1992 gmc truck engine light

My 92 gmc pickup has started showing the service engine soon light.
It only comes on after driving for a while at highway speeds. It stays on until I stop. When I start it back later, no light. Also I can drive for a long time at slower than highway speeds and never see it. Any ideas?

 

you need to chek codes for starters and see what the computer has stored in its memory you can chek codes by jumping the aldl connector a and b pins under the driver side dash when looking at the connector should be the top two pins on the right,now turn key on and count your chek engine light flashes code 12 will be the first code it will flash each code 3 times befor going to the next and then the cycle will repeat. given that it only occurs at highway speeds it could be an egr fault or esc fault but will have no idea until you retrieve codes. you can go to the following website and see if they have your code listed to find out the definition of the code but does not neccasarily mean part is bad for the code listed. you can also post back here if needed.
http://www.cybrrpartspro.com/Manual%...delLookup.html

 

Well, I didn't get around to looking at this until this week
I took the truck in for a state inspection and it failed emmission test.
So, I checked the code, it had stored a 32. I took the EGR valave off and sure enough it was bad, wouldn't hold a vacume. Replaced it and I also went ahead and changed the spark plugs and air filter.
From what I understand, the bad EGR valve could cause some idle problems. What do you think

BTW, the emmission failure was for HC at low speed (676 rpm). The high speed (2100 rpm) passed all readings. The idle speed seems a little low (it has a 5-speed), but not sure if I should try to increase it.
?

I can take the truck in for a re-test for free within 15 days, so I would like to maximize my chances.

The truck runs real well and gets 17mpg in town and 19-20 on the road, so I really didn't think about it having emmission problems.

I appreciate the advice.

 

an egr valve usually only causes idle problems if it is stuck open a little which would also cause a rough idle, i would suggest you check all your vacum lines and replace any that may be leaking, and to change your oil before having it retested, if it hasnt ben changed in the last 1000 miles, cause it will lower your hc readings even if its not due for an oil change, one other thing you may change is the o2 sensor if it hasnt ben changed within the past 60,000 miles , the o2 sensor is located either in the exhaust manifold or exhaust pipe, and shoud be changed as maintence and can cause excessive emissions by not allowing the computer to go into closed loop operation or being slow about going into closed loop operation.

 

OK, the oil was changed about 700 miles ago, but it won't hurt.
The o2 sensor hasn't been done for a long time. Would that affect the HC in the exhaust?

 

yes it can raise hc and will raise co also but may not necassarily fail on both it may be a little high and still meet the requirements of the emissions test.

 

While you're at it, check/change the pcv valve and air filter...They can effect emissions also...