Engine operates smoothly and no acceleration issues before and after this affected rpm range. Best way to describe is it feels like driving over rumble strips in that range, with a low growl (almost sounds like intake growling) and low acceleration power. Almost sounds and feels like trying to drive in 4th gear at 15 mph, but no vibe or sound before 1500 rpm or after 2000 rpm in any gear as well as neutral. I can also hear/feel the intake on the driver side of car rattle some when accelerating or cruising through affected rpm range. It does idle nicely at around 900 rpm when warm, but has a minor miss every few seconds. 64k miles on engine. 89 oct fuel used. Bad EGR? Coil/plugs/wires/rotor? Bad mounts? Harmonic balancer? Bad pulley/belt (Water pump pulley sounds a little noisy but sounds normal through entire rpm range compared to any videos I've watched.)?

Welcome to the forum When the car is idling and you rev the engine up to 1500 rpm do you have any of the same vibration?

Thank you! From idle to 1500 rpm it's smooth. It's barely noticeable while in neutral, but noticeable if I keep it steady between the 1500/2000 rpm. It occurs harder or maybe just more noticeable when in gear, but it's only in that range from 1500-2000 rpm. Before 1500 rpm, and after 2000 rpm in any gear, it goes away and power is fine. It doesn't lose all power, but is noticeably sluggish as it gets through that range. I notice it the most while at a steady 30-35 mph in 4th as it's in that rpm range during that speed for a longer amount of time to hear it. Hoping it's not a bad torque converter, but it's got low miles and seems to have been pretty well taken care of. It's been too cold to really get under it to check any mounts other than the dogbone, and that appears fine. It has been in the negatives where I live for the past week or so, but it's been progressively getting more noticeable.

[This message has been edited by Ginger_Dreadman (edited 02-24-2015).]

I would suggest running some Seafoam through the fuel system and see if that helps. It may take a few days to clean out the injectors.

Would the injector only operate like that within that rpm range? And why do you suggest the fuel system?

I would be surprised if its the injector, there's only one on the 4 cylinder and its pretty bulletproof. I assume from what you say that the car is an automatic? If it is is there any change if you manually downshift to a lower gear when it is making the noise?

4 spd man. I can do it in any gear including neutral. Almost sounds like it's coming from the right rear of the engine bay but it does send quite the vibrations through the driver's seat and vibrates the steering wheel pretty good. It just happens the most in 4th because its at around 1600-1700 rpm at 30-35 mph. I can shift early from 2-3 or 3-4th at 1200 rpm and no vibration or sound until 1500 rpm.Then at 2000+ rpm, in any gear, it completely goes away. I'ts pretty hard to keep that range in neutral or 1st because it revs up pretty quickly above 2000 so its hard to reproduce in the engine bay long enough to even hear or feel from behind the car. I need another person handy i think if I can keep it steady in the affected range i can at least get a general location of the issue. It's weird how I can hear it bog some and shake through my intake from inside the car too. That almost sounds most comparable to a subwoofer cone flapping at a real low frequency.

[This message has been edited by Ginger_Dreadman (edited 02-25-2015).]

Okay, you don't have a torque converter then, that's only automatics. The rear passenger side is where the bottom engine mount is, that could be broken, especially with the cold weather you have had. Every engine has a certain rpm where its vibration is the greatest and it may be vibrating against the frame and reverberating thruout the car.

That's what I thought! Some old dude was tellin' me nope, they all had em' haha. This was my first thought, was an engine mount but I haven't been able to familiarize myself with mount locations so much in this weather. I've been brainstorming all the things that could have possibly been causing it all day. But as soon as I can get space in the shop for some heat I'll start by replacing them and see how she runs. I don't see it being difficult to do but do you guys have any good advice or quick tips for changing them? Best type to get?

I replaced all of the mounts on my 85 4 cylinder and I got them from rock auto.com If you don't have one get a Haynes manual on the Fiero, its fairly easy to replace the mounts, the one on the passenger side is probably the easiest one to do.