Ryann Chandler

93 Corvette, LT1, 6sp, Stock.

Alright, i don't know where to turn to for some help on this issue. I'll start with the symptoms. On my way back in from a drive a few weeks ago, right after turning into my neighborhood, the car started running incredibly rough. It was stuttering under any acceleration. Luckily I was able to limp home to the garage.

The car starts and idles just fine. but if I give it any gas, it bogs down at about 1500 rpm and stutters. I can smell gas, so I can only assume that it's running incredibly rich. The codes that get displayed start out with c12, then modules 1,4, and 9 all show no codes.

What i've done and checked:
The fuel pressure seems good and charges on when the car is on. It'll hang out just below 40 psi. I changed the fuel filter.
I changed the spark plugs. They were all black on the end which should confirm how rich it's running.
The injectors all seem to be ticking along, I did the screw driver trick and listened for the click on all of them.
I changed the map sensor and the TPS sensor.
No blown fuses in the fuse box.
I borrowed my buddies OBD reader which can read OBD1, when I try to use it and pull codes, the cars fans will kick on but the reader will then say that it can't connect to the cars computer.
I bought an aldl cable and aldl android, and it too says it can't connect to the computer.

It seems odd that i'm not able to connect to the computer and i'm wondering if that is an issue, but It's funny that the car will throw the c12. I seems like if there was a computer issue, i wouldn't see the c12 code.
The only other sensor I can think to check would be the 02 sensors, but i also feel like those would throw a code if they were going bad, which again could lead me back to something being wrong with the computer. I'm not totally sure how I can even check the 02 sensors knowing they're behind the cats.

Anyway, wondering if anyone has experienced anything like this before. I might have to find a mechanic who can make a house call.

James93LT1

Try it manually.

https://tech.corvettecentral.com/201...trouble-codes/

dizwiz24

I would start with o2 sensors.

damaged o2 sensors (from an idiot shop getting silicone sealant inside my engine) caused spark plugs to foul out resulting in a no start condition.

since i was using a gimmick plug (ngk iridium) , the tiny electrode (bc they were too cheap to use more iridium) surface completely soot covered preventing enough plugs from firing to start the engine.

if that doesnt fix it, look at optispark also (ie. No or internittent spark causing it to run rich)

stew86MCSS396

To get ALDLdroid to work, you need an ADX that will command the CCM to shutup. It's possible to look at key on engine off but be advised once you start the engine, the CCM will start to chatter on the ALDL line. Workaround to this is to start the engine first then connect to ALDLdroid.

Ryann Chandler

Thanks @James93LT1 Yeah this is how I tried to pull the codes in the first place. Currently it'll flash c12, then cycle through the modules and show no other codes.

Ryann Chandler

@stew86MCSS396 Ok I'll see what I can drum up. After downloading the app i selected the adx for the lt1 hoping it would work. I bet there is probably something better out there. If you have a link that'd be great otherwise i'll post back if I can manage to get things to play nicely together.

Nomake Wan

The ADX for a '93 is attached to this post, inside the ZIP file. Hopefully it helps. If you have a computer, you can grab the program TunerPro RT and use this ADX to do realtime datalogging as well in case ALDLDroid fails you.

As for your symptoms, since there are no codes, we can assume that whatever the fault is, it's not a wiring fault or a totally dead sensor. Instead, it is going to be a sensor that is sending false data to the ECM, or it will be a bad ECM (though this normally pops the SES light), or it will be a mechanical component with no ECM feedback loop which has failed (such as the fuel pressure regulator). These things will not pop codes. You can start diagnosing which of them it is by datalogging. Look at the data you're getting back from the ECM and see if something doesn't make any sense, then go after that component. For example, I dyno tested my dad's '94, then looked at the logs afterwards and noticed the MAF reading was all over the place. Replaced the MAF, car ran great afterwards. Same with O2 sensors--they will have a very predictable waveform when the car is warmed up, and if they don't, they're bad. But they won't throw a code if they are, most of the time.

Good luck!

pbellone

Opti

93Rubie

How does the car run before it gets warmed up?

The ECM ignores the 02 sensors below 140 degrees coolant temperature.

A scan tool to look at engine data would be VERY useful.

Are the fuel injectors original?

If so, they are known to short out and cause similar issues. You can try a OHM test with a DVOM but its not a definitive test. The best test is a Amp ramp with a oscilloscope with the car running but finding a ACTUAL technician (NOT a mechanic) that can has the tools and the knowledge to use it will be difficult.

Ryann Chandler

@93Rubie I'm not sure if the fuel injectors are original but the definitely might be worth testings. I will say that I do smell fuel when pretty strongly.

Ryann Chandler

@pbellone Any way to verify? I have an inline spark tester that lights up, but i'm not sure how to safely test how much current is going through.

Ozzstar

Since there is excessive fuel (running rich) and no codes I would start my diagnosis by checking and testing the fuel regulator. If that passes okay then do a fuel injector balance test. One or more of the injectors may be leaking excessively causing the rich condition. Report back your findings if you can. Sometimes I get lucky giving solid internet auto repair advice. haha I do have a video on the LT1 fuel injector removal/install if that is your problem. Google "Ozzstar's Cars Corvette fuel injection"
Test, don't guess. Good luck -Ozz (Glen)

Ryann Chandler

@Ozzstar I've watched so many of your videos and they've helped me do all kinds of things to my car! Thanks for replying. I'll do some work this weekend to test the injectors and fuel regulator.

Ryann Chandler

Ok so today I tried to check the fuel pressure regulator and it seems to be functioning ok. When the car is idling fuel pressure is about 38-39. When I remove the vacuum hose, it'll shoot up to about 42 and stay steady. The vacuum line was also bone dry.
With the regulator vacuum line attached and I give it throttle, the fuel pressure will jump to about 40 where it starts the hesitation.
I guess what confuses me here is that the car still idles perfectly fine and there are no codes... I guess the next thing to check are the fuel injectors. I can figure out how to check voltage but what other tests can I do?

JimLentz

It is easy to check the resistance of the injectors and could isolate a problem with coils.

Nomake Wan

This sounds normal. As you give the car more throttle, adding load, the vacuum available will decrease. This decrease in vacuum will cause the fuel pressure regulator to add more fuel. Removing the line is the same as telling it you're at full load, which is why it jumps to 42.

So yeah, as above, now that you've confirmed fuel pressure, it's time to look at the injectors and the ignition system.

MatthewMiller

The fact that it idles okay and runs well at very low loads/rpm but isn't throwing any codes makes me think about the coil. IME, when coils start to fail they often do so gradually. The symptom is that they fail to create a strong enough spark to jump the plug's gap when the cylinder pressure gets high. Cylinder pressures rise and fall proportionately to torque. So at WOT, peak cylinder pressure is the same as your torque peak. When a coil initially starts failing, you might only notice misfiring at that RPM at WOT. But as the coil gets worse, it malfunctions at lower and lower cylinder pressures. At low throttle openings (like just trundling around your neighborhood), your torque peak is much lower in RPM and magnitude. So it may be that you weren't noticing the failing coil earlier because you weren't hammering the throttle all the time, but now it's so bad that just hitting "neighborhood peak torque" (yes, I just made the term up!) is enough to make it malfunction. Also, it seems like a failing coil sometimes runs worse as it heats up. I don't have a good way to conclusively diagnose this, short of replacing the coil.

I don't know for sure if a bad O2 sensor is supposed to throw a code on a 93. They don't have downstream sensors to test the cat and sensor function like some later C4s, so I'm not sure how it would know. The key diagnostic here would be if the car runs great at all throttle levels after a cold start, and doesn't start running badly until it warms and goes into closed-loop mode (where the ECM starts referencing the O2 sensors). Someone else already discussed this. If your car runs great before it warms up, then the O2 sensors are a very strong possibility. If you can data log the car, you'll probably see a flatline or some other anomalous response from one of them, but honestly they are maintenance items and if you don't know when/if they were last changed, I'd replace them as such.

PS - On modern cars, where there is one coil pack per cylinder, it's a lot easier to diagnose a failed coil!

Nomake Wan

There are multiple error codes for the O2 sensors in the 1992-1993 ECM. The 'circuit' ones check to make sure the sensors are present and that they aren't shorted to ground (Code 13 Left, Code 63 Right), and there are two extra codes to make sure that they actually fluctuate as expected during closed loop operation (Code 44/45 Left, Code 64/65 Right). However the latter codes will only fail if the 'flatline' is permanent. If the sensor fluctuates every so often but not as quickly as it's supposed to, the ECM will not throw a code--it will adjust fueling.

So as you said, it's still a good idea to replace the O2 sensors if you don't know when they were last replaced. They're cheap and easy to do.

Ryann Chandler

Alright everyone. Thanks again for all the helpful suggestions. No luck yet, but I made a video to show exactly what is happening. The first half is a cold start (directly after a start where I got the bright idea to make a video), and the second half is a closer look at the engine bay. In the second half, I can actually hear what seems to be some kind of knocking that wasn't very audible until I heard it on the video. I'm not sure it's anything to worry about (I'm not sure its a knock at all I just don't know how else to describe it). Here's the link:


I was able to pull off the ignition coil this weekend and check it with an ohm meter. I replaced the coil about 2 years ago on the car after I got stranded when it actually died. This time, everything was in spec with what was listed in the hayes manual for proper ratings on the coil. The video also shows the car running cold and the problem persists. I've read a lot that as the car heats up the coil may misbehave more. Here, the symptoms are the same regardless of engine temp, which again makes me think it might be a sensor. O2's might be the next thing to check/replace, but I'm not totally sure how to get to them.

Thanks again everyone for chiming in. There are still plenty of things it could be, O2's, Opti, ... some other part I don't know about yet. This is the joy of old cars right? I usually spend my week researching how to test/fix something and then try and do it on the weekend.

MatthewMiller

If the problem is the same with the engine hot or cold, it is not the O2 sensors. The PCM ignores them completely until the coolant temp is warm and it goes into closed-loop operation. So I'd say you can scratch that one off for now. That doesn't strike me as a coil problem or opti problem, either, just based on sound...for whatever that's worth.