intermittent rough idle

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Oct 4, 2014 | 10:16 AM

KHD

Thread Starter

Heel & Toe

Joined: Sep 2014

Posts: 22

Likes: 0

From: MALTA NY

intermittent rough idle

HI, new to the forum, first post and 1st vette. I just bought a 1999 LS1 6spd w/83000mi. My concern is an intermittent rough idle, it idles smooth for about 5-10 seconds, then shakes for a couple of seconds, then smooths out again then repeats again. The car pulls hard no skips or anything. There are no codes in the computer. I see no broken vacuum lines. Any info or advice would be appreciated

Oct 4, 2014 | 04:51 PM

flynhi

Le Mans Master

Joined: Aug 2000

Posts: 6,273

Likes: 14

From: Austin TX

Quote:

Originally Posted by KHD

Just had a similar problem with my 99 with 90k miles. Get some spray carb cleaner and clean the throttle body in front of and behind the butterfly blade. Mine was not too dirty in front but filthy behind.
Makes a big difference.

Oct 4, 2014 | 06:16 PM

KHD

Thread Starter

Heel & Toe

Joined: Sep 2014

Posts: 22

Likes: 0

From: MALTA NY

Quote:

Originally Posted by flynhi

THANKS for the input, will do tomorrow. GREAT forum, plenty of info.

Oct 4, 2014 | 08:19 PM

choyt

Instructor

Joined: Nov 2013

Posts: 187

Likes: 13

From: Port Ewen New York

PM sent

Oct 4, 2014 | 10:36 PM

BigGun

Drifting

Joined: Feb 2008

Posts: 1,596

Likes: 67

From: Bahama North Carolina

Be sure to check at the pcv valve. They are known to develop a hole in the bottom of the tube that is hard to see.

Oct 6, 2014 | 06:39 AM

KHD

Thread Starter

Heel & Toe

Joined: Sep 2014

Posts: 22

Likes: 0

From: MALTA NY

Quote:

Originally Posted by BigGun

Be sure to check at the pcv valve. They are known to develop a hole in the bottom of the tube that is hard to see.

THANKS, will do

Oct 6, 2014 | 08:02 AM

Porxter

Instructor

Joined: Sep 2006

Posts: 153

Likes: 4

From: Santa Maria Ca

Quote:

Originally Posted by BigGun

Be sure to check at the pcv valve. They are known to develop a hole in the bottom of the tube that is hard to see.

Good advice there! My technician, while test-running my C5 after changing out my water pump and all cooling system hoses, heard a vacuum leak. He found that rubber elbow at the PCV valve to have a hole in it. When he called me to authorize new parts, he said that elbow wasn't available by itself, but came as part of a PCV assembly with all new hoses, tubes and plastic braces.

He said it was a good thing it came that way, because as he replaced the elbow, the next piece broke, as he replaced that section the next section broke, etc, etc. The whole thing was so brittle it was falling apart as he removed it.

Mine was not idling rough, but would drop to 400-500 when I came to a stop, then the computer would bring it back up to 600-700. He said he was surprised it wasn't throwing lean codes and I said it had just started to, occasionally. I knew my Long Term Fuel Trims were WAY up, too. After replacing the PCV and tubing assy, now my Long Term and Short Term Fuel Trims are back near zero.

It's amazing how the computers can compensate for / mask a problem. I just passed California emmisions several months ago, with a vacuum leak big enough to force the LT Fuel trims to max out. By the way, this is a 203,000 mile LS1.

Steve

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