canadian_blue_fc

So I've owned my 1990 rx7 for about a year now and I've been tackling this issue for just about as long. My car vibrates when the engine spinning at about 2800 rpm to about 4000 rpm regardless of if it's sitting still or if the clutch engaged or disengaged, but only sometimes. The severity of the vibration also seems to be heat sensitive (more severe on colder days until it's been warmed up), and sometimes it almost disappears completely! I've replaced the engine mounts, the transmission mount, tail shaft bushing, and all the exhaust hangers with oem parts. I've also installed a mazdatrix driveshaft. The engine is completely stock and runs great aside from that pesky vibration. So if anyone has any input as to what could be causing this vibration I'd be very grateful.

diabolical1

iTrader: (3)

welcome to the board.

do you know of the engine being rebuilt before you bought the car? can you find any evidence that it might have been?

canadian_blue_fc

Thanks diabolical1!
I don't believe the engine has been rebuilt as it only has 77050 original miles on it. What also leads me to believe it hasn't been rebuilt is the fact that it still has the cover over the crank angle sensor adjustment nut.

WJM ROTARIES

Try running the engine with all the belts removed but not for long some times belts and or pulleys can cause this problem

canadian_blue_fc

Thanks for the reply WJM ROTARIES. I've tried pulling all the belts off as well as the clutch fan but the vibration is still there! I've also swapped my coils with a friend's car that doesn't have this problem and it still persisted. Ive checking the timing with a timing light and everything was ok, so I have my doubts it could be anything spark timing related.

WJM ROTARIES

How many miles has car done ,If high I would say it is the clutch causing your vibration or your spigot bearing in the crank sharft

canadian_blue_fc

The car only has about 77050 miles on it so I've got my doubts that it is clutch related. I've read that that the center nut in transmission mount should be tightened to around 59 ft-lbs and i don't remember tightening that tight when I replaced it, so when I get home tonight I'll tighten it to spec and report back!

canadian_blue_fc

I just torqued the center nut on the transmission mount and nothing has changed. I'm certain that there is no internal engine issues as when you rev the engine with your hand on the upper intake manifold and your knees pressed up against the front tire (you can feel the vibration through the tire) you cant see or feel the engine shake more than it should. My next thought would be that perhaps there could be something resting against the engine or the exhaust, although you can't hear any rattling noise that would suggest that.

Achyllis

Sounds like a balance problem with the engine. Does the engine vibrate pretty severely in the engine bay?
Another possibility could be your driveshaft U-joints. Aaron Cake described a good method of checking them:

BrendanJ

iTrader: (4)

I've heard that wrong series flywheel can cause a slight midrange vibration eg..n370 rotating assembly with an n318 flywheel on there

diabolical1

iTrader: (3)

the nature of the vibration leads me to think it's an internal balance issue (one or more members of the rotating assembly shouldn't have been used with the rest). however, with it being intermittent, it may be something loose. possibly something came loose because of the internal balance issue. however, if you're sure it's never been rebuilt, then those scenarios sort of fade out of likelihood.

it may still be worth your while to at least decouple the engine and tranny and have a look-see if there is something out of order with the flywheel/clutch assembly.

Mototoroto

i have the exact same problem but mine happens around 5800-6300. seems to be temp sensitive as well, its very prominent on hot days but on cold days in the rain its borderline non existent. ive done the belt tests as well and cant seem to pin point it, im guessing its clutch related, but that doesnt explain why it happens regardless of clutch engagement. might be flywheel.

GeneticDrift

Just to share some info:

How do I know if I have a bad crank position sensor?

"You will know that you have a bad crank position sensor by how your engine idles, how your pistons fire, or how you accelerate. It will usually be more noticeable as you raise the RPM's. With a bad or faulty sensor, you engine may misfire causing motor vibration or backfires. You may also experience hesitation while accelerating or shaking during engine idle. In the worst case scenario, you won't be able to start your car."

Read more: Mazda Crank Position Sensor Replacement

Sounds like this could best match your problem.

do a quick check on the electrical lines, and prongs.

dont know if this helps, Lmk

canadian_blue_fc

Thanks for the replies! I'm 90% sure that the engine hasn't been rebuilt, none of the bolts show any signs of being tampered with. I've also had the transmission out of the car fairly recently and the clutch and fly wheel looked to be 100% factory. I've also tested the timing with a timing light and that seemed to be fine, but I'll definitely look into the possibly that the crank angle sensor could be the source of my woes!

lduley

iTrader: (1)

i doubt it, the CAS on these rarely go bad, and if the timing changes you'd notice the engine run different also

since you had the trans out recently, my bet is the input shaft bearing is dry, got damaged, is bad, or all the above