S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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02-22-12 07:05 AM - Post#2193732
Got a 92 S10 pickup, 2.8 5 speed, with 129,890 miles with high idle. When I got the truck had to have a fuel pump and had to replace the wires that go to the injectors on the throttle body, they had been pinched between breather and throttle body. I spliced wires from another truck to my harness to get truck to run.
Truck starts and runs great except for high idle, 1000 rpm after warm up, 1800 when cold, and a slight surge when running a steady speed say in 2'nd gear. Surge is barely noticeable.
After I got truck running it had a code stored, don't remember code, but it was for oxygen sensor so i replaced it. No more codes after replacing sensor.
I have also replaced temp sending unit in head and temp sensor in intake, neither one seems to have made any difference.
I am going to replace thermostat next but thought I would ask you guys what you thought before I did. Seems the temp gauge doesn't register as high as I think it should after warming up but heater seems to work ok. Also thought about timing but haven't checked that yet as my timing light is on the blink right now.
Any thoughts about this?
Thanks 
Edited by S10Fanatic on 02-22-12 07:07 AM. Reason for edit: No reason given.
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oldsman52
Contributor
Posts: 567
Loc: Randy's House.
Reg: 01-18-12
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02-22-12 07:33 AM - Post#2193743
In response to S10Fanatic
http://members.shaw.ca/betterthanyoutoo/how-t o/TBI...
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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02-26-12 05:47 AM - Post#2195318
In response to oldsman52
Thanks oldsman52
That page may be what I am looking for. I haven't had time to do anything to it as of yet but will let everyone know when I do.
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 Chevytech
Subject Matter Expert - Senior Member
Posts: 2692
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
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02-26-12 09:07 AM - Post#2195396
In response to S10Fanatic
Do NOT move the screw that the link is calling the idle set screw yet.
That screw is for changing the Minimum Air rate adjustment.
This is an adjustment that rarely ever needs to be done. If the speed is off it is usually because something else is wrong with the vehicle.
The idle speed is controlled by the ECM and the IAC (Idle Air Control)
There are other steps that should be taken to make a Minimum Air rate adjustment that are not mentioned at the link.
You said you noticed a slight surge which could be caused by a vacuum leak. A vacuum leak is the number one cause of high idle on TBI engines. The most common cause of the vacuum leak is the gasket just under the TBI unit. Next would be a PCV or vacuum hose that is mushy and leaking vacuum.
Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.
The IAC is another possibility.
You are correct in thinking a bad thermostat that causes the engine to run cold can cause the idle to be to high.
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 the it has been altered from stock let us know about that too. |
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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02-27-12 07:42 AM - Post#2195797
In response to Chevytech
Chevytech wrote:
Do NOT move the screw that the link is calling the idle set screw yet.
That screw is for changing the Minimum Air rate adjustment.
This is an adjustment that rarely ever needs to be done. If the speed is off it is usually because something else is wrong with the vehicle.
The idle speed is controlled by the ECM and the IAC (Idle Air Control)
There are other steps that should be taken to make a Minimum Air rate adjustment that are not mentioned at the link.
That is what I am thinking. I have not changed the Minimum Air rate adjustment.
You said you noticed a slight surge which could be caused by a vacuum leak. A vacuum leak is the number one cause of high idle on TBI engines. The most common cause of the vacuum leak is the gasket just under the TBI unit. Next would be a PCV or vacuum hose that is mushy and leaking vacuum.
Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.
The IAC is another possibility.
A vacuum leak was my first thoughts on this. I did a quick look for vacuum leaks but found nothing. I'll go back and do a better inspection for vacuum leaks and pay special attention to the base gasket.
You are correct in thinking a bad thermostat that causes the engine to run cold can cause the idle to be to high.
First thing I am going to do is replace the thermostat to be sure it is working correctly.
After reading your post I at least feel that I am on the right track. Thank you for your reply!
I will post back when I know more.
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oldsman52
Contributor
Posts: 567
Loc: Randy's House.
Reg: 01-18-12
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02-27-12 10:48 AM - Post#2195883
In response to S10Fanatic
Note that I didn't suggest you do anything, just showing you how the system functions. 
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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02-27-12 11:26 AM - Post#2195894
In response to oldsman52
- olicate the pagedrsman52 Said:
Note that I didn't suggest you do anything, just showing you how the system functions.
I understand oldsman52, I appreciate the page.
I'm an old carb guy so I can learn from these pages. 
I got a new thermostat and am going to put it in right now. Will post back shortly with the results.
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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02-28-12 07:02 PM - Post#2196458
In response to S10Fanatic
Well I replaced the thermostat and now idle is down to 900. I don''t know what the idle in rpm's should be but it seems to me it should be lower. Engine still has the surge.
So I pulled the throttle body and found a homemade gasket under it. Didn't look like it was leaking but I replaced it.
While I had throttle body off I removed and cleaned the egr valve, easy to get to with body removed. I also replaced the iac valve with one from another throttle body I had after cleaning the valve and bore. I then did a force idle relearn from info from here.
http://www.fullsizechevy.com/forum/general-discuss ...
It still idles at 900. Replacing the thermostat is the only thing I did that had any effect at all by dropping 200 rpm.
What next?
Soak the throttle body?
Continue looking for a vacuum leak?
Maybe Intake Gasket?
I really feel there is something wrong because of the surge.
I welcome any input and thank you.
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Chevytech
Subject Matter Expert - Senior Member
Posts: 2692
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
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02-28-12 08:42 PM - Post#2196505
In response to S10Fanatic
You’re welcome.
I don’t have a 1992 manual handy so I looked it up in a 1993 manual for a 2.8 Liter with a manual transmission.
The desired idle speed is 800 RPM plus or minus 100 RPM, at sea level, fully warmed up, in closed loop, with all accessories off.
Anything out of the normal range, like a low charging voltage may cause the system to raise the idle speed.
It is within the acceptable range.
As for the surge:
You could disconnect and plug the EGR hose, and take it for a test drive.
Testing the fuel pressure would be a good idea too. Just because you replaced the pump does not mean the pressure is good. There are multiple failures that can causes the TBI trucks to have low fuel pressure other then the fuel pump. It could have a fuel pressure regulator problem.
As these trucks get older it is becoming a common problem for the rubber hose in the tank between the fuel pump and the sending unit to split. Some people notice when this starts happening the truck will run better with more fuel in the tank.
I would only use original equipment parts on the EGR valve and the fuel pump. If an aftermarket fuel pump was used I would be suspicious of it. I have seen so many fuel pumps from other manufactures fail in the warranty period or just after the warranty is up that I suggest only using an OEM pump.
The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.
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 the it has been altered from stock let us know about that too. |
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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03-03-12 09:38 AM - Post#2197908
In response to Chevytech
Chevytech wrote:
You’re welcome.
I don’t have a 1992 manual handy so I looked it up in a 1993 manual for a 2.8 Liter with a manual transmission.
The desired idle speed is 800 RPM plus or minus 100 RPM, at sea level, fully warmed up, in closed loop, with all accessories off.
Anything out of the normal range, like a low charging voltage may cause the system to raise the idle speed.
It is within the acceptable range.
Thanks for the specs Chevytech.
I did replace the alternator from another truck but didn't see a difference in idle but will keep that in mind.
As for the surge:
You could disconnect and plug the EGR hose, and take it for a test drive.
I've been driving it with the hose plugged and it does run better. Does that mean the egr valve is bad or does it have a bad egr control or circuit? It seems that if the valve were bad it would still leak and cause it to run bad with the hose plugged? Don't know.
Testing the fuel pressure would be a good idea too. Just because you replaced the pump does not mean the pressure is good. There are multiple failures that can causes the TBI trucks to have low fuel pressure other then the fuel pump. It could have a fuel pressure regulator problem.
As these trucks get older it is becoming a common problem for the rubber hose in the tank between the fuel pump and the sending unit to split. Some people notice when this starts happening the truck will run better with more fuel in the tank.
I would only use original equipment parts on the EGR valve and the fuel pump. If an aftermarket fuel pump was used I would be suspicious of it. I have seen so many fuel pumps from other manufactures fail in the warranty period or just after the warranty is up that I suggest only using an OEM pump.
The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.
As for testing the pressure I'll do that as soon as I figure out how. As you know these trucks don't have a test port.
I don't remember what kind of pump i used, it came from NAPA, but I know about the problems with them. I replaced the one in my 97 Blazer with a aftermarket one, Bad idea!
Thanks again for your replies.
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Chevytech
Subject Matter Expert - Senior Member
Posts: 2692
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
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03-03-12 10:01 AM - Post#2197918
In response to S10Fanatic
… As for testing the pressure I'll do that as soon as I figure out how. As you know these trucks don't have a test port.
Here is some info about fuel pressure testing I saved from a previous reply I made to a similar post. Hope the links still work.
GM truck TBI systems do not have a fuel pressure test port, but the pressure must be tested with the engine running.
The fuel pressure is tested by using fittings to “T” in a pressure gauge. I like to T in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line and running the pump momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If restricting flow in the return line, using care not to damage the return line hose, makes a low pressure reading surge well above the specification pressure, then the regulator is the problem.
The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.
Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.
I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.
http://www.sjdiscounttools.com/ta37650.html
http://www.actron.com/product_detail.php?pid=16174
http://www.actron.com/product_detail.php?pid=16175
http://www.cfm-tech.com/catalog/fuel_-_cfm-te ch_fu...
More
http://www.tooltopia.com/index.asp?PageAction=VIEW...
http://www.thetoolmaster.com/toolaidta37650gmtbifu ...
customer rating and price chart
http://www.nextag.com/TA-37650-GM-Throttle- 5413925...
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 the it has been altered from stock let us know about that too. |
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Chevytech
Subject Matter Expert - Senior Member
Posts: 2692
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
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03-03-12 10:17 AM - Post#2197922
In response to S10Fanatic
Chevytech wrote:
As for the surge:
You could disconnect and plug the EGR hose, and take it for a test drive.
I've been driving it with the hose plugged and it does run better. Does that mean the egr valve is bad or does it have a bad egr control or circuit? It seems that if the valve were bad it would still leak and cause it to run bad with the hose plugged? Don't know.
Before we go farther we need to determine which EGR system your truck has. My guess is it is a negative backpressure system and the EGR valve is the problem.
TBI trucks are generally Negative pressure EGR or Port EGR systems, except California emission 4.3 TBI trucks had Linear EGR valves.
This is a site that has a photo and an illustration about EGR vale numbers which should help you determine which type of valve your vehicle has.
http://shbox.com/1/EGR_ID.jpg
Here is a site with a good illustration of a negative backpressure EGR valve.
http://shbox.com/1/EGR_valve.jpg
Here are some full size truck threads that will work if your truck does not have a linear EGR valve
http://67-72chevytrucks.com/vboard/showthread.php?...
http://67-72chevytrucks.com/vboard/showthread.php?...
You can also do a Google search with my username and you should be able to find many links where I discussed EGR systems.
If the problem feels totally gone with the with the EGR unplugged test, It is a indication that the EGR valve has failed
NOTE:
A lean condition caused by another problem can be magnified when the EGR valve opens.
ONLY GM valve
If you find it does need an EGR valve, I would suggest only using an OEM (Original equipment manufacturer) replacement EGR valve. These TBI truck are very picky about the EGR valve operation.
Be careful when you buy the new part it is an AC Delco EGR valve. And the number on the part matches the number on the old valve. The number on the box may be different but the number on the valve should match. Be careful as some dealers are now selling aftermarket EGR valves over the counter if you do not ask for OEM. There are several places selling OEM EGR valves online.
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 the it has been altered from stock let us know about that too. |
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S10Fanatic
Contributor
Posts: 101
Loc: Eastern NC
Reg: 09-18-11
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04-09-12 10:53 AM - Post#2213099
In response to Chevytech
Chevytech wrote:
As for the surge:
You could disconnect and plug the EGR hose, and take it for a test drive.
I've been driving it with the hose plugged and it does run better. Does that mean the egr valve is bad or does it have a bad egr control or circuit? It seems that if the valve were bad it would still leak and cause it to run bad with the hose plugged? Don't know.
Before we go farther we need to determine which EGR system your truck has. My guess is it is a negative backpressure system and the EGR valve is the problem.
TBI trucks are generally Negative pressure EGR or Port EGR systems, except California emission 4.3 TBI trucks had Linear EGR valves.
This is a site that has a photo and an illustration about EGR vale numbers which should help you determine which type of valve your vehicle has.
http://shbox.com/1/EGR_ID.jpg
Here is a site with a good illustration of a negative backpressure EGR valve.
http://shbox.com/1/EGR_valve.jpg
Here are some full size truck threads that will work if your truck does not have a linear EGR valve
http://67-72chevytrucks.com/vboard/showthread.php?...
http://67-72chevytrucks.com/vboard/showthread.php?...
You can also do a Google search with my username and you should be able to find many links where I discussed EGR systems.
If the problem feels totally gone with the with the EGR unplugged test, It is a indication that the EGR valve has failed
NOTE:
A lean condition caused by another problem can be magnified when the EGR valve opens.
ONLY GM valve
If you find it does need an EGR valve, I would suggest only using an OEM (Original equipment manufacturer) replacement EGR valve. These TBI truck are very picky about the EGR valve operation.
Be careful when you buy the new part it is an AC Delco EGR valve. And the number on the part matches the number on the old valve. The number on the box may be different but the number on the valve should match. Be careful as some dealers are now selling aftermarket EGR valves over the counter if you do not ask for OEM. There are several places selling OEM EGR valves online.
I have been driving this truck with the vacuum line off the egr valve and the truck runs fine, no surging, except for the high idle. The slowest it will ever idle is 900 rpm. If I pull up to a stop sign it will be at 1200 and takes 10 to 15 secs to slow down to 900. If I plug the vacuum back to the egr the truck surges. I did replace the egr valve with one I had on another truck and there is no change. The #'s were different on the valve but I tried it just to see what happened.
I did find out that it has a port egr system.
I am also hearing some pinging. Does anyone think the pinging would have anything to do with the surging and high idle?
I really need to get me another timing light so I could check the timing.
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Chevytech
Subject Matter Expert - Senior Member
Posts: 2692
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
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04-09-12 11:45 AM - Post#2213129
In response to S10Fanatic
I have been driving this truck with the vacuum line off the egr valve and the truck runs fine...
...I am also hearing some pinging. Does anyone think the pinging would have anything to do with the surging and high idle?
An EGR valve that does not open can cause pinging (detonation) and high Oxides of Nitrogen (NOX) or (NO).
High Oxides of Nitrogen readings, and pinging, are caused by high combustion chamber temperatures.
To much EGR flow can cause a stumble, hesitation, stalling, lack of power…
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 the it has been altered from stock let us know about that too. |
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homer2
Member
Posts: 90
Reg: 02-10-04
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04-16-12 09:49 PM - Post#2215990
In response to Chevytech
I've got a 1990 S10 Blazer. 2 wheel Drive.
4.3 I have a idle problem also.
I can't seem to get it below 1000 RPM.
Tried replacing IAC with some others laying around.
Checked all vacuum lines. Can't find any leaks. Have tried spaying flamible sprays. No difference. Tried a propane torch, not lit, still no difference.
I noticed that the TPI is about 60. Can't get any lower. Runs fine, no miss or surge. Just fast idle. I have replaced the base gasket. Looked OK, but already had it off so I put the new one on.
Where do I look from here?
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 gchemist
Senior Chevytalk Moderator
Posts: 20829
Loc: Austin, TX 78748
Reg: 05-09-00
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04-17-12 12:10 AM - Post#2216008
In response to homer2
Check your temperature probes. Especially the one for the FI PCM. It's on the intake. The head probe is for the temp gauge. Does it idle high at hot temps and cold? Have you checked for any codes?
60 PSI? A TBI should only have about 15 PSI to operate.
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS |
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homer2
Member
Posts: 90
Reg: 02-10-04
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04-17-12 09:35 AM - Post#2216149
In response to gchemist
I have not checked the temp sender. I will. It has the high idle all the time.
TPI is 60 not PSI.
No other codes.
With my scan tool, it says the temp is normal, right at 195 to 200.
It does say my third gear switch is on. Not sure if that means anything. Shifts fine. Just fast idle.
Thanks
Homer
Edited by homer2 on 04-17-12 09:36 AM. Reason for edit: No reason given.
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homer2
Member
Posts: 90
Reg: 02-10-04
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04-17-12 05:59 PM - Post#2216309
In response to homer2
Well, I changed the Temp sender. Did not make any difference. Still high idle.
Would timing do this? Who knows what the person before me did to it.
Thanks
homer
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gchemist
Senior Chevytalk Moderator
Posts: 20829
Loc: Austin, TX 78748
Reg: 05-09-00
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04-17-12 08:46 PM - Post#2216379
In response to homer2
If you have not checked timing, do it! Do you have procedure? I can find one if you need it. TBI are a little tricky. You need to disconnect the input wire and set the timing to zero. A few owners add a few degrees for power and economy over stock timing.
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS |
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homer2
Member
Posts: 90
Reg: 02-10-04
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04-17-12 09:00 PM - Post#2216382
In response to gchemist
Well, that's what I was wondering. If maybe somebody advanced the timing. I will check it out. I guess if that doesn't do it, I will pull the intake and see if the gaskets are bad.
Thanks
Homer
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