Horrible vibration when cold
 

So none of this is "performance" related, but I'm having engine issues. When my 2001 is started after soaking to ambient temp, it has a horrible vibration you can feel in the floorpan and a grumbly sound to match. This is most noticeable on decel in first or second gear. It will also do this right after startup for a few moments, then when letting out the clutch in first or reverse. It becomes less and less noticeable over the course of a few minutes, and by the time the temp gauge is at noon it behaves completely normally. There are no active CELs and it drives fine when hot.

Background: Two weeks ago, I installed new motor mounts, and we jacked up the engine really really high. This past Saturday I installed a roll bar, and Sunday morning this vibration started. There was a check engine light that was intermittently active after installing the motor mounts, but it went away before I could fix my obd port and get a read on it (blew a fuse because of my own terrible wiring work on interior lights...). In the distant past it has thrown a P0300 random misfire, but never ran rough. I accidentally did not hook up the valvecover vent to the intake for a few miles when installing the motor mounts, but it never threw a code. The engine compartment is completely stock.

The recent work has me thinking of wiring, egr, or vacuum hoses on the back of the head getting pinched or pulled. Anyone got any ideas on where I should start looking?

Probably the exhaust rubbing on something. What motor mounts did you get that jacked it up?

Ha! I just fixed this same problem on a coworkers car. Of course, it was a mid 70's Corvette, and all it required was an automatic choke adjustment.

So, some sort of cold start enrichment problem?

The motor mounts are presumably stock replacements. They're the cheapest on rockauto, got them for free off Craigslist. It's not that they raise the engine significantly, it's that we raised it a huge amount when installing them and I'm wondering if anything was damaged from that.

Rleete, that's what I'm thinking as well. But I'm​ trying to figure out if what was done could be the root cause.

During motor mount install (if that's all you did that day) you shouldn't have disconnected anything that would cause this except for the exhaust header. Another possibility is that the mounts are not fully seated or secured to the chassis. Jacking up the engine to install the mounts would not normally produce this (done it myself, no ill effect on the car). Are you use the mounts were new or in good condition? When out of the engine and not been under stress - it can be hard to tell that they are separating or otherwise in bad condition.

Ahh ok gotcha, in that case if you have stock exhaust you probably raised it up and crushed the heatshield against the firewall/tranny tunnel and now it's touching the exhaust when it warms up and making horrible noises. Easy fix.

That would make noise, but hardly any felt vibration.