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]

Can it be possible that the IAC is defective? Although it registers on the Tech 1, the counts are allover a wide range when driving. Sometimes it holds in one area, other times it varies constantly from mid teen counts to as high as in the 80's and 90"s
Should the count be constant in different rpm ranges?

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[XfireZ51]

You mentioned that u blocked airhorn when high idke, and that idle came down and stayed down after unblocking the airhorn. That tells me ur throttle blades are sticking a bit open for a reason. When u block the airhorn, it causes a vacuum that draws the throttle blades in and therefore reducing the amount of air entering the plenum. Idle drops. Move the throttle again, and the blades return to original position.

[Roadster]

I can understand what you are saying, but what I don't understand is why the throttle angle is always at 0% at high or normal idle. Wouldn't that indicate that the blades are completely closed?
I appreciate all the comments and suggestions, but it looks like another plenum pull is in order to make sure everything has stayed attached. Could be that there is a slight vacuum leak. Not sure as you and many others have way more experience than I do with the LT5.
Will have to wait for now as I'm out at the 74....alot of work to be finished here..lol


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