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Old 04-22-2011, 03:16 PM
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92 Chevy 350 TBI idle problems

I have a 1992 Chevrolet K2500 350 tbi that idles high about 1200 to 1500 rpm sometime's will idle about 800 rpm. When i first start the truck it slowly rises from 1000 to 1500 rpm then comes down after five minutes to 1000 rpm then when i drive it for a few minutes then it starts idling at 1100 rpm when the truck is in drive. When i drive on the highway it does fine after i get off the highway the idle is at 800 rpm then wants i get in the city that's when the idle goes up on short distance go and stops. I have changed IAC,TEMP SENSOR, EGR VALVE and SOLENOID, TPS, I have checked hoses for leaks and gaskets. I get a code for the EGR sometimes. I have cleaned the EGR and replaced the gasket on that what else can i look for or check or could the SOLENOID have failed. The EGR Valve moves easy, hoses good.

Thanks for the help.

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Old 04-22-2011, 04:26 PM
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How many miles are on it? If it has high miles, grab the throttle and give it a shake. If it feels loose you`ll need to have bushings installed or get a throttle body that`s not worn out. Why the idle changes is because of the wear, it closes in a different spot each time.
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Old 04-22-2011, 04:58 PM
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Quote:
Originally Posted by Big A's 78 chevy
I have changed IAC,TEMP SENSOR, EGR VALVE and SOLENOID, TPS, I have checked hoses for leaks and gaskets. I get a code for the EGR sometimes. I have cleaned the EGR and replaced the gasket on that what else can i look for or check or could the SOLENOID have failed. The EGR Valve moves easy, hoses good.Thanks for the help.
The money you spent on parts could have been spent getting it on a diagnostic machine to find the problem. The only thing I can see that's left is the passage to the EGR valve in the intake might be blocked with carbon and would require the intake to be removed to clean it. Or, the IAC valve is out of adj. Next time you get a fast idle speed after warm up, remove the air filter lid and listen for a loud hissing noise. If there is, the IAC valve is open. There should also be a small idle speed adj. screw under a cap on the base of the TB just like a carb. After replacing the TPS you may just need to re-set the idle speed. Hope this helps.
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Old 04-22-2011, 05:14 PM
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Thanks for the reply all the parts i have changed needed replaced. I changed these parts over a years time was not all at wants sorry i failed to mention this in the first post, The ECM sent bad signals for them that's why i replaced them. I have had the truck checked out by a mechanic did not find anything. That was $95 wasted. The truck has almost 251,000 miles on it still runs good besides the problem i'm having now. The throttle linking don't feel loose but i was wandering if the return spring might or could cause this problem. If the TBI is bad could that cause a false reading in the EGR or like was mentioned you think i might have to take the intake manifold of to clean it out. Thanks again for the reply
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Old 04-22-2011, 05:20 PM
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What I say to everyone with this tbi and computer control truck. If your keeping the truck then you need the shop manual off of Ebay. SO Knowing what to look for and possible problem is all in there Second a actron scanner to read the sensor to see if they are working right.! With out that your just guess at whats wrong! Did that myself. Sound like an egr problem it could be a couple of other thing but I would be guess! Good site for trucks www.fullsizechevy.com

Craig
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Old 04-22-2011, 05:37 PM
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I would look under the throttle body in the passages in the base.they carbon up and it "confuses" the correct vacuum to the different ports on the throttle body. The pcv should be changed as this is a good suspect for touble. It doesnt pay to clean one. Check the hose to be sure it isnt blocked. Use a new gasket when re installing the throttle body and DONT OVERTORQUE it. Sorry to shout but it is very important.
If you suspect the egr,then cut a blocker plate out of a soda can to slip in between the gasket to block the passages from flowing. If it clears up then you are on the right track.
I agree It sounds like unmetered air entering intermittently.
You may need to reset the minimum air rate also being that the TPS was replaced, that would be the only time you should ever turn thethrottle stop screw that was mentioned earlier.you will need a scanner to perform that adjustment.It can also be done with an accurate Tachometer and a dvom.
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Old 06-18-2011, 10:50 PM
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Egr?

All egr valves that I have seen that are coming on too soon or sticking at idle. Do not create a high idle problem Low rough idles, dieing, and hesitation at tip in acceleration. Are more common signs of a bad egr. Also when replacing one do not get the cheap one at some part store. OEM is the only way to go. They used about 12 different calibrated options. And none from the parts stores are going to match it correctly.

Most likely the issue is a throttle body gasket that it sucked in. With the truck off open the throttle and look down. If you see the gasket pulled in, bingo. Even if you don't see it pulled in, they cost about $5 and not a bad idea to just pull it and look.

More rare, but also common. If this has been going on for awhile. The IAC motor hat can become worn due to having to close all the way against the throttle body constantly. Pull it and look for shiny parts on the hat. If it is, and depending on how bad. Might have to use some valve lapping compound and a used good hat to repair the throttle body passage. So the valve can properly meter airflow.
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Old 06-18-2011, 11:37 PM
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EGR Control Solenoid ???
NAPA/Echlin Part Number 2-19226

... just a WAG, and one of the few components that you haven't listed.
I think you're on the right track with the with the intermittent introduction of additional air theory.

Those TBI units always were a little quirky, and many had a delay between stepping on the gas pedal at idle and the 1500 RPM thing. "On" or "Off" it seemed.

I've also noted that the addition of a throttle body spacer (i.e. Helix Power Tower) seemed to worsen this condition.
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