1997 J80 - EGR VSV Function - Is it supposed to open up when 12V is removed?

Joined
Oct 9, 2018
Messages
117
Location
Santa Rosa, CA
Hello,

I am troubleshooting an issue on my '97 J80. In this troubleshooting I have the upper intake plenum off the engine and am testing the two VSV's on the bottom of the upper intake. One is for fuel pressure, the other is for the EGR. The EGR VSV meets the resistance measurement across the terminals. When 12V is not across the terminals, it flows air between the two ports. When 12V is applied, it holds vacuum. The question I have is if the VSV is supposed to release that vacuum immediately when 12V is removed across the two terminals. Above 10 in-Hg my VSV holds the vacuum when the 12V is removed. From 10 in-Hg to 15 in-HG it will slowly bleed off the vacuum and below 10 in-Hg it will release the vacuum. At 20 in-Hg it holds the vacuum permanently. I let it sit for 20 minutes and it was still holding the vacuum, even with the 12V removed. The fuel pressure VSV does not do this. It opens the valve the second the 12V is removed.

I am thinking I need to replace the EGR VSV. It is original also.

Any comments or help would be appreciated.

Thanks,

Mike
 
Joined
Sep 15, 2016
Messages
1,053
Location
Chattanooga, TN
It should open/close immediately on adding/removing power. Sounds like it's blocked/clogged or failed closed.

If all else in the EGR system is working properly I believe you should be seeing a PO402 code due to the EGR opening more than it should as the ECU has lost the ability to open the VSV and prevent vacuum from building and thus opening the EGR valve. The opposite would be a PO401 when vacuum never builds up and/or the EGR valve doesn't open for whatever reason (leaks, clogs downstream of EGR, failed EGR, etc.).
 
Joined
Oct 9, 2018
Messages
117
Location
Santa Rosa, CA
It should open/close immediately on adding/removing power. Sounds like it's blocked/clogged or failed closed.

If all else in the EGR system is working properly I believe you should be seeing a PO402 code due to the EGR opening more than it should as the ECU has lost the ability to open the VSV and prevent vacuum from building and thus opening the EGR valve. The opposite would be a PO401 when vacuum never builds up and/or the EGR valve doesn't open for whatever reason (leaks, clogs downstream of EGR, failed EGR, etc.).
Thanks for the reply. No codes being set. Issue started up in the Sierra on a trip this week. Rough idle. Stumbles hard when I tip into the throttle. Once I get the RPM up, it smooths out, but there is some fish bite misfires happening Drives pretty good on the freeway. Made it back to Santa Rosa just fine, but had to cut the trip short due to poor engine performance on the trail. On one fire road, in high range, it could not pull itself up a steep incline. Pedal to the floor, engine surging, missing, etc. Does not stall, stays running, just poorly. Put it in 4 low and was able to get it moving again. That is where we pulled the plug on the trip. It is not a transmission slip. Definitely engine related. Reving the engine with the transmission in Drive and foot on the brakes recreates the same problem. Wish I had a code to get me in the right direction.

I will get a new VSV on order. And, continue checking other things. Spark is good, wires meet resistance spec, cap and rotor are newer, coil meets spec fuel pressure is good. The #3 plug is a little dark, does not match the others. Throttle body is clean, IAC valve works fine. Throttle position sensor checks out. Fuel pressure VSV checks out. EVAP VCV and charcoal canister work as per the manual. I smoked the engine before tearing into it and could not find any vacuum leaks.

Thanks,

Mike
 

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