IAC shows fault after running a few minutes

[rsicard]

Running a 350 Vortec with 454 throttle body on engine stand. There appears to be another item that needs fixing. After some number of minutes running, an IAC fault is set. The pintle appears to be nearly fully extended position. Don't know how the 16197427 $0D ECM sets this fault or what is setting it. After shutting down, the pintle appears to be fully retracted. Need suggestions as to what to do for this IAC situation.

[Nasty-Z]

Running a 350 Vortec with 454 throttle body on engine stand. There appears to be another item that needs fixing. After some number of minutes running, an IAC fault is set. The pintle appears to be nearly fully extended position. Don't know how the 16197427 $0D ECM sets this fault or what is setting it. After shutting down, the pintle appears to be fully retracted. Need suggestions as to what to do for this IAC situation.

I know you had mentioned in the past this was a Howell Engineering harness that was made to work with the 454TBI unit , but I have to ask again , did you verify the correct pinout of the 454 IAC valve in relation to the PCM ? By that I mean verifying that all 4 wires to and from the IAC valve leave the correct pin at the valve and terminate at he correct pin of the PCM.

With all of the trouble you are having getting this thing to idle and now setting an IAC code it might be worth a shot to verify.

HTH

TOM

[rsicard]

I know you had mentioned in the past this was a Howell Engineering harness that was made to work with the 454TBI unit , but I have to ask again , did you verify the correct pinout of the 454 IAC valve in relation to the PCM ? By that I mean verifying that all 4 wires to and from the IAC valve leave the correct pin at the valve and terminate at he correct pin of the PCM.

With all of the trouble you are having getting this thing to idle and now setting an IAC code it might be worth a shot to verify.

HTH

TOM

First replaced the IAC motor. Second, checked continuity wire for wire on the 4 pin IAC connector to the ECM. That was found satisfactory. The pintle appears to be fully seated after warmup. After shutdown, appears to retract fully. Don't know what the mechanism would be for the fault.

[tayto]

I'm not sure what BB TBI you are using. One uses the old style "square" connector and the newer style is the "inline" or "flat" connector. I believe I found a late 90s F-body diagram and compared it to my TBI Caprice (EBL upgrade). Don't match colour for colour, just look at the IAC A Lo, IAC A Hi, IAC B Lo, IAC B Hi and pin it accordingly.

[JeepsAndGuns]

Do this test. Unhook the battery to clear the memory in the ecm. Now hook it back up, turn on the ignition but do not start the engine. Let it set for a few seconds. Now turn the ignition off while watching the iac. It should drive itself all the way closed, then open back up slightly.

[Nasty-Z]

I'm not sure what BB TBI you are using. One uses the old style "square" connector and the newer style is the "inline" or "flat" connector.

All of the production light duty BBC TBI's (87-95) use the flat 4 pin connector (Same connector as the 4 pin plug on the ICM) , there was never a production light duty BBC TBI unit with anything other than the 4 pin flat plug.

I posted the correct wiring for the flat 4 pin BBC TBI in one of the OP's other threads.

First replaced the IAC motor. Second, checked continuity wire for wire on the 4 pin IAC connector to the ECM. That was found satisfactory. The pintle appears to be fully seated after warmup. After shutdown, appears to retract fully. Don't know what the mechanism would be for the fault.

What code are you getting ? Code number ? I have had idle problems in the past because of incorrect wiring to the IAC , but never an IAC related code .

TOM

[rsicard]

I'm not sure what BB TBI you are using. One uses the old style "square" connector and the newer style is the "inline" or "flat" connector. I believe I found a late 90s F-body diagram and compared it to my TBI Caprice (EBL upgrade). Don't match colour for colour, just look at the IAC A Lo, IAC A Hi, IAC B Lo, IAC B Hi and pin it accordingly.

The throttle body has the inline oval connector. The square connector is used on the 350 throttle body. The wiring checks out wire for wire.

[rsicard]

Do this test. Unhook the battery to clear the memory in the ecm. Now hook it back up, turn on the ignition but do not start the engine. Let it set for a few seconds. Now turn the ignition off while watching the iac. It should drive itself all the way closed, then open back up slightly.

Will try this procedure to see if it operates properly.

[rsicard]

All of the production light duty BBC TBI's (87-95) use the flat 4 pin connector (Same connector as the 4 pin plug on the ICM) , there was never a production light duty BBC TBI unit with anything other than the 4 pin flat plug.

I posted the correct wiring for the flat 4 pin BBC TBI in one of the OP's other threads.



What code are you getting ? Code number ? I have had idle problems in the past because of incorrect wiring to the IAC , but never an IAC related code .

TOM

Will have to replay the recording and note the code number set and get back with a reply. Just wondering, is there something in software that needs to be modified? Please advise. Thanks.

[Nasty-Z]

Just wondering, is there something in software that needs to be modified? Please advise. Thanks.

No , as long as the wiring is correct , the PCM has no idea nor does it care if it is a SBC IAC or a BBC IAC.

TOM

[sturgillbd]

His IAC is bottoming out and his idle rpm is still above the commanded idle in the bin. The PCM is sensing this and setting the IAC failure code. Either the minimum throttle blade adjustment is off or it is sucking air somewhere. One way to see if it is the minimum throttle blade adjustment is to let the throttle blades close completely and see if the IAC can control the idle. It wont be great for mixture but you should see IAC counts. It may help if the idle is raised from default in the bin.

[fastacton]

His IAC is bottoming out and his idle rpm is still above the commanded idle in the bin. The PCM is sensing this and setting the IAC failure code. Either the minimum throttle blade adjustment is off or it is sucking air somewhere. One way to see if it is the minimum throttle blade adjustment is to let the throttle blades close completely and see if the IAC can control the idle. It wont be great for mixture but you should see IAC counts. It may help if the idle is raised from default in the bin.

rsicard, sturgillbd has a good point. If this is the case, you'll need to drill out the cover on your throttle stop screw and bring it down some. I had to do this on my 454 TB Vortec headed 350 recently.

[rsicard]

His IAC is bottoming out and his idle rpm is still above the commanded idle in the bin. The PCM is sensing this and setting the IAC failure code. Either the minimum throttle blade adjustment is off or it is sucking air somewhere. One way to see if it is the minimum throttle blade adjustment is to let the throttle blades close completely and see if the IAC can control the idle. It wont be great for mixture but you should see IAC counts. It may help if the idle is raised from default in the bin.

Thanks again for chiming in. Don't feel comfortable trying to modify the throttle plates so they can close further. Will temp remove the TPS sensor to see if it is restricting the close position. Otherwise will bump up the desired idle RPM in the binary file via TunerPro. Just thought about it, maybe something can be done with the IAC pintle to provide a better seal against the TB seat. Sealed the 4 barrel adapter and TB gasket as best as possible with Loctite sealer. May smear some sealer on the pintle, and let it dry, to see if that affect the IAC fail situation. Otherwise, the engine is running very well. Taking out some fuel via VE, was the proper thing to do. Now the BLM is above 128 so it is on the lean side. That can be fixed with some additional VE tuning. Need to practice data tracing to figure out which series of cells need to be adjusted. Then smooth from there. Repeat. Repeat till comes near 128 plus or minus 5-8? Need to turn on the other three. Can switch out throttle body for 350 type, yet need to adapt the TPS and IAC connectors. One thing at a time.

[fastacton]

You wouldn't be modifying the throttle plates, just the plug on the front of the throttle body so you can adjust the throttle stop. The attached photo shows what I'm talking about. It's not very likely that the TPS is holding open the throttle any. You also don't want to tune it so that you're at 0 counts on the IAC, you want some wiggle room so the ECM can do it's job properly. In my opinion, you're better off with the 454 throttle body, I've tried the 350 TB on my engine and it couldn't provide the air or fuel needed to get the most out of it.
Attachment 8827

[rsicard]

You wouldn't be modifying the throttle plates, just the plug on the front of the throttle body so you can adjust the throttle stop. The attached photo shows what I'm talking about. It's not very likely that the TPS is holding open the throttle any. You also don't want to tune it so that you're at 0 counts on the IAC, you want some wiggle room so the ECM can do it's job properly. In my opinion, you're better off with the 454 throttle body, I've tried the 350 TB on my engine and it couldn't provide the air or fuel needed to get the most out of it.
Attachment 8827

Man, you hit the bullseye. Just went out and looked at the throttle body. Sure does have the plugged boss, under which is the idle adjust screw. Just have to remove the plug and back out the adjustment screw some to close the throttle plates a little more. Thanks ever so much for the attached photo as that really did the trick. Also found the mostly hidden idle screw. Thanks again. You folks rule.