Quadrajet: Engine Knock, Bad Idle Mixture Solenoid...?!

[G_RIDE_81]

Its a lot easier to work on CCC systems with either a analog dwell meter or a old OBD1 scanner. If its just spark knock you likely have a vacuum leak leaning out the fuel mixture. Disconnecting the M/C solenoid causes the fuel mixture to go full rich which can offset a vacuum leak. The off position for the M/C solenoid is rich, the on position is lean. Finding the vacuum leak is the hard part.

This was my very first thought and test. I've checked every hose and other possible leak points and have not found a leak yet. I'm aware this could still be a possibility.

Does anyone have a "can't believe I finally found the vacuum leak at the ____" story...?

I have a dwell meter. What would I look for? What would that reading tell me is or isn't the problem?

 

[79 USA 1]

Oh, I haven't read this anywhere. So, the reading should climb or drop as I open the throttle? Your problem was the reading zero'd out somewhere in the middle?

Yes, I would get an ohms reading with the throttle closed and as I rotated to full throttle the meter would go to zero then back to reading near 3/4 open. It's been quite some time since having this problem so I can't give specific ohms readings but I do remember the 'Ah Ha" moment when I realized my pinging problem the whole time was staring me in the face.

 

[G_RIDE_81]

Yes, I would get an ohms reading with the throttle closed and as I rotated to full throttle the meter would go to zero then back to reading near 3/4 open. It's been quite some time since having this problem so I can't give specific ohms readings but I do remember the 'Ah Ha" moment when I realized my pinging problem the whole time was staring me in the face.

Ok, makes sense. Will check this.

 

[airboatgreg]

1981 Monte Carlo, V8, TH350, Quadrajet, smog equipment intact.

I've been having some mild knock after the engine is fully warm, during acceleration as I roll into 2nd and 3rd gear then fades away as the gear progresses. It also knocks, a bit louder, if I try to pass so ive been driving like an old lady lately ha ha! Currently running 91 octane because knock is worse with 87. I have been suspecting the idle mixture solenoid might be adjusted wrong. I recently saw a video where someone had a similar problem. He disconnected the solenoid pigtail and his lean knocking went away. I decided to try the same with similar results, no knock no matter how much I mashed on the pedal. Does this test make sense? Is this a logical way to test for a wrongly adjusted or bad solenoid? Of so, does this mean my problem lies with the solenoid and not somewhere else?

I usually take my carb to a shop but they've been letting me down last couple rebuilds. I researched how to open up the carb and adjust it with that "special tool." My wrenching skills are better than they were few years ago. Feel confident I can make this repair myself if this is my problem.

Thanks in advance for any feedback!!

OK. First thing.
Is it a knock or a ping?
Will it do it if you rev up the engine in park?
Trouble codes?
How does it idle?
Do you hear the carb clicking in park at an idle?
Are you using the factory air cleaner housing?

 

[G_RIDE_81]

OK. First thing.
Is it a knock or a ping?
Will it do it if you rev up the engine in park?
Trouble codes?
How does it idle?
Do you hear the carb clicking in park at an idle?
Are you using the factory air cleaner housing?

More of a knock. Only hear it in drive so relying on ears and road echo.

No, not when when reving in park. As mentioned, the knock went away when I tested with sensor disconnected.

No codes.

Idles slightly rough.

I believe the carb clicks in park...

Yes, factory air cleaner. It has all original equipment.

 

[Clone TIE Pilot]

This was my very first thought and test. I've checked every hose and other possible leak points and have not found a leak yet. I'm aware this could still be a possibility.

Does anyone have a "can't believe I finally found the vacuum leak at the ____" story...?

I have a dwell meter. What would I look for? What would that reading tell me is or isn't the problem?

There are 1,000s of places for vacuum leaks to develop. Vacuum hoes are only the first places to check. Vacuum motors need to be checked with a Mityvac, including the vacuum brake booster. Carburetor and intake manifold gaskets can leak. Carburetor and intake manifold castings can develop cracks, warpages, or porosity. Intake manifolds can even leak vacuum on the underside in the lifter valley. Worn throttle valve shafts which requires rebushing to fix. Leaking vacuum NPT fittings. Even a bad or incorrect PCV valve, bad PCV valve grommet, and leaking valve cover gaskets can lean the fuel mixture out. Leaks in the HVAC system will also lean out the fuel mixture.

With a dwell meter connected to the green diagnostic plug near the M/C solenoid plug, you should get a varying reading around 30 degrees. If the dwell stays below 30, its a rich command. If the adjustments have little to no effect on raising a low dwell is a sign of a vacuum leak. With the engine idling you can wave an open but unlit propane touch around possible vacuum leak areas. If the dwell rises when you hit a spot with propane, that is your leak. Sometimes vacuum leaks only appear after a heatsoak when parts expand.

 

[airboatgreg]

More of a knock. Only hear it in drive so relying on ears and road echo.

No, not when when reving in park. As mentioned, the knock went away when I tested with sensor disconnected.

No codes.

Idles slightly rough.

I believe the carb clicks in park...

Yes, factory air cleaner. It has all original equipment.

So when you disconnected the MC solenoid that richened it up and probably the computer reduced the time so the ping(I am assuming it is a ping and not a mechanical knock) I would check initial timing first,
Is the vacuum hose connected to the air cleaner? If it is take for a couple mile drive and with engine running check to make sure the door in the snorkel of the air cleaner is open.
Test the EGR system. It's possible the valve is working but the exhaust passage is plugged.

 

[G_RIDE_81]

If the dwell stays below 30, its a rich command. If the adjustments have little to no effect on raising a low dwell is a sign of a vacuum leak.

So true, vacuum leaks can be anywhere.

Never knew you could use dwell meter to find a leak. Only used the soapy water trick before. Thanks!

 

[Clone TIE Pilot]

Another item to check is the control solenoid for the EGR valve. The solenoid is normally open when deenergized, allows vacuum to pull on EGR valve when turned off by the ECM. When energized, it should block vacuum from reaching the EGR valve such as during cold engine operation and idle. Moreover, if the solenoid leaks, it can allow the EGR valve to operate at times it shouldn't like idle which can cause rough idling. To test, one a couple of jumper leads to the solenoid socket to energize it, then connect a Mityvac to the supply line from the carb to the solenoid to see if it holds vacuum. Supposedly LT1 EGR solenoids can be substituted as replacements. Any carbed Chevy V8 80's car will due as a donor, MCs, GPs, F bodies, and Caprices.

 

[G_RIDE_81]

To test, one a couple of jumper leads to the solenoid socket to energize it, then connect a Mityvac to the supply line from the carb to the solenoid to see if it holds vacuum.

Oh wow, could have used this tip few months ago when the MC wouldn't pass smog. Ultimately, the valve was shot. But I'll definitely try this this weekend.