With regrets to the Beachboys. Okay speculators time for another guessing game round of "What’s my vibe?"
Since replacing the con-rod bearings and vastly improving the oil pressure control in the engine (thanks for all the tutorials, suggestions, and writeups guys, your tips worked really well) I’ve now had the chance to put about 1000 more miles on the car, about 200 of them on track. The car has a pretty bad driveline vibration that I need to sort out now that the season’s ended. I believe there may be as many as three components to it. The first is obviously wheels and tires. It’s strictly speed related and it makes no difference if the engine is running or not. The second is likely a failing input shaft bearing on the tranny. Kyle first suggested this and the thing sounds like gravel in a cement mixer when it’s sitting at idle in neutral. The third is the one that’s got me stumped. I’ve never experienced anything like it and I wouldn’t have been certain that there was really something broken (or actually how badly it was broken) until a friend agreed to take me around RA for a few familiarization laps in his DE car. Rich’s car felt like all the other straight 6’s I’ve owned. It was amazingly smooth, strong and quiet. I felt like I was riding in a car with a completely different (and larger/stronger) motor. Granted he’s got rubber where I have poly and still has sound insulation and carpet. But #57 will shake your teeth out at around 3000 rpm. Driving it to and from RA (over 600 miles) was a rare treat, matched only by the kidney pummeling experience of piloting an empty 26’ box truck between Indy and Atlanta over 1000 miles of potholes. Louisville earns special mention as the city least likely to waste money on interstate pavement repair. The vibration is definitely RPM related. The clutch jutters pretty substantially at engagement so the flywheel and clutch assemblies are suspect. The car has also obviously taken a hit to the left front corner as the sheet metal forward of the bumper shock mount is buckled and has been (half heartedly) straightened. I can’t tell if the radiator is original or a replacement so don’t know if contact was made with the front of the engine but the fan is definitely not spinning in a plane, although the fan clutch is. So I guess the harmonic damper could be suspect. Lastly of course there’s the money pit spectre of a bent crank or rotating assembly. Is there any easy or cheap or reliable way to eliminate this short of removing it from the motor and checking axial runout? Any other tips or suggestions?
Bad, bad, bad, bad vibrations
A noise with the engine idling and the clutch engaged that goes away when the clutch pedal is pressed is from wear in the transmission input shaft bearings. Unless really loud it isn’t anything to worry about.
A vibration that you feel in the seat that gets worse with ground speed will be the from the drive train or rear wheels. Once the possibility of a bent or out balance rear wheel has been eliminated the most likely candidate is the drive shaft, guibo, or center support bearing. Of those the symptoms described sound to me like a bad drive shaft.
So does the wheel/tire vibration go away when you put the track set on?
The fan is mounted on the water pump. If the fan is wobbly you may just need a new water pump. That would also explain where all your water is going. I’ve had them fail a couple times and leak out the front seal. Get rid of the stock fan and don’t over-tighten the belts and the pump will last longer.
Agree with both of these although I’m happy to hear from Jim that maybe I don’t need to rebuild the tranny just yet. And the wheel tire vibration I strongly suspect is from a combination of all the marbles melted to the insides and treads of both track and street wheels and the hideously chunked sidewalls of the street tires from using them for track sessions. They’re low dollar Goodyear touring M&S that have no discernible wet grip and have been thoroughly abused. I’d be kind of shocked if they didn’t vibrate. The mysterious 3rd component is related only to engine speed. It doesn’t matter if the car is going 100 mph or parked. It comes on at a little over 2000, peaks at around 3500, where the car appears to have lots of sympathetic resonances, and settles to a constant drone from 4k - redline. The engine kind of feels like it’s running on 5 cylinders, missing a power pulse on each revolution but it’s not down on power enough to be completely missing a cylinder. Compression readings are adequate. Leakdown tests planned for later this month. I keep waiting for something to break catastrophically but it appears willing to go on ad infinitum. The alternative is to find and fix it first.
Always start with the easy stuff first. Engine pully’s, Harmonic balancer on straight and tight, all cylinders firing and getting fuel, Water pump bearing, alternator bearings. Then think about the fly wheel and if it is on correct, square, tight. Bent drive shaft would be way down on my list of possibilities.
Good call Jim. In the off season I’ve disassembled nearly the whole car. The driveshaft has a frozen U-joint at the differential, the guibo is moments away from being a ball of rubber bands, and I’m not sure the clutch disc is currently balanced anymore.
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Bumping this back up again as now that all of the bad components have been replaced I can accurately report that, although it’s better, nothing I did made the vibration go away. It’s now not speed dependent anymore but there is a strong correlation with RPM with the worst vibrations from about 2500-5000. Doesn’t seem as noticeable at WOT. I didn’t replace the water pump as it was replaced just 5K miles ago by the PO but I did put a new fan in and it still wobbles. Did not replace the uber expensive clutch. Also replaced all of the uber expensive ignition components and had the injectors cleaned and flowed.
Got a dyno pull at Putnam last weekend and the numbers were all over the map. I have no idea how to interpret them or whether to ignore them altogether and try again at a local shop. Since the pulls were asked for by the RD, I only got to look at the sheets, not keep a copy. Numbers were something like 144, 132, 135 RWHP and 132, 121, 125 RWTQ. And spikey as hell.
So what are the potential and likely causes for low(?) output with otherwise good compression over a limited RPM range combined with a nasty midrange vibration? ilateapex suggested:
Throttle position switch
Thermostat sensor
O2 sensor
I think I might add AFM.
Other suggestions? I hate being 6 seconds down. I’d be ready to believe it’s just me and my inability to drive, but I placed 8th out of a field of 40, including GTS3&4 and 944Spec, in the rain. So I get to pretend that it’s the engine.
A noisy curve could be ignition timing, which could be a bad CPS, bad engine harness, bad harmonic balalncer, or a bad DME. Flaky ignition timing might explain the variable dyno results and/or engine vibration.
Hmm, I’m pretty sure you’re on to something Jim. It also doesn’t idle smoothly, although the tach says it does. (In fact it doesn’t idle at all for the first 30 seoonds or so.) When warm I can hear misses and hooking up a timing light shows that the timing will jump by several degrees at irregular intervals a few seconds apart. Seems to oscillate between two fixed positions rather than bounce around. I’ll try the same thing at various throttle openings. I’ll also observe the crank damper. The CPS is tight, secure, and clean. I haven’t measured the gap yet. Any other theories on the failure mechanism with this added info?
If you remove a plug wire, in each case is the change in behavior the same? It would be interesting if removing one of the plug wires made the behavior “pretty bad”, which removing any of the other 6 made it “very bad”.
My lean AFM problem was pretty similar to what you are experiencing, except that I didn’t think of it as a vibration problem but a missing problem. It’s discussed in several threads here but basically the symptoms were
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Engine missed badly at WOT and ~4500-5500 rpm. Not always, but frequently.
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Outside of that rpm, or not at WOT…no symptoms.
Problem turned out to be the AFM. I think the precise cause was that my AFM’s spring had been loosened for “more air”, but there’s some conjecture there. The whole time my F/A meter was telling me that the engine was scary lean but I didn’t believe it. You have F/A info from that dyno run?
Nope. NASA paid for it so NASA kept the results. But I’ve got a dyno shop nearby, to your point it’d probably be worth a few pulls with A/F and documentation. I’ve tried the plug wire test with mixed results. Pull the plug wire once and the engine stumbles badly. Reconnect and repeat, barely changes idle. I’ve replaced the cap, rotor, and plug wires with new parts at great expense…twice (anyone need a set of barely used plug wires?) With no noticeable change except the cold idle went to hell causing me to chase vacuum leaks for a while. Still haven’t found the cause of that problem. I can’t says as how it has a serious miss at WOT. It’s not particularly strong but I don’t sense a miss. Of course it’s running 8 times faster than at idle so maybe I’m just missing the miss.
I’m not much good at sensing a high rpm miss, that takes experience. I’m not sure that testing at idle is the way to test at mid-high rpm. If you can get the symptom to change, you’re on the right track. The problem is that it’s probably not just the rpm, but also the load. Hard to simulate that combo of rpm and load. That’s why it took me a year to figure out what was going on with mine.
Engine management problems can be tricky. A couple yrs ago I floated an idea where we could use a Megasquirt that had a stock tune. This would give us access to big-time engine management logs and help us chase down problems. The MS idea, since it would allow a quick checksum of the tune, also had the charm of being the only real solution to “chipping” DME’s.
The idea was not well rec’d. Except by the folks that wanted to be tuners. They loved it. Sigh.
The WOT switch in the TPS is definitely a factor above 4500rpm when the AFM maxes out. At that point, as far as I can tell, the DME ceases to use AFM data and goes strictly to built in maps for fuel and spark. Below that point a bad AFM will create all kinds of havoc.
While bad ignition wires would result in misfires, their effect on dyno results should be reasonably repeatable. That probably won’t be true in the case of flaky CPS data or a flaky CPS input section in in the DME. Those will tend to cause variable changes in engine operation.
[quote=“jlevie” post=72593]The WOT switch in the TPS is definitely a factor above 4500rpm when the AFM maxes out. At that point, as far as I can tell, the DME ceases to use AFM data and goes strictly to built in maps for fuel and spark. Below that point a bad AFM will create all kinds of havoc.
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A bad AFM can cause problems for engine management anywhere in the rpm range. This is because an AFM problem can screw up long term fuel trim which, like herpes, is forever (rpm-wise).
I would assume you checked this but vacuum leaks(cracked afm boot) will cause the idle problem.
Very true. But removing power from the DME will clear that adaptation. I tend to forget that not everyone has their car configured to only power the DME when the ignition is switched on. Which means that all adaptation is cleared for every start.
Yeah, new AFM boot. New fuel pressure regulator hose. RTV’d the idle air bypass. Haven’t RTV’d the brake booster pulloff yet. The idle seems to be improving so maybe Ranger’s right about fuel trim. Maybe I’ll hit the kill switch overnight and try again in the morning.
Very true. But removing power from the DME will clear that adaptation. I tend to forget that not everyone has their car configured to only power the DME when the ignition is switched on. Which means that all adaptation is cleared for every start.[/quote]
Good point.
I sure wish I knew how fast long term fuel trim takes to make adjustments.
Yeah, new AFM boot. New fuel pressure regulator hose. RTV’d the idle air bypass. Haven’t RTV’d the brake booster pulloff yet. The idle seems to be improving so maybe Ranger’s right about fuel trim. Maybe I’ll hit the kill switch overnight and try again in the morning.[/quote]
If you’re talking about the metal pipes that go into the throttle body, use JBWeld, not rtv goobage.