Fast Idle Issue (Solved!!!)

N2UN

n00b
Jan 24, 2024
4
4
3
Hello everyone. New to the Forum. My car is a 1985 El Camino, 4.3L V6, three speed automatic 200 series. Has about 140k on the clock.

Having a problem with fast idle. Car starts easily but idles in Open Loop at about 1800 rpm. When it goes into Closed Loop it jumps to 2000 rpm and gradually ends at 2300 rpm when warm.

Car drives a quarter mile or so and dies, floods out. Starts again easily.

All sensors are new and functioning, using an Actron 9690 scanner to test.

Recent work done includes new exhaust and catalytic converter last fall (before the issue), and new TBI unit after the issue started.

Thinking a vacuum leak but haven't been able to find any. All vacuum lines are new, including brake booster line and check valve.

New fuel pump, tank, sending unit, and filter in 2022.

Any ideas?

Thanks, N2UN.
 
Last edited:

MrSony

Geezer
Nov 15, 2014
6,726
6,513
113
Des Moines, Iowa
check fuel pressure, might need to adjust the idle speed. im guessing the tbis come preset, but if they're anything like reman qjets theyre usually set wrong.
 
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stew86MCSS396

Greasemonkey
Aug 1, 2022
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Does your scanner rpm match the vehicles tach, well mine has a tach but it reads way too high as compared to having a scanner hooked up. One day I'll get those replacement resistors in there.

Any CEL codes being reported? I'd start there if any.

Sensor reading is good doesn't tell us anything. Can you post pictures of the scanned data or perhaps a datalog from the Actron?

Vacuum leak presumably would give you a high idle accompanied by high MAP reading.

What's the IAC reading? 0 would indicate a vacuum leak or the throttle blade adjusted too far open and the ECM slams IAC shut losing control of idle speed.

What's your TPS volts at idle? If it's too high >1.2v, the ECM won't control the IAC.

Is the new TBI an exact match or did you buy a Holley replacement? Granted that the problem surfaced before you changed it, Holley replacement units do present some issues with revised IAC passages and need to use specific vacuum source to the MAP sensor. Wrong sized injectors could present wrong fuel delivery.

Did you follow the procedure to set the minimum idle speed???
 
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N2UN

n00b
Jan 24, 2024
4
4
3
check fuel pressure, might need to adjust the idle speed. im guessing the tbis come preset, but if they're anything like reman qjets theyre usually set wrong.
Thanks for the reply.
Does your scanner rpm match the vehicles tach, well mine has a tach but it reads way too high as compared to having a scanner hooked up. One day I'll get those replacement resistors in there.

Any CEL codes being reported? I'd start there if any.

Sensor reading is good doesn't tell us anything. Can you post pictures of the scanned data or perhaps a datalog from the Actron?

Vacuum leak presumably would give you a high idle accompanied by high MAP reading.

What's the IAC reading? 0 would indicate a vacuum leak or the throttle blade adjusted too far open and the ECM slams IAC shut losing control of idle speed.

What's your TPS volts at idle? If it's too high >1.2v, the ECM won't control the IAC.

Is the new TBI an exact match or did you buy a Holley replacement? Granted that the problem surfaced before you changed it, Holley replacement units do present some issues with revised IAC passages and need to use specific vacuum source to the MAP sensor. Wrong sized injectors could present wrong fuel delivery.

Did you follow the procedure to set the minimum idle speed???

Does your scanner rpm match the vehicles tach, well mine has a tach but it reads way too high as compared to having a scanner hooked up. One day I'll get those replacement resistors in there.

Any CEL codes being reported? I'd start there if any.

Sensor reading is good doesn't tell us anything. Can you post pictures of the scanned data or perhaps a datalog from the Actron?

Vacuum leak presumably would give you a high idle accompanied by high MAP reading.

What's the IAC reading? 0 would indicate a vacuum leak or the throttle blade adjusted too far open and the ECM slams IAC shut losing control of idle speed.

What's your TPS volts at idle? If it's too high >1.2v, the ECM won't control the IAC.

Is the new TBI an exact match or did you buy a Holley replacement? Granted that the problem surfaced before you changed it, Holley replacement units do present some issues with revised IAC passages and need to use specific vacuum source to the MAP sensor. Wrong sized injectors could present wrong fuel delivery.

Did you follow the procedure to set the minimum idle speed???
Thanks for the reply. I’ll attach a screenshot.

No vehicle tachometer. Base trim V6.
Code 45, Rich exhaust indicated.
Attaching screenshot. Good idea.
Assuming vacuum leak as posted.
MAP hovers just over 1V, ≈ 1.08-1.12
IAC works toward 0 as Coolant temp rises
TPS is set at .53 at idle.
TBI is an OEM replacement same body.
Injectors are same as before issue.
Wasn’t aware of Minimum Idle Speed procedure. If mechanical, wouldn’t apply here as issue started prior to hardware replacement. No change after. Electronic idle is maintained in closed loop by ECM, correct?

Again thinking vacuum leak but they don’t “magically” appear. This issue was acute, one day it just showed up. Have checked for vacuum leaks around the intake and brake booster with starter fluid, can’t find any.

ECM maybe?
 

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stew86MCSS396

Greasemonkey
Aug 1, 2022
167
211
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(808)
You did mention changing the fuel pump...did you fall in the trap of more is better and perhaps installed a TPI pump that could possibly overwhelm the fuel pressure regulator? I think this is rare and the FPR should be able to bleed off the fuel pressure to normal operating but good idea to put a gauge on it to see if it's within spec. Too much pressure will cause the injectors to flow more than what they are rated for.

Some of your screenshot data is useful...is that data at approximately 9.0 sec run time? That would explain the cool temps. I understand that at normal operating temps that's when the problems start but that's where we need to be.

BLM 108 indicate rich but it may be due to cold start enrichment.
Exhaust O2 Lean contradicts what I just said.
IAC position is fine but if it does sink to 0 as it warms up, throttle blades could be closed a bit to make IAC active.
INT 81 indicates rich
MAP volts of 1.08 is ~20" hg which indicates good vacuum at idle.
O2mv 777 well single data point for O2 is pointless. O2mv in close loop should swing back and forth across .450mv.
Crosscounts 0 indicates your O2mv isn't swinging but as mentioned above run time of 9.0 seconds?

Hate to stir you into troubleshooting by parts changing...you can try follow these steps from a f-body TBI FSM for a code 45...

Edit: Removed the TGO links after I realized they were for LB9/L98...pdf file attached of '87 Monte TBI.
 

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N2UN

n00b
Jan 24, 2024
4
4
3
You did mention changing the fuel pump...did you fall in the trap of more is better and perhaps installed a TPI pump that could possibly overwhelm the fuel pressure regulator? I think this is rare and the FPR should be able to bleed off the fuel pressure to normal operating but good idea to put a gauge on it to see if it's within spec. Too much pressure will cause the injectors to flow more than what they are rated for.

Some of your screenshot data is useful...is that data at approximately 9.0 sec run time? That would explain the cool temps. I understand that at normal operating temps that's when the problems start but that's where we need to be.

BLM 108 indicate rich but it may be due to cold start enrichment.
Exhaust O2 Lean contradicts what I just said.
IAC position is fine but if it does sink to 0 as it warms up, throttle blades could be closed a bit to make IAC active.
INT 81 indicates rich
MAP volts of 1.08 is ~20" hg which indicates good vacuum at idle.
O2mv 777 well single data point for O2 is pointless. O2mv in close loop should swing back and forth across .450mv.
Crosscounts 0 indicates your O2mv isn't swinging but as mentioned above run time of 9.0 seconds?

Hate to stir you into troubleshooting by parts changing...you can try follow these steps from a f-body TBI FSM for a code 45...
Thanks for the reply.
OEM AC/Delco fuel pump.
No, you're Ok with the rich assumption. BLM 108 does indicate that the ECM is pulling all the fuel out that it can.
The EXHAUST O2 LEAN oscillates between RICH and LEAN every 1 or 2 seconds in this Dataset.
Hadn't looked carefully at the O2 (mV) and Crosscount data but in the 5 recordings I completed, no Crosscount data was recorded.
Going to replace the O2 sensor, although it is relatively new.
Thanks again for your time and advice.
 
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spongbob

Apprentice
Oct 1, 2022
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Thanks for the reply.
OEM AC/Delco fuel pump.
No, you're Ok with the rich assumption. BLM 108 does indicate that the ECM is pulling all the fuel out that it can.
The EXHAUST O2 LEAN oscillates between RICH and LEAN every 1 or 2 seconds in this Dataset.
Hadn't looked carefully at the O2 (mV) and Crosscount data but in the 5 recordings I completed, no Crosscount data was recorded.
Going to replace the O2 sensor, although it is relatively new.
Thanks again for your time and advice.
Someone's going to say a broke ring but you've have checked for windage problems
 
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N2UN

n00b
Jan 24, 2024
4
4
3
Solved!! Figured out that the IAC valve was wired wrong and was causing the ECU to not see the idle speed correctly. Figured out the correct wiring for the sensor/valve and it started idling correctly. Thanks to all who took the time to respond to this thread. Hope this helps someone fix a similar issue.
 
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stew86MCSS396

Greasemonkey
Aug 1, 2022
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(808)
Wow! Who woulda thunk it!!! Unless someone messed with the wiring... Only time I've heard of something like this is when putting a BBC throttle body on a SBC with the SBC original wiring. Wire colors are correct but pinned in different locations. :friday:
 
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