sluggish and running like something is off. No codes though and the few places I've taken her to to get the timing checked out, I didn't feel confident in what they did. I've heard from a few people that they actually RETARD timing to get a little more performance, and from what I've read that makes more sense than advancing the timing. Retard = delay = more compression before spark correct? Advance would be the opposite, at least the way I understand it... Can someone shed some more light on this please?
    Also how does one go about finding BTDC without taking the number one spark plug out? Is there a way? No shop I took it to removed anything to set the timing, we just jumped A B and they hooked up the timing light to spark #1 and went to adjusting...this didn't look or feel right to me.

    @ Ranger - All pulleys and the tensioner have been replaced a few thousand miles ago while chasing a loud squeal that turned out to be the new power steering pulley I'd put on. Basically the pulley was too far in and caused the belt to be misaligned...should have been the first thing I checked but eh live and learn.

    Thanks all for the help and knowledge

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    drewsdeville is offline Cadillac Owners Master
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    Re: Engine knock - 4.9L w/ 90k

    Ignition timing setting is related to the time it takes for fuel to burn. All engines have the timing set retarded, hence the tern Before Top Dead Center. This is to compensate for the delay between the fuel burning and the piston reaching TDC and beginning the power stroke. More retarded does not = more performance necessarily. If anything, more advanced timing could possibly yield greater potential. However, there are a lot of variables to consider when playing with ignition timing that I'm not going to go through because GM already did it for us. Set it at 10 degrees BTDC, nothing else.

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    Re: Engine knock - 4.9L w/ 90k

    WHOA !!! There's no need to retard timing. Your idea of greater compression/power time is false.

    Think, guys: Ignition is advanced, within fuel octane limits, BTDC (Before Top Dead Center). Retarded is ATDC. Normal ignition timing is initially set at about 6 degrees BTDC (vacuum disconnected, idle), vacuum advance dials in more (maybe another 12 degrees BTDC) depending on engine load (vacuum) and mechanical advance dials in another 8 -16 depending on engine speed. Soooooo.......at highway speeds you're running about 36 - 38 degrees total advance. Put a little load on it, vacuum drops, and some advance comes out to prevent spark knock/ping.

    The timing is set, initially, by mechanically setting #1 piston at TDC on the compression stroke and stabbing in the distributor with the rotor set at #1 wire, TDC center. Then, with vacuum disconnected and plugged, idle at about 650, you use a light to set the timing to factory initial advance, usually 6 - 8 degrees, and tighten down the distributor hold-down. From there on it's design vacuum and mechanical advance at work unless you are really good at working up advance curves for specific engine work..

    Why advance? Gasoline/air vapor does not explode, it burns rapidly. Advanced spark is used to establish a uniform flame travel across the piston so that, as the piston reaches TDC, the combustion chamber pressure is just short of maximum, reaching max pressure at about 20-30 degrees ATDC, thereby extracting greater power from the combustion precess. The flame travel cannot keep up with the downward stroke of the piston if you light it off at TDC, so power suffers greatly.

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    Re: Engine knock - 4.9L w/ 90k

    I only use the timing mark for a reference, and time it from there (advance it from there). When it doesn't get any spark knock, it's the way I run it. Good for power and good for mileage. I mark the timing tab to where I timed it.

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    Re: Engine knock - 4.9L w/ 90k

    12 degrees is just fine and dandy and will help compensate(to an extent) for timing chain stretch. There will be a noticeable increase in power and it will not cause problems provided you run 91 octane or higher. Retarded timing is only beneficial in boost/nitrous conditions. As you drive, the computer actually advances the timing up to 35 degrees or so before top dead center because as the piston moves faster, the mixture needs more time to burn. Advancing the base timing to 12 just adds 2 degrees to the computer's pre-programed 'timing curve'.

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    Re: Engine knock - 4.9L w/ 90k

    Put a little load on it, vacuum drops, and some advance comes out to prevent spark knock/ping.
    Ahh..does that advance come out from the computer? So the 4.9 does have a knock sensor in a way...I have one fella who swore his whole shop (it was a very nice shop) on the fact that the 4.9 does in fact have a knock sensor. I politely told him I'd read otherwise that it didn't and we shared a few opinions, but overall he was still very adamant. I've been watching my timing advance through the computer a lot and notice it always shooting up to 50. I think that's the highest it goes I'm not sure, the hard drive with my alldata stuff is in a bind right now...

    Sounds like this issue has to do with vacuum too...could a vacuum leak, say through the water pump gasket mess with the timing? Sometimes I'll hear the sound of air whistling quite loudly while at idle that I don't recall hearing before... this also brings up my fuel consumption issue. Anything over I'd say half throttle is <5 MPG on the INST readout...could timing affect how rich or lean the engine runs? It honestly sounds like its being flooded in daily driving and my AVG MPG has been going down one sweet tenth at a time.

    Gasoline/air vapor does not explode, it burns rapidly.
    So what does that mean about the overall wear and tear on the engine mechanically? Will running an engine too advanced or retarded have any detriment on the life of the engine, or just affect things like power and fuel consumption? I can stand to put up with it for a week or two as it actually makes me think a lot about what could be happening and I want to learn about it. I really don't want to be putting her through hell every time she's running though either if you feel what I mean...

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    Re: Engine knock - 4.9L w/ 90k

    The 4.9 does NOT have a knock sensor.

    EGR helps to prevent spark knock, so if it is inop, you may get some spark knock.

    There is no vacuum associated with the water pump.

    Forget the instant MPG. It will always drop to near 0 when you put you foot in the throttle. It is basically a digital vacuum gauge. Open the throttle and the vacuum falls. Average is what counts.

    Timing does not affect fuel mixture. It is simply a measure of how far before TDC the plugs fire. Running it out of spec will affect performance and mileage. It shouldn't hurt the engine unless you run WAY out of spec's.

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Engine knock - 4.9L w/ 90k

    Sounds like you missed out on a nice shop. You should have told him to try to order one for you. Timing modifications and performance gains are grossly exaggerated. If you set it where those high paid GM engineers specified, it will run just fine.

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    Re: Engine knock - 4.9L w/ 90k

    I have (very) intermittent Knocking, rattling, bottom end noise in my Northstar with 126k. Car runs strong, smooth, yet I get a knock that goes away when I throttle her. Bad gas? I only use premium of course. Stumped. Thanks.

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    Re: Engine knock - 4.9L w/ 90k

    I run 93-94 oct with my timing set at 12-13 btdc . With 0 problems with 140,000 on the clock.

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    Re: Engine knock - 4.9L w/ 90k

    I would just like to add that I had a problem with an knock in a 4100, detonation is what it turned out to be. But it sounded just like a rod knocking, knock on light acceleration aggravated by the engine being hot. It was the EGR which if working correctly puts exhuast into the combustion chamber which cools combustion reducing detonation. Just a suggestion, easy to check the EGR and if you have a code...

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