engine shakes over 3000rpm

[strokedhatch]

hey everybody, this is my first post on here, so bear with me..! ive got an 85 vette with 20,000 origanal miles on it, i got it with 13k on it about 5 years agao and the car has always ran good, untill an intake gasket went bad and i was geting coolant in the oil, had it all fixed and nowthis...


.car runs good just cruising around, but when you want to pass someone or jump into traffic the car shakes pretty bad..it has good plugs, wires, i changed the knock sensor, checked the TPS, put some fuel treatment in it, still shakes..

any idea?

[Enginerd]

Vibrations can be any number of things... so get ready for some hardcore diagnostics. Firstly, determine if it's engine related. Run the engine up past the RPM that this usually occurs at with the car in neutral. Vibration? It's in the engine itself or something related like the flywheel/flexplate, clutch/torque converter, harmonic balancer, etc. No Shakey? Determine if it's in the rest of the drivetrain by playing around with what gear you're in, verses your speed. Depending on what you find, could be driveshaft (very common), transmission (not so common), rear end, tires, etc. Is it strictly speed related, gear making no difference? That points to anything past the transmission. Experiment, look for patterns, take notes. If you find yourself still pulling your hair out, let us know what you found; with more info, we can help you narrow it down further. From what you said though, it probably is in the engine, so, unfortunately, it will need to be yanked and determined what went wrong with the "repair." And welcome to DC

[Enginerd]

Just realized I brain farted and left out something else... Could be detonation. If, when it was "repaired," the specs were changed, especially the compression ratio, you could be getting detonation, which most commonly occurs at higher RPM and under load. If the vibration seems to have a somewhat, or very, random frequency, and you hear a rattling, or metallic banging noises (you may not be able to though while accelerating), then that may be it. Throw in a bunch of octane booster and/or high octane gas, and see if it goes away. Your repair man may have put the wrong pistons or made some other change, by mistake, or on purpose (for performance), without you knowing.

[strokedhatch]

the engine was not torn down, its still 100% stock, when he repaired it he pulled the intake and valve covers, fushed the milky oil out and put it back together...

it does it will in park also, if you just rev it up it will shake..

its like its floating the valves...? could that be it??

[Enginerd]

Valve float won't usually cause vibrations. And, unless they are damaged, valves shouldn't be floating at so low an RPM. Also, when an engine goes into valve float, it loses power and will not gain much more RPM. And when they do go into float, it permanently damages the springs (which can cause float at a low RPM, but you would have to have this happen at a higher RPM first, then get worse and worse), and will have problems forever after. It can either be a coincidence that the problem manifested itself now (not too likely, but anything is possible), or that something related to it is causing it. Something has probably been damaged from the lack of lubrication; a failed bearing, wrist pin, or something like that. Only flushing the motor out after an introduction of something foreign (whether water, metal partials, etc.) is a gamble, and I hate say it, but it sounds like you got a bad hand. Water in the oil can do a lot of damage in a short amount of time. I think it's time to pull the motor and go for a rebuild, but do some more testing and checking to be sure, as it's hard to give definitive diagnostics through a forum. As I said, it may be just coincidence that it started now, so check things like the balancer. You say it's stock, so it shouldn't be externally balanced, but if the balancer separated, or one of the accessories or the flexplate (check the bolts) is damaged, that can do it too. Good luck, and update us.

[grumpyvette]

Enginerd
has given you good advise,
taking it a step at a time makes sense, but ID also start with the basics here and have you do a compression test and ask you to post clear pictures of each of your spark plugs numbered as to the cylinder they came from as a great deal can be learned from the compression test and the plug condition ID also suggest a tune up and CAREFULLY verifying the firing order and timing advance curve


RELATED INFO, ESPECIALLY IN THE SUB LINKS

http://forum.grumpysperformance.com/...g+noise#p21699

http://forum.grumpysperformance.com/...php?f=50&t=609

http://forum.grumpysperformance.com/...hp?f=50&t=5893

http://forum.grumpysperformance.com/...php?f=70&t=875

http://forum.grumpysperformance.com/...g+noise#p19552

[C5 Dude]

If the engine revs with no vibration in nuatral with no load, and then vibrated when under load in gear, I have found that it normally means a fault in the ignition system. Not knowing your model very well, I can only reference checking the plugs, wires, and plug wells.

I used to have a buddy sit in it with it in gear and hold the brake and give it gas to get it to the vibrating point. You can then see the spark fly from the plug isolator to the side of the head almost in sync with the vibration (sometimes). O

If the compression is good and the ignition is good, then check fuel. A clogged fuel filter could have same issues. Yours doesnt seem to have been driven much, but should probably have been changed due to age any way if it hasn't already been.

[grumpyvette]

the links I posted above have a great deal of useful info, but Id also suggest looking at the drive train and suspension, get the car up on a two post lift . do the basic stuff like checking compression, injectors and ignition, etc.
logical testing or the engine ,drive train, electrical and suspension,components,to isolate and locate the problem,will locate the problem, random swapping of parts gets expensive, takes time and is far less likely to produce the desired results

[92Roadster]

....just a quick note, might not mean anything but I had a
similar situation and found out it was a plug wire burned
on the exhaust manifold and was jumping spark, did not
feel it a low power, it started just about that RPM.....

[g8tr]

Check your pulleys. Same thing happened to me and the top pulley the center hole became out of round and caused the vibration

[87vette81big]

Good tips from everyone above on their posts.

Also check the the bottom crankshaft pulley.
Make sure the center balancer bolt is tight & 3 bolts attaching the crank pulley are tight & not missing.
Take a mirror & check for cracks. Stock GM crank pulley is stamped steel. Sometimes they fail with high mileage & continuous High RPM use.
You can temporarily remove the Serpentine belt & check for vibration present or not.
Sometimes the alternator Rotor will fail, a segment or pole of the spinning rotor will fly off.
Rare but I have seen it happen.

If you have access to an old SUN Ignition oscilloscope or Snap On Counsler CRT Ignition Oscilloscope & knowledge how to use to troubleshoot, it can tell you much without tearing down the engine. Need the GM HEI large cap pickup adapter for Snap On Counselor Scope to work on 1984 to 1991 C4 Vettes.
A few old mechanics like me have them yet & use both of mine still.

BR