First off this is yet another trouble code 33 thread. Second, I have done some homework on this so far. This thread has been especially helpful:

https://www.corvetteforum.com/forums...e-33-help.html

I'm still in the middle of going through all the suggestions on that thread and I've gotten up to supplying 12 volts to the G pin to check the oil pressure switch. Everything else has checked out fine so far. I'm calling it quits for today but there was one odd thing I noticed while trouble shooting.

In addition to throwing the 33 code, the car will not idle for long on its own. When I plug my Moates ALDL connector in however, the car idles fine (a little high at ~1k but stable). When I unplug the cable it sputters out. Could this be a key to diagnosing whats wrong? If it doesn't mean much that's Ok, I'll just keep running through the troubleshooting process. I just didn't want to miss anything that could solve this sooner rather than later.

Pertinent Info:
'86 Auto ECM (recently converted to T56)
The car is dedicated to autocross and road courses so I don't baby it as much as I probably should.
Just set minimum idle before I started troubleshooting. I was able to get it to a stable idle no lower than 750 without sputtering out.
This whole thing started with a surging idle at the track. Replaced MAF sensor and both relays to fix that. Now it just won't idle long before sputtering out.

Thanks ahead of time for any help!


EDIT: Initially though the green wire was reaing 6.9V. Turns out my mulimeter was off by 2V so that wiring is good.

 

Typically it will advance the timing when you hook up the cable. How much depends on the bin. Its called the aldl spark adder, I think. Have to fire up my other laptop to verify this.

 

I checked the ARAP bin and its 8*. An 86 does not come with ARAP but I'd expect it to be similar.

 

Yes, the idle rpm can be affected by plugging in an ALDL cable. There is a parameter in the bin which sets the target idle to 1,000 rpm.

This typically occurs if the cable is plugged in while not scanning.

In $6E bins, this parameter is found at address: D3DA. It is active if MODE WORD 2 bit 5 is set.

C6D9: FDB $0036 ; 54, MODE WORD 2
;
; b0 1 = CARS ACTIVE
; b1 1 = MALF 14 OR 15 THIS START UP
; b2 1 = REFERENCE PULSE OCCURRED
; b3 1 = IN ALDL MODE 8192 LOCKED IN AND MODE 4
;
; b4 1 = DIAGNOSTIC SWITCH IN DIAG POSITION
; b5 1 = DIAGNOSTIC SWITCH IN ALDL POSITION
; b6 1 = HIGH BATTERY VOLTAGE-DISABLE MCU SOL DISABLE
; b7 1 = SHIFT LIGHT ON
;

D3D3: LDAB L0036
D3D5: BITB #$20
D3D7: BEQ LD3DB
; .... else
D3D9: LDAA #$50

 

If you ( the OP) is still interested in the problem that you were trying to resolve...

its a VAC/AIR leak. A pretty good one too...

 

Thanks, Jack and tequilaboy. I guess that answers my original question. But it begs another. What is different about the bin set idle control and normal idle that makes it so stable by comparison?

Leesvet, how do you know its a VAC/AIR leak? It honestly wouldn't surprise me the way I've more or less hacked up the AIR system. Care to elaborate?

Would a large vacuum leak essentially be enducing a false load and requiring more spark advance which the bin provides?

 

Did you check the timing ? Unplug the tan/black est wire first ?

 

By running an original auto trans ecm (more specifically: bin file) in a manual trans car, you're asking for idle trouble.

The programming is different between auto and manual, specifically for idle. The target idle rpm, PID control and spark advance are all different between auto and manual bins.

Manuals need more rpm. For example, Auto trans target idle is typically 550 rpm with a 50 rpm park/neutral offset. Manual trans target idle is typically 700 rpm warm.

You would benefit from tuning. Either start with a manual trans bin, or have a tuner edit yours to make it behave like a manual.

I suspect all of your issues are tuning related. Get an ALDL cable or scan tool and see what is going on at the signal level.

You didn't mention any mods other than the trans. How much does the flywheel weigh? Does it have a cam or intake? Anything else we should know about?

Regarding the Code 33, you may have a short on the MAF signal wire/circuit. Something strange going on if this is > 5 volts with MAF disconnected.

 

Thanks for talking me through this guys. This is great help.

Great info. That would explain why I couldn't get it below 700 without stalling.

Editing the bin requires a Prom burner right?

What should I be looking for? I've got an ALDL cable but I've been having trouble with it connecting with any of my devices lately. Actually it won't connect at all. Before these idle issues I was able to connect my tablet or computer with no problem. Staying connected was typically an issue but I've heard that's common with these 1227165 ECMs. I have an uncle with a pro grade scanner that does just about anything. I could probably use that to snoop around.

Nothing else really. I've removed a lot of stuff like radio, sound system and a/c components. The cats were gutted before I bought the car. Other than that, only suspension components and bushings have been upgraded.

That was my bad. I checked my multimeter and it was reading ~2 V too high due to a low battery. So that should have been right at 5V on the green wire. I'll try to get out there and run through the oil pressure switch circuit today.

Not yet. I don't have easy access to a timing light at the moment.

 

Some ALDL cables have a 10K resistor across terminals A & B. That will cause the ECM to set the idle to 1000 RPM. I believe it also sets the timing to some fixed number. All this stuff is described in the FSM. Some cables have a switch so you can take the 10K resistor out for normal scanning.

 

I never knew that. I thought it just switched between 8192 and 160 baud. I'll take a closer look for that in the FSM. Thanks for that nugget.