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.

 

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

 

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

 

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

 

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.

 

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.

 

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.

 

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.

 

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?

 

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

Just wanted to update. I ended up removing my airpump, which was definitely a substantial contributing factor.

However, I do still have a residual vibration when the car is cold (otherwise as described in OP) that goes away within 15m or so as the car is fully warmed up. Could something be causing a mild misfire or something that goes away with heat? Doesn’t seem to fully go away with reaching operating coolant temps—it seems to take a little longer than that—which makes me think its not an ECU warmup setting. What could cause the car to run smoother as the engine bay heats up?

 

Got video?

 

I could try to capture it but I think video would be unlikely to help. It’s a vibration that’s felt but the car doesn’t sound audibly much different, nor is there dramatic ignition breakup or stuttering or anything like that.

 

Define "mild vibration." Weaker or stronger than a cell phone on vibrate? Strong enough to make an iPhone migrate across the carpet or make it dance? Is it noisy, or just felt?

My car vibrates somewhat starting at 5000-6000, but it's got metal engine mounts, no interior, and a rock hard race seat.

I assume you have a stock seat?

I would suggest getting under the car and visually look for metal-on-metal contact that shouldn't exist, and then using a mechanic's stethoscope and seeing if you can't pinpoint the source.

Are the OEM engine mounts Mazda Competition units by any chance?

 

I started up my 1990 RX-7 yesterday and noticed, (After owning it since 2016), that there was a lot of vibration coming from the engine. Being a rotary engine, which is supposed to be very smooth, it finally donned on me that this was most likely abnormal. When I start it cold, the vibrations are most severe, which is something that is completely felt, not heard. After 15 minutes or so of running, the vibrations are greatly reduced, but certainly not eliminated. When I last took the car out for a drive of 14 miles back in July, I came home and the engine idled rather smoothly and quietly, and the whine or whirring noise coming from the transmission, (Something that existed since I bought the car), was barely noticeable. These vibrations when cold don't go away by stepping on the clutch pedal, and the get more severe when I raise the rpm up from idle to 4,000 and beyond.

Since I purchased the vehicle, I have replaced the water pump and the air conditioning compressor,(To convert the system to 134a), along with a spark plug change, spark plug wire change, and an oxygen sensor change. There is absolutely no smoke or steam coming from the exhaust at start up or idle, other than a little bit of water vapor coming from the catalytic converters,(And there is no smell of anti freeze or anything unusual). Could this possibly have something to do with the clutch and/or pressure plate? I am planning on dropping the transmission this winter to replace the clutch release bearing and input shaft bearing to cure that whine and whirring sound that currently only goes away when I step on the clutch pedal. During this procedure, I plan on checking the driveline for wear,(since I noticed a gentle clink or clunk when I last released the clutch to back out of the garage), change the transmission lubricant, and, of course, inspect the clutch. I wasn't planning on replacing the clutch disc and pressure plate, since I anticipate replacing the engine with a fresh, all-new build within a few years, which includes a new lightweight steel flywheel and all new clutch components at that time. I need some advice, because I missing out on the full, supposedly silky-smooth rotary experience!