Why does my 305 tbi idle so high???!!!

[TruAggression]

On my 87 its got the stock 305 tbi in it, I've replaced pretty much everything sensor related on it including the throttle body base gasket and it still idles at about 1700 when I first start it up and stays there for at least 10 minutes or so then slowly goes down to about 1200 when its all warmed up could it be timing? Theres no check engine lights on, its just getting on my nerves and wondering if anybody else had this problem on a tbi motor. If anybody can chime in and let me know what to look to do that'd be awesome.

[nbpro]

use carb spray to check for vacuum leaks.

Your IAC (Idle air control) valve could be stuck with the plunger retracted.

A bad coolant sensor can make the computer think the engine is cold causing a high idle. Does it seem to run to rich?

Is there any other symptoms other then the high idle



From the Auto Zone site on TBI rebuilding

The easy-to-replace parts that wear out in TBI systems are the same parts that wear out in carburetors: Gaskets, o-rings, and seals, all of which are sensitive to time, fuel, and extreme changes in temperature. Since there are less moving parts involved, rebuilding a TBI system is easier than rebuilding a carburetor. If you have ever built a model airplane, then you most likely can rebuild a TBI system. Armed with a TBI rebuild kit, the task can be performed in just one afternoon. The fuel injectors themselves, however, should be tested and cleaned professionally or replaced if problems are suspected. Finally, as the complexities of TBI systems vary by manufacturer, the first step to a successful TBI system rebuild is consulting a service manual.

[TruAggression]

Doesnt seem to be running rich at all, coolant temp sensor been replaced twice due to me thinking its that as well, the IAC I've went through three of em. It seems to be a vacuum leak somewhere that I can't find. I do work at a shop so I just ordered a smoke machine so I can search for a vacuum leak, when i drove it this morning after a little bit of driving it idle'd at about 900 so seems to be going down after I didn't repalce a thing on it

[nbpro]

vacuum leak yes

[rfmaster]

Originally Posted by TruAggression

On my 87 its got the stock 305 tbi in it, I've replaced pretty much everything sensor related on it including the throttle body base gasket and it still idles at about 1700 when I first start it up and stays there for at least 10 minutes or so then slowly goes down to about 1200 when its all warmed up could it be timing? Theres no check engine lights on, its just getting on my nerves and wondering if anybody else had this problem on a tbi motor. If anybody can chime in and let me know what to look to do that'd be awesome.

Unless you have a gross vacuum leak somewhere or a dead IAC the following procedure allows for IAC reset. Sometimes ECM does not reset IAC position which controls idle speed. You can force reset its position by shorting pins A&B of ALDL connector with a paper clip. This will place ECM into diagnostic mode. While in diagnostic mode ECM will command IAC to continuously move into close position. Without starting engine, ignition in run position you should be able to hear clicking noise from IAC being jammed into full close position. You can see IAC pentacle fully closing idle air passage in the TB (10 o'clock). Remove the connector from the IAC and start engine while monitoring idle speed. Minimum idle speed for a stock engine should be around 450 rpm (can be adjusted). Shut of engine, remove A+B jumper, re-connect IAC connector. Restart engine - idle should be around 600-650 RPM.
//RF

[TruAggression]

Hey rfmaster I just tried that IAC reset and man did it work beautifully then after driving it and shuting her down then decide to start up right back up to 1700 rpm or so do you think my IAC is a bad one again? Still have not seen any vacuum leaks either went through 2 cans of spray already as well. Im guessing my truck may be possessed possibily ???

[rfmaster]

Originally Posted by TruAggression

Hey rfmaster I just tried that IAC reset and man did it work beautifully then after driving it and shuting her down then decide to start up right back up to 1700 rpm or so do you think my IAC is a bad one again? Still have not seen any vacuum leaks either went through 2 cans of spray already as well. Im guessing my truck may be possessed possibily ???

IAC - is dual coil bi-directional stepper motor. They sometimes develop an intermittent open in one of the winding which causes them to stop functioning. This usually happens when IAC is subjected to temperature changes - normal for engine compartment. Temperature intermittent are hard to diagnose, but since you were able to reset IAC and achieve reasonable idle speed I would rule out vacuum leak. So - get a replacement unit - yours is probably tired OE unit. IMHO I do not like to replace parts, but this maybe a good candidate for replacement.

Other possibilities include harness - IAC connector intermittent, ECM driver circuit intermittent.

//RF

[tazman87]

valve cover gasket? i have had that problem.

[TruAggression]

Ok so I replaced the IAC didnt change anything on idle. Did the reset and achieved beautiful idle once again but when I shut it off and then restart it goes right back up. So now I have 2 new IAC's and one of the original ones that was in it did the reset on all 3 of em and got a great idle but once restarted its nice and crappy again. It's like the computer is not remembering what position the IAC was at once restarted correct? Do you think my computer could have a problem? And valve cover gaskets are new due to having a leak like last month.

[rfmaster]

Originally Posted by TruAggression

Ok so I replaced the IAC didnt change anything on idle. Did the reset and achieved beautiful idle once again but when I shut it off and then restart it goes right back up. So now I have 2 new IAC's and one of the original ones that was in it did the reset on all 3 of em and got a great idle but once restarted its nice and crappy again. It's like the computer is not remembering what position the IAC was at once restarted correct? Do you think my computer could have a problem? And valve cover gaskets are new due to having a leak like last month.

When you shut down your engine ECM commands IAC into fully open position - in most calibrations its 160 steps. With engine shutdown the IAC pentacle should be fully retracted. You can shine a light and should see idle air passage open. ECM has no clue where IAC is, but reset procedure forces IAC to fully closed position and this way ECM knows where it last known position was.

During engine cranking ECM will move IAC into intermediate position (110 to 120 steps) depending on temperature. This will result in initial high engine RPM flare up - to prevent cold engine from stalling. Once engine is running ECM monitors RPM & coolant temperature (CTS) and slowly step IAC down restricting idle air thus reducing idle speed (about 25 to 12 counts). This is a simplified explanation of how IAC is controlled. If ECM receives false CTS reading (CTS disconnected = -40C) - i.e. very cold coolant it will maintain high idle around 1100 RPM.

Short A+B and and observe SES light - if there are no pending DTC you should see SES blink code 12 three times and repeat. If there are pending DTC they will follow code 12.

You may have bad CTS - but pull codes first to be certain. Does your engine run rich???

//RF

[Hulk Will Smash]

My R30 is having similar symptoms but also has a burned out check engine light. Got a little list of stuff to mess with this weekend. I'm going to try to reset the IAC since I've already replaced the CTS.

I really appreciate the insight on this stuff guys. I love the truck and want to try to keep the TBI on it rather than convert to carb.

Hey RF, where are you in OC?

Thanks,

Sean

[themecanic]

I have an 87 GMC 305 with TBI and a 87 Chevy 350 with TBI about 8 months ago my idle speed went up so high that it darn near knocked out the U-Joints when shifting in an out of gear as I was doing a tune up and inspection of related components, I found that my o2 sensor was broken off just above where you put the wrench to remove it. I replaced the sensor and my Idle went back to normal so I concluded that the idle was high because of the 02 sensor controls air / fuel mixture to burn off excess fuel for emissions . I may have been wrong about my conclusion because my idle went backup to an unacceptable level and after doing a new new tuneup you know (plugs, cap, rotor, wires, filters, fuel and air and o2 sensor my idle did not go down this time. So I will follow the leader and do the things listed above to see if that might be my problem. thank you all for the support I get from this sight! I will get back with you when I find out what my solution is. or more questions if I dont