Hi all, I have a 92 C4 that randomly throws a “service engine” light, and as soon as it does the car will run rough and hesitate from a stop, sometimes it’ll randomly die and start right back up. I pulled codes, they are...

H74 module 1

H16 module 4
H34 module 4

H64 module 9

now I’ve done quite a bit of work to no avail.

I've replaced the opti, plug wires, MAF, IAC, TPS, MAS, fuel pump, Ignition Module as well as Ignition coil, replaced ground strap under master cylinder, tested and cleaned injectors.

nothing has changed the issue what so ever, I’m officially out of ideas, can anyone help?

 

When in the closed loop? Try vacuum leak or EGR. Does it perform fine after you clear code? How and what make of opti? What about fuel filter?

 

Module 1 H74: Do you have an aftermarket radio, or have you unplugged/disabled the HVAC controls? Those two things would cause this code to appear.

Module 4 H16: Opti Low Res Pulse Failure. You say you replaced the opti. What opti did you use? There are only a handful that are worth a damn, and the vast majority on the market are trash. Did you replace the opti harness at the same time?
Module 4 H34: MAP Sensor Low. According to FSM, this will set when MAP reads less than 16 kPa, RPM less than 700, DTC21 not set; also when MAP reads less than 20 kPa, RPM over 700, TPS over 26% and DTC21 not set.

Module 9 H64: TPS Module Failure (TPS Signal Fault). The TPS signal from the TPS module located next to the blower motor on the passenger side of the firewall is not working properly. If there is no TPS code set in the ECM, this points to the actual module or its connectors failing, not an actual failure of the TPS itself. On a 92, the ABS/ASR computer gets its TPS signal buffered through this module so as not to mess up the TPS signal going to the ECM.

Now, all that said, the 92 ECM is notorious for age-related failure. So if you've already thrown the book at it, it may be worth it to try sending your ECM out to SIA Electronics to see if it's broken. If it is, they can repair it and send it back.

Further, if you don't have a factory service manual by now, get one. They're $22 on Rockauto.

 

It’s definitely not the opti, I got a good brand and it’s literally doing the exact same thing no change, no fuel filter I don’t believe it’s fuel related

it does have an aftermarket stereo that the previous owner installed, the opti isn’t the issue the problem hasn’t changed what so ever and yes I wired a new harness in there, not sure about about ECM unfortunately I think I may just have to find a good corvette mechanic to figure this out, little hard for me especially being OBD1

 

You say you got "a good brand," but you should post what brand that is.

Not sure what the car being OBD1 has to do with anything. Best of luck with your search for a Corvette mechanic (though it does make me wonder why you'd bother posting if that's the solution you automatically jump to after people try to help).

 

sorry I did mean to mention the brand, MSD, and also the OBD1comment was because I have plenty OBD2 scanners and they give a lot of info, I don’t believe OBD1 does

 

I did want to mention I finally was able to clear all the codes. Drove the vette till the service engine light came out, stalled on me at a green light but started back up. Once that engine light was up I got 2 codes

Module 4 code H34

module 9 code H63

can this help narrow it down? The modules get me confused some people say 02 sensor some say TBS and also saw some valve comment as well, I’m lost.

 

O2 sensor failure won't have the car die like that.

Fix the code nearest the front of the engine first, so likely the TPS code, then see what comes back.

 

MSD is not one of the good brands for optisparks. The only decent optisparks on the market right now are from Petris Enterprises, optisparksolutions on eBay, or Sac City Corvettes. All other optis are trash.

You are mistaken about OBD1, since you have a misconception that "OBD1" is actually a monolithic standard like OBD2 is (it isn't). GM's 8192 baud ALDL is actually far more advanced than you give it credit for. For your 92, though, you would need a proper scanner to be able to utilize it. Tech 2 is one, another way is getting an ALDL-to-USB cable and using a laptop. I don't have personal experience with using the latter method with a 92, so while I can tell you how to do it without issue, I can't tell you how useful it will be for your particular issue.

I already explained this earlier in the thread. Module 1 is your CCM, which has nothing to do with the car's operation. Module 4 is the ECM, which is the engine computer. Module 9 is the ABS/ASR computer, which has nothing to do with the car's operation. I also told you already what those codes represent. You also need a factory service manual if you intend to work on these cars by yourself. A Helms or Chilton's or whatever is just plain isn't going to cut it. Anyway an O2 sensor, no matter how bad, cannot make your car die. And while Module 9 H63 does reference the TPS, since you aren't getting a TPS code on Module 4, I believe that right now it's a red herring. I even mentioned that earlier too.

 

It sounds like the car is having trouble with the Oxygen sensor, I would replace it with a new one before going too far. The engine needs to see the O2 Data along with What Temperature the CTS says it is as long as the exact amount of air coming into the combustion process.

When you first buy a C4 you should replace the TPS and IAC or verify that they are good working order. I would also replace the Oxygen sensor just because they "wear out" and a new one is likely to improve the efficiency and performance of a L98.

 

Amazing. Every word of what you just said...was wrong. -Luke Skywalker

The O2 sensor cannot cause the car to die, no matter how bad it is. While replacing one on a car you know nothing about is still good preventative maintenance since they're only $15 each or so, that has nothing to do with the problem at hand (as vader86 and I already pointed out).

Replacing the TPS and IAC is absolutely insane. Especially on the 92-96, where doing so also requires that you recalibrate the ABS/ASR computer to accomodate the new sensor. OP has a '92, and as such there's absolutely no reason to just arbitrarily replace the TPS without having done any diagnostics whatsoever. Same with the IAC. Unless you've actually done diagnostics to rule out other causes, tossing parts at it arbitrarily is insanity.

Oh, and by the by...the '92 is the LT1, not the L98.

 

I have had a Oxygen sensor fail while driving and it caused the car to go into Limp Home Mode, not exactly dead But very close.
This was a Pontiac, Not a Corvette. But a Failed Oxygen sensor will cause a Car to go into the "Limp Home" Mode which put my life in danger on a very busy High Speed highway. Most O2's seem to fail "Rich" to prevent any danger to the engine. The O2 on the Pontiac was going slowly and intermittently, you could be driving and all the sudden it would just drop out forcing the car into Limp Mode. I finally caught it on a scanning software and was able to get the dealer to replace the O2. It was only months old when it died on a new Pontiac. When it failed it was producing no signal at all. It did not fail "Rich" like we see on our Corvettes.

Replacing the TPS and IAC were done on my 1988 C4 with it's L98. I stand by this if you want to have a reliable car after buying a Used L98. Anytime your O2 is over 24 months old and in question I would replace it since your LT1 is still OBD1

My Bad, I do not claim to be an expert on LT1's. I stick with the 1986-91 Corvettes primarily. I have tons of experience with early C3's and early C4's but NONE with your LT1 engine.

 

And as I have told you repeatedly in the past, that is not how it works on a C4. Period. If you read the FSM for any year C4 (especially the 1992, since that's the car we're discussing), you'll find that if an O2 sensor were to fail completely, it would merely set DTC 13 and switch from closed loop operation to open loop operation. And if it did not fail completely, it would be incapable of causing fueling to go out so far that the car would stall. In fact, even if the O2 sensor sticks rich, that will simply set DTC 45 which....also puts the car into open loop operation. Same with if it sticks lean; DTC 44, open loop. Please stop spreading FUD, especially when you know and admit that it applies only to a car that is not the one being talked about. It makes it very difficult to help the original poster when time is wasted correcting misinformation.

 

Got the car back from the paint shop, unfortunately that didn’t fix it (joking) so I’m still absolutely at a dead end, finding a good corvette shop or mechanic in my area seems to be a difficult thing to do, I believe I did most of what you guys said and still have the same issue, was running like crap off a green light I turned right and went to merge onto the freeway and the car stalled, started right back up in neutral and drove about 3 miles and right when I was exiting the freeway the light was back off and the car was running normal again.. such a PIA, any ideas I can check next that I can’t remember?

 

@azmattaz06 I'm having the same problem, did you manage to get this fixed?

 

Last time poster was on site was 09-19-2021. Suggest you start a new thread detailing the issues you are having and things you have done trying to solve it. Include year of car.

 

Good question worthy of a new thread. FYI the OP hasn’t been active on the Forum for three years.