Last try at fixing, or the vette is sold!!! Any help out there!?
10-27-2008, 04:37 PM
#1
Melting Slicks
Thread Starter
Last try at fixing, or the vette is sold!!! Any help out there!?
PROBLEM SOLVED!!! see post 33 for the cure!
I've had it with this idle issue!
Car will not idle under its own power when cold, dies/bucks/stalls. When warm, car idles good, but choppy and dies at times. Runs great with throttle though, perfect!
If i disconnect the est (timing wire), the car idles just fine when its cold or warm, not even close to trying to die. Doesn't respond to throttle as well, because the lack of spark advance.
So...A question I need answered: Does disconnecting the est put the computer in "limp mode"? If so, then i figure I've got a bad sensor somewhere. If not, then something isn't right with the spark advance/retard when the est is connected. I should also note, the car runs rich, but not as bad with the est disconnected.
By making the car run smoothly with just a wire unplugged, I can eliminate air flow, fuel pressure, vacuum leaks, and internal problems from the circle (unless the computer is in fact being put in "limp mode", then airflow is a possibility). It also eliminates spark plugs and wires. It would seem to point to the est module, but it is brand new. Maybe the knock sensor, computer, or a bad ground in the est circuit could be the culprit.
I really need some people here to do some critical thinking, because I am stuck. If I don't figure this out soon, I'm trading it for a mustang and cash....yes...a mustang! Please don't let me do that!!!
If I need to, i can make two short videos of the car to help. One with the est connected and another with it disconnected. Huge difference!
I've had it with this idle issue!
Car will not idle under its own power when cold, dies/bucks/stalls. When warm, car idles good, but choppy and dies at times. Runs great with throttle though, perfect!
If i disconnect the est (timing wire), the car idles just fine when its cold or warm, not even close to trying to die. Doesn't respond to throttle as well, because the lack of spark advance.
So...A question I need answered: Does disconnecting the est put the computer in "limp mode"? If so, then i figure I've got a bad sensor somewhere. If not, then something isn't right with the spark advance/retard when the est is connected. I should also note, the car runs rich, but not as bad with the est disconnected.
By making the car run smoothly with just a wire unplugged, I can eliminate air flow, fuel pressure, vacuum leaks, and internal problems from the circle (unless the computer is in fact being put in "limp mode", then airflow is a possibility). It also eliminates spark plugs and wires. It would seem to point to the est module, but it is brand new. Maybe the knock sensor, computer, or a bad ground in the est circuit could be the culprit.
I really need some people here to do some critical thinking, because I am stuck. If I don't figure this out soon, I'm trading it for a mustang and cash....yes...a mustang! Please don't let me do that!!!
If I need to, i can make two short videos of the car to help. One with the est connected and another with it disconnected. Huge difference!
Last edited by cwyates4; 10-28-2008 at 07:49 PM.
10-27-2008, 05:34 PM
#3
Le Mans Master
Corda, I'm at a loss, I sure wish you could find another ecm just to try it out. Did you see what Vette2Vette wanted for the '85 ecm? They just got the car in and it is a good ecm. Tell Jay I told you to call.
You've went through too much to drop it now.
You've went through too much to drop it now.
10-27-2008, 06:15 PM
#5
Melting Slicks
Thread Starter
Mr. Dave. That really looks like it could be the cause, but then why does it run perfect with the est disconnected? Also, the cts is new, and ohmed out correctly.
Chuck, you are right. I really need to stick with it. And no, I haven't called about the ecm, it kinda slipped my mind. :o I will get to that asap.
10-27-2008, 06:36 PM
#6
Melting Slicks
The spark advance varies as a function of load and rpm. With the est disconnected you essentially have fixed spark advance. This is not ideal, but it is stable at the least.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
10-27-2008, 06:41 PM
#7
Le Mans Master
The spark advance varies as a function of load and rpm. With the est disconnected you essentially have fixed spark advance. This is not ideal, but it is stable at the least.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
10-27-2008, 07:25 PM
#9
Melting Slicks
Here's what we know about the car:
Mods
Forged 383, GMPP aluminum fast burn heads, TPIS mini ram intake, TPIS solid roller cam, Holley 58 mm TB, Bosch 30lb injectors, afpr, TPIS long tubes, built 700r4, 2800 stall, Dana 44 with 3.45s, greenwood body kit, Z51 handling, 11" rears with MT et streets, custom flowmaster exaust, and free stuff.
What we don't know is what is in the tune. I'm assuming it already has a custom tune, but I didn't see it listed.
A cammed car is going to want more idle rpm and more (and relatively flat) spark advance around the target rpm.
I'd guess that this combination will need around 900 rpm and around 28-32 degrees of advance in the region of 800-1200 rpm to idle with any sort of stability.
What is the spark advance and desired idle rpm in the present tune?
For mixture: fuel pressure, MAF calibration, injector constants, the injector low pulse width offsets and battery voltage offsets will also come into play and are possible sources of error.
Mods
Forged 383, GMPP aluminum fast burn heads, TPIS mini ram intake, TPIS solid roller cam, Holley 58 mm TB, Bosch 30lb injectors, afpr, TPIS long tubes, built 700r4, 2800 stall, Dana 44 with 3.45s, greenwood body kit, Z51 handling, 11" rears with MT et streets, custom flowmaster exaust, and free stuff.
What we don't know is what is in the tune. I'm assuming it already has a custom tune, but I didn't see it listed.
A cammed car is going to want more idle rpm and more (and relatively flat) spark advance around the target rpm.
I'd guess that this combination will need around 900 rpm and around 28-32 degrees of advance in the region of 800-1200 rpm to idle with any sort of stability.
What is the spark advance and desired idle rpm in the present tune?
For mixture: fuel pressure, MAF calibration, injector constants, the injector low pulse width offsets and battery voltage offsets will also come into play and are possible sources of error.
Last edited by tequilaboy; 10-27-2008 at 07:32 PM.
10-27-2008, 08:27 PM
#10
Racer
Member Since: Apr 2003
Location: Detroit MI
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The spark advance varies as a function of load and rpm. With the est disconnected you essentially have fixed spark advance. This is not ideal, but it is stable at the least.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
During normal function, as load and rpm vary the spark advance is also varying. The idea is to have the spark advance table designed to provide relatively constant torque in the vicinity of the desired idle rpm, so that small variations in rpm do not result in large torque swings and small changes in airflow through the IAC are sufficient to control the idle.
In your case, this is breaking down somewhere, likely due to a combination of mixtue error and an unstable spark table relative to the engine's mods.
MAF <--- Can we disconnect it and still have timing advance?
TPS <--- You can measure the voltage out the sensor.
RPM <--- Hmmm How could we do this one without scanning?
Any others ??????
If He did get it to scan I think the 85 is 160 baud and the 10k resistor puts the ECM in a diagnostic mode and it stays in that mode unlike the later ECMs that have 8192 baud that return to normal operation and keep sending sensor data.
10-27-2008, 09:21 PM
#11
Safety Car
There are two "coolant" sensors. One is for the gauge and one by the ECM (or at least on the GM cars I had in the 80's). I cannot say for sure, but I think when you disable the timing that it would go into open loop. If you disconnect the MAF, it should go into open loop so that may be another way of verifying.
10-27-2008, 09:47 PM
#12
Intermediate
Member Since: May 2008
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Nice engine mods.
Have you checked the TBS. Should be set to .54 Volt +/- .075 Volt . Another thing to look at might be the minimum idle speed adjustment. If the idle speed is set too low, then the IAC may not be able to pass enough air to keep a cold engine running. If you adjust the min idle speed, you must then re-set the TBS.
Hope this helps…
Have you checked the TBS. Should be set to .54 Volt +/- .075 Volt . Another thing to look at might be the minimum idle speed adjustment. If the idle speed is set too low, then the IAC may not be able to pass enough air to keep a cold engine running. If you adjust the min idle speed, you must then re-set the TBS.
Hope this helps…
10-27-2008, 09:55 PM
#13
Melting Slicks
the injectors may be too small for open loop.
open loop has fixed values, and if it is too lean, it'll be surging.
when in closed loop, the computer will add fuel.
open loop has fixed values, and if it is too lean, it'll be surging.
when in closed loop, the computer will add fuel.
10-27-2008, 10:45 PM
#14
Melting Slicks
Nice engine mods.
Have you checked the TBS. Should be set to .54 Volt +/- .075 Volt . Another thing to look at might be the minimum idle speed adjustment. If the idle speed is set too low, then the IAC may not be able to pass enough air to keep a cold engine running. If you adjust the min idle speed, you must then re-set the TBS.
Hope this helps…
Have you checked the TBS. Should be set to .54 Volt +/- .075 Volt . Another thing to look at might be the minimum idle speed adjustment. If the idle speed is set too low, then the IAC may not be able to pass enough air to keep a cold engine running. If you adjust the min idle speed, you must then re-set the TBS.
Hope this helps…
My 85 with MAF miniram 408 ran great on a stock tune. Very forgiving is that system. I have two distributors, an ecm and a knock module in my drawer if you need to test some things. We can work out an arrangement..
10-28-2008, 11:06 AM
#15
Melting Slicks
Thread Starter
Ok, i'll try and shed some more light here. There is a custom tune, a chip tune by TPIS. The car has only done this for 6 months, but it was fine for a year and a half before hand. The tune would not be the problem then. TPS checks out fine, IAC is new. I've done base idle adjustment a billion times. During the adjustment is when I noticed how well the car ran with the est disconnected. Fuel pressure is good. The problem is only at idle, put a slight bit of throttle in and its ok. The idle is about ~900 in park, 700 in drive. That works fine with the cam, it'll actually idle at ~600 smoothly with the est disconnected.
I still need to know if disconnecting the est puts the car in "limp mode"...
Did have one idea thanks to some suggestions. The introduction of throttle makes the car run fine, but its not the added air that does it, but the fuel I believe. Maybe I could try a higher voltage setting for the tps at idle, say around .65 or so. Or, bump the fuel pressure up from the 39 it is set at. The car already runs rich though...hmm...maybe turn the idle down a hair and raise the FP....worth a shot.
Now having said all that....Why would unplugging the est make the car run just fine if it wasn't ignition related?
That is what I've heard, and the car hasn't always ran like this. I might take a dizzy, the ecm, and the knock sensor off your hands if what I said above doesn't help any. When you say knock module, are you reffering to the sensor, or the est module though? I have a new est module in the car already...
I still need to know if disconnecting the est puts the car in "limp mode"...
Did have one idea thanks to some suggestions. The introduction of throttle makes the car run fine, but its not the added air that does it, but the fuel I believe. Maybe I could try a higher voltage setting for the tps at idle, say around .65 or so. Or, bump the fuel pressure up from the 39 it is set at. The car already runs rich though...hmm...maybe turn the idle down a hair and raise the FP....worth a shot.
Now having said all that....Why would unplugging the est make the car run just fine if it wasn't ignition related?
That is what I've heard, and the car hasn't always ran like this. I might take a dizzy, the ecm, and the knock sensor off your hands if what I said above doesn't help any. When you say knock module, are you reffering to the sensor, or the est module though? I have a new est module in the car already...
10-28-2008, 11:19 AM
#16
You could always take it to the dealership and have them diagnose the problem... Most larger Chevy Dealers have atleast one person who is good with vettes. Shouldn't cost more that $200 to diagnose, and that includes hooking it up to the computer.
-Just a thought...
-Just a thought...
10-28-2008, 11:45 AM
#18
Melting Slicks
Thread Starter
What are they going to see that I won't? All they will do is pull codes, there isn't any to pull. I'm also in a small town, our mechanics around here are jokes, part swappers is all.
And -js-: very good point. The computer stays in diagnosic mode, so I won't see what I need to see even if I can see aldl info.
Anybody want some video to show what the car does? I can have them up tomorrow if it'll help any.
With the est connected, it fires right up and then promplty dies when cold, takes some foot to keep it running until coolant temp reaches about 160, then itll run, but rough. Est disconnected, fires right up and purs all the time.
And -js-: very good point. The computer stays in diagnosic mode, so I won't see what I need to see even if I can see aldl info.
Anybody want some video to show what the car does? I can have them up tomorrow if it'll help any.
With the est connected, it fires right up and then promplty dies when cold, takes some foot to keep it running until coolant temp reaches about 160, then itll run, but rough. Est disconnected, fires right up and purs all the time.
10-28-2008, 11:47 AM
#19
Melting Slicks
Thread Starter
Kidding with you, my fuel pressure is normal throughout. Idle is good, with vacuum disconnected it jumps, and load test is good. leakdown test: holds pressure overnight, no problems with the fuel system at all...except possible adjustments needed.
10-28-2008, 11:52 AM
#20
Burning Brakes
is that what your ended up being!? Told ya to do the fuel pressure test from the FSM!
Kidding with you, my fuel pressure is normal throughout. Idle is good, with vacuum disconnected it jumps, and load test is good. leakdown test: holds pressure overnight, no problems with the fuel system at all...except possible adjustments needed.
Kidding with you, my fuel pressure is normal throughout. Idle is good, with vacuum disconnected it jumps, and load test is good. leakdown test: holds pressure overnight, no problems with the fuel system at all...except possible adjustments needed.
