5.7 Vortec Idle Shake When Warm, out of ideas

[andfern97]

Hello, I have a question for all you experts here! I have a '98 5.7 vortec that has a brand new motor in it 5000 miles ago. It has a vibration/rough idle problem that feels worse when the truck has warmed up. I've already changed all of the following within the last 5k miles:

New L31 Vortec Motor
CPI to MPFI Spider Injector/regulator
Plugs/Wires w/ Delco
Fuel pump
Dist. Cap/Rotor
Knock Sensor
Intake Manifold Gaskets
Carb cleaner check for vacuum leaks (none found)
Cleaned Intake Manifold and Throttle Body

I just had it at a shop for diag. and they tell me that the valves are too tight and MAY need adjustment because vacuum seems erratic on a gauge, but I'm reluctant to pay for that since the motor's brand new from the dealer. Anyone got any ideas of anything else could be? How hard is an adjustment DIY? Thanks a ton!

 

[Supercharged111]

It's an easy DIY procedure and insanely unlikely that a GM crate would come with valves out of whack. Have them show you this erratic vacuum.

 

[m002922]

I'm in a similar situation with being out of ideas for my slightly shaky idle despite doing all the obvious stuff - but my motor isn't brand new (98k). Let me know if you figure it out.

Have you checked for a functioning EGR?

 

[andfern97]

[QUOTE

Have you checked for a functioning EGR?[/QUOTE]

I unplugged the egr valve last night and nothing changed, also tried plugging up the egr tube and nothing happened as well. Mines more of a shudder that comes and goes instead of a consistent shake, it happens more in park than in gear but I can still feel it. What's yours like?

 

[andfern97]

Supercharged, I agree that it seems very unlikely for a brand new motor to come with valves out of adjustment. Do you have any ideas about what else could be causing it? Just talked to the tech on the phone and he says he can see cyl3 misfiring more than the other cylinders and vacuum fluctuations on the gauge, so he thinks the valve above cyl3 is causing the misfire

 

[m002922]

Similar, though I notice mine mostly in drive (and it seems to get slightly more noticeable after driving for awhile). I'll have to check if it is mostly the same in park.

[QUOTE

Have you checked for a functioning EGR?

I unplugged the egr valve last night and nothing changed, also tried plugging up the egr tube and nothing happened as well. Mines more of a shudder that comes and goes instead of a consistent shake, it happens more in park than in gear but I can still feel it. What's yours like?[/QUOTE]

 

[andfern97]

Sounds good, let me know what you find. It's really frustrating given I replaced the motor because it had low compression and misfired... Aargh!

 

[m002922]

I'll let you know, but this is lower on the list as I'm busy chasing down a serious rear brake issue that no one can seem to figure out.

Sounds good, let me know what you find. It's really frustrating given I replaced the motor because it had low compression and misfired... Aargh!

 

[m002922]

I haven't forgotten about this - maybe I can get back to this now that it looks like my brake issue is resolved (and I'll be getting back to having my tools again after I move). Did you ever end up sorting it out?

 

[m002922]

I checked it out tonight. The vibration is the exact same in park.

I'm not getting any check engine lights or indications. The idle is rock solid on the gauge - no movement at all. The motor has 90k miles on it. I just did the lower intake manifold gaskets, and other things that have happened in the last 2-3k miles:

Distributor / Cap / Rotor
Coil
ICM
Plugs
Wires
Fuel Pump

Given the life that I know mine has had (90k miles in a mostly urban area - not a lot of highway miles) I'm starting to wonder if maybe it isn't just the engine mounts going (not sure if OP did mounts when they did the engine).

I'm not really sure of the lifespan of those on this truck - and how much absorbing they do - but given how minor the shake is (but obvious), it seems like it could be a possibility.

The other thing I'm considering is that maybe it's a clogged cat issue causing some back pressure - but I would think that the symptoms would be much worse on that. That said, I've only had the truck like 4k miles, and am not exactly sure how one that is "right" feels . . . so it's possible mine is really sluggish because of that and i just don't know it . . . just throwing that out there, haven't done a lot of research on how a clogged cat performs.