Cadillac STS Forum - 2005 through 2012 Discussion, Rough Idle with no Codes 2005 V8 STS in Past Cadillac Vehicle Discussion; I get a rough idle while the car is in gear and in idle (not as bad when not in ...
I get a rough idle while the car is in gear and in idle (not as bad when not in gear). I think the idle is too low personally but there is no screw to turn out to raise it (old school).
The car has 125K miles.
I replaced the spark plugs at 110. they were not AcDelco.
It has been run on regular for a long time (I just got it back from the wife).
What could be the cause and solutions. Cheapest to most expensive. I have heard EGR,Throttle body, MAF, and Idle control unit (IAC).
Also there are no codes being thrown according to the ODBII reader I have.
I would try in this order...Clean throttle body and MAF, check/new air filter, check/tighten intake bolts. If no change then look into fuel filter then spark plugs (but if these plugs have been fine for the last 15K then probably not the problem.) Also check plug wires and boots. If still no change then you have bigger problems.
Motor mounts have absolutely nothing to do with a rough idle.
So, what was my clue; well, 'idle feels rougher in gear' (load of transmission/drivetrain) putting more pressure on mounts. It still may not be the mounts, but with no engine codes, mounts do needs to be eliminated (as cause).
Update. I did the following: Used Carb Cleaner on the the Throttle body. Used Electrical Cleaner on the MAF. Put in a new Air Filter (needed it no matter what). I also put Sea Foam in the almost full gas tank. The shutter seems to have calmed down but the air temp was 57 and I will have to see how it does later today when the temp goes up again. It is not perfect and I still get 25% of the vibration I had earlier. Although it is not embarrassing now.
To clarify for everyone the car was shaking noticeably while in drive and at idle. It did not sound like it was missing or misfiring. When I would take it out of gear it would lessen. When I press the gas to get the idle to 800 rpm line it smoothed out. Tested with my foot on the brake and in gear also and had a similar result (seemed to become smoother).
It's pretty easy to check for a broken motor mount (though it may not always find a bad one and you need two people); with the engine on and in gear (with the brake firmer depressed), rev the engine and check for the engine to 'torque'(twist) more than what would seem normal (appropriate). The second man, of course is doing the looking (and you don't need to rev the engine too high, just enough to put some load on the mounts).
Again, it may not be the mount(s) (but easy to check); best of luck in your diagnosis.