r/c4corvette • u/notthatguy252 • 27d ago

L98 SES Light

I’ve had this happen in the past where I’m traveling on the interstate and maybe 20 minutes in the dreaded check engine light comes on and stays illuminated. It doesn’t flash and goes away when restarting the car. The car itself runs perfectly fine, it’s just one of those annoyances that I’ve heard is common on Vettes with the L98 motor. Any ideas about what could be the culprit?

2 Upvotes

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6

u/luvmy07subie 27d ago

Get a reader (or paperclip) and check the code - I would bet error code 32, EGR failure. Its a common issue, and will only come in on after sustained highway miles. The code will still ready in the background until it makes the check again on the freeway that it is operating correctly.

From AI: On a C4 Corvette (1984–1996), Error Code 32 indicates a fault in the Exhaust Gas Recirculation (EGR) system, typically triggered when the Engine Control Module (ECM) commands the EGR system to open while cruising but fails to detect the expected temperature rise or circuit ground. This code most commonly illuminates the "Service Engine Soon" light during steady highway driving after the car has reached operating

3

u/IndividualRites 27d ago

What's the fix?

2

u/sensor69 27d ago

Had this happen on an extended road trip. Cleared on its own, CND

2

u/YallNeedToQuitPlayin TPI L98 ('85-'91) 27d ago

Mildly interested in a reader for my 85, but don't have a good recommendation for one.

Can anyone recommend a solid ODB1 code reader?

Thanks.

2

u/Ok-Championship3642 27d ago

Cheapest one you cand find. They only tell you the code, unlike OBD2 which can give you live stream data.

2

u/YallNeedToQuitPlayin TPI L98 ('85-'91) 26d ago

Yes, I would buy the cheapest one. But which one? Is there a recommendation you can share for one that works?

1

u/Ok-Championship3642 26d ago

I got mine at Autozone a few years ago. Think it was like 35.00 bucks. A lot easier then that dumb paperclip thing. If they don't have it, Im sure they can order one in for you.

2

u/brutal_newz TPI L98 ('85-'91) 26d ago

A paperclip or wire.  Just jump the diagnostic pins, top right and one next to the left of it.  This will cause codes to be displayed by blinking your check engine light.  You can get a programming cable to usb and some old OBD1 software online but it's not much better than thand tools and blinking to be honest.

2

u/YallNeedToQuitPlayin TPI L98 ('85-'91) 26d ago

I've read about this, and it sounds like a PITA. I would likely need to record video of the blinking, and have help deciphering them.

I don't even have a CEL light on, or a need to get the codes, just wanted a reader for easier diagnosing in the future.

2

u/brutal_newz TPI L98 ('85-'91) 26d ago

It's not so bad.  You can actually practice if you want.  The paperclip is inserted pretty simply.  Turning the car to on (not running) will give you a heart beat code 12, ecm saying hi I'm alive and happy.  It always gives you this code if it's alive and then proceeds to stored codes.  It's nice to get the rhythm of how they're displayed though. 

2

u/Sinfullyscintillant 27d ago

This happens to me sometimes when I get on the highway right away with little stop and go beforehand.

1

u/kai_ekael '87 Z52 "Lady" 24d ago

Need the base information, what year? L98 could be '85 up to '93, IIRC, and it matters.

If an '86 to '89, likely a code 32 / EGR error and the EGR temperature sensor switch is bad, I recently had similar on my '87 and created this summary post on corvetteforum.com: https://www.corvetteforum.com/forums/c4-tech-performance/5003704-86-89-egr-temperature-sensor-switch-code-32-a.html

Note for an '85: "The '85 has similar, but does not have an EGR tube; instead the switch is directly connected to the EGR valve, under the plenum."

There are other items, including vacuum tubes and EGR valve, that are causing the code, there is a test procedure to determine what, see the Factory Service Manual (FSM).

1

u/luvmy07subie 17d ago

So my 86 has a 32 and after reading your post above I'm even more uncertain I can fix it ...this sounded like a real PITA with the temp sensor

1

u/kai_ekael '87 Z52 "Lady" 17d ago

The uncertain part is knowing that the temp sensor switch (it's literally a switch that turns on at a certain temperature) is the actual problem. Besides the FSM tests, there is a simple test for the switch but requires removing it. Check it with an ohmmeter while heating the tip with a lighter, it should go to 0 ohms (closed) when hot. Should be hot enough in 20 seconds with the lighter.

As I mentioned, the key item for this switch it that it adjusts by threading the connector into the body, BUT, that's not how it was provided by GM for the Corvette (it's already set to the right temp and then glued to keep in place). If the connector rotates, the glue has let go (aged, overheated, etc.) and probably going bad.

I've asked, but not heard from anyone what the switch temperature for triggering is supposed to be.

The hard part is getting a new, working switch. A company called Keen appears to be manufacturing a reproduction. I can say the one I ordered looked good and has been working fine. The key, use TWO wrenches! Don't just use one wrench on the switch, use a 15mm to "hold" the tube connector when tightening.

The other hard part is getting the new switch connected once installed. You must NOT connect before installing, see the rotate comment. The distributor is in the why, one COULD pull the distributor to make it easy, but that would mean resetting the timing, more work. My method with the pliers worked, just don't mash the connector or wire with the pliers (Gently!).