89 S10 4.3 Need help troubleshooting TBI/IAC Idle issue

[ChevyTech]

The fuel pressure regulator is in the TBI unit and testing the pressure must be done with the gauge installed on a T fitting, engine running.

You are just guessing at the pressure because your gauge does not go that high, but 18 PSI is easily reached with a good strong pump when the gauge is on the end of the line, or the return line flow is stopped.

My guess is there is no problem in the tank. If the hose in the tank was split it would not hold pressure after the pump stops. The test you did does not show that the pump is good for sure, but the test leads me to believe the pump is good. You were in the TBI unit already so you would have seen if the fuel regulator spring is broken.

On ODB1 system and these TBU trucks it is a process of elimination, especially working over the internet when I am note using my senses to see how it is running. So far we have not definitively ruled out much.

I suggest you change the oil and get truck on the road to give it some run time after the changes you have made, so it can relearn the IAC position and fuel tables. This will also give it a chance to clean up soot off sparks plugs and the oxygen sensor.

[76C10Stepside]

Originally Posted by ChevyTech

The fuel pressure regulator is in the TBI unit and testing the pressure must be done with the gauge installed on a T fitting, engine running.

You are just guessing at the pressure because your gauge does not go that high, but 18 PSI is easily reached with a good strong pump when the gauge is on the end of the line, or the return line flow is stopped.

My guess is there is no problem in the tank. If the hose in the tank was split it would not hold pressure after the pump stops. The test you did does not show that the pump is good for sure, but the test leads me to believe the pump is good. You were in the TBI unit already so you would have seen if the fuel regulator spring is broken.

On ODB1 system and these TBU trucks it is a process of elimination, especially working over the internet when I am note using my senses to see how it is running. So far we have not definitively ruled out much.

I suggest you change the oil and get truck on the road to give it some run time after the changes you have made, so it can relearn the IAC position and fuel tables. This will also give it a chance to clean up soot off sparks plugs and the oxygen sensor.

Understood. Yes, I verified that the FPR spring is NOT broken (or weak) when I did the rebuild. Will change engine oil/filter and see what happens and report back here. Thanks!

Notes:

I figured a static pressure test was inadequate, but hoped it would do for now. Would like to rig up a permanent test fitting (schrader valve?) so I can do a proper test anytime after I change the oil and filter. The auto parts store did not have anything like that, so I had to settle for an assortment of vacuum fittings with two different sizes on each end. I used two of them, a piece of hose and 3 hose clamps to connect the hose on the guage to the piece of hose added to the fuel lines by somebody else. Any suggestions?

IMO, the (static) reading of 16 PSI was within +/- .5 PSI, because I measured it using an item that is the same width as each 1 PSI gradiation, using the outer/vacuum gradiations to mark each point on a blown-up photo on the computer.

The engine would not even start with the IAC disconnected, but that may be because the crankcase was full of gas by then. After I change the oil/filter, if still having problems (like stalling), I will try the "relearn" procedure again.

[ChevyTech]

Originally Posted by 76C10Stepside

...I figured a static pressure test was inadequate, but hoped it would do for now. Would like to rig up a permanent test fitting (schrader valve?) so I can do a proper test anytime after I change the oil and filter. The auto parts store did not have anything like that, so I had to settle for an assortment of vacuum fittings with two different sizes on each end. I used two of them, a piece of hose and 3 hose clamps to connect the hose on the guage to the piece of hose added to the fuel lines by somebody else. Any suggestions?

...

Here is a link that currently works to what I think is the easiest to use adaptor to test fuel pressure on TBI trucks.
http://www.toolsource.com/fitting-01...y-p-95304.html

You don't have a good gauge so you should buy one like in these links, that comes with the adaptor:
http://www.actron.com/product_detail.php?pid=16175

http://www.autozone.com/autozone/acc...er=232719_0_0_

http://www.toolsource.com/fuel-press...sh0lcunug0t5v7

[76C10Stepside]

Thanks for the links! Current status:

Changed the oil/filter last night (5W-30 synthetic blend, Purolator PureOne filter). Pulled the coil wire. Went to crank the engine to circulate the new oil and nothing. Brand new made in USA Exide battery was DEAD. Put it on the charger overnight.
This morning, put the freshly charged battery in, cranked engine for 15-20 seconds, put coil wire back on and engine fired right it up, went straight to 1750 RPMs and stayed there for about 1 minute. The oil pressure went straight to 60 PSI and stayed there for the 2-3 minutes I ran it. RPMs dropped to 1500 for the next minute or more, oil pressure remained at 60. Checked oil and added half a quart.

Questions:

How long should the ECM take to "relearn" and adjust the idle speed down?

Any suggestions on what to do next? I will not be driving it anywhere until I know I can trust it to keep running and not burn up.

[ChevyTech]

Originally Posted by 76C10Stepside

...
Questions:

How long should the ECM take to "relearn" and adjust the idle speed down?

Any suggestions on what to do next? I will not be driving it anywhere until I know I can trust it to keep running and not burn up.

It won't happen sitting in the driveway.

If I remember correctly one of the conditions for a self learn of the IAC position is a 45 MPH threashold must be met.

You can try this and see if it gets closer.
Using a watch or timer to make sure you hit the times on step 3 and 4:
1) Disconnect battery for at least ten seconds. (with the Key OFF)
2) Reconnect the battery.
3) Start engine letting it run for five seconds.
4) Turn key off for ten seconds.
5) Then start it and see how it idles.

[76C10Stepside]

Thanks, I'll give that a try!

[76C10Stepside]

UPDATE: Looks like I have a bad Idle Air Control (IAC).

I found a paper gasket for the IAC in the remains of the TBI rebuild kit, which proves that I did NOT remove the IAC when I cleaned and rebuilt the TBI. As I mentioned before, the kit instructions and exploded view OMITTED the IAC entirely, though it did say to remove all electrical parts before cleaning (thought I had).

I removed the IAC and measured how far the pintle is extended (just over 1-1/16"), then carefully cleaned it per the instructions in the "IAC maybe?" thread. Then put it back in with the new paper gasket and looked to see if I could see the tip of the pintle sticking through the hole into the TBI. NOPE. Then turn the key on (no sound) and checked again. Still not sticking out at all.

Conclusion: I wrecked the IAC when I cleaned the TBI using carb cleaner. Will buy a new one and see if that cures my fast idle problem. Hope this helps someone else avoid ruining their IAC!

[ChevyTech]

Carburetor cleaner can damage the IAC in a hurry.

They are a high failure part anyway.

They wear.

Drive it a bit to get a self relearn after the new IAC is installed.


Make sure the tip is not out too far when you install the new IAC.

The tip of the pintle must not extend more than 28mm which is 1.10 inches from the shoulder toward the electrical connector, by the threads.

[76C10Stepside]

The new Idle Air Control Valve did the trick. Runs great now! The idle settled down to 750 before I made it to the freeway, but I took on the freeway anyway just in case. Thanks for all the help!

[ChevyTech]

You’re welcome, and thank you for letting us know what fixed it.

[76C10Stepside]

UH-OH! Had a date last night and took the S10. Idle started at 1500, which I think is a bit high for warm weather. Shouldn't it be closer to 1200? It shudders and stalls when in gear and not moving. OK if not in gear. Had to keep my foot on the gas to keep it running. Accelerates fine and runs smooth in non-idle. AND, the TEMP GUAGE has a ZERO readout (see photo) the entire time. Made a stop on the way, and it didn't want to start back up, had to open the throttle and keep cranking it. Same problem starting back up after my date. Once going, it ran fine, except for the idle thing. The idle RPMs were the correct 750. Got gas on the way home, will drive it today and see if it's still acting up. The combination of no temp reading and wanting to stall out at idle is interesting. Bad ECT sensor?

BTW, I checked the ground on the firewall (black plastic thing that has several wires on it), and it was nice and clean (all copper connectors). Didn't find any other grounds for the engine wiring.

[ChevyTech]

The sensor for the computer is separate from the sensor for the gauge.

Like I said early in the thread.
Test the fuel pressure.
To correctly test the fuel pressure, test it with the engine it RUNNING.

Until you cross that off the list, we are wasting time, because it sure sounds like a low fuel pressure problem.

[76C10Stepside]

Originally Posted by ChevyTech

The sensor for the computer is separate from the sensor for the gauge.

Like I said early in the thread.
Test the fuel pressure.
To correctly test the fuel pressure, test it with the engine it RUNNING.

Until you cross that off the list, we are wasting time, because it sure sounds like a low fuel pressure problem.

So, you think the fuel pump works at higher volume/flow, but not at low volume/flow? It sure works fine at anything above 750 RPM when in gear, AND at 750 RPM when out of gear. I'll look for the Actron fuel pressure test rig. I've had an Acton timing light for 30 years and still works great!

And what about the zero temp reading? That can't possibly have anything to do with a failing fuel pump. IIRC, I got that once before, but it started working after I turned it off, then back on again. Records show the digital dash was repaired once, and replace with new (or reman) later on.

[ChevyTech]

Re: The dash gauge reading wrong.
The display like in your truck where a problem before those truck even got old.
They look nice.
They are a high failure item.

If the sender resistance reads even close to where it should, then suspect the instrument panel as the problem part.

So, you think the fuel pump works at higher volume/flow, but not at low volume/flow? It sure works fine at anything above 750 RPM when in gear, AND at 750 RPM when out of gear.

No,
At idle the injector pulse is so short that when the pressure is low, TBI systems go very lean. At high rpms & loads the injector has much longer pulses, spending more time open, and they do better with low pressure then at idle. That's the way it is. Do a search on my threads and see if you think I know what I am talking about. I am giving you my honest opinion. I have no reason to steer you wrong. I know nobody that knows the TBI trucks better then I do.

This is not a carbureted engine you are working on. You are thinking about fuel supply like it is carbureted.

[76C10Stepside]

Originally Posted by ChevyTech

Re: The dash gauge reading wrong.
The display like in your truck where a problem before those truck even got old.
They look nice.
They are a high failure item.

If the sender resistance reads even close to where it should, then suspect the instrument panel as the problem part.


No,
At idle the injector pulse is so short that when the pressure is low, TBI systems go very lean. At high rpms & loads the injector has much longer pulses, spending more time open, and they do better with low pressure then at idle. That's the way it is. Do a search on my threads and see if you think I know what I am talking about. I am giving you my honest opinion. I have no reason to steer you wrong. I know nobody that knows the TBI trucks better then I do.

This is not a carbureted engine you are working on. You are thinking about fuel supply like it is carbureted.

UH-OH! I haven't driven the truck since date night. Just opened the hood and could smell gas. Took the air cleaner off, and there's gas all over the TBI. The left side injector is dripping gas. It was NOT leaking after I installed the new IAC! I need to relieve the fuel pressure FAST to stop it flooding the engine again (if it's not already too late)! Do I have to disconnect the fuel line and run the pump, or what???

[76C10Stepside]

Jacking the front up and disconnecting the fuel line stopped the drip. Probably have a damaged injector. Have to admit that I did not put trans fluid on the injector o-rings when I rebuilt the TBI--I THOUGHT about doing it, but didn't see the line in the instructions that said to do it, and was not sure if the oil might damaged them. Only after I was done did I see that. After installing the new IAC, I snugged up all the bolts in the TBI "to be safe". That may have been the last straw for the injector(s). I only saw the left injector actually leak/drip, but there was gas all over the tops of the parts, like something was spraying.

If I have only one bad/leaky injector, do I need to replace BOTH of them at the same time? Looks like they are $73 a pop or more.

[ChevyTech]

If it has a bad injector you only have to take out the bad one, but you need to look at the aluminum casting very closely.

When you install injectors without lube on the o-rings it can crack the TBI housing.

Injectors are $4 each at the local U=Pull-It where I live.

If the problem comes and goes it is more likely an injector, then a crack, but look to see if you can see a crack before spending money on an injector.

[76C10Stepside]

Originally Posted by ChevyTech

If it has a bad injector you only have to take out the bad one, but you need to look at the aluminum casting very closely.

When you install injectors without lube on the o-rings it can crack the TBI housing.

Injectors are $4 each at the local U=Pull-It where I live.

If the problem comes and goes it is more likely an injector, then a crack, but look to see if you can see a crack before spending money on an injector.

Thanks for both tips, could save me a good chunk of change! I will check the housing first and go from there. The first time you explained why the o-rings need to be lubricated, I had already rebuilt the TBI (before I started this thread). That won't happen again! :-(

Will report back when I identify what broke...

[76C10Stepside]

Hey, ChevyTech. Have not had time to work on the S10 yet. I disconnected the fuel line at the fuel filter, plugged off the side of the line that runs to the tank, and ran the side that runs to the engine into a 1-1/2 gallon gas can. A couple of days later, the can was FULL and overflowing. I emptied it into another can, and now it has another gallon of gas in it! It's a constant very slow dribble.
Where is all this gas coming from??? I don't get it!

If I understand the fuel delivery/return system on this truck (thought I did), there is a hard line to the TBI/regulator, and a return line for returning any extra gas to the tank. Where would 2/1-/2 gallons of gas come from???

Besides being totally confused about this, I am concerned that when I loosen the screws for the fuel meter cover, gas is going to spray all over the place. Also, I'm wondering if I should test the fuel pressure per your directions (using a tee, a correct guage and with engine running) BEFORE I take it apart, OR take it apart and try to identify what it leaking and replace parts as needed first?

I get the feeling there is too much pressure built up inside or something...

[ChevyTech]

Are you parking the truck on a hill, with the front end down hill?

The fuel return line to the tank, in the sending unit, should not be so long that it is down in the gas.

Did you over fill the gas tank?

[76C10Stepside]

Originally Posted by ChevyTech

Are you parking the truck on a hill, with the front end down hill?

The fuel return line to the tank, in the sending unit, should not be so long that it is down in the gas.

Did you over fill the gas tank?

No, it's parked head-in in my driveway, which has a very slight uphill tilt, and the front of the truck is on jack stands! It had less than a quarter tank BEFORE I disconnected the fuel line and 2-1/2 gallons drained out of the ENGINE side. The end of the fuel line that goes to the gas tank is PLUGGED OFF. It's the other side that all the gas is coming from.

It just doesn't make sense. Doesn't the return line come in at the TOP of the gas tank? How could it even get so full that gas goes UP the return line? Anyway, how could it get past the fuel pressure regulator and go DOWN the supply line? Last time I checked, the gas can had another gallon of gas in it. I will check it again today and see if it finally stopped. If not, I guess I'll start selling gas! LOL

The only other thing I can think of is that it must have a non-vented cap, because when it's low on gas, there is a lot of suction when I take the cap off to add gas to the tank. Is that normal for these? It's had the same locking cap on it for years.

[76C10Stepside]

I just checked the gas can, and it had close to 1-1/2 gallons of gas in it. I removed the can, and gas started dribbling out of the hose again at a constant rate. Put a coffee can down to catch it.

I unscrewed the gas cap part way, and a lot of air rushed in. The dribble of gas stopped! For how long, I don't know.

Took a picture showing the rubber hose I'm using to divert the gas into the can where it connects to the fuel line leading to the engine/TBI, but it did not come out. But, I assure you, it's connected to the FRONT end of the fuel line (that goes to the TBI), NOT the back end that comes from the tank. The BACK end is clamped off with a c-clamp and plugged with a tapered dowel, and is not leaking at all (no sucking sounds either).

All this gas MUST be siphoning out of the tank, through the return line??? Does the Fuel Pressure Regulator permit gas to flow BACKWARDS through it from the return line if there is positive pressure? How could the gas even get from the tank to the return line, which I assume is at/near the TOP of the gas tank?

Is this truck supposed to have a VENTING gas cap, OR a NON-VENTING?

[ChevyTech]

Gas caps on today’s automobiles are not vented. It has been that way for decades. The venting is through the charcoal canister.

My guess is you are miss-understanding or miss-stating what line the gas is coming from or which line is which, because it does not make sense, unless someone has modified the fuel sending unit in the gas tank.

Some TBI units do have a bleed hole so gas can bypass the regulator at a slow rate. This would explain the gas getting by the regulator but how it would get up to the return tube in the tank makes no sense.

[76C10Stepside]

Originally Posted by ChevyTech

Gas caps on today’s automobiles are not vented. It has been that way for decades. The venting is through the charcoal canister.

My guess is you are miss-understanding or miss-stating what line the gas is coming from or which line is which, because it does not make sense, unless someone has modified the fuel sending unit in the gas tank.

Some TBI units do have a bleed hole so gas can bypass the regulator at a slow rate. This would explain the gas getting by the regulator but how it would get up to the return tube in the tank makes no sense.

I knew you were going to say that, because just like you said, what is happening DOESN'T make sense! The line that is leaking is the one that has the fuel filter on it, and it's leaking from the fuel filter (front) side.

Thanks for confirming that the gas cap SHOULD be non-venting (just making sure) and explaining that the venting is through the charcoal cannister. I'd forgotten about that. What if the line to the cannister got plugged up? I'll check that ASAP.

I'd like to know more about this weep hole--must have a check valve of some sort, right?--and to confirm that my Model 220 TBI has one. When I dismantle the TBI again, I will look for it, and inspect the new FPR to make sure the diaphragm is intact. Not sure how to test the diaphragm outside the unit, though. It COULD be leaking in the middle where the steel plunger is. I was amazed how flimsy these are, just two thin layers of red rubbery stuff, whatever it is.

I suppose that since somebody altered the fuel line by cutting in a piece of hose, they might also have done other "stuff" to the gas tank. Looks like I'm gonna have to drop the tank, pull the sender and take a look inside and "see what I can see". This will have to wait at least a week.

The only other thing I can think of is if they swapped the fuel lines somewhere in the middle, which seems darn near impossible. As you said, the return line is SMALLER than the supply line, not to mention the fact that these are HARD LINES and resist moving around. I could barely move them where they connect to the TBI, and I've already verified that they are connect properly to the throttle body.

Will report back later. Thanks!

[ChevyTech]

Bleed Passage:
Some TBI units have a hole (Bleed passage) to release the fuel pressure when the pump stops and some TBI units hold the pressure after the engine is shut off. I have tried to find the criteria for which units have the bleed hole and which don’t but have never been able to find GM information on this.

I will see if I have photos of this, but not today. I have more projects then time.