My 87 is stumbling

[f.monroe]

There have been a couple threads on this similar subject lately . I didnt want to hijack anyones thread so I will ask my questions here . My 87 stumbles and I checked it only to find the EGR activates with the opening of the throttle . If you hold the throttle just off idle , it will stumble and surge like it has a large cam or something . I can see the EGR open as the throttle is opened . I unplugged the EGR and the truck runs fine but I get a little detonation under a load . How exactly and when should the EGR operate ? I unplugged the electrical connector from the Egr solenoid( I guess that is what it is ) and the EGr does not activate . Something appears to be signaling it causing it to open prematurely . Temp sensor ? Frank

[ChevyTech]

It is normal for EGR operation when opening the throttle. The tricky part is to determine if it is opening more then it should.

EGR valves have a spring in them that can get weak causing the valve to open to much or to easily.

Exhaust system backpressure can greatly effect the operation of the EGR valves on many vehicles. Modifying the exhaust system can effect how well the EGR works if it changes the pressure in the exhaust system. Exhaust back pressure (from a restricted converter) or no back pressure (from no converter or mufflers) can have an effect on how much the EGR opens.

Keep in mind there are multiple types of EGR valves. You must determine which type of EGR valve the truck has to do diagnostic tests.

Positive back pressure EGR valves - Should have a “P” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Negative pressure EGR valves - Should have an “N” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Port EGR Valves – The EGR solenoid pulses like a mixture control solenoid/fuel injector to control vacuum supply. Port valves have no “P” or “N” and it is blank after the last numbers on the valve.

TBI trucks are generally Negative pressure EGR or Port EGR systems.

I would suggest only using a general motors replacement EGR valve, if you decide to put a new valve on it.

Here is a site with a good illustration of a negative backpressure EGR valve.
http://shbox.com/1/EGR_valve.jpg

This is a site that has a photo and an illustration about EGR vale numbers
http://shbox.com/1/EGR_ID.jpg

This site explains EGR operation. Keep in mind it jumps from one type on system to another.
http://shbox.com/1/4th_gen_EGR.html

Let me know if your vehicle is stock and if it has any exhaust system modifications. Also determine which type of EGR system your truck has.

Is the EGR valve on your truck the original GM EGR valve?

[f.monroe]

My truck has a complete original exhaust system I think . Cat , Muffler , Stainless flex section before the rear axle and factory exit on drivers side . The truck is extremely clean underneath and I believe it was barn/garage kept by an elderly owner who used it to pull a travel trailer . Truck is supposed to be a 56,xxx original mile truck . It runs great , always starts , doesnt idle rough or anything . I did not notice the problem when I bought it . It sits for periods of time without being ran . I noticed the problem after it had set for approx 2 months without being ran . I started it up and drove it . When I went to put it back in the barn , it stumbled on an incline going in . It seemed to go away sometimes but I think I just missed it . If I stab the throttle and take off quickly , I dont notice it . I only notice it when I am using light throttle moving slowly or taking off slowly . If I hammer it , it will spin the 33x12.50x16.5's on pavement so it has plenty of power . I just thought about the problem since there seem to be a couple 87 questions lately . Thanks for the help . Frank