My truck is a 1996 F-150 with a 5.8L. Ever since I bought the truck, it has had a sort of surging stumble at around 1200-1500 RPM. It is more noticeable under a slight load - for example, slowly driving up a hill - but I can reproduce it by opening the throttle with the truck sitting still. I recently replaced my PCM to solve another issue, and I was hoping this issue would disappear as well; the other issue is gone, but this stumble remains.

I did some poking around and discovered that it is definitely EGR related - it goes away if I remove the vacuum line from the EGR valve. I stuck a beer can with a small hole in it (maybe 1/8", I poked it with an awl so not precise) between the EGR valve and the intake manifold as a restrictor plate (similar to the mod the 4.9L guys do), and the stumble seems to be gone.

I know the OBD-II trucks use a DPFE sensor instead of an EVP sensor like the EEC-IV system, so I'm assuming that this will probably set a trouble code once I drive the truck a little more. I would like to fix it correctly, but I'm not sure where to go next. Any suggestions?

 

Sounds like the EGR control solenoid is bad and leaking.

 

Ian, thanks for the response. I have tested that resistance across the EVR solenoid is within range, but it sounds like you are saying that vacuum is leaking through the EVR when it should not be. I suppose I can test that with a vacuum pump/gauge, right?

Is the EVR strictly on/off, or is it pulse-width modulated to control the level of vacuum applied to the EGR valve?

 

The EVR is modulated by the computer but it should be 100% closed at idle so that should be easy to test.

 

My vacuum pump is loaned out and I haven't gotten it back yet, but I was at the boneyard this weekend and picked up a couple of EVR's off other trucks. I swapped one in last night and removed my beer can restrictor; after a quick test drive, it appears that the problem is gone. When I get my vacuum pump back, I will test the new one vs. the old one and see what I find.