Okay guys. We share the same hart. 1993 corvette 42k original miles. Cold start. Fires right up and Runs perfect! Leaves a little rubber on the road under hard pulls. AFTER it warms up , back to the sputtering at 3k.
I suspected a wet opti after the great run this morning. But now it's doing the same thing and no rain in sight.
plug wires look fine. New fuel pump and sending unit too.
new vacuum lines too.
Any thoughts?

 

I know Fred (Injuneer) will be along soon enough. I doubt there’s anyone better at diagnosing LT1 problems. In the meantime, I’ll do my best.

Is the Check Engine light on? Even if it’s not, have you scanned it for codes? If so, please tell us what codes are being reported.

The fact that it works fine when cold and gets bad when warm suggests to me that the problem only exists during closed-loop operation. In case you’re not familiar: on startup, your engine (and nearly all other modern engines) runs in open-loop. This means it’s ignoring the input from several of its sensors and making assumptions instead. The sensors in question are all ones that help it decide how much fuel to inject; it can’t correctly make that determination until some preconditions are met. Ignoring those sensors is called open-loop mode; taking them into account is closed-loop.

Even in open-loop mode, the opti is critical. If I’m right about open vs closed loop, then your opti is fine.

Closed loop mode relies on the O2 sensors, the MAF sensor, and the intake air temp sensor (maybe more? This is where I need Fred’s expertise). One thing you could try is unplugging one of those (MAF is easiest); it’ll set a trouble code, and it will force the computer into open-loop mode, since it doesn’t have one of the necessary inputs. If your problem goes away, that confirms my suspicion, and at least gives you a more drivable car. It’s fairly likely that one of the sensors in question is failing.

At this point it’ll be tempting to start blindly replacing those sensors. Don’t! It’s a waste of both time and money; it could be the wiring for one of those sensors, or any number of other related components. Instead, follow the diagnostic procedure outlined in the factory service manual.

 

I saw your post last night, and was going to ask if you had scanned it for codes. But I didn’t know if the 92/93 Corvette ECM could be scanned by shorting the ALDL pins like the 93 F-Body. From my reference (“Corvette Fuel Injection & Electronic Engine Management”, Charles D. Probst, GM Part # 12497977) it appears that does work: Here's the 93 F-Body instruction:

http://shbox.com/1/dlc2.jpg

Since you have a good cold start, but things go downhill as the engine warms up, two things come to mind:

1 - a problem with the coolant temperature sensor. If the CLT sensor is not accurate, and continues to tell the ECM the engine is still cold, the engine will run excessively rich when it is actually fully warmed up. But the problem would likely result in a very rough idle, and not only show up at 3,000 RPM. Shoebox's temperature sensor test guid (remember, this is based on the F-Body LT1, but I suspect the Y-Body would be the same):

4th Gen LT1 F-body Tech Articles

2 - a problem caused by the transition from open loop to closed loop A/F ratio control. When the engine is cold, the ECM ignores the O2 sensors (they don’t work until they reach 600°F) and uses the “learned” long term fuel trims (LTFT, or BLM). Short term fuel trims (STFT or INT) are not used. The AIR pump is also running at startup, pumping air into the exhaust manifolds, which would cause the O2 sensors to report a lean condition. The ECM stays in open loop until the coolant reaches a minimum temperature (variable, ~120-140°F), the O2 sensors show activity, indicating they are hot enough to work, and a timer (typically 206-seconds run time) times out. The ECM has also turned off the AIR pump at this point.

When the ECM transitions to closed loop, it reads the O2 sensors and starts to adjust the A/F ratio using the STFT's as well as the LTFT's. If you have an issue that is providing incorrect O2 sensor readings, the STFT's wil over-correct, causing the engine to run excessively lean or rich. Engine will run poorly.

Problem #2 can be detected by observing the O2 sensor readings, the LTFT's and the STFT’s, in combination with other factors like throttle position, MAP, RPM, operation of EGR and EVAP systems, etc. We have access to a scanning software that can record numerous engine sensor readings and ECM parameters, and produce a data log that can be reviewed in Excel. Scan9495 was written specifically for the 94/95 F-Nody LT1, and then expanded to include the 93 F-Body LT1. I do not know if it can read the info from the 92/93 Y-Body ECM. I'll check with the author, but I doubt it.

Some real time ECM scanner’s are able to report some of the data, but since the ECM updates very rapidly, the data is only partially captured, and difficult to look at multiple sensors/parameters simultaneously. I've had people copy data to include with a post, and make videos of the scanner screen, but that approach is not always productive.

Let's start w/ checking for codes and checking the CLT sensor.

Additionally:

- is the engine stock? If not, what modifications?

- how many miles on it?

- how old are the O2 sensors and Opti?

- does the engine have aftermarket exhaust headers?

- does the exhaust smell rich?

- have you looked at the spark plugs for fouling or signs of running lean?

- have you checked the fuel pressure?

- can you tie the transition from running OK to running poorly to the 206-seconds required to achieve closed loop?

 

New info. When it does a act up, the SES light comes on, the speedo reads O mph and SYS flashes on the digital display. Today it acted up then normal again then just started running like it had a misfire.
ecm Issues?

"Let's start w/ checking for codes and checking the CLT sensor. "
the shop scan tool FAILED TO MAKE A CONNECTION etc...

Additionally:

- is the engine stock? If not, what modifications?
Yes bone stock

- how many miles on it? 42k original

- how old are the O2 sensors and Opti? O2 sensor are original, likely and the opti is a one year old MSD

- does the engine have aftermarket exhaust headers?
STOCK exhaust

- does the exhaust smell rich? No

- have you looked at the spark plugs for fouling or signs of running lean? Wires look good at night and no cracks. New plugs coming tomorrow.

- have you checked the fuel pressure? I have not

- can you tie the transition from running OK to running poorly to the 206-seconds required to achieve closed loop?
No, it takes much longer. 180 degrees but the problem is not consistent.

 

So your saying shorting the ALDL (DLC - data link connector) pins to indicate the codes did not work?

 

GOT IT! had trouble logging in.


I did replace this but it didn't solve any issues

Jumped it and read the morse code


27. throttle switch open
28. Transmission range pressure switch
43. ESC circuit / knock sensor signal
69. TCC stuck on (4L60-E &4L80-E)
A/C head pressure switch circuit

 

Assuming you counted the flashes correctly, and those are the codes, where did you get the code definitions? They don’t all necessarily match my Corvette fuel injection manual definitions. The Corvette manual definitions match the F-Body definitions, in part.

DTC 27 = Quad driver #2. In the F-Body, that refers to transmission codes (4L60 TCC Control, or reverse lockout solenoid on the manual). Given the Corvette transmission applications may be different than the F-Body, I have no idea what it actually covers. (Is yours auto or manual?). The Corvette and F-Body 6-speeds are a different manufacturer. The 4L60E which you reference later was not used in the F-Body until 1994.

DTC 28 = Quad driver #3. In the F-Body that covers cooling fan relay controls and A/C relay. Again, the Corvette seems to have an outboard module that controls/programs HVAC that the F-Body does not have.

DTC 43 = ESC Circuit. Seems to match the corresponding F-Body code for the knock sensor circuit. When that code sets, the PCM starts to retard the ignition timing almost full time. That will make the engine feel sluggish.

DTC 69 = A/C Clutch Circuit. Matches the F-Body description, but that isn’t going to cause the engine to run poorly.

I have to be honest.... I have no expertise in the fine points of the Corvette LT1 application. I can provide generic info that probably applies, as in post #3, But that's not likely specific enough to solve your exact problem. And that's in spite of the fact I am a 30-year Founding Member of the National Corvette Museum......

 

1) O2 sensors. I’d do a new set of O2’s they are $11-12 a piece

2) Coolant temperature sender in the water pump

3) optispark

 

Found this on corvette central. Is this correct?

 

Opti is new. I'll try the O2's.
and I'll try pulling more codes today

 

Doesn't match the F-Body. Did you even look at the link i posted in #3 above. My Corvette manual indicates the F-Body is the same as the Corvette with regard to pins A and Pin B. Pin A is the ground, and pin B is the ECM diagnostic request. The Corvette manual show pin G (lower left corner in your picture) is for the Fuel pump. Just illustrates the problem of trying to equate the Corvette LT1 to the F-Body LT1.

In your photo the pins are:

F-E-D-C-B-A
G-H-J-K-L-M

That show jumping A to G.

F-Body:

 

Yes I did read it and understood the FUEL PUMP is located there. I watched a Video on a 92 vette. Jumping A to G. Cleared several codes and started fresh. The digital dash prints out the codes without having to count flashes. Cool.
here's what I got. C stands for "current" H for "history"
1993 corvette trouble code C41 "loss of serial data communications"
AND. C72 knock sensor.
So off I go looking for bad connection first.
Thank you for getting me on the right track!