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

[TruAggression]

RF you my friend are the best! So I thought I ruled out it being the CTS. So I took out the newer one put one I had laying around at my shop and wow starts up idles nicely seems like problem solved!!!!!! I feel as if I owe you a beer sir thanks alot bro for helping me diagnose my problem.

[rfmaster]

Originally Posted by TruAggression

RF you my friend are the best! So I thought I ruled out it being the CTS. So I took out the newer one put one I had laying around at my shop and wow starts up idles nicely seems like problem solved!!!!!! I feel as if I owe you a beer sir thanks alot bro for helping me diagnose my problem.

You very welcome. Before you congratulate me please make sure that your truck is running fine (consistently). Open or dead CTS would cause your engine to run rich - so you may have fouled up spark plugs - please check.
BTW, I am not a mechanic, but I like to to modify TBI, since this early EFI system is easy to troubleshoot and can be adapted onto just about any internal combustion engine.

Keep on trucking....

//RF

[TruAggression]

Yeah I drove it for a little bit does seem to have a miss im just gonna go ahead and repalce the plugs tomorrow, its about time for some anyway. Any other tricks for this TBI setup that I need to know. Good gap for plugs I'm gonna throw in tomorrow? Is the throttle body spacer really worth it in your opinion? I am new to this kinda of setup I'm used to carbs. Thanks

Josh

[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

[rfmaster]

Originally Posted by TruAggression

Yeah I drove it for a little bit does seem to have a miss im just gonna go ahead and repalce the plugs tomorrow, its about time for some anyway. Any other tricks for this TBI setup that I need to know. Good gap for plugs I'm gonna throw in tomorrow? Is the throttle body spacer really worth it in your opinion? I am new to this kinda of setup I'm used to carbs. Thanks

Josh

IMHO carbs belong where they should be. Stock TBI equipped engines are performance limited by a tiny cam, restricted stock exhaust system and lousy heads from the factory. Without going into a complete rebuild there are few things one can do to improve performance.

1) Improve fuel volume and pressure. Stock TBI can run with as little as 6 PSI - it will run, but very poorly. Install stiffer FPR spring and adjsutable FPR (from Jet). I like to run my TBI in the 15 to 16 PSI range. Higher pressure improves fuel atomization resulting in better combustion. In my conversions I run 40kV coil and gap plugs at 0.045".

2) I run 1" spacer on my Holley 300-66 intake - keep in mind that stock fuel lines will not clear Holley projection intake without spacer. Spacer improves throttle response.

3) I run headers and true 2-1/2" dual exhaust system on my rig. It sounds nice (as for performance improvement it is hard to tell since I did a lot of work over time)

4) Custom tune - I tune my ECM. So that any flat spots are taken care of.

//RF

[philthethrillohio]

Originally Posted by TruAggression

RF you my friend are the best! So I thought I ruled out it being the CTS. So I took out the newer one put one I had laying around at my shop and wow starts up idles nicely seems like problem solved!!!!!! I feel as if I owe you a beer sir thanks alot bro for helping me diagnose my problem.

ihave same truck same motor same issues. done all the test resets and sensors. my idle was great then start it up after going in the store now its high again? i do have a issue just happened , my temp is stuck at 110 on the gauge. does that have anything to do with my situation? i replaced the temp sensor like 2 or 3 days ago then i just noticed this today. i think its just the gauge. whats all our thoughts?

[rfmaster]

Originally Posted by philthethrillohio

ihave same truck same motor same issues. done all the test resets and sensors. my idle was great then start it up after going in the store now its high again? i do have a issue just happened , my temp is stuck at 110 on the gauge. does that have anything to do with my situation? i replaced the temp sensor like 2 or 3 days ago then i just noticed this today. i think its just the gauge. whats all our thoughts?

Not knowing your truck year and engine details it is very difficult to offer a course of action. The instrumental panel temp gauge uses a separate, single wire sensor which is usually is screwed into cylinder head. The ECM uses a dedicated two wire sensor which is screwed into intake manifold water jacket next the thermostat outlet.

//RF

[philthethrillohio]

Originally Posted by rfmaster

Not knowing your truck year and engine details it is very difficult to offer a course of action. The instrumental panel temp gauge uses a separate, single wire sensor which is usually is screwed into cylinder head. The ECM uses a dedicated two wire sensor which is screwed into intake manifold water jacket next the thermostat outlet.

//RF

its a 87 305 tbi. when i quoted the guy and said i have the same truck figured that make it clear but anyway.i replaced all the sensors so i know where they are. i guess my ne w question is if you replace the iac do you need to resetthe ecm computer? i put one in and went to adjust min idle and it was idling so high, took idle screw so it wasnt even touchingand still high idle. ive heard when replacing you need to disconnect batery for 10min or so to reset ecm or you will have idle around 1500. anyone heard this?

[rfmaster]

Originally Posted by philthethrillohio

its a 87 305 tbi. when i quoted the guy and said i have the same truck figured that make it clear but anyway.i replaced all the sensors so i know where they are. i guess my ne w question is if you replace the iac do you need to resetthe ecm computer? i put one in and went to adjust min idle and it was idling so high, took idle screw so it wasnt even touchingand still high idle. ive heard when replacing you need to disconnect batery for 10min or so to reset ecm or you will have idle around 1500. anyone heard this?

OK - I see what you have. Yes, when ever replacing IAC you need to reset IAC by jumping pins A&B on ALDL connector (inside cab), disconnecting IAC connector and shutting engine down - see full IAC procedure (use search). If IAC was correctly reset resetting ECM is not required, although it does not hurt to do it - it clears all flags, BLM's, etc.

10 minutes with bat cable off is a bit excessive - all it takes about 10 to 30 sec for ECM to loose its memory when battery cable is removed.

high idle speed can be due to a vacuum leak - these usually form between intake and TB especially if that gasket was never replaced. Another reason for high idle is incorrectly set base timing - which must be set EST connector open engine idling around 625 RPM or less. This when you can adjust minimum idle screw.

//RF

[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