87 TBI to Carb conversion

[rfmaster]

Originally Posted by cyberpine

I don't see any difference in that chamber or the hole deep inside in any combination - off, on, running, unpluged. But I do hear a zipping sound when I turn the key to start position.

Always looks like this.

It looks like it is fully retracted - this will result in very high idle!.

To verify, install A+B jumper, turn ignition key into run position but do not start. You should be able to hear a continuous clicking noise from IAC. Disconnect IAC connector, turn off ignition. Take a look - you should be able to see a pentacle head protruding into the chamber. If not - you may have bad IAC, carbon build-up or a bad connection between IAC and harness connector.

//RF

[cyberpine]

I lost a great video of me sticking my finger in there and it whistling like crazy. The hissing sound that I've been hearing is definitley coming from that spot. Also, as suggested I plugged a&b of the ECM and placed in start position and you can hear it trying to extend the IAC constantly.. it sounded like a stuck dc motor.

I have the valve out - from the tip to the base the cylinder is out maybe like 3/4 of an inch. But it's does not manually move. Any way to test it? fix it?

Question, does the head penetrate fully inside that chamber? Is that the open or closed position when it does? Right now(as you see it) now the head appears to be hidden inside the hole - how is that also open? It's unclear what's it's closing.

Factory oem is about $60 + shipping.




Would you recommend this replacement part. $16 with shipping included!

http://www.thepartsbin.com/catalog/?...358+4294963240




Thanks.

[rfmaster]

Originally Posted by cyberpine

I lost a great video of me sticking my finger in there and it whistling like crazy. The hissing sound that I've been hearing is definitley coming from that spot. Also, as suggested I plugged a&b of the ECM and placed in start position and you can hear it trying to extend the IAC constantly.. it sounded like a stuck dc motor.

I have the valve out - from the tip to the base the cylinder is out maybe like 3/4 of an inch. But it's does not manually move. Any way to test it? fix it?

Question, does the head penetrate fully inside that chamber? Is that the open or closed position when it does? Right now(as you see it) now the head appears to be hidden inside the hole - how is that also open? It's unclear what's it's closing.



Would you recommend this replacement part. $16 with shipping included!



Thanks.

JC

IAC is a dual coil bipolar stepper motor. You can look up about stepper motors here

Dealers used to have special test fixture that allowed to test them. Under no circumstances try to operate IAC unbolted from TB. The stepper will eject pentacle - you can not re-insert. If it makes continuous noise when A&B are shorted ECM is commanding IAC to close IA passage. There should be NO air going through. Over time carbon, oil deposits may build up and prevent IAC from closing off this passage. Carb cleaner and good brushing job will remove stubborn deposits. The head should completely close IA passage - amount of protrusion depends on head design.

When you shut off ignition ECM commands IAC to fully open position - wide open IA passage. When A&B are shorted ECM forces IAC to full close. You can disconnect IAC connector - and measure the extension of the pentacle. Re-install, re-connect - and allow ECM to move IAC into full open and repeat measurement. It is a poor mans functional check for IAC motor without resorting to parts swapping.

$16 it is a cheap verification of IAC functionality (parts swapping). In my case I resort to a large collection of used units from my JY trips.

//RF

[cyberpine]

So I just changed the IAC value. No real difference and I still don't see the pinnacle coming through. Truck stalls and heavy smoke and gas smell. I also hear that IAC motor trying to do something with a constant slgiht buzz with the key on start and and ECM A&B shorted.

I have not checked my timing or my RPMs at idle.

Here's the thing thing though, as before, when I disconnect the MAP sensor and that temp sensor it idles a little high, but it runs incredibly strong. I would say there is no way it could run like that if my timing was terribly off.

You may have answered this (need to go back), what's the worse that could happen from running the truck this way (with those sensors disconnected).. high fuel consumption? overheating at idle?

Remembering something the first mechanic told me.. he said he tried to get the timing on the truck but could not explaining the constant fluctuation I'm seeing with the sensors on.

Thanks.

[rfmaster]

Originally Posted by cyberpine

So I just changed the IAC value. No real difference and I still don't see the pinnacle coming through. Truck stalls and heavy smoke and gas smell. I also hear that IAC motor trying to do something with a constant slgiht buzz with the key on start and and ECM A&B shorted.

I have not checked my timing or my RPMs at idle.

Here's the thing thing though, as before, when I disconnect the MAP sensor and that temp sensor it idles a little high, but it runs incredibly strong. I would say there is no way it could run like that if my timing was terribly off.

You may have answered this (need to go back), what's the worse that could happen from running the truck this way (with those sensors disconnected).. high fuel consumption? overheating at idle?

Remembering something the first mechanic told me.. he said he tried to get the timing on the truck but could not explaining the constant fluctuation I'm seeing with the sensors on.

Thanks.

JC

TBI system was designed to use all sensors (TPS, CTS, MAP, VSS, KS, EST) and actuators (IAC, EGRCV, Injectors) for normal operation. Disconnecting them puts ECM in soft fail mode where it reverts to fixed value for a disconnected sensor - enough to get you pull over and limp over to a service bay. Disconnecting and otherwise rendering sensors inoperative will not get this problem resolved. Drivability and performance are compromised.

IAC appears to be functioning. I went to my local parts store and pulled a new TBI replacement IAC and compared to OE GM. OE unit tends to have longer - conical head. This would explain why I can see it from the top. I'll have to assume that it is functioning based on you report - it is clicking and that's what it suppose to do in diagnostic ALDL mode (A+B).

Timing - You must check and set correct timing. It is a very common mistake to incorrectly set timing on TBI systems with end results similar to yours.


How's bidding on Evil Bay?
//RF

[cyberpine]

Originally Posted by rfmaster

How's bidding on Evil Bay?
//RF

I should have all the tools to continue by next weekend.

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[wheels67]

Originally Posted by cyberpine

So I just changed the IAC value. No real difference and I still don't see the pinnacle coming through. Truck stalls and heavy smoke and gas smell. I also hear that IAC motor trying to do something with a constant slgiht buzz with the key on start and and ECM A&B shorted.

I have not checked my timing or my RPMs at idle.

Here's the thing thing though, as before, when I disconnect the MAP sensor and that temp sensor it idles a little high, but it runs incredibly strong. I would say there is no way it could run like that if my timing was terribly off.

You may have answered this (need to go back), what's the worse that could happen from running the truck this way (with those sensors disconnected).. high fuel consumption? overheating at idle?

Remembering something the first mechanic told me.. he said he tried to get the timing on the truck but could not explaining the constant fluctuation I'm seeing with the sensors on.

Thanks.

Time to trade it in for a 70,s model with a carb .........lol. Learning new things can be frustrating but were always better to go thru it. Theres some good repair manuals on computer codes and sensors from chilton or haynes, I would study it and enjoy the adventure if i were you.

Props to rf for his time, im sure alot of people have learned some info.

[rfmaster]

Originally Posted by wheels67

Time to trade it in for a 70,s model with a carb .........lol. Learning new things can be frustrating but were always better to go thru it. Theres some good repair manuals on computer codes and sensors from chilton or haynes, I would study it and enjoy the adventure if i were you.

Props to rf for his time, im sure alot of people have learned some info.

That's how I started with my 75 - trying to get overall performance from a carb. But I got sick and tired of waiting for stupid thing to warm up early in the morning roaring at 2k RPM for about 5 min! With Delta 40 out in the back I was not making friends with my neighbors. Truck never performed up to my expectations until I switched to EFI - fuel consumption when down and it is now a joy to drive.

Both Chiltons and Hanes do not go into fine system details. Only factory service manuals (FSM) have detailed troubleshooting trees. These are very detailed, but do not (always) list sensor parameters which can be found else where.
For extensive details TBI component information, TBI swaps, into early trucks please see:

http://67-72chevytrucks.com/vboard/s...d.php?t=317519


//RF