msilvia

I’ve recently installed and broken in a new crate 13b from Mazda, along with a set of good used OE motor mounts and a new ACT clutch. Most engine accessories carried over from the last engine, as has the transmission, which is certainly getting tired.

I get mild vibration that increases with RPMs, becoming pretty apparent over 3500-4k or so to where you can feel it in the seat. It’s not severe, but you do notice it. This happens even when stationary/free-revving, and with the clutch pedal depressed or released.

The weird thing is that the last engine, which was healthy, did the exact same thing. The car has always been this way, as far as I can recall. ​​​
That level of vibration is abnormal, right? If it’s unlikely to be the engine or motor mounts, what are other possible causes? From searching, I saw some issues with exhaust parts contacting the frame (I haven’t found that so far) and one guy who said it turned out his front subframe was tweaked. Not sure how I’d check for that last one. Other things I should consider?

Can the transmission be a culprit even if the vibration continues with the clutch released? I assume it rules out PPF/diff/etc. I have the common input shaft bearing whirring noise with the clutch engaged (pedal down) in neutral.

I really don’t think the engine is bad, and the mounts looked fine.

gracer7-rx7

iTrader: (16)

what are you running for diff bushings, flywheel and clutch?

msilvia

Poly diff bushings, stock flywheel (ships attached to the crate 13b), ACT street clutch (zx6-hdss).

j9fd3s

iTrader: (3)

does the vibration go with engine rpm or speed?
if its engine RPM you're looking for something that goes engine RPM, water pump, alternator, etc

if its vehicle speed, then its like tires, wheel bearings, etc

if its a noise, noises transmit, so it can be the exhaust rubbing on the body somewhere but you hear the noise from the front. you can still find them, you just need to have an open mind

c0rbin9

iTrader: (2)

I had a similar vibration and the shop said it was the PPF installed out of alignment. It took Rotary Performance in Dallas several tries to get it aligned correctly and eliminate the vibration.

DaveW

Since it happens free-revving with the clutch disengaged, it has to be something attached directly to the engine, and since it happened with the old engine, it is likely something taken from the old engine and reinstalled, like the flywheel, pulleys, etc. that are out of balance. It could also be the new clutch, but having 2 in a row be out of balance is unlikely.

Another thought is that there is something like the exhaust system that is harmonically vibrating at the frequency of (a slight and otherwise not noticeable absorbed by the engine mounts) engine vibration due to how its mounted. The PPF is another thing that could vibrate like that.

msilvia

That was my understanding - that having it happen in free-rev with the clutch disengaged the driveline inquiry need not continue with anything past the pressure plate. Am I right that the trans input shaft should not be spinning with the clutch disengaged, thus ruling out trans issues?

The flywheel and main pulley/trigger wheel come included and installed on the crate engines, so those are new. Power steering, air pump, alternator, and water pump (lazy!) carried over with their respective pullies and the idler. So it could be one of those? Aside from the water pump/alternator, I suppose I could experiment with removing the belts and testing. Is there a better way?
I’ve changed most of the exhaust components around over the years and it hasn’t hone away, but I’ll take a careful look at the system.

DaveW

I think it's unlikely to be a pulley, alternator, or WP since none of those have much rotating mass. As C0rbin9 said, it could be the PPF. As you said, the trans input shaft doesn't rotate with the clutch disengaged.

msilvia

But air pump or power steering are possible culprits?

I’m confused on the PPF. I’d have thought that would come into play only if power were being applied to the driveline. You guys are saying maybe it’s bent/misaligned and applying some kind of pull or torque that’s making the engine vibrate?

When I first got the car (like a decade ago) a shop inspecting it told me the PPF was cracked and re-welded it. So maybe if they botched the repair it’s not 100% straight anymore?

j9fd3s

iTrader: (3)

you could pull the belts off and see if the vibration goes away. if you pull the water pump belt off the water pump won't spin, so don't be like me and run it for 20 minutes! run it long enough to see if the vibration changes

ive had water pumps with failed bearings that were really noisy/vibratey, alternator too.

DaveW

It's not likely MAKING the engine vibrate - It's possible that being bent or cracked somehow normal engine power pulses, etc. are making the PPF vibrate. It's a relatively long narrow structure, so it will have a natural frequency of vibration similar to a guitar string.

Jesturr

I have a hilariously noisy airpump. It gives me a loud "moo" at low RPMs but from the drivers' seat above 2500 you don't really notice it. There is a terminal above the airpump you can disconnect and see if the noise goes away.

scotty305

iTrader: (2)

The symptoms OP is describing might also be a misfire. Try feeling the exhaust coming out of the muffler with your hand while someone revs the engine to make the vibration happen. Don't get too close, the goal is to feel the flow not to burn your skin. The exhaust should be warm with a smooth flow. Noticeable pulses or 'rough' feeling bursts of cold air that accompany the vibration would indicate a fuel or spark problem.

I agree it's not drivetrain or transmission if the vibration still happens when the clutch disconnects the engine from the transmission.

msilvia

Interesting update. I was able to measure the PPF alignment and I’m about 10-12mm above factory specs. I got 87mm (vs 70-77mm spec) when measured from the flat part of the trans crossmember. I’ll attempt to loosen the bolts and adjust it lower when I get some time to wrench. Fingers crossed it’s a simple fix to my issue.

teebeekay

iTrader: (4)

Do you have a differential brace or cradle installed? I put one in mine and it causes vibration just like you described.