800-900 RPM Rough Spot

[jdwk]

So I know it is not just my car, as many have mentioned this rough spot in the rev range. I have inserts on all four mounts and I am tired of the vibration when the car kicks into high idle from either the fans or A/C. In the summer, I often just keep my right foot slightly on the gas to keep it over 1k, but this is a truly awful. I am tempted to just remove the inserts, but I don't want a sloppy engine either. The car is actually very smooth at any other RPM. It's not a V6, but acceptable.

So I've always thought it must just be a harmonic frequency of the FS-DE and there is nothing we can do about it. I've read having the crank balanced helps, but I am not planning on investing the kind of money needed to do that. Maybe if I blow this engine, and need to build up a new one, (although in that case, I'd rather build up a KL instead).

But when I got my car running again last week and the PCM was once again learning how to idle the car, I noticed it vibrated at lower rpms as well, very similar to the normal high idle vibration, and I began wondering if the 800-900 vibration is actually a tuning issue. I know our car is tuned like garbage from the factory, maybe this is also due to tuning.

I am thinking the vibration could also be caused by the VTCS, but I think there are enough people now who have removed the VTCS to know if it causes the vibration to go away. If anyone who has removed it would comment on the affect on the vibration, that would be appreciated.

I should have my megasquirt running the car in the next few weeks depending on how much free time I get. I know tuning for idle is difficult. My plan with the MS was just to make my high idle 1100rpms to relieve my right foot, but if I could get rid of the vibration altogether, that would be even better.

 

[Sport23]

I think this rough spot in the RPM range is a characteristic of the FS engine and not a tuning issue. High idle is closed loop so the car is actively keeping the AFR @ 14.7:1. *Maybe* if you made it run rich in this range you could get it to smooth out a little bit? Also, I'm running the protege garage IM so I have no flaps in my intake manifold at all and I still have a rough high idle like you do (I have all four medieval motor mounts.)

If you do have any luck post up what works! Now that my motor mounts are broken in idle with A/C is the only thing that bothers me. I don't mind cold idle and it's funny when people ask whats wrong with my car lol.

 

[jdwk]

Ok thanks, that at least rules out the VTCS. Yeah, my friends call the A/C button the vibration button.

 

[hornsfan10609]

I have solid motor mounts and I know exactly what you are talking about, the car runs rougher from 8-950rpm. Much smoother at idle around 750 and above 1k rpm.

 

[jdwk]

I am hoping someone with a standalone chimes in at some point. If a standalone still shakes at that rev range then it's a safe bet it has nothing to do with tuning, and we are just screwed unless we want to idle above 1k. I think that is my plan with the Megasquirt.

 

[jdwk]

I am going to keep bumping this until we find a solution or at least a definitive cause. Now that it is over a 100 out here again and I have the A/C on all the time, I am sick of the shaking. I sit at lights now with the e-brake on and my foot slightly on the gas trying to keep it steady at 1k. I just don't understand how an engine can be smooth as silk at 1k or 650, but in between, it wants to rattle my teeth out of my head. I also don't understand why Mazda would program the PCM to high idle right in the absolute worst part of the vibration range.

 

[tweety2300]

The fs is just a bad motor to start with. You could try adjusting the idle to get it to idle higher

 

[Spiced MSP]

Honestly, I would just suck it up. If you use aftermarket inserts or solid mounts, the car is going to vibrate a lot, especially with the rear mount or insert. Raising your idle is just a band aid to the vibration, so either way it will be there. Permanent solution, take the inserts out and be happy. But avoid too much aggressive driving.

 

[jdwk]

Since the car is smooth in all but a small portion of the rev range, it is not the fault of the mounts or inserts. Removing the inserts is just a band aid for the FS.

I know the engine is terrible. Coming from a KL, it's like going from peanut butter to peanut brittle. However, the difference between those 200 or so rpms and the rest of the engine operation is just too severe to not have some sort of explanation. If it was an engine harmonic then we'd feel it again at the 2nd and 3rd harmonics, but we don't.

I am torn about removing the inserts. The car had rear inserts and an AWR front mount when I bought it. I replaced the AWR with a stock mount and inserts a year or two ago and it didn't make much difference. I put the side mounts in it about the same time and that didn't make much difference either. So that leaves the blame mostly on the rear.

I am really tempted to replace the rear with a new one (rubber doesn't last long in AZ) and remove the inserts from it, and hope the other 3 inserts can keep the engine in the bay.

My next daily driver will have a V6 again.

 

[tweety2300]

Honestly, I would just suck it up. If you use aftermarket inserts or solid mounts, the car is going to vibrate a lot, especially with the rear mount or insert. Raising your idle is just a band aid to the vibration, so either way it will be there. Permanent solution, take the inserts out and be happy. But avoid too much aggressive driving.

+1 I have a solid front mount in my Honda and vibrations are horrible but I deal with it.

Also try taking all the inserts out and see what happens. It's also not just a 4cylinder thing. Any motor will do it with stiffer mounts, the reason for the holes in oem mounts is to allow for movement to absorb virbrations

 

[jdwk]

I guess I am just getting old. I am now wondering how I bought the car with it shaking like this and then lived with it so long.

I think I'll take the rear out when I get time and remember to go buy two more feet of ratchet extensions. I have the suspicion it is toast anyway.

 

[jdwk]

Broke down and ordered a new rear mount. Hopefully, get to the install this weekend.