Help - running rough at idle & low RPM - The 1947 - Present Chevrolet & GMC Truck Message Board Network

killthewabbit · 01-13-2013, 01:07 PM

I was hoping you guys could help me. I have a 85 GMC with a 305 and a 350 lockup trans. As far as I can tell, it's stock.

A couple days ago, I heard a weird knocking noise that sounded like a rod knock except it didn't change with RPM and it only happened on deceleration. I popped the hood, and the knocking sounded like it was coming from the rear of the engine. I put my hand on the HEI cap and felt the knocking. I removed the distributor cap and wires but I didn't see anything. When I put it back on, the knocking sound went away.

Here's the problem: Ever since I reinstalled the cap and wires, it runs rough at low RPM. Once I give it gas, it runs great. It runs great on the freeway as well. When I'm at idle, it's rough. When I give it gas off idle, it sputters a bit but always goes. When I'm at 30-40 MPH in lockup and not giving it gas, I can feel it running rough again. I replaced the coil, cap, and rotor this morning, and it made no difference.

Anyone have an idea? This is my daily, so I need to get it running smooth again. I appreciate the help.

snj8198 · 01-13-2013, 01:18 PM

do you have a couple wires backwards on the cap? 1 8 4 3 6 5 7 2

killthewabbit · 01-13-2013, 01:30 PM

Nope, I made sure to label them before I took them off. Took pictures too.

dukcaln · 01-13-2013, 01:53 PM

what about cracked wires ir burnt wires? What about wires that are touching causing irratic spark?

killthewabbit · 01-13-2013, 01:56 PM

I didn't see any, but I'll double check. It runs awesome aside from idle. I also need to go double check vacuum hoses. Maybe something got disconnected there.

dukcaln · 01-13-2013, 02:11 PM

yes vacuum leaks can cause that? What kind of carb? I had a issue like this two times. The first were wires touching sparking on each other and the last was a bad wire. Now I found two wires, one burnt and the other cracked. I just didnt route them well enough after the engine swap last month...

68gmsee · 01-13-2013, 04:06 PM

Don't forget to check the vacuum advance for leaking also.

killthewabbit · 01-13-2013, 08:04 PM

The carb is a stock 80s quadrajet. Stock as it gets. Still leak hunting.

killthewabbit · 01-14-2013, 07:08 PM

Found one leaky vacuum hose. Idles like a champ.

dukcaln · 01-15-2013, 01:22 AM

nice

01-13-2013, 01:07 PM	#1
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	Help - running rough at idle & low RPM I was hoping you guys could help me. I have a 85 GMC with a 305 and a 350 lockup trans. As far as I can tell, it's stock. A couple days ago, I heard a weird knocking noise that sounded like a rod knock except it didn't change with RPM and it only happened on deceleration. I popped the hood, and the knocking sounded like it was coming from the rear of the engine. I put my hand on the HEI cap and felt the knocking. I removed the distributor cap and wires but I didn't see anything. When I put it back on, the knocking sound went away. Here's the problem: Ever since I reinstalled the cap and wires, it runs rough at low RPM. Once I give it gas, it runs great. It runs great on the freeway as well. When I'm at idle, it's rough. When I give it gas off idle, it sputters a bit but always goes. When I'm at 30-40 MPH in lockup and not giving it gas, I can feel it running rough again. I replaced the coil, cap, and rotor this morning, and it made no difference. Anyone have an idea? This is my daily, so I need to get it running smooth again. I appreciate the help.

01-13-2013, 01:18 PM	#2
snj8198 Registered User Join Date: Feb 2007 Location: Star Valley, WY Posts: 2,038	Re: Help - running rough at idle & low RPM do you have a couple wires backwards on the cap? 1 8 4 3 6 5 7 2 __________________ Steve 1971 GMC Longbox. Full Roller Stroker 489 w/ FiTech EFI, Chris Straub Cam/NV4500/205/D60/14B w. Grizzly Locker and 4.11's. 2" Lift on 33's

01-13-2013, 01:30 PM	#3
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	Re: Help - running rough at idle & low RPM Nope, I made sure to label them before I took them off. Took pictures too.

01-13-2013, 01:53 PM	#4
dukcaln Registered User Join Date: Jul 2011 Location: vacaville ca Posts: 1,304	Re: Help - running rough at idle & low RPM what about cracked wires ir burnt wires? What about wires that are touching causing irratic spark? __________________ 04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP *SOLD* 1972 chevy c10

01-13-2013, 01:56 PM	#5
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	Re: Help - running rough at idle & low RPM I didn't see any, but I'll double check. It runs awesome aside from idle. I also need to go double check vacuum hoses. Maybe something got disconnected there.

01-13-2013, 02:11 PM	#6
dukcaln Registered User Join Date: Jul 2011 Location: vacaville ca Posts: 1,304	Re: Help - running rough at idle & low RPM yes vacuum leaks can cause that? What kind of carb? I had a issue like this two times. The first were wires touching sparking on each other and the last was a bad wire. Now I found two wires, one burnt and the other cracked. I just didnt route them well enough after the engine swap last month... __________________ 04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP *SOLD* 1972 chevy c10

01-13-2013, 04:06 PM	#7
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Help - running rough at idle & low RPM Don't forget to check the vacuum advance for leaking also. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

01-13-2013, 08:04 PM	#8
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	Re: Help - running rough at idle & low RPM The carb is a stock 80s quadrajet. Stock as it gets. Still leak hunting.

01-14-2013, 07:08 PM	#9
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	Re: Help - running rough at idle & low RPM Found one leaky vacuum hose. Idles like a champ.

01-15-2013, 01:22 AM	#10
dukcaln Registered User Join Date: Jul 2011 Location: vacaville ca Posts: 1,304	Re: Help - running rough at idle & low RPM nice __________________ 04 CHEVY LLY LS DURAMAX 4WD. MODS TO FOLLOW, EFI LIVE, BUILT TRANS, LBZ MP, LIFT PUMP *SOLD* 1972 chevy c10



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.