I've been having problems with my EGR system on my '88 K2500. 350TBI.
this is what I've found out. the EGR valve itself is fine...
disconnecting the vacuum line and pluging it into the vent on the EGR
solenoid stops the surging. moving up the line, the EGR solenoid is a
brand new NAPA Echlin piece that doesn't leak vacuum when the
metripack connector is disconnected. back probing the metripack shows
an intermittant spike of up to full system voltage (on my digital
VOM... my analog is buried in storage) when you blip the throttle in
park. I've not hooked up the MT-2500 to it to see if the P/N switch
is registering in park or not.

my question is: other than a bad output transistor in the ECM or a
wiring problem, can anything else cause the spike? (which opens the
solenoid, which opens up the EGR valve, which causes jerking and
stumbling)

TIA,
Bret

Just a thought, could a bad TPS cause this? If there's a bad spot and it
suddenly signals full throttle to the ECM (or idle, I can't remember when
the EGR opens) maybe that could cause your spike. A shot in the dark but
what the hell.

WOT is when the EGR opens.

GMC Gremlin

"SBlackfoot" <trypticon (AT) sympatico (DOT) remove.ca> wrote

"GMC Gremlin" <death (AT) becomes (DOT) me> wrote

No, it opens at crusing speed (35 mph and over) with constant TPS signal.
Usually opens on the highway.......................

Doc

Well, I was clarifying that it shouldn't be open at idle.

GMC Gremlin

""Doc"" <noway (AT) nope (DOT) com> wrote

"GMC Gremlin" <death (AT) becomes (DOT) me> wrote

Big difference between "should be open at idle" and "WOT is when the EGR
opens."

Doc

Big difference between should and shouldn't too :-)

Isn't it open at WOT as well?

GMC Gremlin

On Wed, 13 Oct 2004 04:03:02 GMT, "GMC Gremlin" <death (AT) becomes (DOT) me>
wrote:

hooked up, it sounds like I've got about 750HP under the hood w/ the
way the idle lopes and stumbles.

-Bret

Bret
A simple way to test egr open or closed at idle:
disconnect vacume line between egr controlling solenoid and egr
valve(assuming your ride is so equiped) , then run the engine.

If idle has smoothed out, the controlling solenoid would appear faulty.You
could prove this out by sticking vacume gage on the downstream side of the
solenoid.Should be no vacume. Then, with engine still running, connect a
vacume source directly to egr valve, if idle becomes lumpy with vacume
applied and smooths out when released, it would prove egr valve is
serviceable.

Further, I have proven that the egr is controlled open under most part
throttle settings and will close at WOT by connecting (tee in) vac gage
between control solenoid and egr valve. If I'm not mistaken, at WOT, ecu
control goes open loop therefore closing egr.

If you leave the vac line between the solenoid off , go for a drive, chances
are the ecu will set the egr code because map sensor has not sent the
correct value signal back to the ecu.
Ecu is looking for set values based on CTS, TPS, MAP, Knock and VSS(again
assuming all sensors are working properly).

Doc feel free to correct me...

Respectfully submitted
Rolf

"Rolf" <nozel99 (AT) hotmail (DOT) com> wrote

Dunno if that was sarcasm or not, but I agree with everything you said. If
it's loping w/ EGR attached and removing the vacuum source from EGR stops
the loping, the problem is either the EGR solenoid OR the signal being sent
to the solenoid. Even if the solenoid is ok, it's still going to open when
it shouldn't if the ECM tells it to.

I'd hook up a vacuum gauge directly to the EGR solenoid when it's being
symptomatic and see what the hell it's doing at idle. If there is vacuum to
the EGR line at idle, I'd then check to see if the ECM was telling it to do
so by backprobing the signal wire and checking the voltage.

Doc