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HT4100 4.1, 4.5, 4.9 Discussion, Premium fuel? in Cadillac Engine Technical Discussion; Do you guys think premium fuel is really required or has to be used if the manual recommends it? My ...
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    gtm245 is offline Cadillac Owners Fanatic
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    Premium fuel?

    Do you guys think premium fuel is really required or has to be used if the manual recommends it? My caddy DeVille (4.5 V8) doesn't have a knock sensor and the manual recommends 91 octane gas. My mechanic says that the 91 isn't really necessary unless you can hear the knocking or pinging under a load (going up a steep hill or rapid acceleration). Currently I'm using 89 octane (its a gasahol) and I worrying about knocking and pinging. I know knocking and pinging is more likely if you run the vehicle hard, and I do so I don't know if I should be using the 89. When I accelerate hard it makes an unusuall noise but it's not a knocking sound. I don't know how to explain it maybe its the bearings I don't know. I guess I could run this tank of gas until its almost empty and then put 91 octane in and see if the mysterous noise goes away? Any thoughts?

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    Re: Premium fuel?

    We've had this discussion soooooo many times it's almost a cliche. Many here will tell you to use the lowest octane rating you can until you hear knocking. I will tell you (from experience) to use what is required for your engine, PREMIUM. It is what the Cadillac engineers designed it to run on. Many here will say, "well, so what if there is some knocking?" Knocking is detonation, or two flame-fronts colliding into each other causing more pressure on the pistons, etc, and this can cause damage in the way of deformed sparkplugs, piston damage, and maybe cracked cylender walls? etc. Don't be cheap, use the good stuff because you drive a good car. Why take any chances?

    Everyone has heard this from me, but here it is again, before I took delivery of my 1991, for some years my dad ran 87 octane! He is partially deaf and could not hear the knocking. Thank God he never drove it hard or I believe there could have been engine damage. I drove it back then, and around 40 mph, it was knocking very badly! I immediately ran one tankful of premium and all the noise went away. If you are really worried use some octane booster NOW, or wait until it's almost empty, then try one tank of premium, see if the noise goes away. Some will say to play with the engine timing, and a few other tricks, etc. and that's fine too, I'm just giving you my opinion.

    I'm not going to argue with anyone on this one, it's not my car, you can do what you want, but I would like to see you get the max life out of your engine, and I will always use the best gas for the best car in the world! Good luck!

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    gtm245 is offline Cadillac Owners Fanatic
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    Re: Premium fuel?

    So if this noise I'm hearing is a light knock and I switch to premium and the noise goes away, there is a good chance that there was no damage done right? Assuming this is predetonation and it goes away. I just don't know how to explain this noise. It doesn't happen all the time and it doesn't happen just when I'm hard on the car either. It's like a low repetative grumbling sound that happens every once in a while. Anyway 13 more gallons to go and then in goes the 91. I'll post here and tell you if the noise goes away.

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    Re: Premium fuel?

    Quote Originally Posted by gtm245
    So if this noise I'm hearing is a light knock and I switch to premium and the noise goes away, there is a good chance that there was no damage done right? Assuming this is predetonation and it goes away. I just don't know how to explain this noise. It doesn't happen all the time and it doesn't happen just when I'm hard on the car either. It's like a low repetative grumbling sound that happens every once in a while. Anyway 13 more gallons to go and then in goes the 91. I'll post here and tell you if the noise goes away.
    Spark knock is a pinging sound....kind of like marbles rattling in a can with a rag wrapped around it....rattly but somewhat muted and damped.

    If you go to a better grade of fuel (higher octane) and the noise goes away then that is probably the problem.

    Extremely unlikely that you hurt anything driving it like that. Passenger car engines are designed to accept a certain amount of detonation without any damage. Detonation at part throttle like that is relatively benign. Engines can run for tens of thousands of miles with even moderate detonation at part throttle with no problem. Detonation at full throttle is another issue...but on a relatively lightly stressed engine like the 4.9 (low specific output) even then damage is unlikely. We are talking about your passenger engine here....not racing engines or modified engines.


    There is no such thing as "predetonation":

    Detonation is the spontaineous combustion of the end gas in the combustion chamber. It occurs AFTER the spark plug has initiated the normal burn in the chamber and is caused by the heat and pressure exceeding the octane capability of the fuel.

    Preignition is the ignition of the mixture in the chamber by something other than the spark plug BEFORE the spark plug has had a chance to ignite the mixture.

    Related phenomenon no question....but two totally different things.



    There are many many things that affect the octane requirements of an engine. The compression, the amount of carbon buildup increasing the compression, the spark timing, the coolant temp, the inlet air temp , the barometric pressure (and correspondingly the altitude the engine is operating at), the humidity , etc.. It is impossible to say what fuel octane rating is "required" for an engine without knowing all those things. Having said that, the 4.9 was optimized for premium fuel operation. The compression , spark timing, etc. was all optimized assuming premium fuel was used all the time. This covers the worst case situations. However, most all of the other time, a lower grade of fuel is fine. Most 4.9's that I have seen will operate fine on mid grades of fuel, even regular, if the weather is cool, humid, light duty driving, etc. If you are going to tow a trailer thru Death Valley in the summer I would recommend Premium and the best you can find. For Michigan and a great deal of the country in the fall/winter/spring regular or midgrade is fine usually. The key is to listen for spark knock and use a better grade of fuel if it is heard. The factory recommendations for Premium sidesteps this issue by saying Premium because that will work regardless of the situation. If you have a knowlegable ear.....use the lowest grade of fuel that will not spark knock in your driving and you will be fine. Also, understand that the factory requirement for Premium is also basedon the fact that the engine has no knock control system...so it is not protected against detonation like a Northstar would be.

    There is another factor that affects detonation or spark knock a great deal. That is the EGR system and the EGR flow. If you have a 4.9 that is spark knocking on lower grades of fuel in moderate weather then I would suspect that the EGR system is partially restricted, ESPECIALLY if the spark knock happens at light throttle/part throttle driving and then stops when you mash the gas wide open.

    The EGR is easy to check. Remove the air cleaner and hold the throttle blades wide open (engine off). Look down the throttle bores with a strong flashlight. See the two tubes sticking up at you beneath the throttle blades from the floor of the intake manifold?? Those are the EGR delivery tubes and they should NOT be full of carbon. Rod them out with a stiff piece of coat hanger wire until they are clean so the EGR can flow. Perhaps this will fix your noise and/or minize the requirements for premium.

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    gtm245 is offline Cadillac Owners Fanatic
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    Re: Premium fuel?

    "Spark knock is a pinging sound....kind of like marbles rattling in a can with a rag wrapped around it....rattly but somewhat muted and damped."

    Thats a pretty good description of what I'm hearing except it is really a dull sound; not like a ping. Nevertheless that is pretty much what it sounds like. It's odd because it happens spontaniously. One day it will happen at WOT and a couple days later it will happen when I'm driving a steady 30 MPH through town and anywhere in between. But it is very muted and barely audiable. I guess I'll try the premium next tank and see what happens.

    BTW didn't the 4.5's have bad main bearings that were common to make noise and sometimes fail? I remember hearing something about that and there are revised bearings for it.

    (sorry about "predetonation" as I did mean preignition)

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    Re: Premium fuel?

    Quote Originally Posted by bbob
    Spark knock is a pinging sound....kind of like marbles rattling in a can with a rag wrapped around it....rattly but somewhat muted and damped.

    If you go to a better grade of fuel (higher octane) and the noise goes away then that is probably the problem.

    Extremely unlikely that you hurt anything driving it like that. Passenger car engines are designed to accept a certain amount of detonation without any damage. Detonation at part throttle like that is relatively benign. Engines can run for tens of thousands of miles with even moderate detonation at part throttle with no problem. Detonation at full throttle is another issue...but on a relatively lightly stressed engine like the 4.9 (low specific output) even then damage is unlikely. We are talking about your passenger engine here....not racing engines or modified engines.


    There is no such thing as "predetonation":

    Detonation is the spontaineous combustion of the end gas in the combustion chamber. It occurs AFTER the spark plug has initiated the normal burn in the chamber and is caused by the heat and pressure exceeding the octane capability of the fuel.

    Preignition is the ignition of the mixture in the chamber by something other than the spark plug BEFORE the spark plug has had a chance to ignite the mixture.

    Related phenomenon no question....but two totally different things.



    There are many many things that affect the octane requirements of an engine. The compression, the amount of carbon buildup increasing the compression, the spark timing, the coolant temp, the inlet air temp , the barometric pressure (and correspondingly the altitude the engine is operating at), the humidity , etc.. It is impossible to say what fuel octane rating is "required" for an engine without knowing all those things. Having said that, the 4.9 was optimized for premium fuel operation. The compression , spark timing, etc. was all optimized assuming premium fuel was used all the time. This covers the worst case situations. However, most all of the other time, a lower grade of fuel is fine. Most 4.9's that I have seen will operate fine on mid grades of fuel, even regular, if the weather is cool, humid, light duty driving, etc. If you are going to tow a trailer thru Death Valley in the summer I would recommend Premium and the best you can find. For Michigan and a great deal of the country in the fall/winter/spring regular or midgrade is fine usually. The key is to listen for spark knock and use a better grade of fuel if it is heard. The factory recommendations for Premium sidesteps this issue by saying Premium because that will work regardless of the situation. If you have a knowlegable ear.....use the lowest grade of fuel that will not spark knock in your driving and you will be fine. Also, understand that the factory requirement for Premium is also basedon the fact that the engine has no knock control system...so it is not protected against detonation like a Northstar would be.

    There is another factor that affects detonation or spark knock a great deal. That is the EGR system and the EGR flow. If you have a 4.9 that is spark knocking on lower grades of fuel in moderate weather then I would suspect that the EGR system is partially restricted, ESPECIALLY if the spark knock happens at light throttle/part throttle driving and then stops when you mash the gas wide open.

    The EGR is easy to check. Remove the air cleaner and hold the throttle blades wide open (engine off). Look down the throttle bores with a strong flashlight. See the two tubes sticking up at you beneath the throttle blades from the floor of the intake manifold?? Those are the EGR delivery tubes and they should NOT be full of carbon. Rod them out with a stiff piece of coat hanger wire until they are clean so the EGR can flow. Perhaps this will fix your noise and/or minize the requirements for premium.
    Hey b-ski, I'm sure you covered this before but I didn't print it out. I have a 92 Eldo 4.9 and I want to change the timing from 10 to 12 BTDC. Can you give me a quick rundown on procedure ? I know the computer insists on getting involved if some things aren't disabled right. Thanks
    Bob

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    100
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    Re: Premium fuel?

    Quote Originally Posted by bob2231
    Hey b-ski, I'm sure you covered this before but I didn't print it out. I have a 92 Eldo 4.9 and I want to change the timing from 10 to 12 BTDC. Can you give me a quick rundown on procedure ? I know the computer insists on getting involved if some things aren't disabled right. Thanks
    Bob
    Well, I am not bbob...
    As far as I know, changing the timing on 4.9 doesn't require any
    computer tricks. Assuming that your car is tuned right at 10 degree,
    I believe all you have to do is to loosen the distributer and rotate in
    clockwise (to advance) 2 degree.

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    Re: Premium fuel?

    You need to put it in base timing mode first. I think it is jump pins A and B on the ALDL, but your Emissions label should have that info on it.

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    100
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    Re: Premium fuel?

    OK, I have a question on knocking.

    My DeVille has been knocking all the time. The timing is right.
    The EGR valve seems to be OK. The way I checked it was; I
    manually opened EGR with my finger at idle and the engine almost
    stalled, concluding that the EGR is OK.
    O2 sensors are working fine, passed the emission just a couple
    of months ago, so I assume the A/F mixture ratio is not too off.
    The thermostat is working, the coolant temperature never
    exceed 110 degree C.
    No codes for intake air, coolant temperature sensors, none for
    outside, intake air pressure sensors. The fuel pressure regulator
    is not leaking. When I measured the fuel pressure at idle, it was
    40 psi (maybe a bit too low?).
    By the way, the engine is a rebuild one, replaced before I bought it
    and it has only about 20K, so I think carbon deposit is unlikely.

    I can't find anything wrong so far. I don't know what else I should
    try. Actually I am making a custom timing controller (I don't know if
    it will work or not). Any suggestions?

    I should have made a new thread...???

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    Re: Premium fuel?

    Quote Originally Posted by 100
    OK, I have a question on knocking.

    My DeVille has been knocking all the time. The timing is right.
    The EGR valve seems to be OK. The way I checked it was; I
    manually opened EGR with my finger at idle and the engine almost
    stalled, concluding that the EGR is OK.
    O2 sensors are working fine, passed the emission just a couple
    of months ago, so I assume the A/F mixture ratio is not too off.
    The thermostat is working, the coolant temperature never
    exceed 110 degree C.
    No codes for intake air, coolant temperature sensors, none for
    outside, intake air pressure sensors. The fuel pressure regulator
    is not leaking. When I measured the fuel pressure at idle, it was
    40 psi (maybe a bit too low?).
    By the way, the engine is a rebuild one, replaced before I bought it
    and it has only about 20K, so I think carbon deposit is unlikely.

    I can't find anything wrong so far. I don't know what else I should
    try. Actually I am making a custom timing controller (I don't know if
    it will work or not). Any suggestions?

    I should have made a new thread...???
    Per your earlier post....there is a "computer trick" to setting the distributor reference setting....if you didn't use it when you checked/set your timing then possibly it is off. You have to go to the ALDL connector under the dash beside the steering column and jumper pins A and B together. If the engine is idling and the trans is in park this will force the computer to deliver a fixed 10 degrees in the "set timing mode" and you can now check/set it with a timing light.

    I would double check the timing and make sure that it is correct. If the spark knock persists then retardt the timing a degree or two (set it to 9 or 8 instead of 10) and see if that helps.

    There are two other possibilities that I can think of. The check you did on the EGR is good and shows that the system is capable of flowing EGR...but...it doesn't prove that the EGR solenoid is turning on and sending vacuum to the valve. You could tee in a vacuum gauge between the solenoid and the valve and double check that the EGR valve always sees vacuum supply from the solenoid when the throttle is opened. If not, there is likely a problem with the EGR solenoid/vacuum supply that is preventing the EGR valve from opening. That would cause spark knock.

    The other way to check that the EGR is opening is to have a helper start the warmed up engine and put it into drive and hold the brake. Have them GENTLY crowd the throttle while holding the brake to load the engine a little against the torque converter. This should turn the EGR on. You can reach under the hat of the valve and feel the diaphragm with your finger tips while the helper loads the engine lightly. You can feel the EGR valve diaphragm open slightly as the vacuum signal hits it. If nothing happens then that is a clue that the EGR may not be working even though it is capable of flowing if it gets the vacuum signal.

    The other possibility is that the mark on the harmonic damper is off. The timing mark for TDC is part of the outer ring of the damper. If the elastomeric bond between the hub and outer ring of the damper is broken the outer ring will migrate and the mark will be meaningless. This can often happen on older, high mileage dampers on all cars and especially if the damper in question has been hammered on or imporperly installed in the past. If you have a replacement engine then likely the damper was removed and reinstalled at some point and it may have been damaged. The only way to check this is to use either a positive stop or a dial indicator and find TDC in #1 and verify that the mark is at TDC on the timing tab. This is not really hard to do by it is time consuming.

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    Re: Premium fuel?

    Thank you for very quick reply, bbob!

    Well, the base timing was inspected right by shorting A and B
    terminals at ALDL, but as you suggested if the mark is off, it
    is off. You are right about it, I didn't think about this.

    I tried to retard the timing by 2-3 degrees, it helped a bit, but
    not enough to eliminate all knocks. Now the timing is set at
    about minus 6-7 degrees off just to stop knocking (but it still
    knocks sometimes).

    About the EGR, you are right about it. But I noticed that when
    the vacuum pipe was disconnected at the EGR valve and I unplug
    the electric wires from the EGR solenoid while idling, the EGR
    valve side of the vacuum pipe started sucking! Is the EGR solenoid
    "normally open" solenoid?? I thought it would have been a
    "normally closed valve" type solenoid. Well, when I plug the
    electric wires back again while idling, it stopped sucking, so the
    solenoid is somewhat OK, I think, am I right about it? Well, I will
    put the vacuum gauge and see if it is really pulling vacuum well
    at higher revs.

    If nothing turned out to be the case, I suppose that I will have
    to mechanically see if TDC is right on No.1 cylinder...

    Again thank you!

    Quote Originally Posted by bbob
    Per your earlier post....there is a "computer trick" to setting the distributor reference setting....if you didn't use it when you checked/set your timing then possibly it is off. You have to go to the ALDL connector under the dash beside the steering column and jumper pins A and B together. If the engine is idling and the trans is in park this will force the computer to deliver a fixed 10 degrees in the "set timing mode" and you can now check/set it with a timing light.

    I would double check the timing and make sure that it is correct. If the spark knock persists then retardt the timing a degree or two (set it to 9 or 8 instead of 10) and see if that helps.

    There are two other possibilities that I can think of. The check you did on the EGR is good and shows that the system is capable of flowing EGR...but...it doesn't prove that the EGR solenoid is turning on and sending vacuum to the valve. You could tee in a vacuum gauge between the solenoid and the valve and double check that the EGR valve always sees vacuum supply from the solenoid when the throttle is opened. If not, there is likely a problem with the EGR solenoid/vacuum supply that is preventing the EGR valve from opening. That would cause spark knock.

    The other way to check that the EGR is opening is to have a helper start the warmed up engine and put it into drive and hold the brake. Have them GENTLY crowd the throttle while holding the brake to load the engine a little against the torque converter. This should turn the EGR on. You can reach under the hat of the valve and feel the diaphragm with your finger tips while the helper loads the engine lightly. You can feel the EGR valve diaphragm open slightly as the vacuum signal hits it. If nothing happens then that is a clue that the EGR may not be working even though it is capable of flowing if it gets the vacuum signal.

    The other possibility is that the mark on the harmonic damper is off. The timing mark for TDC is part of the outer ring of the damper. If the elastomeric bond between the hub and outer ring of the damper is broken the outer ring will migrate and the mark will be meaningless. This can often happen on older, high mileage dampers on all cars and especially if the damper in question has been hammered on or imporperly installed in the past. If you have a replacement engine then likely the damper was removed and reinstalled at some point and it may have been damaged. The only way to check this is to use either a positive stop or a dial indicator and find TDC in #1 and verify that the mark is at TDC on the timing tab. This is not really hard to do by it is time consuming.

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    Re: Premium fuel?

    Quote Originally Posted by gtm245
    So if this noise I'm hearing is a light knock and I switch to premium and the noise goes away, there is a good chance that there was no damage done right? Assuming this is predetonation and it goes away. I just don't know how to explain this noise. It doesn't happen all the time and it doesn't happen just when I'm hard on the car either. It's like a low repetative grumbling sound that happens every once in a while. Anyway 13 more gallons to go and then in goes the 91. I'll post here and tell you if the noise goes away.
    In my 1988 seville (4.5tbi) a low grumbling noise can be heard when the "smog pump" line is disconected from the air cleaner assembly and you hit the gas, I would describe the noise as something like a shop-vac on steroids. I would check the "smog pump' hose as well as the other suggestions.

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    Re: Premium fuel?

    Thanks for the procedure guys.
    Bob

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