TBI running rich*update post 23*fix post45*

[dyeager535]

Well how are the injectors done? I suppose if they are PWM, then the EGR can be as well.

Problem with most testing equipment is that it simply can't respond fast enough. Even on the injectors a light is only so useful....it barely starts to dim before it's triggered again.

What I'm thinking is that it's just doing a very fast (when working correctly) on/off, sort of like the O2 sensor where under cruise conditions it goes rich/lean/rich constantly, never achieving a consistent mixture. The EGR needs to be just as responsive, so I'd expect even if off/on, the commands come so fast from the ECM that the EGR solenoid is constantly being commanded on/off.

If it was PWM, that would mean a variable output signal or variable ground, correct? The manual page I link to shows a switched ground, with straight 12V input. I would ASSuME that if it were PWM, that would be reflected in the diagram?

 

[BGKYK5]

You can have switched ground PWM control. In fact it is very common. Anything that lists a duty cycle is likely PWM controlled. Injectors are sort of PWM controlled, but the carrier frequency changes with engine speed. Usually PWM has a constant switching frequency. You could make a resistor/capacitor filter circuit to see the "average" voltage, that would give you some indication of how the PWM signal varies.

 

[dyeager535]

That sounds fairly complex for early 80's technology. Is it, or is it still likely?

 

[RedBrute]

I think I got a definitive answer last nite. I tried the light trick and during the day it appeared the light stayed lit all the way to work. Coming home in the dark was pretty cool. The light stayed lit solid no variation whatsoever. It did go out based on throttle position just as the EGR should shut off at idle or during heavy throttle acceleration.
I'm going to state right now that there is no PWM for this style of EGR...at least not as far as my simple testing can determine.

At this point I think I understand how it should work, that the command to the EGR solenoid is good, the soledoid reacts to the commands, I'm fairly certain the problem is the EGR is not doing what it's supposed to so I'm going to replace that today and see how it runs.

My thoughts on the noid light are pretty basic, if it can detect the pulse rate of an injector it ought to see the rate for the egr circuit which should be slower since all the components are huge in comarison to the tiny little injector assemblies. I wwasn't trying to determine what the "frequency" was, just IF it had a "frequency".

 

[RedBrute]

Problem solved... new EGR did the trick. Sytem hooked up completely and functions fine.

The original EGR tested as good with a vacuum gauge and manually actuating it, not sure why it didn't work, but it didn't work right. I got the new one from NAPA, thier website lists OEM numbers so I could get an exact match. I found the numbers on mine only after I removed it and wire brushed the rust out of the stamped in numbers.

The test drive yeilded interesting results. Right off I noticed the BLMs down around 112 and the INT about 124. After driving for maybe 10 miles the BLM had crept up to 120 and the INT had moved up to 127. My guess here is the ECM was slowly adjusting to the now functional EGR? Oddly with no EGR function at all both BLM and INt were running 128-130 pretty reliably...not sure what to make of this...

So why didn't I throw a new EGR on it the day I unplugged it and noticed the problem was gone? simple...for all I new, it tested as a good part , and there was just too much I didn't understand about the system to be certain

Thanks to all that contributed to this solving little problem. Hopefully this was a learning experience for all

 

[dyeager535]

Good job.

It may take longer, but it pays to not just throw parts at the problem. Odd that the EGR tested good but didn't work right. Any common failure I can think of would be found by testing.