Elusive RS Vibration
 

Hello,

I am going to appeal to the board again on this issue that has been plaguing me for over two years now. I have a 2.7RS motor that has a nasty "harmonic" type of vibration that is robbing somewhere in the range of 25% of my horsepower. This vibration is present sitting still, in neutral, and in all gears under power.

A quick history is as follows:

o Mechanical overrev from missed shift - 8 bent valves
o Rebuilt MFI Pump
o Replaced Distributor, Cap, Rotor, Wires, Plugs
o Cleaned and Inspected all wiring
o Complete Topend overhaul - valves, machining, etc...
o Entirely new clutch

While doing the top-end, I found a number of things that I thought for sure were contributing to this vibration. For example:

1) a broken tappet in the MFI pump
2) 32mm instead of 36mm intake gaskets between the throttle bodies and heads on one bank of cylinders
3) Eight bent valves

With this kind of damage, I was certain one or more of these things were causing this vibration. Add to that a professional top end job done by German Precision (Ted Robinson of the Warhorse Gang), and I thought my RS motor would be screaming once again. I decided not to go into the bottom end, since I had no broken head studs, and the motor had been rebuilt some 30k ago.

Once the motor was back together with all of the above repaired, and the new valve job, the dang thing runs almost identically to how it did before I started!! Can you imagine that! One, the above repairs did not really improve things, and two, the vibration is still present!! The car feels like it has around 160 hp, when in actuality is has around 210 and runs incredibly!!

So, I am planning some more exploratory surgery. What I am asking the board, is what should I be looking for? I am thinking that something is bent rather than broken, since everything seems to be tight and there is no noise. Just the vibration and resulting lack of power. Some ideas:

1) Bent intermediate shaft
2) Bent connecting rod
3) Bent crankshaft

How will I identify this when I get it opened up? Standard tolerance measurements usually check for wear, but I don't think that approach will be revealing in this case. I guess it is possible that I will be able to see something out of whack with the naked eye, but I am also concerned about the possibility that everything appears to be in order.

Any ideas of what to look for would be appreciated.

Thanks,

JA

Im glad thats your problem and not mine ! For starters if you have a bent crankshaft you would likely have even worse symtoms than you are currently experiancing. Also a bent rod would not likely cause the extreme power loss or vibration(unless we are talking really bent) . I cant imagine you could bend a rod or crankshaft without even hurting the pistons. So I would put those 2 ideas to rest. As far as the intermediate shaft ? I cant imagine that it could get bent either but it could be a possible suspect allthough it wouldnt cause a large loss in HP(assuming your cams are properly timed). My guess is the flywheel . I would check the installed(checked on engine with dial indicator) runout on the flywheel as well. But then your problem seems to require a hands on diagnoses. Good luck with it.


Kurt Williams

Are you sure its not misfiring on one (or more) or the cylinders? I'll admit that I had two ignition wires crossed once (1 & 3 I think) and it wasn't immediately obvious that there was a problem until I got on the gas hard. It wasn't that dis-similar to what you are describing...

Very possible(what matt said). Sometimes its the simple things and they get easily overlooked. Have you done a plug reading ?


Kurt Williams

I would also start eliminating good cylinders from bad ones. Ground a spart plug lead and drive for a short while. If the problem is no worse, you've identified an offender. As this will be dumping raw fuel into the cylinder, you don't want to do it for very long, and it wouldn't be a bad idea to change you oil afterwards either.

Thanks for the suggestions Kurt and Mr. B.

Regarding the firing order and wire placement, I do all of the engine work myself, and have had the distributor, wires, plugs, cap, etc... removed probably 25 times since the overrev incident occurred. I felt 150% confident in the firing order of 1-6-2-4-3-5. But, since I know how these things can happen, and many, many seemingly difficult problems turn out to be something very simple, along with the fact that your posts gave me just an ounce of uncertainty, I just went to the garage and verified one more time and all was in order.

As for the flywheel, I forgot to mentioned that it was also resurfaced during my clutch job. That is one of the better improvements I have found, as my clutch is now butter smooth. The runout on the original flywheel was signficant prior to machining, but the vibration was present both before and after that repair.

Kurt, I am with you on the bent shafts/rods. I just am at a loss. Any other thoughts would be appreciated. I will religiously check any suggestion that comes from anyone from this board.

Thanks in advance.

By the way, here is the motor...

http://forums.pelicanparts.com/uploa...1061771578.jpg

http://forums.pelicanparts.com/uploa...1061771602.jpg

Hello,

After reading Souk's message about his problems, another idea occurred to me. What are the possibilities that a baffle has come loose in my muffler restricting my exhaust? Another complaint that I didn't talk about in my post above is that the car sounds like crap. There is a lot of rattle and resonance coming from the exhaust, and kind of a nasty rapping sound while under power. Some have told me to check headstuds, but I have checked and rechecked torque on all the heads.

I am running a two-in, one-out Ansa muffler. It was supposedly new a couple of years ago, but the mechanic who sold it to me has been more than a little questionable in the past.

Any thoughts on this?


Thanks,

JA

What about the alternator/fan?

Chris,

With the fan and alternator, I had them out when I did the top end. I checked the fan for cracks and overall integrity, as I had it powdercoated during the process. It seemed to be in very good condition. I inspected the alternator as well, and it spins freely and quietly with no play in the bearings.

I am really starting to wonder about this muffler. It is certainly one common element that has been with the motor since the vibration began. There was a large backfire during the overrev and I am wondering if the muffler could have become a casualty as a result of that, and just freakishly lodged one of it's baffles in the direct path of the exhaust flow. Sounds like a longshot, but given the crappy engine sound, may be worth looking into.

Any other thoughts?

JA

Do a thread check on non OEM mufflers on MFI cars. I remember some subtle driveability and "power-loss" problems being resolved by re-installing the OEM MFI exhaust. A "harmonic" or "pulsing" type of feel might be caused by incorrect back pressure in the exhaust system. MFI engines seem to be more sensitive to this. It seems to me that the OEM system is "always" favored over any other aftermarket system.

Just a friendly guess. Good luck. Beautifully prepped engine.

Thanks Scott.

I believe ANSA is OEM. Plus, this is the same muffler I have been running before and after the vibration. In other words, it is not possible that this muffler "introduced" the vibration.

So, should I try a new muffler, or should I go for the deep dive exploratory?


JA

I would do some more testing first. Pull one plug wire at a time & see if one or more cylinders aren't firing. I would also think compression & leakdown tests are in order.

I agree with the others, the vibration and lack of power sure sounds like a dead cylinder to me.

Dad and Kurt,

Thanks, but multiple compression tests all check out. Here are the latest results done just a couple of weeks ago:

#1 172
#2 168
#3 170
#4 164
#5 169
#6 171

Again, fresh top end with all new valves, seals, etc...

You can see why I am at a loss. All of these things have been checked and re-checked and come out clean.

I have not tried to pull a plug from each cylinder and drive it to see if there is no degradation. I will definitely do that test.

No one seems suspicious of the muffler. Would it be that rare to have one's innards come apart and block the exhaust path? Anyone have a cut-away view of a stock OEM or sport muffler?

JA

John,

It does sound like a very suspicious situation concerning the muffler! Exhaust aberrations can certainly affect the responsiveness and perceived power on an MFI engine, and eliminating this as a source could save a lot of time and expense ... if it isn't an internal engine problem afterall!

It certainly would be worth the effort to pull off the muffler a give it a buzz down the street to see if the vibration is gone ... though megaphones or a dual-out polished Dansk would be my preferred test setup! Even cobbled-up glasspacks or SuperTrapps could resolve the situation ... if the Ansa is at fault.

One easy check is remove muffler , and run it with just headers as a test ..only take 30 min to find out if muffler is the problem, or at least eliminate it with out buying new muffler

And also see if you have a sweet tone at 4500 + rpm

- Joe -

Thanks Warren and Joe!!

Ahh... just enough support and encouragement from the two of you for me to go ahead with the muffler test.

Now, just how bad will it be for me to blast down the street with open heat exchangers? I assume it will be completely obnoxious, thus explaining Warren's recommendation on the mega's or Dansk.

I may pop into a local P shop and see if they have an MFI "suitable" muffler laying around that I could borrow.

Thanks guys!

Wish me luck!

I'll post results as soon as I can conduct this "test".


JA

Do you have a buddy whose stock early muffler you could borrow for a couple of hours? MFI motors are reputed to only run optimally w/ stock or factory sport mufflers, backpressure is crucial. Still, an incorrect muffler would not likely cause vibration and massive power loss. If you had a plugged muffler it would be obvious, just put a banana or one of those tennis balls in the tail pipe, you'll see what I mean. ;)

If it was me, at this point I would just pay a good Porsche shop/wrench to check your car for you. Even though you obviously have mechanical skills, sometimes just a second doctor taking a fresh look at the patient will reveal new info. When the pros are baffled at the shop, they do not spend hours scratching their heads and staring at the car alone. They all gather around it, and a Powow ensues. Good luck! :cool:

John,

Open heat exchangers will be loud ... similar to 'pre-noise-abatement' levels at an SCCA regional! Definitely loud enough to get a ticket if there is a cop in attendance ... but, I suspect your 'war-whoops' of joy will be louder than the HE's at 7300 rpm -- if the vibration is gone throughout the rev range!

This is fascinating, although over the Internet it's frustrating, as Tom and I could probably find the problem if you brought it by and we could hear it.

Checking the plugs is a good idea, but doesn't always give you the full equation. For example, if you switch plug wires, then the plug will be firing, but at the wrong time. I prefer to feel the manifolds either with your hand when the car is just started (only when it's cold, just beginning to warm up), or with a temp gauge later on. Severe temperature differences will tell you what is firing and what is not.

On that same note, analyzing the temps on the exhaust pipes will also tell you if you have a clog in your muffler. One side may be hotter than the other (the side with the clog). I would do this simple test before looking at anything else.

A couple of questions. If you pull the harness to the CD box, and try to start the car with the starter, can you feel any vibrations? How is the car starting when cold? The same vibration problem (exactly) was there before you replaced all of this stuff? If yes, then I would rule out flywheel and clutch, as you changed these). Intermediate shaft probably a no, but the crank may be slightly bent - although you can't easily turn a bent crank in the engine case, plus the cranks are pretty strong. If all else checks out, I would guess a slightly bent rod.

-Wayne