Piston Slap: The U Body, The Relay Bad Idle

Sajeev Mehta
by Sajeev Mehta
piston slap the u body the relay bad idle

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

TTAC Commentator APaGttH writes:


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.

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2 of 53 comments
  • StudeDude StudeDude on Jan 07, 2015

    I owned a '96 Intrepid, 3.5, 125K miles and and had the identical issue. Did a complete tuneup, changed the motor mounts, even changed out the vibration damper---no real change. Car ran fine under load, got 26 MPG on the highway, etc. The issue was a leaking intake manifold gasket. I found this possible solution on an internet forum and since the gasket kit cost $6 @ Advance Auto, decided to give it a shot. I used Permatex Copper forma-a-gasket in addition to the Felpro kit, let it dry for 24 hours and Presto, the car idled smooth as silk--problem fixed. I would also point out that a defective check valve at the power brake booster can give similar symptoms, not to mention the booster itself if it has an internal leak. Hope this helps.

  • Lonborghini Lonborghini on Jan 07, 2015

    "...about 55K of those 151K miles have been in my garage." That is one big garage. I'm green with envy. I don't have a garage.

  • Aidian Holder I'm not interested in buying anything from a company that deliberately targets all their production in crappy union-busting states. Ford decided to build their EV manufaturing in Tennessee. The company built it there because of an anti-union legal environment. I won't buy another Ford because of that. I've owned four Fords to date -- three of them pickups. I'm shopping for a new one. It won't be a Ford Lightning. If you care about your fellow workers, you won't buy one either.
  • Denis Jeep have other cars?!?
  • Darren Mertz In 2000, after reading the glowing reviews from c/d in 1998, I decided that was the car for me (yep, it took me 2 years to make up my mind). I found a 1999 with 24k on the clock at a local Volvo dealership. I think the salesman was more impressed with it than I was. It was everything I had hoped for. Comfortable, stylish, roomy, refined, efficient, flexible, ... I can't think of more superlatives right now but there are likely more. I had that car until just last year at this time. A red light runner t-boned me and my partner who was in the passenger seat. The cops estimate the other driver hit us at about 50 mph - on a city street. My partner wasn't visibly injured (when the seat air bag went off it shoved him out of the way of the intruding car) but his hip was rather tweaked. My car, though, was gone. I cried like a baby when they towed it away. I ruminated for months trying to decide how to replace it. Luckily, we had my 1998 SAAB 9000 as a spare car to use. I decided early on that there would be no new car considered. I loathe touch screens. I'm also not a fan of climate control. Months went by. I decided to keep looking for another B5 Passat. As the author wrote, the B5.5 just looked 'over done'. October this past year I found my Cinderella slipper - an early 2001. Same silver color. Same black leather interior. Same 1.8T engine. Same 5 speed manual transmission. I was happier than a pig in sh!t. But a little sad also. I had replaced my baby. But life goes on. I drive it every day to work which takes me over some rather twisty freeway ramps. I love the light snarel as I charge up some steep hills on my way home. So, I'm a dyed-in-the-wool Passat guy.
  • Paul Mezhir As awful as the styling was on these cars, they were beautifully assembled and extremely well finished for the day. The doors closed solidly, the ride was extremely quiet and the absence of squeaks and rattles was commendable. As for styling? Everything's beautiful in it's own way.....except for the VI coupe....it's proportions were just odd: the passenger compartment and wheelbase seemed to be way too short, especially compared to the VI sedan. Even the short-lived Town Coupe had much better proportions. None of the fox-body Lincolns could compare to the beautiful proportions of the Mark V.....it was the epitome of long, low, sleek and elegant. The proportions were just about perfect from every angle.
  • ToolGuy Silhouetting yourself on a ridge like that is an excellent way to get yourself shot ( Skylining)."Don't you know there's a special military operation on?"