Intermittant Rough/Serging Idle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**nitrosport_5** · 10-18-2012, 05:24 PM

been trying to fix this problem for quite a while, still have not come up with a solution. I thought that it only did it when it was hot outside, but it was doing it to me this morning after i got to class.

Its a 92 k1500, 210,000 miles. has a fairly recent gm crate 350 (about 30-40k). I beleive the throttle body was replaced when the engine was replaced. Working on it in the passed, i thought it was fuel pressure because if you pinched the return line, it would smooth out. Replaced the fuel pump and filter with same results, tested the fuel pressure and it came out to be in spec. Replaced the IAC valve because i had heard that could cause the surging. didnt fix it. Checked the readings of the CTS and they seem to be within spec. Had it hooked up to a snapon computer of a friends and it couldnt see anything wrong. I never have a check engine light or anything.

This morning i went out and started the truck to warm it up, was about 32* outside. Does its normal high-er idle until it builds a little heat and then idles itself down. assuming this is the choke more or less. I drive about 45 minutes to get to class, pull in the parking lot and it is idling about like its got a cam of a 1000hp big block. i can rev the engine to a normal 2k rpm and hold it steady and it runs great, let it idle back down and it will keep the rough idle. If i wing the throttle to about 4k rpm or more, it will idle normally again. after class i get back in it and it is running just fine, drive a few blocks to my apartment and shut it off.

does anybody have any more ideas? Im out of things to try on this and it seemt o be somthing that has everybody stumped. My dad was a mechanic that put the engine and trans in there and he was never able to figure it out. His old boss that is an old school mechanic was not able to figure it out. Hopefully someone else has run into this issue before and is able to help me out.

ChevyTech · 10-19-2012, 08:57 AM

It could be an EGR problem.

You could try disconnecting the hose to the EGR valve when the problem is occurring and see if it helps.

No vacuum should get through the EGR solenoid when the engine is at idle.

On a negative back-pressure EGR solenoid there is one port which is the vent and should be left open, but people find a spot they think should have a vacuum hose, so they plug the vent port. The vent port does not have barbs on it like the one that is supposed to go to the EGR valve.

**nitrosport_5** · 10-26-2012, 06:56 PM

ill try taking that hose off this afternoon. it has gotten worse. will hardly idle at all now. sometimes it runs just fine, but seems now more often than not it wont idle. ill letbyou know what disconnecting the egr does.
Posted via Mobile Device

wayfastwhitey · 10-27-2012, 11:34 AM

I had this same issue on my last 95, I contributed it to a number of things, mainly a chopped wiring harness, but eventually after I swapped in an iac off an older truck that had the original iac it seemed to clear itself out. I just made sure it was clean and reset it properly. Try blocking the hole in the tb above the iac with your finger while it idles and see if it gets any better
Posted via Mobile Device

**nitrosport_5** · 10-29-2012, 09:42 PM

The wire harness is unmolested. Only thing changed on it is a remote start, and i had the issue before i put the remote start on.

Well, before i had the chance to try any of the above, it partially cleared up. Put some Heet in the tank because i suspected water. after i ran half the tank through it, she cleared up. been running great since. Now that it is cold out, it wont do the idle surge. once it starts getting warmer outside is when it will start the surging. so i guess i will have to postpone any of these tests until after winter. unless someone has any other ideas.

what kind of milage should i be getting? Used that half of tank this weekend, i calculated 12.5MPG. Most of the driving is at 60 with the cruise on. Not really straight flat highway driving, but more rural roads between small towns.

wayfastwhitey · 10-30-2012, 03:41 AM

I couldn't say on mileage, my gas gauge on the last truck didn't work and out would start bouncing around and the truck would throw a fit once it started to get low so it got fuel every time it left the driveway and my speedometer was off so I couldn't track my mileage, but I will say even in town, which is where 90 percent of my miles were (and they were quite spirited miles) it seemed to go quite a long way on ten bucks at a time.
Posted via Mobile Device

**nitrosport_5** · 01-02-2013, 04:28 AM

Well it is back at it again. Took a trip this weekend about 200 miles. when i got to my destination, it was idling rough. 100 miles later when i was home, it was still idling rough. put it in the heated shop and worked on taking out the grille and headliner (installing a compass/temp mirror and a new headliner). after it sat in there for a few hours, i fired it back up and it was still doing the same thing. I did notice the exhaust was black, and that tells me the thing is running rich. while it is sitting there surging, it it puffing back smoke out, and when i rev it, it will still smoke black.

Im thinking possibly the coolant temp sensor inst reading right. I still hadnt changed it. I have one reserved at napa and will pick it up tomorrow. Not going to drive the truck till i get the CTS. Going to start it before, see if it is doing it still, and then immidiately change it and see if that helps at all.

here is a video so you guys can see how it sounds. Sorry the video is kinda crappy. hard enough to start the pickup from my chair and wheel around the thing, let alone start it and wheel around it with a phone taking video. haha

and no, this isnt a cammed up engine. it is a stock GM crate 350. the idle is usually smooth.

http://s23.photobucket.com/albums/b3...ningcrappy.mp4

CSGAS · 01-02-2013, 06:37 AM

You said it's a TBI? today's gas can jam the injector needles and damage them very easily. If it's been over 6 months pull the throttle body and have the injectors (I am guessing it is a stock GM 2-barrel throttle body) checked out, maybe even sonic cleaned as preventative maintenance.

If you have black smoke but your check engine light hasn't come on, then the overall mixture is still within programmed limits (14ish to 1)--but the spray pattern is causing a lot of UNBURNED fuel. There are also many other reasons for an incomplete burn--check your plugs against a chart like the one on Autolite's web site to figure out what possibilities you may be contending with. You'll want to fix it before that kills your catalytic converter (it may already have done so).

**nitrosport_5** · 01-02-2013, 02:07 PM

Quote:

Originally Posted by CSGAS

You said it's a TBI? today's gas can jam the injector needles and damage them very easily. If it's been over 6 months pull the throttle body and have the injectors (I am guessing it is a stock GM 2-barrel throttle body) checked out, maybe even sonic cleaned as preventative maintenance.

If you have black smoke but your check engine light hasn't come on, then the overall mixture is still within programmed limits (14ish to 1)--but the spray pattern is causing a lot of UNBURNED fuel. There are also many other reasons for an incomplete burn--check your plugs against a chart like the one on Autolite's web site to figure out what possibilities you may be contending with. You'll want to fix it before that kills your catalytic converter (it may already have done so).

Yes its a tbi. I will see about having it cleaned. And as far as the cat, its long gone. No emissions tests here.
Posted via Mobile Device

ChevyTech · 01-02-2013, 02:33 PM

If you think it is running rich, the coolant senor is the first thing to check.
Also make sure all the ground wires that connect to the front of the intake manifold are in good condition, because this is where the coolant sensor ground terminates.

These threads have specifications and instruction for testing the coolant sensor:
http://67-72chevytrucks.com/vboard/s...d.php?t=322811

http://67-72chevytrucks.com/vboard/s...d.php?t=379142

Also make sure the vacuum hose to the MAP sensor is in good condition and is supplying vacuum to the MAP sensor.

**nitrosport_5** · 01-03-2013, 02:19 AM

Everything looks in good shape. replaced the CTS this afternoon and it did not help at all. Im almost sure it is the fuel around this area. There are a few others that are having issues with it as well. Just recently i installed a stereo in a 94 k1500 with the 4.3, and it ran horrible. THe owners said it was after filling up that it started.

Is there anything i can do to make the truck able to stand the ethanol fuel? Or should i just start paying the extra for the ethanol free fuel in the next town over. Was looking at a Holley 502-9 replacement throttle body. Would this tollerate the fuel any better, or will i have the same issues?

ChevyTech · 01-03-2013, 11:46 AM

I have noticed that many TBI trucks get a strange rhythmic misfire or stumble at idle when the fuel has ethanol in it. The higher the ethanol content, the more noticeable it is.

The dealership bought a test kit/beaker for testing the ethanol content of fuel samples, and we found different ethanol content levels from various local gas stations.

You may want to try buying gasoline somewhere else and see if it helps.

My guess is, this is going to be a bigger problem as higher ethanol levels are mandated.

10-18-2012, 05:24 PM	#1
nitrosport_5 Senior Member Join Date: Jul 2012 Location: St. John, Washington Posts: 648	Intermittant Rough/Serging Idle been trying to fix this problem for quite a while, still have not come up with a solution. I thought that it only did it when it was hot outside, but it was doing it to me this morning after i got to class. Its a 92 k1500, 210,000 miles. has a fairly recent gm crate 350 (about 30-40k). I beleive the throttle body was replaced when the engine was replaced. Working on it in the passed, i thought it was fuel pressure because if you pinched the return line, it would smooth out. Replaced the fuel pump and filter with same results, tested the fuel pressure and it came out to be in spec. Replaced the IAC valve because i had heard that could cause the surging. didnt fix it. Checked the readings of the CTS and they seem to be within spec. Had it hooked up to a snapon computer of a friends and it couldnt see anything wrong. I never have a check engine light or anything. This morning i went out and started the truck to warm it up, was about 32* outside. Does its normal high-er idle until it builds a little heat and then idles itself down. assuming this is the choke more or less. I drive about 45 minutes to get to class, pull in the parking lot and it is idling about like its got a cam of a 1000hp big block. i can rev the engine to a normal 2k rpm and hold it steady and it runs great, let it idle back down and it will keep the rough idle. If i wing the throttle to about 4k rpm or more, it will idle normally again. after class i get back in it and it is running just fine, drive a few blocks to my apartment and shut it off. does anybody have any more ideas? Im out of things to try on this and it seemt o be somthing that has everybody stumped. My dad was a mechanic that put the engine and trans in there and he was never able to figure it out. His old boss that is an old school mechanic was not able to figure it out. Hopefully someone else has run into this issue before and is able to help me out. __________________ "Rusty" 1972 K20 Cheyenne Super, 496, A/C, Tilt, Tach, Buckets, 4l80e/NP205, 4" Rancho Lift, 35" BFG AT "Ramp Truck" 1972 C30 Ramp Truck 350, Air Bags, Factory A/C. "1968" C10 Shortbed project

10-19-2012, 08:57 AM	#2
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: Intermittant Rough/Serging Idle It could be an EGR problem. You could try disconnecting the hose to the EGR valve when the problem is occurring and see if it helps. No vacuum should get through the EGR solenoid when the engine is at idle. On a negative back-pressure EGR solenoid there is one port which is the vent and should be left open, but people find a spot they think should have a vacuum hose, so they plug the vent port. The vent port does not have barbs on it like the one that is supposed to go to the EGR valve. __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too.

10-26-2012, 06:56 PM	#3
nitrosport_5 Senior Member Join Date: Jul 2012 Location: St. John, Washington Posts: 648	Re: Intermittant Rough/Serging Idle ill try taking that hose off this afternoon. it has gotten worse. will hardly idle at all now. sometimes it runs just fine, but seems now more often than not it wont idle. ill letbyou know what disconnecting the egr does. Posted via Mobile Device __________________ "Rusty" 1972 K20 Cheyenne Super, 496, A/C, Tilt, Tach, Buckets, 4l80e/NP205, 4" Rancho Lift, 35" BFG AT "Ramp Truck" 1972 C30 Ramp Truck 350, Air Bags, Factory A/C. "1968" C10 Shortbed project

10-27-2012, 11:34 AM	#4
wayfastwhitey right below above average joe Join Date: Mar 2009 Location: palm bay, fl Posts: 107	Re: Intermittant Rough/Serging Idle I had this same issue on my last 95, I contributed it to a number of things, mainly a chopped wiring harness, but eventually after I swapped in an iac off an older truck that had the original iac it seemed to clear itself out. I just made sure it was clean and reset it properly. Try blocking the hole in the tb above the iac with your finger while it idles and see if it gets any better Posted via Mobile Device __________________ i flush the of weakness with mushroom clouds at hand '95 short bed x cab c1500... just another old truck someone else beat up, abused, hacked and didnt respect... my turn to turn it into a beat up old truck into a beat up and reliable pleasure to drive... them- "when you gonna put an exhaust on that thing?" me- "what do you mean? The exhaust comes all the way to the bumber and sounds great!" them- "straight pipes isnt an exhaust man..." me- "meh... who needs a fancy muffler when you've got straight pipes? its got a cat, doesnt resonate, and I like how it sounds, and to be perfectly honest, it makes me behave myself when I'm out and about!" ^^^ favorite conversation about my truck to date

10-29-2012, 09:42 PM	#5
nitrosport_5 Senior Member Join Date: Jul 2012 Location: St. John, Washington Posts: 648	Re: Intermittant Rough/Serging Idle The wire harness is unmolested. Only thing changed on it is a remote start, and i had the issue before i put the remote start on. Well, before i had the chance to try any of the above, it partially cleared up. Put some Heet in the tank because i suspected water. after i ran half the tank through it, she cleared up. been running great since. Now that it is cold out, it wont do the idle surge. once it starts getting warmer outside is when it will start the surging. so i guess i will have to postpone any of these tests until after winter. unless someone has any other ideas. what kind of milage should i be getting? Used that half of tank this weekend, i calculated 12.5MPG. Most of the driving is at 60 with the cruise on. Not really straight flat highway driving, but more rural roads between small towns. __________________ "Rusty" 1972 K20 Cheyenne Super, 496, A/C, Tilt, Tach, Buckets, 4l80e/NP205, 4" Rancho Lift, 35" BFG AT "Ramp Truck" 1972 C30 Ramp Truck 350, Air Bags, Factory A/C. "1968" C10 Shortbed project

10-30-2012, 03:41 AM	#6
wayfastwhitey right below above average joe Join Date: Mar 2009 Location: palm bay, fl Posts: 107	Re: Intermittant Rough/Serging Idle I couldn't say on mileage, my gas gauge on the last truck didn't work and out would start bouncing around and the truck would throw a fit once it started to get low so it got fuel every time it left the driveway and my speedometer was off so I couldn't track my mileage, but I will say even in town, which is where 90 percent of my miles were (and they were quite spirited miles) it seemed to go quite a long way on ten bucks at a time. Posted via Mobile Device __________________ i flush the of weakness with mushroom clouds at hand '95 short bed x cab c1500... just another old truck someone else beat up, abused, hacked and didnt respect... my turn to turn it into a beat up old truck into a beat up and reliable pleasure to drive... them- "when you gonna put an exhaust on that thing?" me- "what do you mean? The exhaust comes all the way to the bumber and sounds great!" them- "straight pipes isnt an exhaust man..." me- "meh... who needs a fancy muffler when you've got straight pipes? its got a cat, doesnt resonate, and I like how it sounds, and to be perfectly honest, it makes me behave myself when I'm out and about!" ^^^ favorite conversation about my truck to date

01-02-2013, 04:28 AM	#7
nitrosport_5 Senior Member Join Date: Jul 2012 Location: St. John, Washington Posts: 648	Re: Intermittant Rough/Serging Idle Well it is back at it again. Took a trip this weekend about 200 miles. when i got to my destination, it was idling rough. 100 miles later when i was home, it was still idling rough. put it in the heated shop and worked on taking out the grille and headliner (installing a compass/temp mirror and a new headliner). after it sat in there for a few hours, i fired it back up and it was still doing the same thing. I did notice the exhaust was black, and that tells me the thing is running rich. while it is sitting there surging, it it puffing back smoke out, and when i rev it, it will still smoke black. Im thinking possibly the coolant temp sensor inst reading right. I still hadnt changed it. I have one reserved at napa and will pick it up tomorrow. Not going to drive the truck till i get the CTS. Going to start it before, see if it is doing it still, and then immidiately change it and see if that helps at all. here is a video so you guys can see how it sounds. Sorry the video is kinda crappy. hard enough to start the pickup from my chair and wheel around the thing, let alone start it and wheel around it with a phone taking video. haha and no, this isnt a cammed up engine. it is a stock GM crate 350. the idle is usually smooth. http://s23.photobucket.com/albums/b3...ningcrappy.mp4 __________________ "Rusty" 1972 K20 Cheyenne Super, 496, A/C, Tilt, Tach, Buckets, 4l80e/NP205, 4" Rancho Lift, 35" BFG AT "Ramp Truck" 1972 C30 Ramp Truck 350, Air Bags, Factory A/C. "1968" C10 Shortbed project

01-02-2013, 06:37 AM	#8
CSGAS optomistic ah-so Join Date: Mar 2009 Location: Charlotte, NC Posts: 544	Re: Intermittant Rough/Serging Idle You said it's a TBI? today's gas can jam the injector needles and damage them very easily. If it's been over 6 months pull the throttle body and have the injectors (I am guessing it is a stock GM 2-barrel throttle body) checked out, maybe even sonic cleaned as preventative maintenance. If you have black smoke but your check engine light hasn't come on, then the overall mixture is still within programmed limits (14ish to 1)--but the spray pattern is causing a lot of UNBURNED fuel. There are also many other reasons for an incomplete burn--check your plugs against a chart like the one on Autolite's web site to figure out what possibilities you may be contending with. You'll want to fix it before that kills your catalytic converter (it may already have done so). __________________ Rubbing is Racin' Ribbing is Bench Racin' 1970 C-10 lwb Fleetside. Originally 350 2-bbl 3-on-the-tree, m/b, m/w, m/s no a/c. Currently running on a '76 Camaro 305.

01-02-2013, 02:33 PM	#10
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: Intermittant Rough/Serging Idle If you think it is running rich, the coolant senor is the first thing to check. Also make sure all the ground wires that connect to the front of the intake manifold are in good condition, because this is where the coolant sensor ground terminates. These threads have specifications and instruction for testing the coolant sensor: http://67-72chevytrucks.com/vboard/s...d.php?t=322811 http://67-72chevytrucks.com/vboard/s...d.php?t=379142 Also make sure the vacuum hose to the MAP sensor is in good condition and is supplying vacuum to the MAP sensor. __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too.

01-03-2013, 02:19 AM	#11
nitrosport_5 Senior Member Join Date: Jul 2012 Location: St. John, Washington Posts: 648	Re: Intermittant Rough/Serging Idle Everything looks in good shape. replaced the CTS this afternoon and it did not help at all. Im almost sure it is the fuel around this area. There are a few others that are having issues with it as well. Just recently i installed a stereo in a 94 k1500 with the 4.3, and it ran horrible. THe owners said it was after filling up that it started. Is there anything i can do to make the truck able to stand the ethanol fuel? Or should i just start paying the extra for the ethanol free fuel in the next town over. Was looking at a Holley 502-9 replacement throttle body. Would this tollerate the fuel any better, or will i have the same issues? __________________ "Rusty" 1972 K20 Cheyenne Super, 496, A/C, Tilt, Tach, Buckets, 4l80e/NP205, 4" Rancho Lift, 35" BFG AT "Ramp Truck" 1972 C30 Ramp Truck 350, Air Bags, Factory A/C. "1968" C10 Shortbed project

01-03-2013, 11:46 AM	#12
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: Intermittant Rough/Serging Idle I have noticed that many TBI trucks get a strange rhythmic misfire or stumble at idle when the fuel has ethanol in it. The higher the ethanol content, the more noticeable it is. The dealership bought a test kit/beaker for testing the ethanol content of fuel samples, and we found different ethanol content levels from various local gas stations. You may want to try buying gasoline somewhere else and see if it helps. My guess is, this is going to be a bigger problem as higher ethanol levels are mandated. __________________ For those of you that are wondering why you are not getting replies to your thread: Did you give the model, year, engine, fuel system type, and transmission information? If it is modified from what came stock from the factory, let us know that too.



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