Iac maybe?

[xs-style]

Just did a rebuild on an 87 tbi. Prior to the rebuild, idle was normal at around 800 rpm. After cleaning it up and doing the o rings and gaskets, I can't seem to get the idle at less than 2k. I've checked for vacuum leaks, and I'm using map that has tested good. I bought a new Iac and did the two reset methods I've seen on the forums. Any other things I could look at?

[homemade87]

That seems to be a definet vacuum leak . Be sure to check all your vacuum hoses that you have not dislodged one or cracked and the fitting . Also take a good look at the wire connector to be sure a pin has not been damaged and the iac is not functioning .

[xs-style]

I double checked all the vacuum ports tonight. They were all where they should be, and no evidence of any cracking. The pins for the Iac looks fine. Tried it in reverse, neutral, and drive. Park and neutral are just over 2k rpms, reverse and drive drop it to about 1200. I didn't touch the idle screw while doing the rebuild, so I dunno what's up.

[Desert1957]

Coolant sensor will make it idle high if its bad, but not 2000 rpm high.

What relearn procedure are you using on the "IAC" , FYI you should never manually force the plunger in by hand if its been removed. right?

IAC can do many funny things with TBI, It basically is a controlled vacuum leak anyway.

Look down the throttle bore while the engine is off and make sure throttle plates look closed.

Do not try and adjust the screw you think is the idle screw, that's not what its for. IAC does all controlling of the idle.

[xs-style]

I haven't adjusted the idle screw, though I do know where to look. The factory plug is still in place so it should be ok.

The throttle plates are closed and have good spring resistance.

As far as what steps to use in the relearn, I've found a couple different ways. The process I've used most is to clear any codes. Not that there have been any codes flashing, just doing it as a part of the process. Then i jumper the diagnostic ports, and turn the switch on. Once I've heard the iac click and waited 30 seconds, I go unplug the connector. Then go turn the switch off and plug the connector back in.

I've used the original iac and two new ones. As high as the rpms go, my first thought was vacuum leak at the base gasket. Sprayed around everything with carb cleaner, no changes.

The only time I got a change in rpm was spraying into the TBI. That actually lowered the rpms. In my mind that backs up a vacuum leak.

[homemade87]

Try this procedure . I could not remember how I did mine . I found this from another member here .

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

The difference here is you are plugging the connector back in before you start the engine and set the idle if needed .

[Desert1957]

Originally Posted by homemade87

Try this procedure . I could not remember how I did mine . I found this from another member here .

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

The difference here is you are plugging the connector back in before you start the engine and set the idle if needed .

^^^^^^^This^^^^^^

[Dayton601]

My first thought would be to put back in your old IAc (idle air control sensor) but make sure you clean the area were it seats up against on the throttle body, sometimes carbon can build up not letting it seat good, also the Throttle position sensor can go bad causing it to rev up a little, just recently on my 91 TBI it was idling up higher than normal,turns out that the hard black vacuum line going to map sensor, was cracked, and letting air in, you might also make sure the wires are plugged in good to the map sensor, sometimes the little rubber boot will keep it from going all the way in,

[xs-style]

Well guys I tried that procedure. It's one of the ones I've tried before and unfortunately it didn't help. With the Iac unplugged, switched, or even removed completely and plugged with my finger I'm still between 2-3000 rpm. I am beginning to think the iac had nothing to do with it. Which leaves a gross vacuum leak.. but where? Spraying carb cleaner all around doesn't make it rev any higher... And where ever it settles into idle, it stays steady.

[Dayton601]

Take the intake back off, and check for intake gasket leakage, the biggest headache is resetting the distributor, that and about 20 bucks, maybe it didn't seat good,

[xs-style]

Well I took the thing back apart and found a leaking fuel pressure regulator.

Is there anyone who has recently redone their TBI that could tell me what the seal under the fuel pressure regulator looks like? By this I mean once you put the spring in and the cap on, then put in the four screws in the cap.. the seal between the bottom of the regulator there and the other portion of the throttle body

[xs-style]

I tried to attach a picture of where I mean.

[RUN GMC]

Just went through everything you just mentioned and read all related threads to try to fix the issue over a 2 month period. I had ordered and swapped out parts and also I had changed spark plugs and wires about 6 months ago.....so out of desperation I decided to change spark plugs again since new parts were not changing the condition. I end up founding 3 spark plugsthat were fouled a bit and having laid them out in order to know which cylinder was being affected by the fouled plugs and proceeded replacing them with new spark plugs I fired up the truck and I could feel the bog and heard a faint miss firing with new plugs...so I then ordered a new set of AC Delco spark plugs wires on-line from Advance Auto (via the discount code method) and replaced the 3 suspect wires and to my happy surprise the truck ran like a new one. Drove it to work and it ran great so 2 days later I replaced the rest of the wires with new ones....2 sets of wires in just over 6 months....the ones I took still looked good and brand new and I couldn't tell what was wrong with them other than the plugs were fouled a bit that they were connected to........so I would advise you to check that your plug wires are on properly and seated on the cap and plugs if they are then I would change plugs and check for any that look less than good to see if have any bad wires that are not allowing proper firing. I replaced a lot of parts including distributor, iac, egr, temp sensor, oil pressure on back of intake, vaccuum lines (made a smoker out of paint can and hickory chips and incense)(just found 2 small leaks with it), check valve and hoses to brake booster, IAC reset 3 times, almost rebuilt TBI, and some more parts I haven't mentioned.....good thing now I have got a lot of spare parts on the shelf for my truck, bad thing is I spent a lot of money also....but in the end it was $16 spark plugs and $30 spark plug wires(Advance Auto online discount code prices).....and in the threads it was mentioned a few times to recheck spark plug wires to see if some were bad new ones out of the box.......finally I read it takes 40 crank cycles for ECU on my 87 GMC to build up a proper parameter profile to get it back to running efficiently again which is probably true because the more I drove it the better and better and better it ran........I was able to drive it Indianapolis on that thursday to the Power Tour and join in on the last day drive to Bowling Green. I hope this gives something to think about and hopefully help you out of your jam......laterz......

[xs-style]

I dunno. It's got new delco plugs in it.. new like within the last month. Store brand wires, but that's the same combo I've run in my 78 for ten years or more. I may at least pull them and check the condition.

Good news is that I fixed the leaking fuel regulator. Bad news is rpms in park are up to 3300. What's redline when your sitting still? Seems like this mofo just wants to blow itself up. I'm of half a mind to let it lol

[Desert1957]

3300 RPM? sure seems like a throttle plate issue in the TBI itself, Iv'e never seen anything related to sensors or a vacuum leak keep the idle that high. Still think its related to a worn, bent or stuck throttle shaft not letting the plates close all the way. very unusual.

Only other thing (I'm sure you have checked) is the throttle body mounting base gasket, these are notorious for sucking air.
spraying around the base doesn't always find the leak, take it off and inspect it.

[xs-style]

Well just to eliminate...I swapped base gaskets. That did the trick, rpms are back to normal. The one that was on there was new, and the one I swapped it to was new as well. No visable differences in the two, but there was obviously something. I found out just in time to run out of work time for the day lol I guess it will be tomorrow before I find out if the TBI rebuild solved my dieing in gear issue or not.

[wilkin250r]

Originally Posted by xs-style

I guess it will be tomorrow before I find out if the TBI rebuild solved my dieing in gear issue or not.

Be careful what you wish for. I'm pretty sure a 3300rpm idle is plenty high enough to keep it from dying when you put it in gear...

[xs-style]

This is true lol I put it in gear while I had the other gasket in there and drive was around 1200 rpm sitting still foot on the break. Shoulda tried to drive it just to see what would happen

[Desert1957]

Originally Posted by xs-style

Well just to eliminate...I swapped base gaskets. That did the trick, rpms are back to normal. The one that was on there was new, and the one I swapped it to was new as well. No visable differences in the two, but there was obviously something. I found out just in time to run out of work time for the day lol I guess it will be tomorrow before I find out if the TBI rebuild solved my dieing in gear issue or not.

So it was the base gasket?

[xs-style]

I guess the answer to that is yes and no. Cold start it, and you'll idle at about 800 rpm. Let it warm up and it holds steady. Put it in reverse or drive and idle starts surging from around 500 to about 2000. Put it back in park and your idles gonna hang around 1800.

/Rant I dunno what to say fellas. I'm sick and tired of trying. I was in a car wreck over a year ago that took out my main transportation. I looked at it as a blessing. It paid off a car payment I kinda wanted out from under anyway and provided me a chance to drive one of my old rides. But here I am over a year later, 3000 dollars in three different trucks that I haven't even made it to the other side of town in. I pin one problem down and something completely different shows up. This is getting retarded. /Rant