1992 LT1
How do I diagnose the reason my "Service Engine Soon" light comes on erratically. I drive on the highway for about 20 to 30 minutes then the light comes on. After about 20 minutes the light goes back out. This activity is very erratic. What can I do to figure out the problem?

I know this light is linked to fuel, ignition, and emission controls. How can I narrow it down?

 

could be emissions-related

check the codes

easy to do with a paper clip... there's a how to about it on the forum

 

Welcome to the club. My 92 does the same thing but have not found the cause. I noticed it more when I was on the Freeway kicking back about 85-90, it came on then the wifee starts getting nervous that we might break down, hell it was running way to good to break at this point. So I pushed it a little more and it went out. If you find it let me know.

 

How do I locate the code connector to short out so I can get the code?

 

The DLC connector pin out to pull the codes is:

___________________
| M L K J H G |
| A B C D E G |
______________

Best I could do to draw the connector. And its upside down from the drawing in the manual because the connector is mounted that way. This is about how it will look.

Ignition OFF
Using a paper clip Ground Pin G to Pin A
Ignition ON

Press Trip Reset until 4.0 is displayed on the trip monitor.
Press Trip/Odo until 4.1 is displayed

Press Eng/Met to display the codes.
Use Fuel/Info to backup

---- indicates the end.

After repairs are done, repeat above except press Trip/Odo until 4.7 is displayed. Press and hold Eng/Met until --- is displayed. Codes are cleared.

From what your describing I betting you'll get a 32 EGR. It could be the EGR valve, vacuum harness or EGR solenoid. All three are not too expensive.

 

94 cpe, blk/blk Just curious, how long will a code stay and be available if the error light is no longer on?????

 

It varies. Most codes will clear after "X" number of cycles with no more problems (x determined by the general), some (not all) will clear if you disconnect the battery, and all will clear with the proper clear reset procedure.

 

my 91 convert has done the same thing since new, light illuminates periodically for no apparent reason and then goes out. It's the most annoying feature on the car. Three dealerships have checked out the car and can give no explanation other than - "it just does that".

 

Since EGR is mostly emissions related, the engine for most instances seems to be operating normally. To get this DTC to set, the vehicle must be above 50 mph, the MAP is as listed in its table (depends on altitude), TP sensor is between 8% and 30%, and no more than 4% change in TP sensor.

You'd probably detect the issue if you were monitoring emissions on a cold start.

That's my guess without knowing what specific DTC is being set.

 

damn... service engine soon light... time to sell...

 

I had same problem w/ my '95 - intermittent "Service..." light, and no help from the shop, even after keeping it for a week - "no problem found", I think they said.

So, I bought an AutoScan and set it to monitor mode. When the "Service..." came on, I punched the button and it captured an O2 sensor locked up. That one was bad and I replaced the pair. However, the next time it occured, it the other manifold O2. The third time an O2 again had locked up on the opposite manifold - 3 in 2 years.

In any case, when the O2 "froze" and the "Service..." light came on, the computer went into "open loop" mode and the light came on. But, when conditions were such that when the operating O2 reading was near that of the one not opperating (default?), the light would go out and the system returned to "closed loop" operation until next time the two were no longer in agreement for a period of time.

Neither of the sensors replaced showed any signs of oil contamination - not black/sooty/oily; normal. I thought 3 sensors was a bit much, but since I have done some research on the O2s, and it isn't all that uncommon to have problems w/ O2s (in particular). Knock on wood, so far they are working fine now and have for over a year and 14,000 miles.

Just a thought!

P.

 

We need to wait until he gets a chance to pull the DTCs.
Reg if you haven't done this before its pretty straight forward. One thing that surprised me was when you insert the paper clip into those two pin slots, the fans come on. I was not aware that would happen the first time I did it but from what I understand its normal. It was a little surprising.

 

We need to wait until he gets a chance to pull the DTCs.

Reg if you haven't done this before its pretty straight forward. One thing that surprised me was when you insert the paper clip into those two pin slots, the fans come on. I was not aware that would happen the first time I did it but from what I understand its normal. It was a little surprising at first.

 

Here are the two code numbers I received for my 92 vette.

Code 26

Code 32



What do you all think should be my next step? Can I still drive the car?

 

Code 32 is as I described above. The EGR PN 17113382 is 48.37 from Superior, the solenoid #1997223 is 16.03. If your vacuum harness is leaking that is a discontinued part. But I know where they are to be had I just have to look it up.

The 26 is Quad-Driver Module #1 circuit. The ECM is used to control several components such as the air pump, its relay, EGR solenoid valve and air bypass solenoid valve. The ECM controls these devices through the use of a Quad-Driver Module (QDM). When the ECM is commanding a component ON the voltage of the output circuit should be low (near 0 volts). When the ECM is commanding the output circuits to a component OFF the voltage potential of the circuit should be high (near battery voltage). The primary function of the QDM is to supply the ground for the component being controlled. Each QDM (there are three) has a fault line which is monitored by the ECM. The fault line signal status can be displayed on a Tech 1. The ECM will compare the voltage at the QDM. If the QDM fault detection circuit senses a voltage other than what is expected, a DTC 26 is set.

The diagnosis is a little complicated. If you want to do it yourself I can get you photos of the diagnostics procedure to run which will help isolate the fault.

Yes you should be able to still drive it, gas mileage may suffer some.

I cleared my 32 by replacing the EGR, solenoid and vacuum harness. The reason I replaced all three pieces is the first two relatively obvious but the harness had become hard and brittle from heat and it was probably time to replace it anyway. It was like $8.00. plus shipping. You can run all the diagnostics to isolate which part quite detailed or just spend 70-80 and replace the parts.

Personally I would deal with the 32 first since its pretty straight forward then clear the codes and see if the 26 is still being set. Or drop by the dealer and have them monitor the voltage on the QDM with a Tech 1. (Or do it youself with a DVM and the procedures).

Hope this helps.