Hi, folks. I'm going to give plenty of details in a moment, but my basic question is this: I suddenly developed a rattle/vibration in my exhaust and am having trouble figuring out the cause. It varies with rpm and load, and at some frequencies almost disappears. It is quite a bit louder under the car than above the engine, and almost not present at idle (which is part of the issue with diagnosing it by myself.) I'm trying to get suggestions on possible causes that I can look into.
Now for the specifics: I had the exhaust redone last year from the flange that bolts to the manifold back (they kept the original flange and welded the new pipe to it) and I did not have this issue before or after they returned it to me. So far as I can tell, the exhaust is still sturdily mounted, although the first mount behind the flange is at the carrier bearing support. I just had the engine rebuilt (thanks to Dale Cooper and Doug Wildrick!) and installed it a couple of weeks ago. I did not unbolt the intake from the exhaust manifold when I bolted them back to the block, but I used a normal 9/16 wrench to tighten the bolts and didn't tighten them as hard as I was able. When I first started the engine, the exhaust was quiet and seemed to be sealing well -- no rattle. I spent the next 15 minutes keeping her around 1700 rpm, as advised by my machine shop, to break in the cam/lifters, and everything sounded good. I then took it for a drive, and maybe 2 miles down the road I instantaneously developed an exhaust leak and heard the rattle/vibration. Car did not run any differently, however. I brought it back, unhooked the flange from the manifold and replaced the gasket (I had reused the old one, and it was now pretty burned up.) I tightened down the new gasket but didn't make much difference. Bought new bolts and nuts for the flange and installed them, it got a bit better. I took it to the shop that did my exhaust, the owner listened to it with a stethoscope ending in an open tube and declared that he didn't hear any leaks from the runners, heard a tiny bit from the flapper in the manifold, and heard leaking from the flange gasket. He recommended that I use high temp RTV to seal it, said it wasn't uncommon on older vehicles, especially with 2 bolt flanges. He was very unconcerned about the rattle/vibration, but it sets my teeth on edge. I unbolted the exhaust again and looked as carefully at the exhaust runners from underneath as I could (still haven't put the hood back on yet) and I don't see any cracks or signs of soot on the runners or the body of the manifold, nor have I really heard exhaust leak from same. I also checked the little pipe that protrudes above the surface of the flange and was unable to make it move. I slathered RTV on the flange gasket and reinstalled. Today, the exhaust leak seems almost entirely gone, but the rattle/vibration persists. I used a stethoscope earlier with a metal tip and heard it most clearly where the flange meets the recently welded pipe, and I can hear it all down the exhaust and in the exhaust note. I hear a ticking near #1 and #2 on the exhaust manifold with the stethoscope, but it sounds like slightly louder valve noise compared to the rest of the cylinders. The engine idles smoothly and mostly pulls smoothly as well (I think I'm a little off on ignition timing and it could use some new plugs.)
So, I'm kinda stumped. I'm imagining everything from a cracked manifold to cracked flange to cracked block at the exhaust seat (not sure what, if any, noise that would make, and I should note that the block was checked before machining and was not cracked.) I'm wondering if this vibration/rattle sounds familiar to anyone on the board, or if I'm overlooking something to check. I'm going to be pulling the whole drivetrain back out in a couple of weeks (leaking rear main) so there's nothing I won't be able to get to, but I'm just trying to be sure to check every possibility while it's out and try to get it right before I put it back in.
So, anyone have any great ideas? I appreciate any and all responses, and sorry if this is clearly a cracked manifold -- I just don't see any cracks on top or below it!
Thanks in advance
Try wiring the valve in the open position and see if that eliminates the problem. With the weight towards the engine block.
I was hoping you'd have a suggestion on this! I am absolutely hoping it's the valve in the manifold but wasn't sure how to test. Your idea is simple and elegant, and therefore will probably prove to not solve my problem (ha, ha). I'll cross my fingers and try this tomorrow. If this does prove to be it, is the solution to cut the valve out?
Thanks a bunch!
[quote="Geoff Clark " post=24077]Try wiring the valve in the open position and see if that eliminates the problem. With the weight towards the engine block.
So, I tried this, and it may be my imagination but I do believe that it decreased the intensity and level of the vibration, but did not eliminate it or change the conditions under which it intensifies or vanishes (most intense from about 1300 - 1500 rpm under load, not noticeable under deceleration). I'm still wondering if there are any other possible causes that I could test, and what the recommended course of action would be if it's entirely the valve in the manifold.
Thanks to all.
Extremely difficult to diagnose from a distance. Is the front of the flapper shaft tight in it's hole? Is there an anti-rattle spring there?
Yeah, I do realize that this is a big ask; I've got it in front of me and am struggling to diagnose it, you're in New Zealand!!! -- I'm mostly trying to figure out if there are any possibilities that I've overlooked. The spring is there (and the coil). I can wiggle the shaft via the front or the back by hand, enough to hear it hit the opposite side of the hole. It was rotating without much hangup before I wired it open (against spring tension), but I don't know if it was different before. I guess one question I have is: could the wobble in the hole have advanced from not making the rattle to this point in an instant? I don't see any significant carbon blackening around either side of the shaft as it exits, although my view is somewhat obstructed by the coil, etc.
Thanks again for taking the time to respond.
Hey John,
I had a similar situation some years ago on a different vehicle.
It turned out to be a washer that had somehow gotten in to the exhaust pipe when it was installed!
It only did it sometimes and I had similar symptoms to you. Sounds weird but once I figured out the problem and removed it
everything was fine.
Jeff
Thanks, Jeff. I hadn't considered that, so that's super helpful. I will definitely check for that when I pull the motor back out, including sticking a magnet down the pipe as far as I can to try to fish anything out. Having now taken a look at the flapper valve on my spare manifold, I'm guessing that one of the welds that secures it to the shaft has popped and that it's vibrating against the shaft like mad, but I'm far from confident in this.
Finally got closure on this. I'm running a glasspack instead of a regular muffler, and the interior packing retainer on my glasspack had broken loose and was rattling in the exhaust! Was diagnosed by the shop that installed it, they said it occasionally happens with glasspacks as the retainers are pressed in during manufacture. He melted a small hole in the front and rear of the housing and welded the retainer into place, then welded up the holes. Voila! Rattle gone!
Of course, it turned out that my exhaust manifold had cracked as well on #4 cylinder (I replaced it with a 2bbl manifold I had lying around). So I was trying to chase 2 exhaust issues! Thanks to all who offered up suggestions! And, btw, I will acknowledge that the glasspack was made by Magnaflow, but I'm not upset with them -- I have 2 of the same glasspacks on another car and they've performed flawlessly for over a decade.
John, you also did not mention the needed support bracket located at the rear engine support plate. This bracket needs to be there to hold the exhaust secure to the engine as you accel. If the bracket isn't there, it can allow movement in the pipe. This movement can and usually cause the elbow and/or manifold to break. Obviously you can't see the manifold as you drive down the highway, but it is glowing hot at cruise speeds- Under normal operation-, thus allowing the torque to make slight movement. That slight movement over time,without the bracket can blow the gaskets or break the casting.
If you look, there is a hole at the bottom of the support plate. The bracket is a very simple piece and has a curve to the portion that contacts the exhaust pipe and has a clamp that goes around it. From the factory, the bracket is attached to the support plate, lays directly against the pipe, then has a wide clamp that goes around it. Someone here surely has a picture of this. Sometimes the factory support bracket doesn't work very good because the exhaust has been changed and is too far away from the support plate, so it isn't connected or was discarded. I usually simply weld a piece onto it, re-drill the hole to secure it.
** - Finding a crack in between #3 and #4 exhaust manifold, especially if equipped with a 308 and Twin-H is common, some worst than others. I had found manifolds that have a real loose heat riser shaft, usually have a cracked or damaged manifold. When the shaft is worn badly, it allows fresh, outside, oxygenated air into the manifold when you let off the throttle pedal, sort of like an air pump does for emission controls. When this occurs, the manifold gets super hot because of this. We have found the heat riser is not needed for proper engine running/tuning. Basically we are not driving our vehicles in the winter or the cold. So if and when you have the manifolds removed, we remove the lower manifold elbow and remove the shaft and heat riser. I think the actual shaft is made of stainless steel, and does cut with the torch. A die grinder or air powered saw works well. After the shaft and old worn out bushings are removed, a couple of 7/16th's bolts with torque prevailing nuts in each side work perfect to seal the holes. Doing this resolves the extra unwanted heat being generated under the hood as well as the exhaust leaking/clicking as you accelerate.
Thank you again, Doug. The new exhaust does not have the bracket attaching the pipe to the support plate (I don't think it was there before I took it in to have the new exhaust made, either) but if you say it matters, I'll fabricate a bracket in that area. As always, thanks so much for your time and knowledge.
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