Core issue: While driving 96' LT1 M6 (mods below), the car will:
1) accelerate itself (IE when the accelerator is not being depressed)
2) not idle down when coming to a stop
3) rev when shifted

Not sure what to test or do next to resolve the issue.


Symptoms: Once at running temperature (and it 65*-75*f ambient temps)...

- Above roughly 30 mph...when I get off the accelerator engine holds rpms.
- Car will accelerate when no pressure is applied to pedal.
- When taken out of gear while moving (typically while shifting) the engine revs. How much it revs seems related to current rpm before I take it out of gear.
- When I come to a stop the rpms do come down, thought not to desired idle (program for ~1K) . Sometimes it only comes down to 3k and other times all the way to 1400. If the idle stays at 3K when at a stop, I will slip the clutch to bring the rpms down to 1k and then let off the clutch...at which point the car will idle back to 1500-2k rpms.

Diagnostics:
- No check engine light. Do not see any pending codes either.
- IAC are in acceptable ranges.
- Vacuum ranges from 13hg all the always to 21hg.
- MAF appears to be measuring incoming air.
- TPS is showing below .9 volts when throttle body is closed. When the engine is holding a review the TPS does show voltage relative to that position.
- MAP is roughly 7 hg at 1,200 rpm.
- Throttle body set screw is correct.
- Throttle body plates move without binding or resistance.
- Throttle body plates spring back well when the engine is off.
- Throttle cable moves without resistance. When idle issue is observed while parked, the cable does not feel like it has tension that is holding the throttle plates open.



1996 Z. Manual.

Engine
Arizona Speed & Marine 58mm Throttle Body
Port & Polished the MAF sensor
throttle body by-pass
!CAGS
SLP Cold Air Induction
Comp Cams 306 cam (230 / 244, .510 / .549 112 LSA)
LT4 Timing Chain
Port and Polished Heads
Mr. Gasket Head Gaskets (.026 thick)
Port and Polished Intake (and 58mm Throttle Body ready)
Comp Cams 1.6 Roller Rockers
Comp Cams Hardened Push Rods
Comp Cams "R" Series Hydraulic Roller Lifters
Crane Dual Valve Springs
Comp Cams Guide Plates
Taylor Pro Spiral 8mm wires
1LE Elbow
LT4 Knock Module
Fuel Pressure Regulator (T.P.I.S. Top Hat Kit)
CSI Electric Water Pump
MSD 6AL ignition box

Exhaust
Hooker Long Tubes with JetHot 2000 coating
Pacesetter Y-Pipe
MagnaFlow 59959 catalytic converters
Hooker Aero Chamber Cat-back

 

The IAC counts is the position the PCM is telling the IAC motor to move the pintle to. There is no feedback as to whether the pintle has moved to that position. When you come to a full stop, in neutral, you indicate RPM could be anywhere from 1,400-3,000 RPM. Is that as reported by the PCM scan, not the dash tach? If it is above the target RPM, how does the IAC count respond? Does it try to close down the pintle to bring RPM down to the target ~1,000 RPM? Why do you use the “approximate” symbol in front of the 1,000?

When the idle is in the range of 1,400-3,000 RPM, what is the MAF reading in grams/second? Obviously idle above programmed has to involve more air flow. Is it seen in the MAF reading? If not, have you checked for a vacuum leak? The low MAP would appear to confirm a vacuum leak, although with that cam, I would expect 15” Hg or less at idle. Surprised to see 21”Hg... that's in stock cam territory (unless that is the number you are seeing on closed throttle decel).

On the other hand, 7”Hg MAP at 1,200 RPM seems unusually low. Vacuum = MAP - Barometer, so if you are at sea level with a typical 30” Hg barometer, you have 7 - 30 = 23”Hg ..... that’s more vacuum than a stock cam. What is your elevation above sea level?

When the vehicle is moving (VSS not = to 0) the PCM elevates idle RPM. Confirm VSS reading when vehicle is stopped.

When the CLT temp is “cold”, the PCM elevates idle. Have you confirmed the accuracy of the CLT sensor from the scanner readings?

What brand Opti do you have? If an MSD, how old is it? Using Scan9495 data logs, I was able to trace several cases of erratic, surging (as high as 2,700 RPM) and elevated idle to faulty signals from the MSD units built over the last couple years. After providing the data to MSD via one of their customer service advisors on another site, and providing them with one of the defective units, MSD eventually agreed with my analysis, and reportedly made changes to the optical module and the cap.

Do you have, or can you produce a data log (in csv or Excel format) of the engine exhibiting the above idle issues?

 

Diagnostic drive was performed city/county roads with speeds up to about 55 mph. Sensor readings below are during the high idle event unless noted otherwise. The pedal was not depressed at this time. I had just pulled into a residential driveway and came to a stop.

My responses are in bold.

1) Confirm RPM from PCM during high idle at at stop. Note PCM and dash tach readings.

PCM and dash tachometer vary by about 200 rpm.

2) When at a stop, and the idle is higher that programed value, how does the IAC respond? (is it closing the pintle to bring down the rpm?)

Dropped to 14. While VSS at 4mph (last reading before 0mph), IAC was at 108.


3) Why do you use the “approximate” symbol in front of the 1,000?

I used the approximate symbol because depending on temp the idle has a range and for this given temperature it is near 1,000. 1,012 to be exact.


4) When the idle is between 1,400 – 3,000, what is the MAF grams/second?

MAF values are certainly changing and seems to be responding.

At 2,337 RPM, the MAF sensor was seeing 2.696 #/min.
MAP was 10.1 Hg at the time.
TP sensor was reading 2% (reads 0% when throttle body is “closed” resting on the set screw) even though I was not depressing the pedal.


5) Is the MAF seeing the additional air that is coming it from being above idle. If not, check for vacuum leak.

I assumed sufficient vacuum equates to no vacuum leak. I did search for one (visually and with propane) and did not find one.


6) MAP expect to see 15” Hg or less at idle. 7”Hg MAP at 1,200 RPM seems unusually low.
Vacuum = MAP - Barometer, so if you are at sea level with a typical 30” Hg barometer, you have 7 - 30 = 23”Hg ..... that’s more vacuum than a stock cam.

@2,337 rpm, MAP is 10.1Hg.
During this drive, MAP read as high as 29.3 Hg and read as low as 9.5 Hg.


7) I am a pretty much at sea level, 602 feet.

8) VSS reading when vehicle is stopped is 0 mph.

9) The coolant temp sensor’s values do change as the car warms up.

At startup for this data logging, start temp was 120* F (95*F ambient air temp [pcm], and the car had been driven 4 hours prior) and after 3 minutes and 50 second of running the temp reached 180*F as reported by the pcm.

During high idle event, temp from pcm was 192* F.

10) Opti, I believe it is an AC Delco unit. Not the original…it has been replaced, but cannot put a date to the replacement.

11) A data log of the high idle event is attached. If you would like to see the larger data set of the entire drive, let me know. The way Car Code OBD-II scanner (obd-2.com) logs to a csv can makes large data sets hard to read.

The high idle event starts on Row 2.

Note to self: Data points 92 & 93 on EDscan37.csv via Car Code OBD-II scanner (obd-2.com)

 

I guess my questions weren't real clear in all cases.

1) When you reported 1,400 - 3,400 RPM, were you looking at the tach or the PCM? Fairly normal for the two readings to differ, that's why I asked.

2) When the IAC counts dropped to 14, did the RPM return to the programmed target value? Or did the idle, as reported by the PCM, remain above the programmed target value - and the PCM did not lower the IAC counts to try and reduce the high idle?

3) What value is entered into the program for the target idle, with the coolant fully warmed up? 1012?

4) MAF flow seems OK for that high idle. Nothing bypassing it.

If the TPS % is not = 0, the idle will not return to the programmed target value. PCM appears to assume you have your foot on the throttle, and “idle” is no longer the issue. This problem requires more investigation. May also move the ignition timing from the idle table to the 0-4,000 RPM (non-idle) timing table. See comments in 6) below.

5) My comment regarding the low MAP indicating a vacuum leak was incorrectly worded. Ignore. The question remains, how can the engine with that cam be pulling more vacuum (7” MAP - 30” Barometer) than a stock cam?

6) At wide throttle openings, MAP should approach Barometer, as long as you have a decent air supply system, clean filter, etc.. So 29.3”Hg is not unusual. And if you saw 9.5” Hg on closed throttle decel, that might be normal. But the 7”Hg you reported earlier at 1,200 RPM idle seems irrational. Seems like the sensor calibration could be off. With larger cams, tuners will play with the idle ignition advance table to try and stabilize cam surge. One axis on the timing table is MAP, the other is RPM. A faulty MAP reading might throw off the tuner's strategy.

7) to 10) - no comment

11) I'll see what I can do with the data log. I haven't seen one from a Car Code scanner before.

 

1) Primarily the tach, but glancing at the PCM to confirm.

2) When the IAC counts dropped from 108 (at about 4mph from VSS) to 14 @ 0 mph, the idle rpms did not change. It did not return to the target idle of the PCM.

3) With ECT @192*F, the PCM (via scanner) indicates the Idle Speed Desired RPMS is 1,013 rpms. This was the recorded desired RPM at the time of 2,337 RPM being reported as the actual rpm, and the IAC counts at 14.

Likely not necessary, but I have LT1 Edit and the current program loaded to the pcm so if I need to look up any programmed values/tables I can do so.

5) I do not know. lol.

When the high idle event happens, if I manually push the throttle linkage to the closed position (hitting the idle set screw)...if feels like something is holding it open the throttle open. It almost feels like the throttle is being held open by the flow of air.

When the engine off, the throttle blade snap back in place real nice. Using a pull scale, I measure about 2.6lbs of force is needed near that 2%-3% TPS reading to open the throttle blades. See photo. As I open the throttle further that pressure required to pull it open goes as high as 3lbs.


6) At this point legs assume the 7Hg at 1,200 was user error. I have reviewed 3 data logging sessions and cannot find this value. This is likely a case of my quickly looking down at the screen while logging and misreading the values.

All the values in my first response to you are from readings of data logging done will sitting at a computer desk for accuracy. AKA no "glancing" at the laptop while logging.

 

Does the car have traction control - RPO “NW9”?

 

Yes it has traction control.

During high idle event, throttle cable does not appear to have tension between it and the tractions control assembly. For traction control, one cable goes from the TB to the traction control assembly and then a second separate cable goes from the traction control assembly to the pedal.

Traction Control is disabled by default on startup.

 

The traction control servo on the driver side valve cover may be interfering with the throttle cable operation.

 

Any recommended diagnostic test? When the high idle even occurs there does not seem to be tension on the cable from the TB to the TC servo.

 

While investigating permanent removal of the Traction Control (physical parts and software disabling), I found myself in LT1 Edit and decided to grab a few screenshots of the PCMs programed values.

 

SOLVED!

The issue was the spring on the throttle body no longer had the strength to properly close the butterflies when the engine was running. I was able to add more spring tension and drive it for a few days to verify the revving issues were gone.

Arizona Speed & Marine has been top notch in helping me with getting the spring repaired. Their TB's come at a premium cost, but their quality and customer service is well worth the cost.