Further idle diagnosis....

[Roadster]

So I been having this slight idle problem since January. More annoying than an actual problem. But something I can live with for now...

As mentioned in another thread when I start the engine (has Marc's chip) if revs high for a few seconds and then come down to 650-700 r's at idle. Driving out of the development and stopping along the way, the idle comes back down to the 650-700 range. Now when on the road going through the gears and then coming to a complete stop the idle can sometimes be around 900-1000 r's. Now what I don't understand is that with my foot on the brake and I let up on the clutch pedal, I can get the engine to drop to the lower rpm range and stay there. On further stops it can be a hit or miss situation on where the idle will be, unless I use the same procedure as mentioned.

Talking to Yun, as he mentioned it could be the shaft in the throttle body that may be worn. Also did the airhorn test and used the FSM as my block in front of the airhorn. At high idle 900-1000 r's after blocking the airhorn, the idle came down to normal after about 3-4 seconds. And on Marc's site it also mentions the problems with the TB. So a lot to absorb.
I also cleaned (sprayed) the outside throttle springs and whatever part of the shaft I could get to with WD40. Amazing the amount of grime that ended up on the paper towels. I also sprayed lightly inside the TB, even though it isn't recommended per Marc's papers. Didn't really matter, because it acts the same. I always start off with a normal idle and even revving the engine to around 1500-2000 rpm's the engine will come back down to normal. Now when driving, it is a different result.
Hooked up the Tech 1 on a warm engine and the IAC was reading about 20 with normal driving. Backing off in 3rd gear @ 2000 r's, the IAC would be much higher and fluctuate at higher readings. Come to a complete stop whether the idle is normal or higher, reading has always been "0".
From Marc's site:
"When the IAC goes to zero the idle speed can not be reduced by the engine control system. Then the engine idles at more than 650 rpm." What I don't understand is when the idle is normal, I still get the "0" IAC reading when stopped.

TPS is set @.54v And the throttle stop screw is adjusted correctly. The throttle cable has play, so it isn't too tight that it would hold the throttle off of the idle stop. I changed the IAC from before, but it did not make a difference. I need to verify that the IAC fully closes as per Marc's papers.
Other then the above the engine runs great especially with the secondaries fully operational.

So short of sending out the unit to Marc for a rebuild, guess I will keeping trying to find an easier if possible fix...

[secondchance]

There is an idle set screw (throttle plate stop) that can be adjusted. We might try backing off a bit. Worth a try...
Also, giving it another thought, blocking the air horn maintaining 650 may be due to just enough air leakage somewhere other than the TB.

[XfireZ51]

Originally Posted by Roadster

So I been having this slight idle problem since January. More annoying than an actual problem. But something I can live with for now...

As mentioned in another thread when I start the engine (has Marc's chip) if revs high for a few seconds and then come down to 650-700 r's at idle. Driving out of the development and stopping along the way, the idle comes back down to the 650-700 range. Now when on the road going through the gears and then coming to a complete stop the idle can sometimes be around 900-1000 r's. Now what I don't understand is that with my foot on the brake and I let up on the clutch pedal, I can get the engine to drop to the lower rpm range and stay there. On further stops it can be a hit or miss situation on where the idle will be, unless I use the same procedure as mentioned.

Talking to Yun, as he mentioned it could be the shaft in the throttle body that may be worn. Also did the airhorn test and used the FSM as my block in front of the airhorn. At high idle 900-1000 r's after blocking the airhorn, the idle came down to normal after about 3-4 seconds. And on Marc's site it also mentions the problems with the TB. So a lot to absorb.
I also cleaned (sprayed) the outside throttle springs and whatever part of the shaft I could get to with WD40. Amazing the amount of grime that ended up on the paper towels. I also sprayed lightly inside the TB, even though it isn't recommended per Marc's papers. Didn't really matter, because it acts the same. I always start off with a normal idle and even revving the engine to around 1500-2000 rpm's the engine will come back down to normal. Now when driving, it is a different result.
Hooked up the Tech 1 on a warm engine and the IAC was reading about 20 with normal driving. Backing off in 3rd gear @ 2000 r's, the IAC would be much higher and fluctuate at higher readings. Come to a complete stop whether the idle is normal or higher, reading has always been "0".
From Marc's site:
"When the IAC goes to zero the idle speed can not be reduced by the engine control system. Then the engine idles at more than 650 rpm." What I don't understand is when the idle is normal, I still get the "0" IAC reading when stopped.

TPS is set @.54v And the throttle stop screw is adjusted correctly. The throttle cable has play, so it isn't too tight that it would hold the throttle off of the idle stop. I changed the IAC from before, but it did not make a difference. I need to verify that the IAC fully closes as per Marc's papers.
Other then the above the engine runs great especially with the secondaries fully operational. And when stutting down the engine there is no pre-ignition at the higher rpm range, which is a good thing.

So short of sending out the unit to Marc for a rebuild, guess I will keeping trying to find an easier if possible fix...

How do you know that the Min Air Adjustment is correct? With a 0 IAC, sounds like it isn't.

[Roadster]

Originally Posted by secondchance

There is an idle set screw (throttle plate stop) that can be adjusted. We might try backing off a bit. Worth a try...
Also, giving it another thought, blocking the air horn maintaining 650 may be due to just enough air leakage somewhere other than the TB.

I did see that but did not make any adjustments. I imagine I'll make a mark on it to keep track of any adjustments made. I checked all of the hose connections that I could get to and always make sure any screw seen is tight.

Originally Posted by XfireZ51

How do you know that the Min Air Adjustment is correct? With a 0 IAC, sounds like it isn't.

I don't know, but if it is out of adjustment then why after starting the engine the idle will be normal or close to normal and not at the 900-1000 range?

[Roadster]

Ok, just looked up one of my previous threads dated 12-21-16 where I used the Tech 1 for the first time.
Here are a the results from those readings....


It will ask you for the 8th digit of your VIN#, which is "J" and will then ask you if a 94 Corvette. Press "YES" and you are in.....
The Menu....
F0....DATA LIST
F1....FIELD SER
F2....DTC
F3....SNAPSHOT
F4....MISC TEST

Started with F0, these results are with the engine running.....

ENG SPEED
870-890......as engine warmed up......830-815 rpm's
DESIRED IDLE
850.............as engine warmed up......812-800 rpm's

Engine coolant reading is off by 2 degrees from the IP with the tech 1 giving a higher reading by that amount.

MAP (KPa,V)
39 1.50-1.54-1.56

FUEL EVAP PURGE
0%

THROTTLE POS
0.52 VOLTS increased as throttle was depressed slightly .62 VOLTS
THROTTLE ANGLE
0%................................................ .............................2%

LEFT H025 BNK 1
53 mV
LOOP STATUS
OPEN LOOP

RIGHT H025 BNK 2
44-53 mV

ST FUEL TR BNK 1
128
LT FUEL TR BNK 1
115

FUEL TRIM CELL
16 CELL
FUEL TRIM ENABLE
NO

SPARK ADVANCE
12 degrees MPH 0 km/h 0

KNOCK RETARD
0
KNOCK SIGNAL
NO

SYSTEM VOLTAGE
14.1 VOLTS
FUEL PUMP VOLTS
13.5 VOLTS
EGR DESIRED POS
0%
EGR ACTUAL POS
0%
EGR PINTLE POS
0.60 VOLTS
EGR DUTY CYCLE
0%

IDLE AIR CONTROL
22
ENGINE SPEED
678-700 RPM's

FAN 1
OFF
FAN 2
OFF
A/C FAN REQUEST
NO
A/C REQUEST
NO
A/C CLUTCH
OFF
AIR PUMP RELAY
OFF
AIR CONTROL
DIVERT

MPH
0
Km/h
0

1-4 SHIFT ACTIVE
NO
ENG OIL TEMP
153 degrees by tech 1........149 degrees by IP
SEC VAC (kPa, V)
0 0.58

POWER SWITCH
NORMAL.....w/key turned....FULL
SECONDARY SYSTEM
OFF
FEDS/PASSKEY
OK
SEC AIR INLT SOL
OFF

CALIBRATION ID 9011

ENGINE SPEED @200degrees
691 RPM's
DESIRED IDLE
662 RPM's

After totally warmed up received these readings.....

LEFT H025 BNK1
as low as 75 mv to as high as 735 mv
LOOP STATUS
CLOSED LOOP

FUEL TRIM CELL
16 CELL
FUEL TRIM ENABLE
YES

Notice the IAC and idle readings warming up to full operating temps.
It appears that the idle may have not always been at the 650 range, but it was never at the 900-1000 range either.
So something changed after the plenum pull one month after these readings. It has to be a minor adjustment or minor vacuum leak, I just can't wrap my head around that the throttle shaft would develop that much play to increase the idle at times in one month.

[XfireZ51]

Why did u pull the plenum?

[Roadster]

Originally Posted by XfireZ51

Why did u pull the plenum?

Found out that I had various vacuum leaks. Figured if we're under there, just do it all and be done with it. And the funny thing about it is, that even with the vacuum leaks, I never had a problem with the idle, go figure!!!
All of the items were completed....
1-to get secondaries fully operational
2-replace starter
3-replace coils
4-replace vacuum lines
5-replace check valve
6-replace filter
7-replace PCV valves
8-replace PCV connectors
9-replace MAP connector
10-replace Fuel Regulator hose
11-clean the valley
12-unclog the drain
13-clean Plenum and other parts
14-replace Fuel Injectors
15-replace alternator
16-changed spark plugs at a later date

I have to say the engine runs great, and I thought it was great before. What did I know??? Once the idle issue gets sorted out, it will be perfect for me. The only future mod I would do is the exhaust when the time comes.

[Roadster]

Been giving the idle issue some thought, spoke to a relative over the weekend who is into heavy equipment for a livelihood and at one time had a performance oriented business years ago.

This is his theory....
He doesn't feel that it is a vacuum leak or the throttle body shaft for the following reasons. If it was a vacuum leak, then the idle would be higher all of the time and not just some of the time. As mentioned before, when I first start the engine after it comes down off high idle, it is around 650-700+ r's. When driving it will either stay the same when completely stopped or be around the 900-1000+ r's. And when I put a load on the engine, with me completely stopped, foot on the brake, in 1st gear and letting up on the clutch pedal, the r's drop down to 700+ rpm's and stay there until the next complete stop. Then the idle will be back up to 900-1000 rpm's. And yes the IAC is @ 0 when stopped. His feeling is that the TPS may be defective. Saying that when you first start the engine and on restarts your idle is normal, or somewhat normal. In those situations the TPS is sensing all of what is should for the correct idle. When you drive and are coming to a complete stop, the TPS goes through "steps" as the engine de-accelerates and may not be sensing that last "step" it needs to sense for the proper idle, when completely stopped. But again, if you shut the engine down and then restart, the TPS is fine. He also feels that if it were the throttle shaft, you would not be able to get the normal idle during restarts.

Now that is just his thoughts and it does make sense to an extent. So your thoughts are welcome on this issue, and very much appreciated.

Also took my wife out today along with the Tech 1 so she could record some of the readings during our ride.

Here they are.......I also have Marc's chip as mentioned....

Start-up...cold engine
Desired idle...850 rpm's.............Actual idle...890 rpm's
IAC 25

Engine warming up with temp @135*
IAC 2-1-0....................idle 695 rpm's

All other reading were with the engine at normal operating temps.

During our ride the IAC would be 0 @idle and would jump up during acceleration and de-acceleration from 5-11-16-18-14-15-17-29-23-47-42-5-0
The numbers would increase during acceleration and slightly change when shifting, but liked to be around 16-17 in the 2700-3100 rpm range.
When backing down is when you would see the higher 47-42 and sometimes higher than that (as high as 90-87-85) depending on the speed when backing down.
Then when coming to a stop the count would decrease and be @ 5 then 0 when completely stopped.

Even when the engine speed @idle was as high as 1125'r's the TPS was still @.54v

MAP (KPa,v)
36 1.36-1.40

TPS @ all idles .54v
Throttle Angle 0% @idle

LHB 1 Closed Loop
range from 98-750 mV

RHB 2 Closed Loop
same as above

ST Fuel Trim Bank 1...122
LT Fuel Trim Bank 1....126

Fuel Trim Cell......16 cell
Fuel Trim Enabled.....Yes

Spark advance......9-10* and in the high 20's when accelerating

When engine speed was @850's during idle, the desired idle was listed as 662 r's

When shutting the engine down, and key off the Tech 1 read IAC 135 and engine speed was @ 884 r's. Don't know if this reading has any meaning or value.

Upon hot restart the idle was @740r's and the IAC @ 0


I did replace the IAC as mentioned before after the plenum pull. And I know that it is possible for more than one part to go bad or a newer part to be deflective. I have been there done that before with other vehicles....lol

I also want to add that the idle stop screw did not move at any time one way or another during regular or high idle stops.

Took pics with the engine running and @ normal and high idle, you would think that at high idle if the blades were opened slightly that the idle stop would not fully make contact, no so as shown in the pic, it was the same for both idles...

[XfireZ51]

If there is no "hunting idle", the IAC count is 0 because the bypass air is being introduced thru the throttle plates and not the IAC. Close the plates, and the IAC count will increase. U will need to reset or at least recheck the TPSv whenever you do that. Everything else u logged is normal and accountable for in the calibration.

[Roadster]

Originally Posted by XfireZ51

If there is no "hunting idle", the IAC count is 0 because the bypass air is being introduced thru the throttle plates and not the IAC. Close the plates, and the IAC count will increase. U will need to reset or at least recheck the TPSv whenever you do that. Everything else u logged is normal and accountable for in the calibration.

Ok, a few simple questions......

1- I am assuming that I can use the Tech 1 since it reads the voltage when I reset the TPS, therefore eliminating the use of a mutimeter, correct?

2- are you saying to adjust the idle stop nut & thread and make sure the blades are completely closed?
From what I can determine, they are!!! And wouldn't normal idle suggest that they are fully closed?

3- The idle has never "hunted" but has not always been rock steady according to the Tech 1, it does fluctuate 30 to 40 rpm difference whether normal or higher idle.

4- Can you explain more that if the IAC is 0 and my idle is normal, then how can air still be introduced through the throttle blades as opposed to a higher idle which would seem more logical?

5- Then if the IAC count increases as you start off and accelerate, does that mean the IAC is working properly at that time and allowing the proper amount of air into the TB?

Thanks for any additional info......appreciated........