89 S10 4.3 Need help troubleshooting TBI/IAC Idle issue

[76C10Stepside]

I could use some help troubleshooting a problem with my '89 S10 4.3 with model 220 throttle body injection. Engine has been running a little lean for the last year, idles a little funny. Suddenly started running very poorly, but no trouble codes. Rebuilt the TBI. It ran a lot better, but would stall suddently, or not want to start after being run. Several hours later, it starts right up, like it was flooded, but there is no strong gas smell. At idle, there is enough gas being sprayed to coat the butterfly valves, does not look like a lot to me. What does a normal spray pattern look like at idle?

Just replaced the fuel filter (gas looked clean). It's still not running right. Idles too fast, 2000 rpm when started cold, revs go up and down for awhile, then down to 1500 for awhile, then 1000, and finally 750, but is not idling smoothly.

Followed Step 10 in "Troubleshooting your TBI Fuel Injection System" (http://www.73-87chevytrucks.com/tech...TBISystems.pdf) for when engine is revving up and down. I don't hear any sucking sounds, no loose vacuum lines. Again, no trouble codes. With key on, I hear a "clacking sound that I don't remember ever hearing before. When I pull the Idle Air Control (IAC) plug, the clacking stops, so it was the IAC (is that normal?). Then remove the jumper from ALDL connector (key still on), turn off for at least 15 seconds, and restart the engine. The engine refused to start. I sort of got it started by pumping the gas pedal, but it would not run. Not sure what to make of this. Plugged IAC back in, but engine still won't start/run. Again, no strong gas smell, but acts like it's flooded.

Another weird thing that started happening today is the oil pressure reading is very low at idle. The "oil can" even started flashing for a bit. Engine has a fresh oil/filter change and is topped up. This truck has the digital dash with the led bar readouts, and has been replaced once. Sometimes it acts up a little, but turning off and back on makes it work right again. This oil pressure thing kept up for numerous on/off while testing.

The only other thing is one of the rubber insulator things that go between the injector and the plug is missing, pretty sure it was missing when I got the truck, does not seem to affect running.

Not sure what to do next. Suggestions?

[ChevyTech]

Check out these threads. Then post your thoughts.
I would check for problems in the order I posted the links.

I would think you replaced the base gasket under the TBI when you rebuilt the TBI, but if you didn’t you should of, it is a common spot for a vacuum leak.

High idle thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=439984

Coolant sensor testing specifications and instructions:
http://67-72chevytrucks.com/vboard/s...d.php?t=379142

Fuel pump testing information; truck had split hose in tank-
http://67-72chevytrucks.com/vboard/s...d.php?t=375132

[76C10Stepside]

Thanks for the suggestions and links (had already read the IAC thread).

Yes, I did a full rebuild on the TBI using a kit for Rochester model 100, 200, 220 and 295 TBIs. Kit included new gaskets, injector filters and o-rings, and new pressure regulator. Kit did not include a new regulator dust seal. I tested the injectors using a D-cell battery (good). The kit diagram and instructions did NOT mention the idle air control (IAC)! IIRC, I removed it before cleaning the throttle body and installed a new paper gasket.

The symptom before I rebuilt the TBI was that at idle, the left bore was dry, and the right bore had a film of gas. When I tore it down, the only sign of dirt/debris was what looked like engine oil in the fuel meter body. I assumed this was oil vented from the crankcase.

Now, the IAC is making clicking sounds when ignition is turned on (engine not running and running). Does this sound like a dirty IAC?

The brakes work fine, so pretty sure brake booster diaphragm is not leaking.

Also tested the fan clutch recently (good), but it is engaged for cold starts and most of the time in general, only rarely disengages.

Coolant sensor: The temp reading has always stayed right on the normal mark, and still does, so i think it's good. Do you agree?

Fuel pump: After rebuilding the TBI, truck ran great on the test runs, great throttle response, and made good power (better than before).

Just went out and tried starting the engine again, and it fired right up, ran to 1800 rpm for 20-30 seconds, then down to 1200. The fact that it starts right up after sitting and today's very low oil pressure sounds to me like it's flooding. Do you agree?

Originally Posted by ChevyTech

Check out these threads. Then post your thoughts.
I would check for problems in the order I posted the links.

I would think you replaced the base gasket under the TBI when you rebuilt the TBI, but if you didn’t you should of, it is a common spot for a vacuum leak.

High idle thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=439984

Coolant sensor testing specifications and instructions:
http://67-72chevytrucks.com/vboard/s...d.php?t=379142

Fuel pump testing information; truck had split hose in tank-
http://67-72chevytrucks.com/vboard/s...d.php?t=375132

[95 S_Trucker]

Originally Posted by 76C10Stepside

Thanks for the suggestions and links (had already read the IAC thread).

Yes, I did a full rebuild on the TBI using a kit for Rochester model 100, 200, 220 and 295 TBIs. Kit included new gaskets, injector filters and o-rings, and new pressure regulator. Kit did not include a new regulator dust seal. I tested the injectors using a D-cell battery (good). The kit diagram and instructions did NOT mention the idle air control (IAC)! IIRC, I removed it before cleaning the throttle body and installed a new paper gasket.

The symptom before I rebuilt the TBI was that at idle, the left bore was dry, and the right bore had a film of gas. When I tore it down, the only sign of dirt/debris was what looked like engine oil in the fuel meter body. I assumed this was oil vented from the crankcase.

Now, the IAC is making clicking sounds when ignition is turned on (engine not running and running). Does this sound like a dirty IAC?

The brakes work fine, so pretty sure brake booster diaphragm is not leaking.

Also tested the fan clutch recently (good), but it is engaged for cold starts and most of the time in general, only rarely disengages.

Coolant sensor: The temp reading has always stayed right on the normal mark, and still does, so i think it's good. Do you agree?
Fuel pump: After rebuilding the TBI, truck ran great on the test runs, great throttle response, and made good power (better than before).

Just went out and tried starting the engine again, and it fired right up, ran to 1800 rpm for 20-30 seconds, then down to 1200. The fact that it starts right up after sitting and today's very low oil pressure sounds to me like it's flooding. Do you agree?

There are 2 coolant temp sensors. 1 for the gauge(on the drivers side cyl head.) and 1 For the PCM/ECM(close to the thermostat housing)

[ChevyTech]

Coolant sensor: The temp reading has always stayed right on the normal mark, and still does, so i think it's good. Do you agree?

No
As trucker stated there is a separate sensor for the gauge and a different sensor for the coolant temp reading that the computer uses.

Just went out and tried starting the engine again, and it fired right up, ran to 1800 rpm for 20-30 seconds, then down to 1200. The fact that it starts right up after sitting and today's very low oil pressure sounds to me like it's flooding. Do you agree?

All I get from this is that it sounds like the IAC still works at least some.

Now, the IAC is making clicking sounds when ignition is turned on (engine not running and running). Does this sound like a dirty IAC?

I there is any difference, a dirty IAC is more quite.
After the engine is shut off, and before it is started, the IAC should open somewhat. This makes noise. The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position. If the IAC does not open at all, the engine will usually “load up” or “flood”.

When you replace, remove and clean an IAC, or unplugged the IAC with the key on, the ECM may need to relearn the IAC position. This will not happen without driving the truck and meeting specific conditions or taking other steps the cause it to relearn the idle.

Fuel pump: After rebuilding the TBI, truck ran great on the test runs, great throttle response, and made good power (better than before).

The only way to know the fuel pressure is good is to test it. TBI trucks have no test port, which was a poor design choice by the Penney pinchers at General Motors.
The most noticeable symptom of low fuel pressure on TBI trucks is poor idle quality. The most common comment from an owner after a fuel pressure problem is fixed is "it has so much more power now".

[76C10Stepside]

Originally Posted by 95 S_Trucker

There are 2 coolant temp sensors. 1 for the gauge(on the drivers side cyl head.) and 1 For the PCM/ECM(close to the thermostat housing)

OK, thanks. Do you think this has anything to do with my problem?

==> Can anyone tell me if I should be able to see gas in the throttle bores at idle?

If not, it's flooding, but I don't know WHY. The pressure regulator does not leak gas when engine is not running.

[ChevyTech]

Two of us (at least) think it could be a temperature sensor problem and that is why we mentioned it.

It is very common for the temp sensor to read the wrong resistance. You should test it.

When the engine is running there should always be fuel getting sprayed in the throttle bores. It should look equal on both sides. No fuel should drip out the injectors once the engine is turned off.

[speedygonzales]

Not being familiar with non-vortec 4.3's I have to ask if the FPR may be leaking? The vortec ones are notorious for that. Gives you poor performance and bad idle quality and it can effect the oil.

Does the oil smell like fuel or did it ever before?

Fuel contaminated oil would also give you the intermittent "oil can" light you spoke of earlier.

But again back to what someone said earlier to check the fuel pressure is the key to several questions.

[95 S_Trucker]

Originally Posted by speedygonzales

Not being familiar with non-vortec 4.3's I have to ask if the FPR may be leaking? The vortec ones are notorious for that. Gives you poor performance and bad idle quality and it can effect the oil.

Does the oil smell like fuel or did it ever before?

Fuel contaminated oil would also give you the intermittent "oil can" light you spoke of earlier.

But again back to what someone said earlier to check the fuel pressure is the key to several questions.

The TBI fuel pressure regulator diaphragm is replaced when the TBI is rebuilt

This is what it looks like

Fuel On one side, Spring pressure and Vacuum on the other

[76C10Stepside]

I'd like to thank all who have offered me help so far!!!

Just confirmed that the oil is contaminated with gas. I could actually SEE gas on the dipstick, too! Dang, just changed the oil and filter a couple weeks ago. Since my problem is still not fixed, I don't want to change the oil/filter until I fix the cause (fingers crossed).

I installed a new FPR when I rebuilt the TBI. When I turn the key on, I hear the fuel pump cycle the normal amount, then total silence. When I replaced the fuel filter, there was still a TON of pressure in the lines. So the FPR is probably not leaking, right?

So, I guess testing the coolant temp sensor that feeds the computer is my next task.
Problem: Don't have a shop manual for this truck! So, here come the dumb questions...

Where is the sensor on this engine (4.3 Vortec with non-Vortec intake)?

Can I test it without removing it?

If I have to remove it, do I need to drain the system first?

How do I test it?

[ChevyTech]

Originally Posted by 76C10Stepside

I installed a new FPR when I rebuilt the TBI. When I turn the key on, I hear the fuel pump cycle the normal amount, then total silence. When I replaced the fuel filter, there was still a TON of pressure in the lines. So the FPR is probably not leaking, right?

Turn the key on, to built pressure, without cranking the engine, and look at the TBI unit to make sure no fuel is leaking from the injectors, regulator, or gaskets.

Originally Posted by 76C10Stepside

So, I guess testing the coolant temp sensor that feeds the computer is my next task.
Problem: Don't have a shop manual for this truck! So, here come the dumb questions...

I posted a link.

Originally Posted by Chevytech

Coolant sensor testing specifications and instructions:
http://67-72chevytrucks.com/vboard/s...d.php?t=379142

Here is another link:
http://67-72chevytrucks.com/vboard/s...25&postcount=7

You could also search my username and coolant sensor.

Originally Posted by 76C10Stepside

Where is the sensor on this engine (4.3 Vortec with non-Vortec intake)??

On the front of the intake by the thermostat housing.

Originally Posted by 76C10Stepside

Can I test it without removing it?

Yes

Originally Posted by 76C10Stepside

How do I test it?

With an ohm meter.
The instructions are in the links posted.

Does anyone look at the links I post???????

[76C10Stepside]

Thanks, ChevyTech! Yes, I read the links. I'm just clueless, lol! ;-}
But seriously, hot weather and the worst allergies I've ever had are making it real hard to think/concentrate.

I found the post where you list the temp sensor approximate resistance for various temps. It's noon and 76 degrees F outside. I went out and found the temp sensor on the right cylinder head. Set my DVM to the 20000 ohm range and got .004. This looks to be about right, given the ambient temperature and using the closest two resistance value from your list:
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
There is no sign of corrosion or leaking on/around the sensor.

Then I turned the ignition on and checked for leaks and found none. The entire area in and around the TBI is totally dry and no sounds to indicate a leak inside.

What should I do next? I don't want to run the engine if I don't have to.

[ChevyTech]

Originally Posted by 76C10Stepside

I went out and found the temp sensor on the right cylinder head.

No, No!, NOOOOOOOOOO
That is not the sensor you should be testing!!!!!

Originally Posted by 76C10Stepside

Where is the sensor on this engine (4.3 Vortec with non-Vortec intake)??

Originally Posted by Chevytech

On the front of the intake by the thermostat housing.

You better stop taking those allergy meds!

[76C10Stepside]

According to my phone, it's 76 degrees F outside. The engine is COLD. My DVM is set to the 20k ohm range. The sensor on front of intake (for ECM) reads 005. For comparison, the OTHER sensor (for temp guage) reads 004.

What's the verdict? Is my ECT sensor BAD?

[76C10Stepside]

Please ignore my last post and read this one (ran out of time to edit it)! I had the DVOM set to the wrong setting. Here are the CORRECT readings for both sensors (to see if they agree), both with outside temp of 76 degrees F:

ECT sensor: 3900 ohms
GUAGE sensor: 3670 ohms

The actual readouts were 3.90 and 3.67. I did the conversion to ohms on a scientific calculator by entering the readout value (3.90), then pressing the "Exp" button followed by the "3" button.

Your list of approximate sensor resistance specs says I should get 2796 ohms at 77 degrees F: 2796 ohms @ 77 deg. F. or 25 deg. C.

I'm not sure what this means. Are BOTH sensors bad? Did I do something wrong?

[ChevyTech]

The engine will not warn up as quickly as the air does during the day.

The sensor readings look close enough.

Another test you can do is run the engine up to operating temperature (195), shut it off, and see what the resistance is when warm.

My guess is, the sensor is not the problem.

[76C10Stepside]

Originally Posted by ChevyTech

The engine will not warn up as quickly as the air does during the day.

The sensor readings look close enough.

Another test you can do is run the engine up to operating temperature (195), shut it off, and see what the resistance is when warm.

My guess is, the sensor is not the problem.

Thanks, ChevyTech. I was hoping all I needed was a new ECT sensor.
I would test the sensor with the engine at operating temp, but I have a crankcase full of gasoline. If I change the oil/filter now (so I can get it up to operating temp), it's just going to dump more gas in the sump. I don't have a thermometer or temp tester, or I would remove the sensor and test it in a pot of hot water on the stove. What would you do now if you were in my shoes?

[ChevyTech]

The sensor is showing a resistance that is not far off. The engine should start and run they way it is supposed to with the reading you got from the sensor.

If the crankcase is getting fuel in it, there is another problem. Could the fuel have gotten into the crankcase before you did the TBI work and maybe you already fixed the problem like fuel leaking from the TBI?

The sensor can be good, but if the wiring to it is bad, the computer will get bad data.

Resistance on the wire, or wire connections, will cause the computer to think it is colder, so more fuel gets injected.

If the ground wires have resistance the same thing can happen. The most important grounds are the ground wires at the front of the intake near the thermostat or manifold studs.

The TBI trucks don't leak fuel into the oil like the central port injected trucks do. I would change the oil, because if that much fuel is getting in the oil, it should be obvious if you look at the TBI with it running.

[76C10Stepside]

Originally Posted by ChevyTech

The sensor is showing a resistance that is not far off. The engine should start and run they way it is supposed to with the reading you got from the sensor.

It's been one weird thing after another. It ran great for the last year, then all of a sudden...

Originally Posted by ChevyTech

If the crankcase is getting fuel in it, there is another problem. Could the fuel have gotten into the crankcase before you did the TBI work and maybe you already fixed the problem like fuel leaking from the TBI?

Definitely not. Or, maybe I should say, "not to my knowledge". :-\

After rebuilding the TBI, it started right up and ran GREAT. I was on the way to get gas, and it died about a mile down the road. Thought it was empty, but when I added 2 gallons of gas, it would not start. Came back next morning and it fired right up, but died again as I was backing into my parking spot at home. I didn't even smell gas until I went to warm it up to run tests. This is when the crankcase started filling with gas, and the oil pressure dropped WAY down. I was worried, about that, but let it run for awhile, trying to get it up to operating temp.

Originally Posted by ChevyTech

The sensor can be good, but if the wiring to it is bad, the computer will get bad data.

This seems unlikely on this particular truck. It was well cared for all its life (except for being parked outside). All the wires and connectors look great, no corrosion anywhere.

Originally Posted by ChevyTech

Resistance on the wire, or wire connections, will cause the computer to think it is colder, so more fuel gets injected.

If the ground wires have resistance the same thing c an happen. The most important grounds are the ground wires at the front of the intake near the thermostat or manifold studs.

That's one I didn't know and haven't checked yet. Will check these.

I replaced the starter and the battery right after rebuilding the TBI, because I couldn't get it to start and the battery tested "good" (but it was bad), so I replaced the starter first (it was an old rebuilt and was rusty anyway). I also removed and cleaned the battery ground wire at the engine (it was fine).

Originally Posted by ChevyTech

The TBI trucks don't leak fuel into the oil like the central port injected trucks do. I would change the oil, because if that much fuel is getting in the oil, it should be obvious if you look at the TBI with it running.

Good to know that the TBIs don't leak that way!

Yes, you would think it would be obvious, but it's not and I don't know why! This is why I asked how much gas I should be seeing with engine idling. BEFORE I rebuilt the TBI, I could see a thin coating of gas on one butterfly, and the other one looked DRY. At that time, the truck was running too lean and never flooded, EVER.

After rebuilding the TBI, there was a thin coating of gas on the bores and butterfly valves, but I could not see the injectors actually SPRAY gas while engine was idling. Note I said "idling". Did not check for spray at higher RPM because all hell broke loose before I could get to that.

Thanks for not giving up on me yet! I really need this truck back on the road. My other driver got wrecked last month (not my fault).

Would it help to take pictures and post them? Should I test the Idle Air Control? Is it time to dismantle the TBI again? Should I get the injectors tested by a qualified shop? Seems like the ONLY possibilities are a) Injectors are dumping too much gas in the engine (reason still TBD), or b) Gas is LEAKING into the engine from somewhere (besides the injectors).

When I replaced the fuel filter, I didn't cycle the fuel pump to relieve the pressure. First I disconnected the fuel hose between tank and filter. Then I disconnected the incoming fuel line from the TBI, and not much more gas dribbled out. Then I started disconnecting the outgoing fuel line (return line for when there's too much pressure), and it just kept on leaking and leaking gas. I finally gave up and tightened it back up again. Does this tell you anything? Is there any way that a plugged up return line could result in gas being dumped into the engine?

[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!