When does the egr operate?

Thanks to an overzealous smog technician, my Comanche failed its smog test because there was no vacuum at the egr transducer. I had to take apart my egr and egr transducer circuit to clean out the little backpressure tube, which was clogged, and now I have to take my truck back to have it rechecked. Problem is, while the backpressure tube is now clear I'm worried the egr/transducer system is still not up to snuff.

My first question: is the egr valve open or closed at idle? It appears that the transducer actuates the egr valve (the valve stem pulls away from the manifold) when I rev the engine but then reverts once the engine attains the higher rpm. My question is, when exactly should the egr valve actuate? Should it open only while the engine is revved or should it also stay open at, say 2000-3000 rpm? Should it be open at idle? My Haynes and Chiltons manuals don't explain when precisely the egr valve should actuate.

What's so frustrating about this is that I live in an area of California where oxides of nitrogen aren't even tested for. My truck passed the emissions test...it failed the "functional" test because the technician took it upon himself to hook a mityvac to the transducer and found that it didn't have 5lbs of vacuum. Sheesh.

Any insight on this is much appreciated.

Erik
88 Comanche
4.0 engine

condensed from Haynes manual:

The solenoid should be receiving vacuum all the time. The solenoid's default position (not-energized) is open, sending vacuum to the egr valve. At idle, the ECU should send a signal to the transducer to energize it and close the egr valve. It also should be closed when the engine is cold.

A quick functional test of the system should be to disconnect the solenoid at idle. The egr valve should open and the engine should stall.

If this condition is met, you ought to be able to go back to the tester, pull the wire, and demonstrate that the EGR system works when the computer tells it to. It sounds as if the tester was assuming that the transducer should always see vacuum, and may have been unaware of the solenoid. If this is so, he needs to be better informed.

What Haynes doesn't adequately explain is that as far as I know most EGR valves will also close again when you gun the engine and the vacuum goes low. EGR usually operates only during high vacuum conditions. In addition, I think that the transducer is designed to modulate the vacuum depending on back pressure. The early 4.0 versions had no transducer, but a back pressure valve in the EGR diaphragm, so that it simply leaked vacuum until back pressure reached a certain level. It looks as if the transducer has replaced that function.

By the way, for any other readers of this, this means that if you have an early 4.0 with no transducer on the EGR, it will fail a vacuum test even if there is nothing wrong with it.

The EGR should open when the throttle is bounced open - like for a passing maneuver. The EGR proper can be tested with a vacuum pump and gage, and system function can be checked by watching the diaphragm while revving the engine manually. Help?

5-90
(Gotta go do another smog to-day...)

Oh - where in NorCal are you? I'm down in San Jose...

5-90

This EGR situation is all very interesting. Here's what's happening:

With the engine warm (190 degrees), the EGR solenoid is receiving a constant vacuum signal from the manifold. The solenoid is shut, prohibiting vacuum from reaching the EGR transducer.

When I unplug the solenoid from its electrical lead, it opens and vacuum is passed through, but the engine doesn't stumble or quit. Blipping the engine causes the EGR pintle to move somewhat.

When I apply vacuum manually and directly to the EGR, the pintle moves significantly and the engine stalls. Coincidentally, I notice the EGR holds vacuum (so its diaphragm seems ok).

As for the EGR transducer, is it basically an on/off valve or does it simply lessen--but never shut off completely--the vacuum signal to the EGR? I ask this because even when exhaust backpressure modulates the transducer diaphragm, the transducer continues to pass along vacuum to the EGR albeit at a somewhat different level.

Does any of this make sense? Thanks.

Erik

5-90, I'm up here in Sonoma. Strangely, the particular zip code I live in is exempt from the new Bay Area enhanced smog test, so luckily I get to pay only $35 instead of $95 or so (except for the fact that I'm gonna have to pay again for a retest).

I'm not entirely sure when the transducer does (read: I've not been arsed to figure it out yet) but it otherwise sounds like you are in good workin' order. I see two possibles -

1) The Smog guy didn't know how the system on your XJ works, and goofed on the test.

2) Your shop doesn't do free retests, and needs to make money to pay for installing the Dyno machine.

BTW - are you sure you aren't going to SCII? I went to a seminar on it a few months ago, and it was my understanding it was going to be statewide, and the only way out of it was to be AWD (which is a conversion I have in mind sometime over the next two years...)

5-90

Never having had one of these that actually worked, I'm not sure whether yours is right or not, but I wonder if the solenoid is failing to open all the way. Could it be clogged?

I'll check the solenoid. The lines running to and from it appear intact.

Erik
88 Comanche