John, "Stock" and OEM are interchangeble in my brain. As Denis and Warren point out, correct backpressure is important to MFI tune.

The MFI pump was rebuilt with new tappets (pistons) ? Maybe the bad tappet damaged the pump internally? Hopefully an E or T spacecam wasn't installed by accident! Other quick MFI checks: make sure the pump is correctly timed to crank just in case the belt jumped a tooth.

But, definitely start by pulling plug wires out one by one while it runs(wouldn't necessarily drive) to try and isolate one cylinder or another.

(I'd still swap out for a Dansk or sport muffler ;) )

Scott, Warren, Wayne and Denis...

Thanks! Thanks very much.

Wayne, if I was close to you guys, I would sure bring it by. And to Dennis' point, five doctors are always smarter than one. Believe me, if I thought there was a remotely competent shop here in Kansas City, I would go there. I have been through a few and if you search my posts you will see some of the charades I have been through. Imagine Auto is here, and I know Stephen has an incredible reputation. I just think he is interested in building high dollar turbo motors, and doesn't really want to mess with the odd RS problem. Can't say that I blame him.

Also, I like the idea of feeling the exhaust headers while warming up. That could be revealing. That one, and the pulling of the plugs to notice a difference will certainly be tested.

Regarding the MFI pump... it was rebuilt by Pacific Fuel injection, and again, the vibration was there before and after the MFI pump rebuild. And I have gotten fairly good at setting the MFI timing, so I am sure this is good as well. I checked it after I got it all back together just to make sure.

This is why this is so puzzling... there were many things that COULD have easily caused this vibration, but when all were resolved and the vibration was still there, I was shocked and disappointed tremendously.

And Warren, you are right!!! You have been around since this started a couple of years ago, and the joys and whoo hoo's will make paying that ticket a pleasure if it fixes the problem!

I truly appreciate all the thoughtful insight. Keep them coming, and I will perform the checks and report back.


JA

I still think there's a dead hole and that it's not due to something out of balance.

Start pulling plug wires (making sure to ground them) and narrow it down to a specific cylinder. I'm still betting on a fuel or ignition problem. If it was a cylinder that was plugged by the exhaust system to cause such a big problem, I would think it would be backfiring through the stack when reving in neutral.

Triple check your firing order. Rotate the engine and watch the distributor rotor move, checking each successive wire/plug combination.

Can you record the sound the engine makes? That may help us diagnosis the problem.

Good luck!

Update - Sort of.

Tonight I tested two things:

1) Feeling the heat exchanger header pipes during warm-up to detect cylinder not firing. ** PASS ** All pipes heated up immediately and at a seemingly equal pace.

2) Pull spark plug wires to detect change (or no change) in running condition. ** PASS ** Each wire when pulled exhibited a noted drop in RPM and corresponding stumble in running.

Also, I thought I might try to make an audio clip, but the quality turned out poorly and would not relate the problem. I noticed that the vibration is more of something you would feel. Unfortunately, no help for you long distance diagnosers.

But I noticed again that this vibration is SIGNIFICANT. It is not something my wife might notice, but anyone familiar with engines, probably not even just Porsche engines, would surely sense is not right.

I have not checked the muffler yet, but noticed again the resonating rattle that comes from it when accelerating and decelerating. I am still suspicious of that as the culprit, but my confidence level in that diagnosis is not high - maybe 10%.

I wish there was an expert on hand...

I am going to call around and see if I can locate a suitable "test" muffler. Beyond that, I am getting low on ideas.

One question that is bothering me... what caused the broken tappet in the MFI pump? The broken tappet actually occurred at a track session AFTER the overrev, but before most of the other work. It's a long story, including a poorly repaired valve by a local shop, but the point is that tappet broke, seemingly of its own accord. Here is a picture:

http://forums.pelicanparts.com/uploa...1061861296.jpg

This happened while I was in a high speed sweeper in third gear and the car just fell on its face. I drove it into the pits, and eventually home, before finding the problem. Basically, the broken tappet siezed in the raised position in the pump and was no longer pumping fuel to cylinder number three. I sent it out to Gus and he rebuilt the whole pump, so all should be good there. Just wondering what would cause that kind of failure.

Other than these things, I am at a loss. I do feel that one of the real experts, as Wayne suggested, would be able to diagnose this problem from a seat of the pants feel of the motor running. Me, I can just guess and ask for assistance from eager and ambitious Pelican posters!

Thanks very much.

JA

So it _never_ backfires through the intake or exhaust? Have you driven it with the lid off so you can hear it better?

When sending the MFI out for rebuild, did you send everything? Perhaps its a system problem. Ideally the whole system could be run from pump to injectors to ensure the fuel flow is equal across all the cylinders.

This didn't happen until the over-rev, right?

Check to see if you have a bent input shaft in your transmission, it is one of the few things I have not noticed yet in this thread and it is also one of the few parts - not in the motor that also turns at crankshaft speed, in neutral and in any gear.....

Put the transmission in gear and depress the clutch pedal, this will stop the input shaft and isolate the vibration outside of the transmission if it still exists during this test.

Regards

Hayden

Mr. B,

I have flow tested the electric fuel pump that delivers fuel from the tank to the filter console. It flowed at approximately 1000 cc's in 30 seconds - GOOD. I had all of my injector bench tested at a local shop and they all came back good. I have replaced all of the rubber fuel hose from the tank up through and including the ones that attach to the filter console and MFI pump. I have cleaned and inspected the hard aluminum fuel lines that go into the top of the MFI pump. Beyond that, I have not tested the "system" as a whole.

No backfiring, intake or exhaust really. Occasionally I will get a slight intake pop when I quickly blip to full throttle in the driveway under no load, and there is no exhaust popping to speak of. I have not really had a chance to tune the MFI system since I got it back, so it is right where Gus left it at the moment. I would like to do some tuning on it, but not until I get this other issue squared away.

Yes, the MFI tappet gave up the ghost in a different track session than the overrev. In between the two, I had a local shop do what turned out to be a shoddy repair of a bent exhaust valve in cylinder number two. This shop is the one who put the wrong size intake gaskets on, and left half of my engine accessories including hoses and wires unhooked when returned to me. Bottom line is, I did another track day and that is when the tappet failed. But, truth be told, the power has never been the same since the overrev, and the vibration began immediately after the overrev, so it has been there through all of this.

Hayden, that input shaft is actually something I had considered, but failed to mention here. I think I did the test you describe, but I will do it again and report back. Thanks.


JA

Thanks for the pix and info on the MFI tappet - I was hoping the plug wire test would lead to something... like debris in an injector or line but you've got that covered. Rocker arms ok?

Too bad you can't stick it in a different car, just to reduce trans / electrical / fuel system issues. What about an out of the car dyno run? If it vibrates out of the car you'd know it was internal....

Sounds like you'll have take it apart and check mechanical tolerances of rods, etc, anyway and at least the motor would already be out of the car.

So the vibration started after the valves hit the pistons, and you didn't check the crank and/or rods? Sounds like you know what I'm going to say next...

-Wayne

I gotta agree with that. And I know a fraction of what Wayne knows.

But here is a story that happened to me 10 or so years ago. I had an Andial complete rebuild in a mid year (2.7). I never could feel the engine in the car after the rebuild, only hear it. At about 10,000 miles, one of the flywheel bolts decided to pop off and lodge itself in the clutch locking it up. Damage to the flywheel necesitated replacement with another. They had a used one and I had them put it on. From that point on, I had a vibration coming into 3000 rpm and smoothing out a little before 3500. Was never like the original rebuild.

So, where flywheels are not a cause of power loss, they are most certainly a possible cause of vibration. My engine machine places want to see the pressure plate as well to see if it is balanced and then index it to the flywheel. Even the disc should be looked at. It will end up in a different orientation each shift, but it could cause a vibration. Combinations of things can do weird things. Every bit of this stuff has to be rechecked with each engine assembly.

All what i read above sounds like a good place to start....but who did the rebuild? you may have an orfan rod or piston in there and it's out of balance. I just had my motor balanced at my rebuild and it is unbelievable smooth....UNBELIEVABLY smooth.

I hear ya Wayne and Zeke... It only makes sense to check what happened down below when a valve introduced itself rather rudely to a piston. I'm with ya, and in hindsight, I should have gone in.

I am sort of subconsciously preparing for bottom end surgery. I just wanted to see if there were any other possibilities that would make sense to check while I still have it together. I want to know that I have eliminated all of the easy stuff, and I am getting more and more comfortable with that feeling as a result of this thread. Thanks all.

I'll probably still try the muffler. Cheap, easy, quick test. Probably only a 10% chance of success, but at least that's something.

As I sat in the driver's seat last night and revved it, this vibration just feels like it is coming from the bowels of the motor. I am thinking crank and/or rods.

Assuming some moving part is slightly bent, do you think this will be something that is visually apparent?


Thanks,

JA

Moving on past the muffler issue ...

No, I don't think a rod will necessarily 'appear' bent! Since you had eight bent valves, that means the 'usual' six bent intakes, right, plus two more exhaust valves, OK??? The cylinders with the two bent exhaust valves are the ones I would suspect to have very slightly bent rods. It will take a special measuring jig and dial indicator to find/confirm the bent rod ... of couse it might be as simple as laying the rods on a Blanchard-ground plate, and looking for a slight misalignment at the 'little' end. In this case, slight could be 0.001" or 0.002" ...

I'm a big fan of "conventional" wisdom. Despite a lack of sophisticated testing equipment and fancy certifications, some mechanics can just get to the problem faster than others. There is no substitute for experience and linear thinking. This is part of what I have always admired in Warren. And JW.

My engine has a vibration. JW admits this. When I mention stuff like rods, he insists that, while that is theoretically possible, in his experience Porsche engines are nearly always sweetly balanced, and problems are almost certainly caused by flywheel/pressure plate issues. I installed a NEW flywheel and PP. I had noticed that the new flywheel had quite a few balance drill marks in just one location. As though it was fairly out of balance before the balancing/drilling operation that is part of manufacturing. Next time I have the engine out, I will put the old flywheel in (it's still good), and I bet the problem will be solved. I will balance the PP and flywheel first, just to be sure.

My point: Very possibly you have an out of balance flywheel/PP.

John,

Super brings up a good point, so let's take a closer look at the nature of the vibration ...

Is it evenly progressive (worse in amplitude or harshnes) as rpm's rise, or does it have worse 'periods' in the rev range? A static crank/flywheel/pulley imbalance will be very progressive in nature, as in the case of a friend's Ford 302 rebuild from a '70 Bronco ... the parts store gave him the wrong or bad flywheel, and the engine had a bad vibration at idle after installing back in the truck, and it got progressively worse up to 4000 rpm, and he didn't feel like pushing it further! The store had given him the wrong flywheel, and installing the correct flywheel fixed the vibration problem.

If the vibration has peaks and lulls through the rev range, then it is a bit more difficult to surmise the possible cause, though you have a strong case for a rod or crank problem!

Supe,

Thanks, and I agree on the conventional wisdom. That is why I always respect experience and reason found in those who have been there day in and day out.

Regarding the flywheel / clutch, this vibration was present in its identical form both before and after a machined flywheel and new clutch were installed. I am pretty confident that those items did not "introduce" this vibration.

Secondly, a somewhat major "collision" occurred in my engine, that could certainly explain corrupting that "sweet" balance that the Porsche reputation certainly deserves.

One thing that bothers me, but may be explainable, is the power loss. Can a vibration be so severe that it robs power? Significant power? Again, it is hard to describe the magnitude of the vibration and/or the power loss, but let me give it another try...

The vibration is one that if you set a glass of water on the dashboard and revved the engine, I think you would see major turbulence in the water. It does not make the whole car visibly shake, but you can feel it in the pedals, the steering wheel and the seat.

Regarding the power loss, I think my car is generally speaking a 5.5 second car 0-60 (based on comparison's with a friend's 5.2 second 993). My car now feels more like a 7 second car. My car is light, and normally has 210 HP, so you can get a feel for the difference. Is it a total dog? No. But it feels like it has lost around 40-50 HP or gained around 500-600 lbs. Take your pick.

Could a bent rod or bent crank, or someother abnormality in the engine that is generally "quiet" cause this kind of power loss? It truly does FEEL like the vibration is hampering the throttle response and power generation, I'm just curious if that makes sense from a mechanical point of view.

Thanks,

JA

Yes, vibration and out-of-balance will indeed rob power? Why? The engine will be fighting itself to remain on track.

I ruled out the clutch components because the vibration started after the incident, and didn't subside when the components were replaced. If it was the clutch, then it would have changed when you replaced the components...

-Wayne

John,

Yes, I believe that a slightly bent rod ... putting significant side thrust on a piston ... could affect piston-to-cylinder friction and hence, power output, and also cylinder temps in operation. Perhaps a stainless thermocouple probe on a digital multimeter or thermometer ... could be inserted up next to the fore and aft fin area around the 'midpoint' area to see if there are any aberrant temp readings on a cylinder or two???

I would find it a little incomprehensible that a "50 h.p. bent rod" would still rotate freely in your motor. But that's just me. There is not a hell of a lot of room for redesigned measurements inside of there, if you know what I mean. :cool:

Something in the motor assembly is out of balance. Severely, it sounds like. Even more than something like a bent valve, it sounds like the reciprocating masses do not add to zero. Something weighs more than its mate on the other side, including, still potentially, flywheel or PP halves. I under stand that the vibration did not change when flywheel and PP were changed, but I have seen motors vibrate less than you describe, even when they are slapped together without regard for balancing. So, it sounds like something is severely out of what WEIGHT-wise.

At any rate, to address this problem the engine comes out of the car and the flywheel/PP assembly is checked first. If they are in balance, you will need to remove the crankshaft. Then and only then will we find facts.