Too dumb to figure out last steps to troubleshoot EGR

[GetitBilly]

I've got an 87 V2500 w/ a 5.7. It's a 8600 GVW truck w/ NA4 code. I'm to the point where I know that with the EGR hooked up, I've got ~21 in. at idle but revving up to 3000RPM and holding will drop to a steady vacuum of 15 in. I then unhooked my EGR solenoid and see that vacuum stays good @ 21 in both at idle and @ 3000rpm. I did have the MAP unhooked when I did these tests though, because I've also got a rough and surging idle when MAP is hooked up. With MAP unhooked, truck idles good, just a bit higher at around 1000rpm. I also double checked my parts numbers and confirmed the correct valve and solenoid for a 8600 gvw truck.

I read through the great info @ https://www.gmsquarebody.com/threads/exhaust-gas-recirculation-egr-system-1989-tbi.6902/, but I don't quite understand if I've got enough data points to pinpoint either the EGR valve or the solenoid as the culprit. I did all the multimeter tests you can do against the solenoid and think they came out OK, although I won't rule out that I botched something on those tests.

Based on my vacuum readings, does that mean the valve is opening up when it shouldn't? I know that the valve does hold vacuum, meaning the diaphragm doesn't drop with my finger over the port. Thanks for any help!

 

[Charlie]

 

[GetitBilly]

Thanks for the welcome...still working on this thing.

Current status:
-New Map Sensor
-New IAC Valve
-New cap, rotor, plugs, wires
-EGR capped at solenoid so it won't open up for the time being. Pretty sure solenoid (or ECM?) is bad and it was opening up at low RPMs when it shouldn't, causing a hesitation
-Timing set at 0 degrees
-Fuel pressure reads steady at 11PSI. Kinked the fuel line and confirmed fuel pump is capable of putting out 17+ psi
-It idles decent, although sometimes hunts for idle. Responds ok to opening the throttle up when in park (used to hesitate in park, but that went away when I took EGR out of the equation).
-Doesn't throw codes.

Here's the truck going around the block...hesitating, missing, and almost dying:

https://photos.google.com/share/AF1...?key=RjVDRElHNGhodE42c1BaQjlmbG9EN1ZpcVh4VmF3

I'm still learning, what's left on my list to investigate:

-Something wrong in distributor
-Maybe a bad ECM?
-Could injectors be an issue? I can rule out the fuel pressure regulator given that pressure remains at 11psi at various throttle positions, right? Spray pattern looks decent at idle.

Anything else I should investigate?

 

[Honky Kong jr]

Easy way to fix it, is to delete it.

 

[4WDKC]

did you calibrate/relearn the IAC when you replaced it? Have you checked values of the tps when this happens? These old systems dont do a self diagnostic when **** goes wrong.

 

[GetitBilly]

Yes, I've gone through the IAC relearn.

Last night I found the ground strap between the firewall and back side of the passenger engine wasn't connected. I've read that this ground is pretty critical to ensure the ECM does a good job of getting and interpreting clean readings. I can't easily see where this ground strap was supposed to be bolted to - somewhere on the passenger rear block or head? It looks like it would be tough to get a wrench back there and sand down for a clean connection, so I'm just going to try an easily accessible bolt on the intake.

 

[73c20jim]

Anywhere on block or head to anywhere on firewall as short as possible would be my input.

 

[GetitBilly]

Got the braided ground strap from the firewall connected to the same bolt on the back of the engine that has a single ring terminal with 2 ground wires running to it from the wiring harness.

New distributor didn't change anything. But no regrets on replacing it since old one was crusty and rusty and pickup coil was in bad shape.

Where I'm at now:

With MAP sensor plugged in (which is brand new and verified via OBD scanner to be showing proper readings), I'm getting about 15-16in of vacuum at idle. Watching the timing mark as I increase rpms at the throttle and hold them steady, I see timing go all over the place. It dips down quick to around 8 degrees and goes up past the end of the timing mark gauge where it should be...then keeps bouncing back and forth as vacuum bounces up and down, too.

Unplug the MAP and vacuum at idle goes to 21 in. As I increase rpm, I see timing advance increase, no more dips...it seems to be doing what an engine should do.

So if you unplug the MAP, the ECM goes to some default program, right? What does it mean...bad ECM, bad ground???? I'm running out of ideas.

 

[Dbelk33]

Got the braided ground strap from the firewall connected to the same bolt on the back of the engine that has a single ring terminal with 2 ground wires running to it from the wiring harness.

New distributor didn't change anything. But no regrets on replacing it since old one was crusty and rusty and pickup coil was in bad shape.

Where I'm at now:

With MAP sensor plugged in (which is brand new and verified via OBD scanner to be showing proper readings), I'm getting about 15-16in of vacuum at idle. Watching the timing mark as I increase rpms at the throttle and hold them steady, I see timing go all over the place. It dips down quick to around 8 degrees and goes up past the end of the timing mark gauge where it should be...then keeps bouncing back and forth as vacuum bounces up and down, too.

Unplug the MAP and vacuum at idle goes to 21 in. As I increase rpm, I see timing advance increase, no more dips...it seems to be doing what an engine should do.

So if you unplug the MAP, the ECM goes to some default program, right? What does it mean...bad ECM, bad ground???? I'm running out of ideas.

Did you ever get this resolved? Mine is doing the exact same thing.