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Old 03-13-2019, 02:01 PM   #1
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Exclamation Heavy Engine Vibration at 900, 2k, 3k. Just replaced front moto + trans mounts. DMF?

I've gotten some great advice from people here before. So here's my current issue I'm digging into. Before I go all in and replace the DMF I'd like to check here and see if people have any idea what's going on.

Here's the symptoms
- Engine speed unstable at idle, 3k (def need to clean MAF, TB, intake and I'll report back after)
- Shaking engine vibrations at idle, 2k and 3k marks during warm idle
- No vibrations during acceleration or deceleration
- Cold idle seems to have much less vibrations
- I replaced both the engine mount and the transmission mounts a few days ago
- The clutch was replaced ~4 years ago but DMF was not done at that time as the car only had 50k miles on it (90k now)

Likely the unstable engine speed is due to the throttle body, MAF, or intake being contaminated with oil. I have the AOS go out about every 2 years in this car and haven't cleaned those parts out in a while

About two weeks ago I started noticing heavy vibrations at startup. Happened all of a sudden. So I got in there and replaced the engine mount and transmission mounts which helped dampen this vibration but it's still heavy enough to shake the seat when sitting during warm idle. Interestingly cold idle seems to be much much smoother. When in gear I don't feel any of these vibrations at all which leads me to believe it's the DMF.

If you've seen this issue or have any ideas I'm all ears

Thanks!

Last edited by dmboxster; 03-13-2019 at 08:29 PM.
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Old 03-14-2019, 07:37 AM   #2
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Do a comprehensive tune up, plugs, coils, etc. (tubes if they're leaking)
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Old 03-14-2019, 09:13 AM   #3
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Thanks for that suggestion. I'm planning on doing plugs as part of the DMF swap. It's fairly easy to replace the coils in my parking spot, so I'll do that if necessary at a different time. It's about $400 to get all new coils and I'd rather not spend the money on that unless it's necessary.

To follow up on the vibrations, the engine has been having unstable idle for a while now. It never caused these vibrations before (even with the old engine mount)

I checked the OBD and I don't see any codes or concerning issues. I did take a readout of the MAF vs Engine Speed and throttle position. Throttle position is solid at 2% during idle so that's not moving The MAF seems to bounce around some (26-36 lbs/min) during idle. I see the engine speed bounce around between 656 and 715 RPM.

Here's my final theory. The unstable engine speed is causing the failed DMF to slam back and forth +-15mm causing the engine vibrations. As the engine accelerates one way it slams, and then as it slows down it slams the other direction causing more engine speed instability. I see engine instability at all three points (idle, 1k, 1.8k, 2.8k) which means that the slamming will be worse in those areas. Here's the kicker, because when the car is in gear there is a constant force on the DMF it can't vibrate back and forth anymore, this is why when driving I don't feel any vibrations and the idle is smooth.

I'm writing this out in hopes that someone else will read this and find it informative. Here's all what I'll be changing:

- Full clutch kit from pelican parts (includes throw out bearing, DMF, etc)
- Spark plugs (already did tubes at 55k)
- Air filter
- Clean MAF, Intake
- Replace AOS, upper AOS to Intake tube (part of my regular 2 year maintenance as they fail every two years on my car)
- Replace inner rear CV joints (might as well since I'm taking them off)
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Old 03-14-2019, 09:32 AM   #4
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Thanks for that suggestion. I'm planning on doing plugs as part of the DMF swap. It's fairly easy to replace the coils in my parking spot, so I'll do that if necessary at a different time. It's about $400 to get all new coils and I'd rather not spend the money on that unless it's necessary.

To follow up on the vibrations, the engine has been having unstable idle for a while now. It never caused these vibrations before (even with the old engine mount)

I checked the OBD and I don't see any codes or concerning issues. I did take a readout of the MAF vs Engine Speed and throttle position. Throttle position is solid at 2% during idle so that's not moving The MAF seems to bounce around some (26-36 lbs/min) during idle. I see the engine speed bounce around between 656 and 715 RPM.

Here's my final theory. The unstable engine speed is causing the failed DMF to slam back and forth +-15mm causing the engine vibrations. As the engine accelerates one way it slams, and then as it slows down it slams the other direction causing more engine speed instability. I see engine instability at all three points (idle, 1k, 1.8k, 2.8k) which means that the slamming will be worse in those areas. Here's the kicker, because when the car is in gear there is a constant force on the DMF it can't vibrate back and forth anymore, this is why when driving I don't feel any vibrations and the idle is smooth.

I'm writing this out in hopes that someone else will read this and find it informative. Here's all what I'll be changing:

- Full clutch kit from pelican parts (includes throw out bearing, DMF, etc)
- Spark plugs (already did tubes at 55k)
- Air filter
- Clean MAF, Intake
- Replace AOS, upper AOS to Intake tube (part of my regular 2 year maintenance as they fail every two years on my car)
- Replace inner rear CV joints (might as well since I'm taking them off)
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Old 03-14-2019, 10:36 AM   #5
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I had a similar issue...I checked the coils and 3 of them were cracked .I replaced them along with the plugs and problem was solved.
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Old 03-14-2019, 05:55 PM   #6
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Thanks

I'll give replacing the all the plugs and the coils. The car's got 90k on it and I doubt the coils have ever been replaced. If this doesn't work then I'll try the DMF.
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Old 03-14-2019, 09:46 PM   #7
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What transmission and motor mounts did you use?
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Old 03-15-2019, 09:46 AM   #8
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The transmission mounts are Rein Automotive. The engine mount is a Lemfoerder. It was pressed into the original bracket in the correct orientation.

Before the updated mounts the entire car was shaking. Itís noticeably better now. Thereís just continued vibrations.

The parts are ordered and should be here next week. Iíll update after I install the coils + plugs and clean the intake, MAF
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