How To: Vacuum Solenoid Diagnostics and Leak Repair

The vacuum advance solenoid valve is part of the ignition advance and emission control system. It sits behind and below the fuel distributor and has vacuum hose connections to the ignition distributor and the thermal vacuum control valve on the coolant Y-pipe.

This How To covers a recent diagnostic sequence and the subsequent leak repair on the (original) vacuum solenoid valve on 0934.

A diagnostic intake system smoke test on our car over at mark w’s place last weekend showed smoke coming from somewhere on the vacuum solenoid. We used his Redline professional smoke machine pumping into the right rear port of the intake manifold with air filter box removed and the metering unit intake blanked off. He thought the smoke came from the electrical fitting on the valve.

Noting here that the diagnostics below can be done on the vac solenoid in case you don't have a good smoke machine and want to see if your old valve is OK or not.

The vacuum solenoid on this car is the original. While replacement units are available I opted to go for a DIY repair job to start with.

Back home I removed the vac solenoid, its connector and its 2 hoses noting which hose went to which port (see emissions label on engine cover for confirmation) and not worrying about the electrical connector polarity since it doesn't matter in this case.

I ran bench tests with the valve out of the car using a 12V power supply, a brake bleeder pump/gauge, soapy water sprayer and hoses/plugs:
1. Verified clicking sound when 12V was connected-disconnected at the two small male spades (PASS)
2. Center port held vacuum with valve activated/closed and outer port (distributor advance) open (PASS)
3. Center port didn’t hold vacuum with valve deactivated/open and outer port capped closed (FAIL) - consistent with the smoke test.
4. With outer port still capped and valve deactivated a soapy-water leak test showed bubbles at/around the electrical connector while lightly blowing into a hose to the center port proving the seal at or around the elec connector had failed.

DIY remedy
1. Prepped the face of the valve and the electrical connector base with a wire brush and parts cleaner plus an alcohol wipe.
2. Applied silicone sealant where the connector base meets the face of the valve. Also sealed the small gap between the two raised black plastic connector bases.

After the silicone cured I repeated tests 3-4 to verify PASS, then reinstalled the valve.

Sending a shoutout to mark w for the pro smoke test and further advice on this little job.

Thankyou for the write up.

Did the vacuum leak have any symptoms? (Hunting idle....) Or was it too small at that point?

Quote:

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Originally Posted by Helirich

Thankyou for the write up.

Did the vacuum leak have any symptoms? (Hunting idle....) Or was it too small at that point?

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Good question. No, there were no performance symptoms. The smoke test was purely for preventive/diagnostic purposes. It's a 40 year old air induction system so it was worth a look since mark w offered to smoke it for me.

As a good-size vac leak only in off-idle conditions it means the engine was trying to run lean while driving. That issue wasn't noticeable.

The vac solenoid valve closes when idling. There was no vac leak in that bench test condition - test #2. The valve body leaked only when valve was de-activated/open yet plugged on the 'distributor' port, the non-idle condition. The smoke test can only be done with engine off, so although it was "at idle" the valve was open (de-energized) in that test. Simulated in bench test #3 (fail).

In retrospect we could've also run the smoke test with engine off, ignition on, which would have closed the vac valve and thus stopped that leak, the real life bench test #2 situation (pass).

Hope it's ok to ask a question in your thread here as it's related to what you were discussing...

I wanted to know what the difference was between the vacuum solenoid and the vacuum advance unit? What does each of these two parts do? And how might the car perform if either of them were failing or had failed?

Vacuum advance unit: https://store.delorean.com/102684-va...ance-unit.html

Vacuum solenoid: https://store.delorean.com/101116-vacuum-solenoid.html

Quote:

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Originally Posted by Jonathan

Hope it's ok to ask a question in your thread here as it's related to what you were discussing...

I wanted to know what the difference was between the vacuum solenoid and the vacuum advance unit? What does each of these two parts do? And how might the car perform if either of them were failing or had failed?

Vacuum advance unit: https://store.delorean.com/102684-va...ance-unit.html

Vacuum solenoid: https://store.delorean.com/101116-vacuum-solenoid.html

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The advance unit mounts on the ignition distributor and can rotate the internals to change the ignition timing.

The solenoid is a "switch" for steering the vacuum to advance ignition timing or not. When no vacuum is driven to the advance unit it also vents that line so the spring loaded advance unit returns to no advance.