Loud knock after vacuum hose change

Yesterday, I replaced all the vacuum lines on my 1995 LeSabre. Afterwards, I started the engine to check for leaks.

Of course, my ear was sure it heard one, but I think it was just the power steering pump making a new noise. Either that, or there's a component that now has vacuum that hasn't before. Now that is does I'm hearing the vacuum pressure applied to it... My ear is sure it's a hissing noise, but I was unable to find it by running my ear all along the top of the intake manifold.

It's louder toward the front of the engine I believe, so it's either power steering pump or the PCV valve. (I recently removed the PCV just to check that it rattled. I'm sure I re-installed it correctly... but perhaps I did not get it seated all the way. Perhaps it was not working before, and shaking it made it start. Perhaps shaking it broke it and I should just spend $3 to replace it to be on the safe side.)

However, there now a loud knock that is extremely disturbing. It only occurs when the engine is cold. After the engine is at full operating temp, I cannot make it to reoccur. As the engine warms up, the intensity of the sound steadily decreases.

The knock is not always present and it does not occur randomly. It only occurs when the engine is trottled up hard and then released when the engine is still below full temp. To replicate the symptom, I can rev the engine twice to 3000 RPM (approx), then let it drop to idle. About 2 seconds after the engine drops to idle, there is loud "clunk" from the engine.

Really loud, sounds like a piston rod impacting straight into the side of the engine. The noise is likely similar to what a loud cylindar detonation sounds like.

Since it started after I changed the vacuum hoses, those components are the ones I investigated. My theory being, I corrected vacuum to one of those components and diphram is engaging hard because it's not used to having full vaccum. I placed my on the fuel regulator, transmission modulator, master cylinder, and cruise control one at a time and replicated the problem. I was unable to feel a hard vibration or knock coming from any of them when I heard the sound.

Since the noise ceased after the car warmed up, today I am going to investigate if maybe it is the EGR solinoid opening and closing. The EGR is electronically controlled, but still. The EGR added exaust to the intake, which is where the vacuum is generated. So changes to the vacuum system would cause it to react differently or start functioning. Plus, changing the vacuum pressure will also change the readings that are monitored by the PCM, which controls the EGR valve.

So anyway... I'm going to check the EGR out in a couple minutes, but I'm somewhat doubtful. Wanted to find out if anyone else has ever experienced this, or if anyone else has any other good guesses and expliations as to what it could be.

Just to sum up:
-Engine below temp.
-Noise decreasing as engine temp increases.
-Only occurs when engine is reved hard (usually twice).
-Occurs 2 seconds after engine drops back to idle.

This clunck sounds like a backfire. Something vacumn controlled must be somehow messing up the spark timing. I would check the PCV. Also see if there is any carbon on the wrong side of the air filter.

As for the EGR valve, unless the operation of one of these changed between 1986 and 1995, and I'm not saying it hasn't, there shouldn't be any EGR at idle. According to my service manual;
-engine must be above idle, less that WOT
-code 33 or 34 not present
-Engine coolant temp. 42.5degrees celcius or greater
-transmission in gear.

I would like to know if that is different on the newer cars, so I don't keep thinking it isn't. Even the old carbureted vehicles didn't have vacumn at idle. But, those are old...this is newer stuff.

My 1995 must have vacuum at all times. As the vacuum controls the fuel regulator and environmental controls. Both of which work at idle, and both of which draw vacuum from the same vacuum manifold.

The PCV valve on this model engine does not use any vacuum tubes. Instead, it is fitted into a capsule located at the rear-left of the intake manifold. Vacuum created by the intake manifold draws blowback gases up into the manifold for burning. Pressue in the crankcase opens the PCV valve to allow the removal of the gases.

I see no reasons why the EGR valve wouldn't open at idle. The engine is still cold, which would cause a low quality air fuel mix. So the EGR should open to inject heated air into the intake to better the air/fuel mix. The only reason I could think why you'd not want the EGR open at idle, if because the pressure of the exaust would be several times greater than the flow of fresh air being sucked in by the intake. (Due to the car being stationary.)

This could feasibly cause some problems, I may be hearing the EGR closing... but it's way louder than I think it should be... most likely because if it has not been functioning prior to this, the action is a little sticky and inhibited.

Spark timing and the ICM should be controlled entirely by the camshaft and crankshaft sensors. However, these sensors are not read when the engine is cold. So the engine would be running on just known timing varibles, and not retarding or advancing the timing at all.

So that could be a problem... and it may of corrected itself alright. After the engine warmed up, I took the car for a good 15 mile drive. No problems at all. This likely would give the PCM enough data to adjust the timing to correct for any backfire. Thus, the problem may be gone when I go start the car today. We shall see.

The EGR system is used to lower the NOx emissions. By diluting the air/fuel mixture combustion temps are reduced. The egr valve is activated when the engine is in closed loop and above idle speed.

My 92' LeSabre started doing the very same thing, now I did the same thing that you did with the vacuum lines, but I can't say there was a connection because I don't remember hearing the knock right after I fixed the vacuum lines. Now I had two knocking sounds that sounded very similar, one was simple and the other I never found. Both of these noises occured after revving the engine up and then instantly letting it fall back down to idle. One of them was the stupid belt tensioner smacking back and forth as the engine re-adjusted it's idle, but it took me a while to figure that out. The other sound almost sounded as though it was coming from the transmission casing on the drivers side where the vacuum modulater was located. I think the second knock sound is more like what you have, it almost sounds deep in the engine or tranny, just like you say, as if some internal part is really clunking. If it helps you at all, I can vouch for the fact that this was not an EGR sound because my 92' didn't have EGR at all, lol. There was a period of a year or so when this engine was not at all equipped with any sort of EGR system, I did of course have the EVAP system for fuel tank vapors. The knock sound though is too loud to be caused by anything vacuum related. Have you ever revved the motor like this before? Perhaps you never had occasion to rev your engine or pay attention to this until you wanted to check for vacuum leaks. On my new 2000 I have heard a fainter version of the same knock sound upon revving the engine in the same manner, I have heard some people talking on other forums about this being a drive chain symptom, but I don't think the drive chain should be moving at all when the car is parked.

PcMos- Yeah, your second knock sounds similar to mine. I was actually planning to post that the knock is not so much loud as it is deep. Sounds like it is coming from inside the engine, which is where my "piston rod impacting the cylinder" analogy came from.

I don't think I've ever reved the engine like this when under the hood before. I've never had cause too. Since I had just changed the vacuum lines, I suddenly noticed this whistling sound and was trying to find it. (I reved the engine to create more vacuum pressure trying to make the noise louder. Didn't work, just scared the crap out of me.

Taking into account what BuickMasterMind and JedRule said about the EGR system... if could possibly be the EGR valve snapping shut. The engine is cold (that's the only time it occured), and by pressing down on the throttle of would of increased the RPMs far above idle. In theory, the EGR would of opened. When I let go of the throttle, it would snap shut.

However, I held my hand on the EGR valve and attempted to recreate the noise. It only happened once (maybe because it had already warmed up outside), but I could not feel any vibration from the EGR, so I don't think it's that.

BuickMasterMind suggested a back firing condition. Which is certainly plausible, as fixing the vacuum system would change enough engine varibles for the timing to be out of adjustment. That would also explain why the second time I played with the engine when it was cold, I could only get it to occur one time.

After taking the car for a good 15 mile drive through varying speeds and conditions, the PCM should of gathered enough data to correct the timing. Though, as many will testify, sometimes it takes a couple driving cycles for the PCM to completely readjust the varibles. (I haven't played with the engine again since. Decieded I'd drive it for a couple days and see if it corrects itself before I get too worried. I'm sure I'll check again tomorrow.)

As far as it being the transmission drive chain. That's a plausible explaination (as I cannot say that the engine did not make this noise previously). With the engine running the flywheel will turn no matter what, the flywheel turns the torque converter, which mates to the transmission spline, which turns the transmission fluid pump and the drive spocket, which turns the drive chain, which connects to all the clutches, gears, and BS in the rest of the transmission.

So the drive chain would be spinning even when the engine is in park. The clutches, solinoids, and various gear assemblies are what cause the transmission turn the wheels. They are all just disengaged when the car is in park. (That's what the transmission assembly diagrams reflect, and I'm pretty sure I'm interpretting them correctly. I found the park gear particurally amusing, a whole standard spocket like gear who's entire purpose it to simply provide a way to jam the drive shaft from turning. LOL)

So... since other people have commented that this is the cause of the noise... I shall convince myself that it is the cause and all is well for the time being. I'm sure I'll go mess around with the engine some more and look for the noise. I'll be putting my hand on the top of the transmission next. LOL

I guess what this post was really seeking is if anyone knew of something directly affected by the vacuum system that would be causing such a deep, hard thunk. That's really what it is, just a single hard thunk. Not a continous knocking, rattling, whinning, or anything, just thunk and that's it.

(Like a vacuum diaphram the 10 feet in diameter snapping shut... Though it would almost have to be filled with water to cause the deep resonance quality of this thunk. Or, you know, occur inside an engine filled with oil/coolant or a transmission filled with hydralic fluid. LOL)

If you try it a few more times and rev it harder I'll be you can get it to clunk more than once when you let go. I can't think of anything vacuum related that could cause the noise. If I could have ripped into the tranny case though on my old one I would have looked at the sprockets that the drive chains ride on, sounds like a broken keyway that is letting one of the drive sprockets slip on its shaft a little bit. Or maybe excess slack in the chain is making it contact the casing. I just wish I could hear someone say "yes I had that problem and it was fixed by doing such and such". Another idea is the balance pulley on the left side of the engine, the one attached to the crackshaft. I've heard that this has been known to fail and cause this sort of sound.

I had a harmonic balancer come loose on 1979 Chevy. Snapped 2 of the 3 bolts that hold it one while driving through town. It broke the crank shaft in 3 places. The harmonic balancer is a 15 pound weight that spins at the same RPM as the engine. If it was loose, I expect it would be causing a whole lot more than a simple clunk...

With the drive chain, I expect it has to do with the transmission pressure and clutches inside of it. When the engine is reved, the increased vacuum acts on the transmission modulator. Which would cause the fluid flow to change in the engine, just as if you reved the engine in gear.

I'll have to read some more to find out exactly how it reacts. But I'm betting that the transmission spins up and engages this and that inside. When you let go to the throttle, the engine RPM drops relatively fast back to idle. So the transmission engages gears and clutches inside, and then the engine speed suddenly drops and the transmission disengages those things.

There may also be some sort of brake in the transmission that kicks in the slows part of the transmission down inside, while at the same time such-and-such clutch disengages. I could see that when that clutch disengages from the part that is being braked, thereby allowing one section to freewheel back down, there would be a clunk. Who knows though...

Maybe it's listed in the transmission troubleshooting section. Most transmission noises are, as that's the primary way you troubleshoot a automatic transmission. The transmission documentation is insanely detailed though, so I haven't read barely any of it.