How do you adjust the IAC?

[get me on the road]

According to my scanner, my 89 5.7 TBI, at operating temp and idleing, has an IAC reading of 78, the scanner also says that it should be between 20 and 40, and under 20 is preferable. The truck, when at operating temp, is idleing at 1000-1025 rpms. Any ideas?

[ChevyTech]

has an IAC reading of 78

If the ECM was trying to slow down the RPMs, would keep lowering the count (pulsing the IAC) until it got the idle where it wants the RPM, or until the count hits zero.

Conclusion: The ECM is not trying to slow down the engine.

Have you looked in the distributor for oil coming up the shaft or a loose shaft?
The ECM gets its RPM signal from the distributor.

The truck, when at operating temp, is idleing at 1000-1025 rpms

Is this in gear or in neutral?
Is the scanner showing this RPM or is this from a dash or separate tachometer?

What does the scanner show for a coolant temperature reading?

Does the TPS look ok (0% open or around .5 volts) so the ECM knows to control the idle?

Does your scanner show Park neutral switch data, and does the truck have an automatic or manual transmission?

[get me on the road]

Originally Posted by ChevyTech

Have you looked in the distributor for oil coming up the shaft

I checked under the cap and under that plate that the cap seals against, and all was dry.

Originally Posted by ChevyTech

or a loose shaft?

As far as pulling up and down I had around, but less than an 1/8". Side to side movement less than a millimeter. Twisting clockwise and counter clockwise around, but less than an 1/8"

Originally Posted by ChevyTech

Is this in gear or in neutral?

Park

Originally Posted by ChevyTech

Is the scanner showing this RPM or is this from a dash or separate tachometer?

Scanner

Originally Posted by ChevyTech

What does the scanner show for a coolant temperature reading?

193 F

Originally Posted by ChevyTech

Does the TPS look ok (0% open or around .5 volts) so the ECM knows to control the idle?

At 120 F with key on engine off .7v then with throttle wide open 4.62v
At 193 F key on engine on, idleing at 1000rpm, in park, all accessories off, I had .68v, took the rpms up to 1225 and had .76v

Originally Posted by ChevyTech

Does your scanner show Park neutral switch data,

Yes and it does work, but when I put it in neutral from reverse I sometimes have to wiggle the shifter to get it to show P-N-- on the scanner. As far as going into drive, I do NOT have to wiggle the shifter for it to show D on the scanner.

Originally Posted by ChevyTech

and does the truck have an automatic or manual transmission?

Auto with over drive.

Sorry I forgot to mention that I just tuned up the TB and added a TB spacer.
I have 15.5 pounds of fuel pressure key on, and the same(15.5 pounds) while its running. When I shut the truck off it drops to 0. Is that normal?

Here are some of the readings I have. If you need different ones the scanner has more, just ask. The scanner also has a graphing mode if you want some more accurate/in depth readings.

Key on engine off at 120 F
TPS .7v
IAC 145
MAP 101 kpa

Key on engine on at 120 F, in park at idle, all accessories off
RPMs 1025
TPS .68v
IAC 104
MAP 31 kpa

and the scanner says High Gear......YES

Key on engine on at 193 F, in park at idle, all accs off
RPMs 1000
TPS .68v then at 1225 RPM/ .76v
Block Learn 132, at 1225 RPM/ 127
IAC 78, then at 1225 RPM/ 72
02 cross counts, truck idleing at 1000 RPM/ 18-21
MAP 29 kpa
MAP .98v

and again the scanner is saying High Gear.......YES, and it says that no matter what gear I put the truck in.

[ChevyTech]

I have 15.5 pounds of fuel pressure key on, and the same(15.5 pounds) while its running. When I shut the truck off it drops to 0. Is that normal?

That fuel pressure is a bit high. 13 PSI is the factory spec max allowable pressure.
Is this the truck with the aftermarket fuel pressure regulator spring that looks shorter but heavier duty? I am going by memory here and might be mixing your truck up with another posters vehicle.

The system pressure should not drop to zero when the truck is shut off. It should hold pressure for at least several minutes.

On MAP readings I watch the voltage. Your warm reading voltage is good. The normal range for kilopascals readings just don’t stay in my memory.

The Block Learn is in the normal range which surprises me with the high fuel pressure. Maybe your fuel pressure gauge is off???

Check and see what the IAC count is in drive, at idle, at operating temp, with all accessories off. Also see what the RPM is.

Test the voltage at the battery with it running, and test the voltage at an ECM fuse with it running. If the system running voltage is low the ECM will alter settings.

and the scanner says High Gear......YES

and again the scanner is saying High Gear.......YES, and it says that no matter what gear I put the truck in.

??? I do not know.

Does this truck have a custom chip (prom) in it?

[ChevyTech]

At 193 the coolant temp is good also. My first thought was the engine may be running cold so the ECM is holding the fast idle for cold operation.

[get me on the road]

Originally Posted by ChevyTech

Is this the truck with the aftermarket fuel pressure regulator spring that looks shorter but heavier duty?

Yes. Can I adjust the pressure somehow?

Originally Posted by ChevyTech

The system pressure should not drop to zero when the truck is shut off. It should hold pressure for at least several minutes.

Any thoughts on that?

Originally Posted by ChevyTech

Check and see what the IAC count is in drive, at idle, at operating temp, with all accessories off. Also see what the RPM is.

Once I put it in drive the IAC will go from 78 to 145 within about 3 seconds.
The RPMS vary between 700-900, and you can feel the truck lerching forward, like your slightly brake torqueing it. So I have to step on the brakes harder
Then something even odder happens, I put it in park, the RPMs slowly(within 2-3 secs) climb to 1500 and the truck just quits. ?????????
I also noticed after I enter the vin on the scanner and choose the Codes&Data menu, while the scanner is connecting to the computer the trucks idle raises probably 200 RPMs.

Originally Posted by ChevyTech

Test the voltage at the battery with it running, and test the voltage at an ECM fuse with it running.

Battery 14.19
ECM fuse 14-14.02

Originally Posted by ChevyTech

If the system running voltage is low the ECM will alter settings.

I took the truck for a drive to warm it up, after about 5 miles I came to a stop. I saw my dash lights were dim, and my volt meter on the dash was about 1/2 way between 9 and 14. I had the heater on high, subwoofers thumping and just the head lights, no brights, on. I shut everything but the truck and headlights off as I was accelerating up to 45 mph. Long story short, I got home put my MAC battery/starting/charging system checker on the battery and ALL tests passed. This checker has a total of like 6 or 7 tests and a few load tests while at idle and at 2000 rpms, and it checks for any ripples in the alternator. I can turn all accessories in the truck on, heater on high, brights on and stereo system all the way up and my dash gauge says like 13, but when I put it in drive and sit there with my foot on the brake, the gauge starts to drop and everytime the subs hit it drops even more, but wont drop past 1/4 way above the 9 (1/4 of the way between the 9 and 14). I'll load test the battery first thing tomorrow when its at its coldest.

Originally Posted by ChevyTech

Does this truck have a custom chip (prom) in it?

I dont know and I dont think so, I have never messed with the computer. Is there any tests that you know of that will tell if there is a chip in it?

[ChevyTech]

Originally Posted by ChevyTech
Is this the truck with the aftermarket fuel pressure regulator spring that looks shorter but heavier duty?

Yes. Can I adjust the pressure somehow?

There are aftermarket adjustable regulators for TBI trucks. There are also articles on how to make the stock regulator adjustable. On the factory setup the spring is what determines the pressure – that is why I asked about the spring.

Originally Posted by ChevyTech
The system pressure should not drop to zero when the truck is shut off. It should hold pressure for at least several minutes.

Any thoughts on that?

Most of the time this is caused by a poor quality aftermarket fuel pump letting the fuel drain backwards through it as soon as the pump stops running.

The battery voltage and the voltage at the ECM fuse looked good.

Scan tools have different mode of operation. Look in the scanner manual and it should discuss this. The scan tool could be causing the truck to run fast.

Does the truck run ok with a normal idle when the audio system is off with no scan tool on the truck?

I am not to into giant subwoofers but if you are running a huge system you should consider a separate battery and alternator for the sound system or some isolation electronics with a second battery.

[get me on the road]

Originally Posted by ChevyTech

There are aftermarket adjustable regulators for TBI trucks. There are also articles on how to make the stock regulator adjustable. On the factory setup the spring is what determines the pressure – that is why I asked about the spring. .

Could a malfunctioning fuel pump also give it that high pressure?

Originally Posted by ChevyTech

Scan tools have different mode of operation. Look in the scanner manual and it should discuss this. The scan tool could be causing the truck to run fast..

Will do.

Originally Posted by ChevyTech

Does the truck run ok with a normal idle when the audio system is off with no scan tool on the truck?.

I dont know what the idle, RPMs, should be on a 'perfect' vehicle at operating tempurature. It does idle very smooth though. The only problem I noticed was the exhaust has a rich, hot, sting your nose smell, and it appears to be getting worse. The truck does not have a converter on it, but I know the exhaust didnt smell like that when I got it this past summer. Any suggestions on where I should go from here, as far as the high IAC and the bad smelling exhaust?

Originally Posted by ChevyTech

I am not to into giant subwoofers but if you are running a huge system you should consider a separate battery and alternator for the sound system or some isolation electronics with a second battery.

Its not a HUGE system, just 2 12's being pushed by an 800 watt mono amp.

[ChevyTech]

Could a malfunctioning fuel pump also give it that high pressure?

NO


Originally Posted by ChevyTech
Scan tools have different modes of operation. Look in the scanner manual and it should discuss this. The scan tool could be causing the truck to run fast..

I knew I had this written down somewhere, it just took me a while to find it:

Data link connector mode
(Also referred to as the 10K or special mode)
While in this mode all obtainable data is readable.
The operating characteristics are modified as follows:
Closed loop timers are by passed
Distributor ignition is advanced
PNP restrict functions are disabled
IAC valve controls engine idle to 1000 plus or minus 50 RPM (if applicable)
Some engines the canister purge solenoid is enabled

I dont know what the idle, RPMs, should be on a 'perfect' vehicle at operating tempurature. It does idle very smooth though.

Without looking it up I would guess the ECMs target RPM should be in the range of 500 to 550 RPM, in gear, warm, with all accessories off.

The only problem I noticed was the exhaust has a rich, hot, sting your nose smell, and it appears to be getting worse. The truck does not have a converter on it, but I know the exhaust didnt smell like that when I got it this past summer. Any suggestions on where I should go from here, as far as the high IAC and the bad smelling exhaust?

The high fuel pressure is going to cause a rich condition at times. That will make the exhaust smell.

If it still idles to fast with the scan tool off it, the coolant temp correct and the voltage ok, here are some possibilities:

There is a vacuum leak and the IAC count is really zero with the scan tool not connected to the system, so the IAC can not slow the engine any farther.

The IAC is failing to move the way it should at times and the ECM is losing track of its position. (Bad IAC)

Wiring problems between the ECM and the IAC.

The ECM is toast.



If I were working on this truck I would look down in the TBI from the top on the passenger side and see what the position of the IAC plunger is. If the plunger is fully extended, and still idling to fast, there is a vacuum leak somewhere.

If the IAC was not fully extended, I would try another IAC on it. If that did not help I would swap out the ECM.

[get me on the road]

Originally Posted by ChevyTech

Originally Posted by ChevyTech
Scan tools have different modes of operation. Look in the scanner manual and it should discuss this. The scan tool could be causing the truck to run fast.

I knew I had this written down somewhere, it just took me a while to find it:

Data link connector mode
(Also referred to as the 10K or special mode)
While in this mode all obtainable data is readable.
The operating characteristics are modified as follows:
Closed loop timers are by passed
Distributor ignition is advanced
PNP restrict functions are disabled
IAC valve controls engine idle to 1000 plus or minus 50 RPM (if applicable)
Some engines the canister purge solenoid is enabled

I was reading the manual and it said that if I put it(scanner) in drive mode it should drop the RPMs. So we'll see.

Originally Posted by ChevyTech

Originally Posted by ChevyTech
Without looking it up I would guess the ECMs target RPM should be in the range of 500 to 550 RPM, in gear, warm, with all accessories off.

The high fuel pressure is going to cause a rich condition at times. That will make the exhaust smell.

If it still idles to fast with the scan tool off it, the coolant temp correct and the voltage ok, here are some possibilities:

There is a vacuum leak and the IAC count is really zero with the scan tool not connected to the system, so the IAC can not slow the engine any farther.

The IAC is failing to move the way it should at times and the ECM is losing track of its position. (Bad IAC)

Wiring problems between the ECM and the IAC.

The ECM is toast.



If I were working on this truck I would look down in the TBI from the top on the passenger side and see what the position of the IAC plunger is. If the plunger is fully extended, and still idling to fast, there is a vacuum leak somewhere.

I did look at the IAC while the truck was idleing, and slowly opened the throttle and all appears to be well, but I cant tell if the plunger is all the way extended. I'll check for vacuum leak.

Originally Posted by ChevyTech

Originally Posted by ChevyTech
If that did not help I would swap out the ECM.

huh, $$$, Well thank you sunshine j/k
If I find no vacuum leaks, I should probably start by getting the fuel pressure right and get a GM fuel pump. Correct?
I'll see what readings I get with the scanner in drive mode and let you know.



Any ideas about the voltage dropping like I described? I did the bat/start/charging tests again and all was fine. Could the alt or bat have an intermitant problem? Or could the ECM cause this as well?

[get me on the road]

Heres the new readings with scanner in drive mode

Key on engine off
Temp 73 F
IAC 145
MAP 4.86v
TPS .68v
Block Learn 128

Key on engine on in drive all accessories off
RPMs 500-525
Temp 198
O2 crosscounts 0-20
IAC 50
MAP 1.60-1.66v
TPS .68v

Key on engine on in park all accessories off
RPMs 500-550
Temp 194
IAC 41
O2 crosscounts 10-25
TPS .70v

[ChevyTech]

Originally Posted by get me on the road

Heres the new readings with scanner in drive mode

Key on engine off
Temp 73 F
IAC 145
MAP 4.86v
TPS .68v
Block Learn 128

Key on engine on in drive all accessories off
RPMs 500-525
Temp 198
O2 crosscounts 0-20
IAC 50
MAP 1.60-1.66v
TPS .68v

Key on engine on in park all accessories off
RPMs 500-550
Temp 194
IAC 41
O2 crosscounts 10-25
TPS .70v

Much better readings!
IAC counts are higher then desired but much better.
RPMs very good
TPS good

Seeing cross counts tells you it is controlling and switching rich and lean. A zero count could be the system loosing control. Being I am not used to your scanner I don’t know if it actually hit zero cross counts or not.

[get me on the road]

If I find no vacuum leaks, I should probably start by getting the fuel pressure right and get a GM fuel pump. Correct?
I'll see what readings I get with the scanner in drive mode and let you know.



Any ideas about the voltage dropping like I described? I did the bat/start/charging tests again and all was fine. Could the alt or bat have an intermitant problem? Or could the ECM cause this as well?

Should I replace the IAC, or wait till after the fuel pressure gets straightened out?

[get me on the road]

Originally Posted by ChevyTech

A zero count could be the system loosing control.

What 'system' would that be?

[get me on the road]

Would a Bosch fuel pump be good enough ChevyTech, or should I go to the dealer and get a GM?

[ChevyTech]

If I find no vacuum leaks, I should probably start by getting the fuel pressure right and get a GM fuel pump. Correct?

No need to worry about vacuum leaks now that we see the system able to control the idle. With 500-550 at idle in drive and an IAC count of 50 the system could actually use a little more air. Before I changed anything I would drive the truck and to let it go through several learn cycles.

When you had the TBI off did you clean the throttle bore just at and below the throttle plate? A buildup of deposits here can cause a higher IAC count.

The high fuel pressure concerns me more then the drop of pressure when you shut the engine off. If it has trouble starting, or drivability issues, then consider a pump but before you condemn the pump I could give you some more tests to make sure the regulator is not the problem.
The high pressure can lead to a rich condition – like the burning eyes you were speaking of. The system may also set a rich code with the fuel pressure to high.

Any ideas about the voltage dropping like I described? I did the bat/start/charging tests again and all was fine. Could the alt or bat have an intermitant problem? Or could the ECM cause this as well?

The ECM will not cause this voltage problem. But a voltage that drops low will mess with the ECM. Check all the ground wires especially at the front of the intake. It would not hurt to put a volt meter on the grounds and make sure there is no voltage reading with the engine running.

[ChevyTech]

Originally Posted by get me on the road

What 'system' would that be?

Fuel mixture

The mixture should constantly swing back and forth from a little rich to a little lean. That is what the cross counts are showing.

[get me on the road]

Would a Bosch fuel pump be good enough ChevyTech, or should I go to the dealer and get a GM?

Originally Posted by ChevyTech

No need to worry about vacuum leaks now that we see the system able to control the idle. With 500-550 at idle in drive and an IAC count of 50 the system could actually use a little more air. Before I changed anything I would drive the truck and to let it go through several learn cycles.

So that says NO NEED TO WORRY ABOUT VACUUM LEAKS?

Originally Posted by ChevyTech

When you had the TBI off did you clean the throttle bore just at and below the throttle plate? A buildup of deposits here can cause a higher IAC count.

I cleaned every part of the TB very good, and cleaned the barrels in the intake pretty good.

Originally Posted by ChevyTech

before you condemn the pump I could give you some more tests to make sure the regulator is not the problem.

I'll do them, just let me know how.

Originally Posted by ChevyTech

Check all the ground wires especially at the front of the intake. It would not hurt to put a volt meter on the grounds and make sure there is no voltage reading with the engine running.

The bat ground and the intake ground with the bat connected? Any others?

[get me on the road]

Originally Posted by ChevyTech

Fuel mixture

The mixture should constantly swing back and forth from a little rich to a little lean. That is what the cross counts are showing.

I cant recall what the scanner called it, but there was something switching from rich to lean and back and so on. It would switch every 1-2 seconds

[ChevyTech]

Originally Posted by get me on the road

Would a Bosch fuel pump be good enough ChevyTech, or should I go to the dealer and get a GM?

I am not familiar with Bosch pumps. It is possible that it is a good brand but with all the bad aftermarket pumps I have seen, I will only use GM pumps.

I know several people that regularly buy their parts at NAPA. After the son of a friend had his second pump from NAPA fail in the warranty period, I had him take the NAPA pump apart and we compared it to an OEM GM pump. After he saw the difference he went and bought a GM pump. Then he remarked the price was not much more for the GM pump then he paid at NAPA.

I will help a friend diagnose his bad aftermarket parts, but when it comes to a fuel pump that is in the tank, I would buy a GM OEM pump.

[ChevyTech]

Re: Ground testing

The bat ground and the intake ground with the bat connected? Any others?

The wires that are under the studs that hold the front of the intake on are the grounds for the ECM and ECM sensor circuits.

On the rear of the cylinder head on the passenger side there should be a ground.

The battery should have a wire that grounds to the inner fender.

Don’t assume any thing has a good ground. Take your meter and touch everything you think should be grounded.

[get me on the road]

So touch 2 grounds at the same time with voltmeter while truck is running, and look for voltage, correct? Thank you very much for your time and efforts, I greatly appreciate it.

[ChevyTech]

Originally Posted by get me on the road

So touch 2 grounds at the same time with voltmeter while truck is running, and look for voltage, correct? Thank you very much for your time and efforts, I greatly appreciate it.

Clamp the Black meter lead on the negative battery post/bolt. Touch the red lead to everything that should be grounded. Some test may show a couple tenths of a volt, depending on how mush currant is flowing as the alternator is charging


Then touch the black lead to the engine and touch the red lead to the ECM grounds on the intake. I should not show ANY voltage on the ECM grounds.

[ChevyTech]

Originally Posted by get me on the road

I'll do them, just let me know how.

I find it very unlikely the regulator would cause high pressure running and bleed off the pressure when the engine is turned off. I would not say that it is impossible.


This is to test to make sure the fuel pressure is not leaking though the regulator after the engine is shut off. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure.

With the fuel pressure gauge connected by using fittings to “T” in a pressure gauge.

Pinch off the return line, using care not to damage the return line / hose.

Cycle the key on just long enough to get fuel pressure then turn the key off.

If the pressure still drops when the pump stops, and fuel is not leaking past the injectors, the fuel is leaking backward through the pump.

[get me on the road]

Originally Posted by ChevyTech

Pinch off the return line, using care not to damage the return line / hose.

Cycle the key on just long enough to get fuel pressure then turn the key off.

If the pressure still drops when the pump stops, and fuel is not leaking past the injectors, the fuel is leaking backward through the pump.

Can I pinch the metal braided flex hose that comes out of the TB and not cause internal damage? Or should I just unscrew the return line at the TB and plug the TB with a piece in my fuel pressure test kit?