View Full Version : Knocking on 95 FWB, looking for input

04-05-05, 09:46 PM
I have a 95 cadillac fleetwood. The car knocked just slightly last winter one really cold days. This past winter though it was bad enough that I tried to fix it. I went through all the problems I could find on this forum and managed to dampen it but not fix it. I'll first describe the symptoms then list what I did to try and fix this. It is strongest when it is cold out. The colder the harder it knock (although it is still knocking now in the 60s which is why I decided to post this). It knocks more closer to start up. And I can usually only feel it at lower speeds and right before downshifts. It seems like it either is or isn't knocking; one day it will do it and the next it simply wont. And it NEVER knocks when I have my foot on the gas even a little. Only when I'm going slowly and not accelorating.

To try and fix it I've
-put in high mileage synthetic 5w30 mobil motor oil
-checked ac, alternator, and flushed the power steering pump to make sure that there was no tention on the pullies
-I put on new header gaskets and bolts (had some break which caused some exhaust leak). I also continued to check for any other leaks
-checked to make sure the cat's flowed properly
-installed new engine mounts to stabilize the engine better

If anyone has had a problem like this and/or knows the solution I'd greatly appriciate the help. Or if anyone knows other common issues that cause knock and ways to fix it I'd be willing to try that. I'm about ready to go to the local cadillac dealer and have them hook it up to their computer and see what it says. They charge way more then I can afford but if it fixes the problem them it might make my life easier. Any advice though before I take it in?

04-05-05, 10:08 PM
I had pretty bad knocking (i.e. spark knock) problems with my '93 for quite a long time last year. Didn't knock at all when idling, when my foot was off the gas, or when I was heavy on the gas, but did so basically any other time--light when cruising and very noticeable under normal acceleration and light throttle. It was at its extreme when it was cold out, also. Of course, after a while, I also noticed the Check Engine light would come on when I was on the highway, and because everything else seemed in order, I figured both the light and the noise were connected.

Had the GM dealer run diagnostics and pretty quickly found the EGR valve was very dirty and pretty much stuck in one position (versus it normally being able to move). They replaced that and the accompanying solenoid, and haven't had a single knocking sound (or any weird sound for that matter) in the last 3 months.

Don't know if this is your issue, but it was the cheif cause of mine.

04-05-05, 11:43 PM
What fuel are you using 87 octane in NY 10% ethanol is added for the winter months which makes my car run crappy. Have you tried 93 or 94 octane to see if its the fuel your using. Caddycruiser is right with the egr valve might be worth taking off and looking at it to see if its stuck...

04-08-05, 11:37 PM
So this EGR valve seems as if it causes a minor dampener on performance but is worth its while. I've looked around online for a while and considered just blocking it off. I could counteract this with a 160 thermostat and with the airflow around my headers it should keep things cool enough. The real trouble in blocking it off would be with the computer. I've read that the EGR is programed into the computer to operate under specific conditions. This means that removing it would cause a timing retard and not only would negate any potential power gains but also cause a trouble code. I'm also worried about it causing the engine to maybe idle higher as that the ratio of oxygen would be higher. Would the engine even be able to compensate (inject more fuel) for the additional oxygen level? Is this even worth doing or should I just replace the EGR and keep everything running stock?

04-09-05, 07:18 AM
I's start by removing the serpentine belt, and running the engine to verify that the noise is definately coming from the engine itself. Then, I'd run some "Top-Engine Cleaner" through it to verify that the knock isn't simply a "carbon-knock". Beyond that, chances are good that it may actually be a bearing knock.

04-30-05, 08:45 PM
DanThe Man,

Your car may have two kinds of knock. The first one sounds rather like piston slap which disappeares in a few minutes as soon as the engine warms up. Do not worry about it at all.

Another knock you hear may be actually spark knock. It is more pronounced when the engine gets hot (over 180) under slight acceleration. Just stop somewhere when the engine is really hot (some 200 degrees) and barely depress the gas pedal while in Drive and applying the brake. Carefully increase the RPM and hear . At some 1200 RPM you should hear realy loud knock sounding like sewing machine. That's spark knock.

The reason you get is that your EGR system does not function properly. Under slight acceleration when the engine is hot EGR valve should enable exhaust flow through the combustion chamber to cool it down. At the same time the ECM advances ignition timing assuming where is exhaust flow. But you actually do not have that flow. As a result you get detonation - sprk knock. As soon as you depress the gas pedal harder the ECM returns the ignition timing to normal and detonation goes away . So does the spark knock.

If you connect a vacuum hose from the trottle body directly to the EGR valve chances are you'll get rid of the knock but your idle will be really rough. This is just a test to prove you hear detonation and not a bearing or something like that.

05-01-05, 09:04 PM
Thanks for your input. I haven't had time to look into it here the last few weeks because of school work. Once finals are over here this week I'll take it out and see if it isn't the EGR. It has gotten a little better now that it is warmer out but it is still definatly pronounced.