Piston Slap: The U Body, The Relay Bad Idle

U Relay Got A Problem? (photo courtesy: APaGttH)

TTAC Commentator APaGttH writes:

Sajeev!

Long time listener, first time caller. The patient: a 2005 Saturn Relay FWD3 with 151K miles, GM 3.5L V6 and 4-speed automatic. I am the fifth owner and this is our grocery getter and general abuse vehicle. I’m the longest owner – 4 years and about 55K of those 151K miles have been in my garage. I can’t comment on how well it was cared for from year 2 to year 6, but I have done everything by the book since 96K miles.

The Saturn developed a rough idle of sorts about a year ago. I swapped out the spark plugs, changed the fuel filter, and flushed the fuel injection. The car continues to idle rough. No CEL and no CEL history.

Rough idle doesn’t start until the engine is warmed up. There is an occasional difficult hot start, but nothing that falls into, “crap, I’m going to be left for dead because this won’t start,” hard. The rough idle isn’t shown on the tachometer, which is as steady as a rock. But the engine definitely feels like it is vibrating for a lack of a better way to describe it, and far beyond the general coarseness of the GM “high value” 3.5L V6.

The hood is resonating and making metal vibrating noises from it – which isn’t as alarming as it sounds as the fit and finish on the panels isn’t as tight as it should be. Put a few finger lightly on the hood and the noise goes away. The vibration is worse under load, like when the AC is running, and reaches a point you can feel it in the car seats. Acceleration is perfectly normal, driving is perfectly normal, and fuel economy is perfectly normal. No issues outside of idle. There is no rotten egg of burning match smell, and van recently passed emissions. Another thing we’ve noticed – living close to sea level, the problem is worse. If we cross the mountain passes and go to altitude, say about 2.5K feet or higher, the problem goes away. If I give the car light gas at idle, to say 1.2K RPM, the vibration goes away.

I’m stumped. Possible candidates in my mind include the IAC has gone, or is going bad, vacuum leak Hell, bad motor mount, or a dirty MAF and/or throttle body. Given the book value is under $4K and my plan is to drive it to 200K or death, whichever comes first, I’m not interested in pouring a ton of money into the van at this point, but I would like to get this addressed as it feels like something I can’t ignore.

Sajeev answers:

You “relay” should scan the computer to see if there’s a latent code that isn’t triggering the CEL. And U “relay” should consider the smooth idle of LS4-FTW instead of this High Value LX9’s bad idle.

The engine vibration turning into body (hood) problems means that the mounts to the body are the problem. But perhaps new mounts aren’t in your budget, as you’re saving up for LS4-FTW, obviously. In the meantime, there was a TSB with a recalibration resolution to address the hot-restart issue. Get that done first, if applicable.

If that doesn’t cure the bad idle, or if it isn’t applicable? Well… maybe the Relay “relay” needs new mounts, especially the torque mounts (I forgot their name) as seen in the photo below.

Very professionally highlighted in red. (photo courtesy: gtcarlot.com)

Basically, LS4-FTW is the only answer. We gotta believe, we must ensure the notion of a bad-ass V8 U-body actually turns into a trend.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.