Dear God, where should I begin? Let me start with the combo. 1969 Nova, speed density tpi being run with a 730 ECM and a painless harness with a T5. My timing is 6* BDTC and fuel pressure is 43 PSI with vacuum disconected. I read codes with a paperclip, so data loggin is not exactly an option.

Here's the problem: Flucuating idle, between 550-1350 RPM, which seems completly at random. Sometimes it runs fime, idling at 650RPM. Idles high when in warm up mode ~900-1100 RPM. When it goes into closed loop is when the flucuating idle begins. I did all the general diagnosis, checked volatges to various sensors and systematically began disconnecting one sensor at a time to see if the computer might go into a limp home mode. Behivor may go away for a small ammount of time, but comes back, kicking the correct trouble code. Plug in sensor, reset ECM, problem resumes and no codes are set except 12 (OK). I have replaced the following with no remarkable change in symptoms:
Distributor (Swapped with buddy who took it out of his running car)
Map sensor
CTS
TPS
IAC
Injectors
Fuel Regulator
O2 Sensor has 25,000 on it

The only things left are the Air Intake Temp Sensor and the ECM. I was told that if I remove my prom that the engine can run, but mine does not. The fuel pump primes, but no ignition. The Air Temp Sensor does not set off a code, but will if unplugged. Am I looking at an ECM Or am I simply going crazy?

 

'502

You need to post a data log (WinALDL or eq) of what your engine is doing. First, what are your engine particulars (L98 was a starting point) - i.e. cam, heads, compression, etc???
Since you are running speed density which MAP sensor do you have and what is your idle vacuum levels (or MAP voltage)??

From what you are describing it appears that you have is a classic large overlap camshaft with low idle vacuum (say about 14 to 10 in-Hg range) and due to excess overlap raw fuel ends up in exhaust manifolds. Unburned fuel causes O2 to report lean condition forcing ECM to dump more fuel which engine does not need at all!!

One possible solution is to disable close loop at idle RPM's - this would require n-Alpha programming of your idle area fueling and speed density elsewhere.

//RF

 

Mine was doing the exact same thing and then after resetting it for the third time it gave me the rapidly SES blinking which means new computer for me.

Might be want you need to try

 

It is a corvette L98. Bone stock save for Sandersen Shorty Headers. The cam is stock. As I mentioned before, I do not have the capability to data log; I assume a vacuum gague would serve the same purpose? As fas as disabling closed loop mode, I haven't the equipment. Do you have any reccomendations for people, either a shop or thirdgeners who could do this?

 

'502

It is tough to say what ever your ECM is defective or not. Most likely it is doing what it is suppose to do.

Since you have headers - where did you install your O2 sensor and what type is it? What might be happening is that O2 gets cool enough during idle and begins report lean condition - lower O2 voltages. The easiest solution is to install a heated O2 - AFS-74 with ignition activated relay kit.

Get a DVM and measure MAP voltage vs. your idle vacuum..

Sorry, I do not know of shops in SB area - I do my own EFI tuning .

//RF

 

I am running a heated 3 wire and have been for some time now. I am actually not in Santa Barbara anymore. My wife got an awesome job and now we live in Hollywood! Anyone near the 10 and 405?

 

Take a look at this tech article. Maybe it will help your situation, or maybe not.

IAC and TPS Adjustment
Tom Keliher Mar 31 2006 - 3:07pm
Idle Air Control
Tools needed:
Torx bit # T-20
Paper Clip
Small Punch
Take the paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' pins.
Turn on the ignition, but don't start the engine. Wait 30 seconds. Now, go remove the connector from the IAC.
Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine.
Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars).
If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the car for 20 minutes to allow the ECM to relearn your driving style.
Throttle Position Switch (TPS)
Tools needed:
Digital Volt-Ohm-Meter (VOM)
Jumper Wires (make your own)
Auto Xray Scanner (if available) will eliminate the need for VOM and jumper wires.
Turn on ignition, but don't start the engine.
With a scanner: plug in the scanner and read the TPS voltage. It should be 0.54Volts +/- 0.075Volts
With VOM and jumper wires: disconnect the connector from the TPS. Using your jumper wires, make a connection allowing some room for the VOM terminals to contact the jumper leads and read the TPS voltage.
If out of spec, loosen the two screws holding the TPS to the throttle body, and slightly rotate the TPS up or down, reading the voltage until it comes into specification. Tighten screws. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, that could mean a bad TPS switch and could be a cause of stumbling and driveability problems.
After setting the correct voltage, turn off ignition switch. Remove jumpers/scanner and reconnect the TPS connector as required

 

I thought I reset the IAC, but I did not manually adjust the idle stops. I will give that a shot in the morning.

 

Well what happened

 

carguy

you may also want to check the pickup coil, i had a similar problem as have many others, and a new pickup coil took care of it. the easiest way to find out would be to swap the distributer out from your buddy's car.

 

Well, I tried the IAC reset and it made things considerably worse . It would not idle anywhere below 1200 rpm, so I reset it again (link ALDL, pull plug, ect) and I now have a somewhat normal idle with some flucuations, just as before. I have been running my friend's dizzy for nearly 6 months now with the same issues as with my old dizzy. No codes and now I am pinging like a bell under hard acceleration. Has anyone done a Mega Squirt?

 

Believe it or not when I first purchased my 89 it began idling something like you describe. Even shut off once or twice at a stop light. Eventually found carbon build up under the IAC. The passage inside the TB also required cleaning. Replaced all the gaskets and new IAC run until it was pulled this past summer..

 

Should the engine be warmed up to operating temperature before doing the IAC adjustment? Or can it be done when the engine is cold? I'm leaning to warming it up first, but not sure if this is correct.