The check engine light comes on only when I cruise on the freeway. What would cause that?

 

If it's an L98 (what year Vette) my guess is an EGR problem.

Jumper ALDL pins A&B (two upper rignt), turn On the ign key and count the SES flashes. After three "12"s will come each stored trouble code three times, followed by another three "12"s.
If and EGR problem you will get a "32" code.

 

You need to find any codes stored in the computer.

File in your profile so we can see what YEAR CAR you have.....or if you don't want to do than.....PLEASE make the year of the car part of your post....IT MAKES A DIFFERENCE on the advice we can give you. Your asking for advice....give us a little help here.......

 

Yes sorry it's a 1988. Thank You!

 

Okay..on the 88 do as 65Z01 indicated....take paperclip and bend into a "U" and short out terminals A & B as indicated above. Count the flashes. It will always flash #12 three times to indicate your connected, then it will flash any other stored codes three times each. After all the codes hae been flashed...it will flash 12 again three times. The codes will give you the starting point to trouble the SYSTEM where your problem(s) is occurring.

Remember that the codes such as the 32 for EGR indicate a system. There are several things that could be wrong within the system. But there are places to start your trouble shooting the problem and narrow it down.

IF you don't have a listing of codes, post what you find and we will get back to you.

 

Thanks John!

 

Pull the codes using one of these methods:

http://forums.corvetteforum.com/1574918379-post18.html
http://corvettephotographs.com/c4vettes/codes.htm

What they mean:

http://forums.corvetteforum.com/1576116036-post35.html

I expanded on the conditions that cause the error codes to be set here:

http://home.earthlink.net/~cliff_har...onditions.html

 

Mind came on after it heated up and the fan came on.

 

I have the same issue but only once in awhile. I thought it was originally the temp sensor next to the distributor. Since it only happens when the mood strikes it to happen I have never bothered to change the sensor. Any ideas on why it would only come on once in awhile versus every time I am on the freeway ?

 

If the light comes on after about 10 minutes of driving, it's most likely the EGR valve needs replacing, verified by the 32 code.

 

I replaced EGR, Temp Sensor, and pig tail but still have the issue.
I read a forum that stated the fan sensor switch and the radiator thermostat not working together that cause this issue.
How to check to make sure the correct parts are install?

 

I have two possible solutions for you...

Really hard to say without pulling up the codes, but I had a very similar and frustrating scenario like that with my '95 so first and foremost, I have to ask...are you running stock sized tires/wheels on your '88?

On my car, the check engine light behaved and would never come on during around the town driving. However, the instant I got on the freeway and drove a bit, the check engine light would come on and stay on. After I reached my destination and shut it off, the light would remain off for the next restart and then would not come on again until I hit the freeway again. It caused me no end of grief until I gave up and then accidentally figured it out.

Years ago when I first bought my car, the previous owner had put on non-factory sized front tires (for a '95) with a lower profile (255/40/17 vs the correct 255/45/17) resulting in a smaller overall diameter front than what originally came on the car. That by itself was fine. The problem appeared when I bought a set of used 18" wheels for it and decided just to try the big ones in the rear. The rear wheels that I bought were not only one inch larger in diameter than what came with the car (18" vs 17"), but they were also mounted with really tall 275/40/18 tires on the back. As such, the fronts were too small and the backs were too tall, compared with what originally came on the car. This resulted in a big discrepency in revolutions per mile between the front and back tires.

One day, when I had the money, I used the other two 18" front wheels I had purchased and installed 285/35/18 tires all around. The instant I did it, the light stopped coming on. After checking all kinds of engine possibilities for the illumination of that light in vain and pulling my hair out, it self-corrected when I changed the tires.

Turns out that the ABS sensors didn't like the previous difference in front and rear diameters. It noticed the difference in revolutions per mile between the smaller front and larger rear diameters, but only triggered the light once I got rolling on the freeway. Once I put the same diameter tires front and rear onto the car, there was no longer a difference and the sensors no longer sensed a problem.

That one drove me nuts for a while...live and learn, I guess!

Now, if that is not the case, then I may have an answer for you anyway.

That same car, a couple years later started giving me a check engine light that would come on only after driving 15 to 20 minutes. The light would stay on, even after a restart, but miraculously would not come on again if the car had a chance to sit for a few hours or overnight. Then the next time it was driven again, it would illuminate, but only after driving 15 or 20 minutes again.

I ran the codes and although it was a couple years ago, pretty sure it threw a code 32. I also recall cursing that the code was so vague as I struggled to diagnose it. As John A. Marker posted, it was a SYSTEM code so it did not point to a specific component, just a problem area.

Anyway, after changing the EGR and the EGR stop solenoid, I still had the same problem...the light kept coming on after 15 to 20 minutes of driving. Car ran great and no big deal except for the fact that in California, a check engine light is an instant fail during our lousy bi-annual smog inspection.

I then changed my next component on the list, an evaporative vapor canister purge valve and that fixed it instantly. All of those parts were genuine GM/AC Delco that I got off Amazon for a great price at the time. It was frustrating as hell until I figured it out though.

Hope this helps

 

What next on the list if that does not work?

 

Check the EGR temperature sensor wire. It is very near the EGR hot line and can burn and short if your EGR insulation is gone. If this is the case, you should be getting the 32 code.

 

Could be a sticking EGR Valve.
Also, the system depends on manifold vacuum to activate the EGR Valve so a problem in that area could cause problems too.

Unless you drive in the desert, a 160 deg F T-stat will solve the problem as the ECM only calls for EGR when coolant temp is >175deg F and you have been at steady speed for a while.

 

These are the conditions to get an error code 32 while "cruising down the road":

EGR temperature switch not low when EGR is commanded
Coolant temperature > 194°F (89.8°C)
EGR duty cycle commanded by ECM > 51.9%
TPS > 5.8%
TPS < 30%
Error codes 21, 22, 33 and 34 not present
EGR temperature switch input on pin C15 not low after EGR commanded
All conditions must be met for > 256 seconds (4.27 minutes)

The last one is what causes the SES light to come on or not, seemingly at random. For example, if you take your foot off the gas and the TPS goes below 5.8% then the timer is reset and starts over.