1991 Cali EGR plumbing

[Scota4570]

I did not pass smog because the EGR was not functioning. I have replaced everything related. The OG tech noticed that Port "P" on the vacuum modulator was not receiving vacuum. He refused to work on it because he was to busy. I have found nobody else who has the first clud on these old systems.

The EGR is functioning correctly. IT is clean inside. I spent some time looking and diagnosing with a vacuum gauge. I found that the two ports on the throttel body act differently. The back port is open and free, it pulls vacume nicely. The front port seems moslty blocked. I ran a pipe cleanet in the tube with alcohol and it was not super dirty. I had the poer P on the modulator plumbed to the front moslty blocked tube in the front of the throttel body. That may have been a mistake on my part.

I have two modulator and they both test the same.

Do any of you know if modulator "P" port goes to the fron or rear tube on the throttel body?

Being in California I must make this work to register the vehicle.

See attached for "schematic" and drawing of syutem.

Thanks in advance.
Scot

 

[Scota4570]

Some time later...........

I guess a new day brought me luck. To got inside the throttel body. The front port on the throttle body was plugged. I unplugged it. I was hesitant because I was not sure if there was something fragile inside the throttle body. They were both just holes into the intake barrel.

I figure out the plumbing of the modulator. I think. The schematic is not a schematic, just a drawing showing parts layout.

Mr. T could have been a little less cryptic and shown what actually connects to what from the hard tubes. Does modulator P go to the front or rear port in the throtel body? I am used to electronic schematics. They give more infornation than a parts layout drawing.

Both P and R on the modulator are getting vacuum from the throttle body.

The EGR works when I manually pull a vacuum on the Q fitting of the modulator.

I am not sure how to see if the EGR is working when connected to the modulator. How do I determine that?


Regards,
Scot

 

[Marco Lau]

I think P and R ports are interchanged. They both pull air from the intake tube.

 

[Scota4570]

Does that mean they are interchangeable, or I have them hooked up backward?

 

[Marco Lau]

Does that mean they are interchangeable, or I have them hooked up backward?

Yes, they are pulling metered air from the same place.

 

[tech_dog]

My 1994 throttle body has tiny raised letters at the vacuum line connections, indicating which line goes where. I remember the schematic not having enough info, but the letters on the TB completed the mapping.

 

[jonheld]

I am not sure how to see if the EGR is working when connected to the modulator. How do I determine that?

Starting on page EC-6 of the FSM there is a test procedure for the entire EGR system.

 

[Scota4570]

It passed smog with flying colors. The emissions are lower than they ever have been while I owned the truck. It runs much smoother too.