Engine Knock

Home  \  Repairs & Maintenance  \  Engine Knock

I have a 1997 Chevy Suburban with a 5.7 V-8. It has roughly 115k miles on it. I have had it since it was new and I've took great care of it. I've changed the oil every 3000 miles since mile 0. Anyway, I sometimes get a knocking noise in the engine when I start the motor after it has been sitting say overnight. It doesn't do it everytime but it seems that if the temperature is below 40 degrees it seems to knock until the engie warms up to at least 120 degrees. Also, my oil pressure gage acts very funny at the same time. Until the engine is warm, the gage moves almost like a tachometer. When I first take off, the gage spikes as high as it goes 80 then as I drive longer, the knocking goes away and the gage returns to the normal position and it doen't move near as severly as it does when the motor is just warming up. Do I maybe have a clog in my oil lines someplace or does this problem seem to be more severe than that. Any help would be great thanks.

posted by  sceptre

You have introduced us to your problem, but you failed to introduce yourself..Please do so.. Don't pass go, don't collect $200... :laughing: :laughing:

posted by  R34RB30DETTV

Just like when you visit a new place for the first time you should introduce yourself. We have a section just for that purpose and it's very well marked. That should be your first step, especially if you're looking for help or advise.

posted by  vwhobo

Is the start-up knock only at idle or at all engine speeds? What is the oil pressure on initial start-up idling? Did this start suddenly or gradually? Possibly right after an oil change or some other work was accomplished? Let's start there.

posted by  vwhobo

I believe it is at all speeds however, I can't hear it as much as the engine rpm's increase. Normally at idle the oil pressure is at 40 or just under. It seemed to just start all of the sudden, I'm not sure if it was just after an oil change but I can say that the oil was changed no more than two or three weeks before I first started hearing it. It is hard for me to do much more diagnosis becuase I live the the mid-west and it is spring time and the weather will most likely not get much below 40 for a long time. When I changed the oil, I put a new filter on. Is it possible some sort of clog in the filter? I checked the oil level and it seems OK.

posted by  sceptre

What weight oil and what brand of oil filter do you use.

posted by  vwhobo

I use 5W-30 as recommended by GM and an AC Delco filter. I believe the brand of oil was valvoline high milage if it wasn't that then it was trop artic by Phillips 66. I have used WIX and Fram filters in the past also. 115k mile = lots of oil changes. :laughing:

posted by  sceptre

Wow. In all honesty this is a hard one to call without seeing and hearing it. Low oil pressure might be accompanied by engine noise but not likely with high pressure. Also the filter would possibly cause low pressure, not high.

Maybe we should go after it this way. Try explaining the noise. I'm not trying to run you around, I'm just drawing one of those rare blanks.

posted by  vwhobo

What kind of gas are you running? If you're using regular, try going to higher octane; 91 perhaps.

posted by  Sonreir

The first time that I heard the noise, it was during this past winter. It sounded like to me like the fan was hitting a piece of ice or something. As the car warmed up, it went away so that's what I thought it was. It did it again maybe a week or two later and I noticed that my oil pressure acted as I described previously. I can't really explain the knocking noise anyway else besides the fan hitting something. I've looked in the fan shroud and I couldn't find anything. Besides the funny oil pressure pretty much rules out the fan hitting anything. I'm just afraid that this could be a serious problem and if I just ignore it, it will just get worse. I don't have the money for a new vehicle right now. The oil guage acts like a tach when the engine is not warm. It doesn't do this every time I start the engine either. It seems to only do it when it is cold outside. I could see if I was using a higher weight oil that a problem could occur. I did put 10W-30 in last fall, mostly by accident. But when it was the coldest outside, it had 5W-30 in. Anyway, thanks for the help that you have given me.

posted by  sceptre

For the price of a filter and maybe some peace of mind, WHYNOT replace your oil filter? Just a thought :doh:

posted by  lectroid


Any sign of oil leaks and what is your oil consumption between oil changes?

Has their been an increase in fuel consumption ?

After changing your oil , did you look at your old oil?

Was there any signs of metal in the bottom of the old oil (i know its kind of late to ask)?

Also take a look at your pcv system, your looking for excessive oil in the top end (no drain back).

posted by  hitchhiker

No leaks that I'm aware of. I don't use or burn any oil. My fuel consumption is fairly constant. I watched the oil drain into the enclosed pan so I couldn't really get much of a look. As I remember, it looked like normal 3000 mile oil, dark brown almost black. I don't really remeber any signs of metal. The plug is magnetic so it should catch some of it if there is any. I'm coming up on an oil change anyway so I will look for any of those signs. Wouldn't I be hearing a knock because of lack of pressure instead of excessive oil in the top end? How do I check to pcv system?

posted by  sceptre

What i was trying to get to was the fact that your oil pressure is very high with the knock. The only way i can think of that can happen is that the oil ain't going where it outta be. I was thinking that excessivly high oil pressure and a knock could be a drain back problem where the oil galleys are blocked.If this happened you would probably get a leak or two.

Also i had thought a of the pcv system being clogged. Positive Crankcase Ventilation if you didn't know, is the way an engine equalizes the pressure between the top (Valve covers) and the bottom of the engine ( oil pan). Removing the vacuum hoses ( Usually 3/8) and checking for clogs or leaks and making sure the pcv is free and clear. Lastly making sure the vacuum source is working.

All the care you put into you truck does not rule out the case that there may be mechanical engine failure, nor prohibit the fact you may just have to drop the pan and take a look. A spun rod bearing could have this effect.

If it is rod-knock, how could the oil pressure be so strong?
The rods are at the absolute end of the oil path; if you took #6 rod
out entirely and let the crank bleed, it wouldn't affect oil pressure
that much. What you're measuring is back pressure at the pump. It's
not a direct relationship to the pressure at any other place in the system.
Don't forget that an engine's oiling system is open ended. The oil eventually dumps back into the pan.

Other possibilitys could include a cracked piston skirt. Piston slap would occur when your oil was at its lowest flowability rate. ie. when its cold out and the vehicle has sat overnight. The increased oil pressure in this case would be due to blow by from this cylinder in the crankcase. This doesn't seem likely as the obvious running problems and oil/fuel contamination.

The last thing i can think of is a failure in the esc knock sensor. This system retards the timing to eliminate knocks in the engine. The old knock sensor code is 43 for open or failed knock sensor . We could attribute the oil pressure flucuation due the engine advance curve being way off. This will require you to get a code reader to see if the code is in the system. I am not sure if this code will trip the check engine light.

posted by  hitchhiker

That last thread is so full of misinformation that somebody has to set the record straight.

The oil drain backs are nothing more than than holes cast into the heads and block that allow oil to gravity feed back into the sump. While a blockage could in fact cause leaks it in no way affects the oil circulation system which is pressurized.

The PCV system has nothing to do with equalizing pressure throughout the engine. The crankcase, lifter valley and rocker boxes are all connected by the above mentioned drain back holes. Just like an inflated balloon pressure in one area will be the same as in any other area. Keep in mind though that a running engine does have varying pressure due to movement of the reciprocating assembly and any combustion blow by.

Finally something that is more or less correct. :thumbs:

For reasons previously covered, blowby will not increase oil pressure. It will increase crankcase pressure. And a piston/cylinder interface with excessive clearance may quite down as the engine warms and parts expand. A cracked piston is a cracked piston, the engine doesn't really care what the temperature is.

What are the chances of a bad knock sensor causing detonation at idle? The computer isn't commanding enough advance for that to happen. And we can't attribute fluctuating oil pressure to timing advance. For example, if an engine is running at 2000 rpm at 12 degress BTDC or 2000 rpm at 25 degrees BTDC, how does that affect the flow or pressure output of the oil pump? I don't know either.

posted by  vwhobo

As some may believe that my last post in the thread was “Full” of misinformation , let me clarify a few things.

If the oil filled both valve covers and the pump was still trying to pump up oil without relief from drain back, would that not in fact increase oil pressure? I know when I put my finger over the end of a hose the pressure does increase. A bit extreme for this scenario, I know. Would this to occur, you would have some mighty big leaks (more like spouting geysers). Or they could end up
forcing oil through the only other available orifice, the pcv hose if on that side.

check out this link,

and i quote " At higher engine speeds, blow by gases increase crankcase pressure that can cause oil leakage from sealed engine surfaces"

and here,

So, the pressure from blow by emanates in the crankcase case. That pressure moves past the oil flowing down through the oil galleys and into the valve covers and is relieved via either an open breather system or the pcv. The top and the bottom of the engine are always at a pressure differential except when the engine is a operating temp and at idle this will be very low.

The exhaust gases caused by blow by often are full of unburned HC from the combustion process and harmful acids. The pcv by using vacuum in the top of the engine to pull these gases out of the crankcase and with a closed system , reburns them. The side effect of this is the pcv becomes the controlling factor into when and how much vacuum is introduced into the engine to alleviate positive pressure. The very thing that allows a pcv to operate is its ability to equalize pressure from a positive to a negative state. Allowing more vacuum at higher rpms and less at lower. Thus the pcv can be said to equalize pressure throughout the engine. Taking positive from one portion of the engine and introducing negative from another. (Btw nice reference to Bernoulli’s principle.)

I believe this is completely correct regarding oil flow. If I am incorrect please let me know in what area.

What are the chances of a bad knock sensor causing detonation at idle? The computer isn't commanding enough advance for that to happen. And we can't attribute fluctuating oil pressure to timing advance. For example, if an engine is running at 2000 rpm at 12 degress BTDC or 2000 rpm at 25 degrees BTDC, how does that affect the flow or pressure output of the oil pump? I don't know either.

Probably slim, but if he has a bad knock sensor, wouldn't this by definition be proof.But for want of a better idea it’s a direction to walk in. Since you oil change is coming up anyways, how about sending your oil out to be analyzed? They could tell you if there was a high metal content and that may eliminate internal engine damage. You could also try the scan tool angle and perhaps glean some info there, looking for that code 43.

Apparently vwhobo will spit in a glass and tell you it’s full.

posted by  hitchhiker

And if everyone in America flushed their toilet at the same time both the Atlantic and Pacific oceans would flood simultaneously. Both scenarios are to much theory and not enough reality.

So what you just did was change what you said and found a much more involved way of explaining what I said about a balloon. You can run an open vent or catch can and the pressure will still self equalize. What the PCV does is place negative pressure on the crankcase. Congratulations for being a fast typist and good at cut and paste.

The rods are the end of the line for the bottom end of the engine. Because of their size and loading they require far more flow than the top end. If "if you took #6 rod out entirely and let the crank bleed" it would bleed pressure so fast you would first damage the top end due to lack of oil. Hey, you asked.

Proof of what? Are there any codes? If the knock sensor is bad, why does the noise go away when the engine is warm and under load when it would be most likely to detonate? And why did you forget to point out that what I said about oil pressure being dead on, or would that hurt your pride.

Apparently you feel if you attack some logical reasoning with bullsh*t and then follow up with a personal attack it will make you appear to be correct. The only thing it has done is shown that you are way above your head on this subject.

posted by  vwhobo

Very informative, I will make sure all my plumbing questions are posted here

Actually , if you had read what I wrote with an opened mind you would have realized I expanded my line of thought and gave reference to support what I had written. “Thus the pcv can be said to equalize pressure throughout the engine” sound familiar? Its what I said in my original post. I didn’t reference your balloon analogy because I thought it was a poor example. A better analogy would have been you have a balloon with an intake and exhaust . You endeavor to keep the balloon the same size. The inlet has varying pressures coming in and you use the outlet to control the size by letting out air .
Sound a bit better? Objectively speaking that is.
Btw I type with one finger, maybe two on a good day.

So, my post was only partially correct because I forgot to mention the fact if you remove engine bearings it would be bad for the engine. Right, writing that down now.

Proof that a knock sensor being bad can affect oil pressure, its what I was talking about. Why did I forget the oil pressure thingy? Let me rectify that now, finally something that is more or less correct.

Oh I see, if I lead with a personal attack and then present facts it would be fine , writing that down as well . And quite often when I am in over my head I try a unique method only known to me . I swim.

posted by  hitchhiker

Good for you.

I did read it and it was only somewhat more correct the second time than the first. The crankcase pressure will equalize all by it's lonesome. It did it for years before the PCV valve came along. The purpose of the PCV is to reduce unburned hydrocarbons by pulling them through the engine instead of venting them to the atmosphere, not to equalize pressure.

You are the one who is apparently unable to read. That bullsh*t sentence is nothing more than a weak attempt at tap dancing around the fact that you were proven wrong.

Again, another weak attempt. My comment on the oil pressure is exactly correct.

You should have used the screen name Fred Astaire with all the dancing you're doing. Why is it that small minded people like you can't make at least a reasonable attempt at getting their facts straight BEFORE they post. If you're going to provide advice, you have a responsibility to give the best information possible.

posted by  vwhobo

Hitchhiker - the oil pump on most cars is driven in some form or fashion by the crankshaft. The crankshaft sits there and spins at a given rpm. The crankshaft does not care if the 2000 rpm it is spinning is occurring if the ignition is timed @ 20 BTDC or 25 BTDC - 2000 rpm is 2000 rpm.

Sceptre - I am stumped on this one = let me think about this some.

posted by  tbaxleyjr

Does the noise sound like a rapid clicking (like someone is holding a card next to bycycle wheel spokes) or is more of a metal to metal clanging noise?

Can you tell from what part of the engine assembly it is coming from? If it as a clicking sound, you may want to listen to the area around the valve cover either with a 3/8 in. hose or a mechanics stethoscope to try to pinpoint the area where the noise is originating.

posted by  tbaxleyjr

Hello, I am Nicholas from Trinidad in the West Indies and I have a similar problem as Sceptre with a Subaru Impreza Wagon imported used from Japan. The mileage on the dash states 34,000km (but this could have been adjusted in Japan) and the year of manufacture is 2001. I just wanted to know what you did to solve the problem?

posted by  Supersonick

Hello, I am Nicholas from Trinidad in the West Indies and I have a similar problem as Sceptre with a Subaru Impreza Wagon imported used from Japan. The mileage on the dash states 34,000km (but this could have been adjusted in Japan) and the year of manufacture is 2001. I just wanted to know what you did to solve the problem?

posted by  Supersonick

Ok, trying changing the oil filter again. The oil filter could be defective. This happened in our shop the other day on a Chevy truck with a Vortec V8 engine. The filter was defective.

posted by  KNTRDR

Your Message