Idle problems

neighbor · 07-03-2008, 05:46 PM

I recently got my 91 s10 back on the road after a yearlong haitis. I replaced pretty much the entire top half of the engine. It took a while but I finally got it running pretty well. My main problem is that it has a tendency to stall, especially when first started. Though it will do it at other times as well. I've replace most of the sensors as well as the IAC and nothing seems to help. Also my mileage sucks. I've replaced the O2 and coolant sensors and at one time I had it up to 20mpg but it's dropped back down to about 15 to 16. I'm about to replace the O2 again just out of curiosity. Any ideas?

Yogi · 07-03-2008, 05:59 PM

Are you sure that the timing is set correctly?

neighbor · 07-03-2008, 09:14 PM

Quote: Originally Posted by Yogi

Are you sure that the timing is set correctly?

To the very best of my knowledge it is. I followed the manual to the T. I was surfing through the website and a few other suggestions to similar problems said to check the T B Gasket, EGR and a few other things I hadn't thought of.

baggedzq8 · 07-04-2008, 11:30 PM

i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good

id try that and see how it goes

the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it

Bill Wheats · 07-05-2008, 10:23 AM

On my 91, I've had the same issue. 3 things that have caused mine.
1.Sticky thottle cable: lube it real good while the engine is off and the intakes are covered real good with a plastic bag. Don't let any silicone get into the intake or you will have to replace the O2 sensor.
2.Had a leaky injector and 2 intermitent plug wires that carbonised the top end till it got carbon balls holding the EGR plunger open. Fixed the cause, blew all the carbon out including the EGR, and never had that problem again. You don't need to use the EGR, but the plunger has to be seated.
3.IAC; I have to reset mine every so often. On the 91 I disconnect the neg batt for a while. Then after reconnecting it, turn the key to run for at least ~10 seconds, then turn the key off. Wait a while, then start the engine.

The 91 has some easy tests. While the engine is idling, Pull the front pass side vac line that runs strait back to a tee with one end going to the black ball on the pass side fender well. When you pull the line at the intake manifold, the idle should rise fairly high.
If it doesn't, then you have a big vacuum leak.
That's also the port that you want to connect a vacuum gauge to, because you don't need that Vac line for the engine to run. It only feed the heater controls.
A ~$15 vacuum gauge and a volt/ohm meter are a must for trouble shooting that motor.

Good Luck

4.One other thing that will screw it up. The friend that I bought it from had the oil changed at Jiffy lube just before I picked it up. They put ~4.5 quarts of oil in it. It only takes 4 w/ new filter. If you over fill the oil just a little it will run like shit.

neighbor · 07-05-2008, 11:48 AM

Thanks for all the help. On the subject of oil, I've been putting 5 qts in since day one and never knew that could cause a problem. I had the intake manifold off and cleaned it out when I redid the top end but it's still the original egr. I'll trace it back to the controller.

neighbor · 07-05-2008, 11:52 AM

Quote: Originally Posted by baggedzq8

i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good

id try that and see how it goes

the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it

I get the same thing. My check engine light comes on till I restart it. Usually it's a lean code, haven't check it lately. Mine runs fine at start up but stalls when you put it in gear and the idle drops. Then it stalls when yo slow and the idle drops again. I've checked for vacuum leaks and my gauge holds stead.

Bill Wheats · 07-05-2008, 12:21 PM

Quote: Originally Posted by neighbor

Thanks for all the help. On the subject of oil, I've been putting 5 qts in since day one and never knew that could cause a problem. I had the intake manifold off and cleaned it out when I redid the top end but it's still the original egr. I'll trace it back to the controller.

Well, that's probably one of your problems. I doubt GM used 2 different oil pans during in 91. Try draining it down to just below the full mark and try it out for a while.

Quote: Originally Posted by neighbor

I get the same thing. My check engine light comes on till I restart it. Usually it's a lean code, haven't check it lately. Mine runs fine at start up but stalls when you put it in gear and the idle drops. Then it stalls when yo slow and the idle drops again. I've checked for vacuum leaks and my gauge holds stead.

New info

OK; 3 common things

Wornout O2 sensor; Replace it if it has over 70K miles on it.
Carbon buildup in the intake or cylinders, or coolant leaking in to the combustion chamber, an exhust leak before the sensor, bad O2 wire connection can cause the system to fail and send that code.
O2 sensors don't smell gas fumes, they look for oxygen in the exhust. Water, coolant, and carbon, will cause the O2 sensor to tell the computor to raise the fuel mixture. When it can't raise it any higher it will through the code.
Next time it thows the code don't shut the engine off. Just park and go look at your tail pipe.
Does it smell like gas (running rich)?

What is the vacuum reading holding at? Mine usually sits ~20 while I'm at sea level.

lesterl · 07-06-2008, 09:35 AM

I always ran 4.5 qts, the one I had the manual said 4 qts, 4 1/2 with filter.
Of course I always made sure the dipstick was on full and not over/ under.

Yogi · 07-06-2008, 10:24 AM

I always run 5 qts. in my '98 and I've never had a problem. Guy at my oil change place has a Vette and he says that Chevy recommends overfilling by 1 qt. when using a Vette for Autocross or road racing.

Bill Wheats · 07-06-2008, 10:42 AM

Seriously. Mine will run like shit with over 4 quarts.
Maybe the factory dropped an air wrench in it or something.

I do know that it's clean inside, and I've changed it more than 10 times and the oil pan looks small and I don't have engine oil lines going into the radiator.
With way too much oil your oil pressure will drop and on many engines you will see shorter life on head gaskets.

lesterl · 07-06-2008, 11:17 AM

Bill, do you have a remote oil filter? The blaz does and that may be the difference.

Bill Wheats · 07-06-2008, 11:22 AM

No. 91 S10 pickup

lesterl · 07-06-2008, 03:06 PM

That could be your difference, the remote filter and lines may take more oil in the system to keep the same ammount in the pan?

gscrippler · 07-06-2008, 07:54 PM

Quote: Originally Posted by baggedzq8

i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good

id try that and see how it goes

the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it

why dont you pull the egr and clean the carbon off it replace gasket with the one with a screen and you won't have to go through all that BS

lesterl · 07-06-2008, 11:17 PM

Quote: Originally Posted by gscrippler

why dont you pull the egr and clean the carbon off it replace gasket with the one with a screen and you won't have to go through all that BS

Oh, come on that would be too easy.

07-03-2008, 05:46 PM	#1
neighbor Registered User Join Date: Jul 2008 Posts: 4 Location: User is: OffLine	Idle problems I recently got my 91 s10 back on the road after a yearlong haitis. I replaced pretty much the entire top half of the engine. It took a while but I finally got it running pretty well. My main problem is that it has a tendency to stall, especially when first started. Though it will do it at other times as well. I've replace most of the sensors as well as the IAC and nothing seems to help. Also my mileage sucks. I've replaced the O2 and coolant sensors and at one time I had it up to 20mpg but it's dropped back down to about 15 to 16. I'm about to replace the O2 again just out of curiosity. Any ideas?
	[Reply w/Quote]

07-03-2008, 05:59 PM	#2
Yogi Registered (Ab) User Join Date: Feb 2008 Posts: 2,530 Location: League City, TX (Houston) User is: OffLine	Re: Idle problems Are you sure that the timing is set correctly?
	[Reply w/Quote]

07-03-2008, 09:14 PM	#3
neighbor Registered User Join Date: Jul 2008 Posts: 4 Location: User is: OffLine	Re: Idle problems Quote: Originally Posted by Yogi Are you sure that the timing is set correctly? To the very best of my knowledge it is. I followed the manual to the T. I was surfing through the website and a few other suggestions to similar problems said to check the T B Gasket, EGR and a few other things I hadn't thought of.
	[Reply w/Quote]

07-04-2008, 11:30 PM	#4
baggedzq8 Registered User Join Date: Mar 2006 Posts: 519 Location: User is: OffLine	Re: Idle problems i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good id try that and see how it goes the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it
	[Reply w/Quote]

07-05-2008, 10:23 AM	#5
Bill Wheats Registered User Join Date: Apr 2008 Posts: 1,044 Location: Maine User is: OffLine	Re: Idle problems On my 91, I've had the same issue. 3 things that have caused mine. 1.Sticky thottle cable: lube it real good while the engine is off and the intakes are covered real good with a plastic bag. Don't let any silicone get into the intake or you will have to replace the O2 sensor. 2.Had a leaky injector and 2 intermitent plug wires that carbonised the top end till it got carbon balls holding the EGR plunger open. Fixed the cause, blew all the carbon out including the EGR, and never had that problem again. You don't need to use the EGR, but the plunger has to be seated. 3.IAC; I have to reset mine every so often. On the 91 I disconnect the neg batt for a while. Then after reconnecting it, turn the key to run for at least ~10 seconds, then turn the key off. Wait a while, then start the engine. The 91 has some easy tests. While the engine is idling, Pull the front pass side vac line that runs strait back to a tee with one end going to the black ball on the pass side fender well. When you pull the line at the intake manifold, the idle should rise fairly high. If it doesn't, then you have a big vacuum leak. That's also the port that you want to connect a vacuum gauge to, because you don't need that Vac line for the engine to run. It only feed the heater controls. A ~$15 vacuum gauge and a volt/ohm meter are a must for trouble shooting that motor. Good Luck 4.One other thing that will screw it up. The friend that I bought it from had the oil changed at Jiffy lube just before I picked it up. They put ~4.5 quarts of oil in it. It only takes 4 w/ new filter. If you over fill the oil just a little it will run like shit.
	[Reply w/Quote]


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07-05-2008, 11:48 AM	#6
neighbor Registered User Join Date: Jul 2008 Posts: 4 Location: User is: OffLine	Re: Idle problems Thanks for all the help. On the subject of oil, I've been putting 5 qts in since day one and never knew that could cause a problem. I had the intake manifold off and cleaned it out when I redid the top end but it's still the original egr. I'll trace it back to the controller.
	[Reply w/Quote]

07-05-2008, 11:52 AM	#7
neighbor Registered User Join Date: Jul 2008 Posts: 4 Location: User is: OffLine	Re: Idle problems Quote: Originally Posted by baggedzq8 i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good id try that and see how it goes the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it I get the same thing. My check engine light comes on till I restart it. Usually it's a lean code, haven't check it lately. Mine runs fine at start up but stalls when you put it in gear and the idle drops. Then it stalls when yo slow and the idle drops again. I've checked for vacuum leaks and my gauge holds stead.
	[Reply w/Quote]

07-05-2008, 12:21 PM	#8
Bill Wheats Registered User Join Date: Apr 2008 Posts: 1,044 Location: Maine User is: OffLine	Re: Idle problems Quote: Originally Posted by neighbor Thanks for all the help. On the subject of oil, I've been putting 5 qts in since day one and never knew that could cause a problem. I had the intake manifold off and cleaned it out when I redid the top end but it's still the original egr. I'll trace it back to the controller. Well, that's probably one of your problems. I doubt GM used 2 different oil pans during in 91. Try draining it down to just below the full mark and try it out for a while. Quote: Originally Posted by neighbor I get the same thing. My check engine light comes on till I restart it. Usually it's a lean code, haven't check it lately. Mine runs fine at start up but stalls when you put it in gear and the idle drops. Then it stalls when yo slow and the idle drops again. I've checked for vacuum leaks and my gauge holds stead. New info OK; 3 common things Wornout O2 sensor; Replace it if it has over 70K miles on it. Carbon buildup in the intake or cylinders, or coolant leaking in to the combustion chamber, an exhust leak before the sensor, bad O2 wire connection can cause the system to fail and send that code. O2 sensors don't smell gas fumes, they look for oxygen in the exhust. Water, coolant, and carbon, will cause the O2 sensor to tell the computor to raise the fuel mixture. When it can't raise it any higher it will through the code. Next time it thows the code don't shut the engine off. Just park and go look at your tail pipe. Does it smell like gas (running rich)? What is the vacuum reading holding at? Mine usually sits ~20 while I'm at sea level. Last edited by Bill Wheats : 07-05-2008 at 12:23 PM.
	[Reply w/Quote]

07-06-2008, 09:35 AM	#9
lesterl Wire Splicin Fool :-) Age: 28 Join Date: May 2008 Posts: 860 Location: Kansas City Area User is: OffLine	Re: Idle problems I always ran 4.5 qts, the one I had the manual said 4 qts, 4 1/2 with filter. Of course I always made sure the dipstick was on full and not over/ under.
	[Reply w/Quote]

07-06-2008, 10:24 AM	#10
Yogi Registered (Ab) User Join Date: Feb 2008 Posts: 2,530 Location: League City, TX (Houston) User is: OffLine	Re: Idle problems I always run 5 qts. in my '98 and I've never had a problem. Guy at my oil change place has a Vette and he says that Chevy recommends overfilling by 1 qt. when using a Vette for Autocross or road racing.
	[Reply w/Quote]

07-06-2008, 10:42 AM	#11
Bill Wheats Registered User Join Date: Apr 2008 Posts: 1,044 Location: Maine User is: OffLine	Re: Idle problems Seriously. Mine will run like shit with over 4 quarts. Maybe the factory dropped an air wrench in it or something. I do know that it's clean inside, and I've changed it more than 10 times and the oil pan looks small and I don't have engine oil lines going into the radiator. With way too much oil your oil pressure will drop and on many engines you will see shorter life on head gaskets. Last edited by Bill Wheats : 07-06-2008 at 10:46 AM.
	[Reply w/Quote]

07-06-2008, 11:17 AM	#12
lesterl Wire Splicin Fool :-) Age: 28 Join Date: May 2008 Posts: 860 Location: Kansas City Area User is: OffLine	Re: Idle problems Bill, do you have a remote oil filter? The blaz does and that may be the difference.
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07-06-2008, 11:22 AM	#13
Bill Wheats Registered User Join Date: Apr 2008 Posts: 1,044 Location: Maine User is: OffLine	Re: Idle problems No. 91 S10 pickup
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07-06-2008, 03:06 PM	#14
lesterl Wire Splicin Fool :-) Age: 28 Join Date: May 2008 Posts: 860 Location: Kansas City Area User is: OffLine	Re: Idle problems That could be your difference, the remote filter and lines may take more oil in the system to keep the same ammount in the pan?
	[Reply w/Quote]

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07-06-2008, 07:54 PM	#15
gscrippler Registered User Join Date: May 2008 Posts: 24 Location: User is: OffLine	Re: Idle problems Quote: Originally Posted by baggedzq8 i bet it is the egr, my egr controler or whatever it is called that actually pulls the vacuum on the egr diaphragm is messed up, and what happens is on startup it idles really rough like it has a HUGE cam in it(does sound sweet), but then it dies and take a minute to start back up...i unhooked the vacuum line and runs like a champ, the check engine light comes on after about 5 mins of driving, but resets everytime i start the truck, so once a month or so i jump the connector to make sure no other codes are set and im good id try that and see how it goes the part i am talking about is above the passenger valve cover about 1"x2.5" and has 2 vacuum lines coming off it why dont you pull the egr and clean the carbon off it replace gasket with the one with a screen and you won't have to go through all that BS
	[Reply w/Quote]

07-06-2008, 11:17 PM	#16
lesterl Wire Splicin Fool :-) Age: 28 Join Date: May 2008 Posts: 860 Location: Kansas City Area User is: OffLine	Re: Idle problems Quote: Originally Posted by gscrippler why dont you pull the egr and clean the carbon off it replace gasket with the one with a screen and you won't have to go through all that BS Oh, come on that would be too easy.
	[Reply w/Quote]