pgtr

Negative Pressure EGR Valve Diagnosis - 84 2.8L

I have an '84 Firebird w/ 2.8L V6 Auto. I've suddenly started experiencing considerable spark knock (detonation) after replacing the intake manif gaskets. I had to remove the EGR stuff and reinstall but didn't pay any attention to it during the process. According to the manual one of the areas to investigate is the EGR system remaining closed causing spark knock.

I have the original EGR valve (180k mi) installed part # 17084729. It has an "N" stamped on it which is supposed to indicate it's a negative pressure EGR valve (vs a positive or ported EGR valve). Supposedly it will open up if vacuum is applied while the engine is turned off. I tried this with my mightyvac pump and it doesn't move. It will however move relatively easily by hand. When I do actuate it manually by hand - it does run rough and stumble. I also hookedit up directly to manifold vacuum for kicks and this time w/ the engine running - no movement detected at various RPMs while in neutral w/ engine warmed up.

Q: Sound like a bad unit, should be replaced? (I ask because there are different EGR systems in this year/model and was curious about experienced advice regarding the '84 2.8L carb motor specifically)


Also I tried to test the EGR control solenoid (electric switch mounted on valve cover w/ wire plug and 3 vac ports...). I noticed it's always one (energized) when the engine is running in park regardless of temperature, RPMs. But virtually no vacuum is present at the EGR valve regardless of the state of the solenoid.

Q: Why is the EGR valve solenoid energized all the time? Cold? Hot? Various RPMs (in Park, not under load)... Shouldn't it only be energize under certain running conditions after the engine is warmed up?

Q: Does this also sound suspect? If a new EGR valve solenoid were installed - it would seem to have a 'new' EGR valve open all the time...?

2_point8_boy

The EGR valve does sound like it's got a faulty vacuum diaphram. I would replace the valve.

The EGR solenoid is always powered as long as the key is on. The ECM applies ground when it wants EGR operation. if you snap rev the engine, you should get a vacuum signal at the EGR valve itself.

The question is, are you testing this system because of a check engine light or because of something else? The car self-diagnostic system does not test to see if the valve itself works, it just checks the solenoid/vacuum sensor for open, shorts to power, and shorts to ground, and cehcks to see if vacuum is seen at the vacuum sensor when the solenoid is commanded on. It never has any idea if the EGR valve itself is opening.

pgtr

Yes - the switch it grounding at all times. I'm running a test light completely through the backside of the plug.

I'm investigating because of considerable audible spark detonation. No codes. The detonation is bad enough that I don't want to drive it anymore other than quick test drives.

I'm thinking I'll go ahead and buy a new EGR valve and probably solenoid switch too (I'll pull the old switch and bench test it).

But even so that still leaves the issue of the switch always being energized (grounded) when the engine is running... Which I predict will then give me the opposite problem - it then being open all the time.

Ideas...?

pgtr

After sleeping on this had a thought... wonder if I'm also encountering an out of tolerance TPS. I believe a throttle sensor is somewhere on the carb. Maybe it's confusing the ECM and the ECM in turn is grounding the EGR solenoid switch... I don't have any trouble shooting guides in front of me but I wouldn't be surprised if at some point the EGR troubleshooter gets to checking the TPS at some point...

Q: Could the TPS or anything else 'fool' the ECM into activating the EGR at idle etc?

Q: Would a mis-adjusted TPS eventually cause a vintage '84 ECM to throw any codes? What are other symptoms of a TPS out of whack?

I recall replacing the TPS way back when the car had less than 100k on it due to a code. I vaguely recall an adjustment procedure when installing. But it's been so long I may have confused it with another mid 80s GM vehicle we had with the same 2.8L...?

Again it has about 180k mi, 1-owner, all stock/orig and is about 25 years old; throwing no codes (EGR, TPS or otherwise, just "12...").

2_point8_boy

Using a test light to backprobe the ground side of the EGR solenoid is not the right way to test it.

1. By using a test light you are creating a low resistance path to ground and in turn activating the EGR solenoid. This is bad for your diagnosis.
2. You are going to always have a lit test light with the EGR solenoid plugged in and key on since it wil light with power, which is always there.

The correct way to test will be to use a Digital multimeter backprobed into the ground side of the solenoid. If you have a 12V reading, then the ECM IS NOT activating the EGR solenoid. When the solenoid is being commanded on, you will seea slightly lower voltage because it's being pulsed and the DMM will average the voltage reading for the display.

The EGR solenoid is ALWAYS pulsed, never fixed on. The ECM does this to control the amount of vacuum to the EGR valve. For instance, if the ECM is commanding the EGR solenoid to be on for an equal amount of time as it is off (on for 100ms/off for 100ms) then you'll read approximately 7V with the engine running...assuming your charging system is working properly.

If you continue to use a test light, you will get inaccurate diagnosis and could cause other damage due to current draw. Don't get me wrong, test lights have their place and I use them everyday, but you have to know when, where, and what you're looking for.

pgtr

Thanks. What you're saying makes sense. I was following the '84 FBird's FSM's EGR System Diagnosis Chart C-7A which states to use a test light. It says to observe a steady light, off light, on light or 'FLICKERING' light. Per the chart, w/ the 4 wire EST disconnected at the distributor and the test terminal grounded on the ALDL, I did indeed get a 'flickering' test light at the solenoid. As you posted I also tried it with my DVM and got about 6.3V.

Assuming my EGR solenoid switch is OK (bench testing it shows it to be switching the ports correctly) and getting properly signalled - this leads me to two(2) problems:

A) the EGR valve itself is not functioning (I applied vacuum via a MightyVac pump - NO movement)

And...

B) I am NOT getting vacuum at the EGR solenoid! The FSM states I should be seeing at least 10" of vacuum. I'm not.

I've checked this over repeatedly. All the vac hoses are 100% correct per the emissions label. And they are clear and unblocked. There are multiple ports off the main carb body that can potentially supply vac to the solenoid. Not one of them produced any vacuum. They are all ported I presume as they are on the main body and not down at or below the throttle plate. I tried 'blowing' into each of the various ports and they all blow clear (minimal resistance and I can 'hear' air passing inside the carb...) so again no blockage here... I've tested them w/ the engine running at varous speeds and none of the vac ports off the carb main body produce any measurable vacuum...

This one has me a bit stumped as to the lack of vacuum at the solenoid. I may T into a vac source off the manifold and bring it up to the solenoid (though I shouldn't have to that!).

FWIW an identical 2nd solenoid is used for the high idle, it gets good vacuum and when the switch is 'on' it kicks up the E2SE carb by several hundred RPM as it should. Though it kicks on under various circumstances as it should - it does not kick on when the AC is turned on - a separate issue I believe.