Testing Egr Valve On A 1988 Gmc C2500 Tbi

88 GMC Truck · April 17, 2009

I have an EGR Valve in my 1988 GMC C2500 TBI Truck. There are different EGR Valves for that application I am told. The one way to test a good EGR Valve is to do a vaccume test at 10 LBS and if it holds its good. The one that was put on my truck when a new enging was installed has a vaccume line running from an electronic valve (next to the TBI) and my present EGR Valve. I have been told that this style EGR Valve cannot be tested by vaccume. Does anyone have a clue as to what my truck should be using as far as an EGR valve is concerned? The valve I have had vaccume applied to it like normal valves and it acts like it has an air leak all the time and the diaphram doesnt move at all! Any Suggestions? Got me stumped.... By the way I have a rough idle and a miss at crusing speeds. Replaced all Plugs, Wires, AIC valve TPS distributor electronics - Cap and Rotor Exceteria Exceteria Exceteria !!!

rodram · April 18, 2009

I have an EGR Valve in my 1988 GMC C2500 TBI Truck. There are different EGR Valves for that application I am told. The one way to test a good EGR Valve is to do a vaccume test at 10 LBS and if it holds its good. The one that was put on my truck when a new enging was installed has a vaccume line running from an electronic valve (next to the TBI) and my present EGR Valve. I have been told that this style EGR Valve cannot be tested by vaccume. Does anyone have a clue as to what my truck should be using as far as an EGR valve is concerned? The valve I have had vaccume applied to it like normal valves and it acts like it has an air leak all the time and the diaphram doesnt move at all! Any Suggestions? Got me stumped.... By the way I have a rough idle and a miss at crusing speeds. Replaced all Plugs, Wires, AIC valve TPS distributor electronics - Cap and Rotor Exceteria Exceteria Exceteria !!!

Hi.... I am no mechanic, so I checked my newly acquired Haynes manual... (while still written in greek and gibberish ) the manual says that there are two procedures to vacuum test the EGR on 1988-1995 TBI engines.

1. vehicles under GVW 8500

Turn ignition OFF. Disconnect vacuum hose on the intake manifold side of the EGR solenoid valve and hook up a hand vacuum pump. Apply vacuum with the pump. The vavle should hold vacuum. If it doesn't, replace the valve.

2. vehicle over GVW 8500

Turn ignition ON, but do not start engine. Disconnect vacuum hose on the intake manifold side of the EGR solenoid and hook up a hand vacuum pump. Apply vacuum with the pump. The EGR valve should NOT move. If the EGR valve moved, disconnect the electrical connector and repeat the test. If the EGR valve moved, replace the solenoid valve.

The manual says that is the limit of what you can test for the EGR because the operation is intrically related to the operation of the ECM (engine control module)

88 GMC Truck · April 18, 2009

Thanks for checking this. But I think there is more to the storyin that the local auto parts store checked on different valves and one vale stated that it cannot be checked by vaccume. So since my truck is a 3/4 ton with a GVW of 8500 LBS I really need to know. Personally Id like to go with the one you can test. Seems to be more straight forward thats for sure. Thanks again and Ill let the forum know when I get this resolved. Just in case it makes a difference the intake on the new engine is new and for a 1992 engine. This is a duel plane intake. Maybe that can help diagnose the EGR valve situation Hope So !!!!!!

88 GMC Truck · April 18, 2009

UPDATE - I just inquired about a new EGR Valve and the one that is for an 8500 GVW Vehicle is called a "BackPressure" Valve. Apparently the valve operates on exhaust back pressure combined with an electronic signal from the ECM to an electronic controlled valve which inturn activates the valve which then allows vaccume to go to the back pressure EGR valve at a given determined time. This is controlled automatically by the ECM.

OK Now for the hard question for the group. I have a new BackPressure EGR Valve to install. The only problem is there are eleven washers with different ID diameters. Since I do not have the original EGR valve and I have a "new different engine" in my truck, how do you determine which washer to put into the new EGR valve? Is there some way to determine this?

VERY IMPORTANT: When changing your original EGR valve write down the number from the old valve because in the kit with the new EGR valve you can match the original number and the paperwork will tell you which washer to install with the new EGR valve. Go Figure!!