Vacuum solenoid

[Bitsyncmaster]

I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

Any body ever see this problem before?

[Delorean Industries]

I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

Any body ever see this problem before?

the valve has failed internally. I have gotten away from these completely with our EFI conversions and use a pwm solenoid to control vacuum via the ecu for the distributor. Nice feature with this is that it is based off a CLT map and can be adjusted for different operating climates.

[DMCMW Dave]

I was helping Christian with his engine work and found the advance vacuum was only 10 inches. Now the center hose on the solenoid was the normal 20 inches (manifold vacuum). Only thing I could think of is the solenoid has a vent stuck open and the vent "orifice" is the same size as the inlet resulting in half the vacuum.

Any body ever see this problem before?

Yes. They just go bad (leaky) internally. Was it still closing when powered? The vacuum port should be blocked when powered, and the distributor port is open to the vent at that time so that the vacuum advance snaps back to no-advance. When unpowered, the two ports are connected together and the vent is unused.

If you happen to hook it up backwards the vacuum advance (timing) will "hang" for a short time since it can't bleed off through the vent, and you'll have a slight but un-noticeable vacuum leak at idle. I've had this cause emission test failure.

[Bitsyncmaster]

Yes. They just go bad (leaky) internally. Was it still closing when powered? The vacuum port should be blocked when powered, and the distributor port is open to the vent at that time so that the vacuum advance snaps back to no-advance. When unpowered, the two ports are connected together and the vent is unused.

If you happen to hook it up backwards the vacuum advance (timing) will "hang" for a short time since it can't bleed off through the vent, and you'll have a slight but un-noticeable vacuum leak at idle. I've had this cause emission test failure.

It still worked, idle no vacuum advance, some throttle had 10 inches vacuum advance.

[Trstno1]

It still worked, idle no vacuum advance, some throttle had 10 inches vacuum advance.

I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

Thanks in advance. hehe.....see what I did there...

[David T]

I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

Thanks in advance. hehe.....see what I did there...

Idle 'lumpiness" or uneven idle can be caused by a vacuum leak but is most often due to some kind of imbalance between the firing of the cylinders. A vacuum leak can affect the cylinders closest to the leak by leaning out the fuel mixture. A mis-gapped spark plug, a bad ignition wire, a valve adjustment, all can affect a cylinder's efficiency. On one car the gaps of the reluctor to the impulse coil were not all the same and that made the idle "lumpy". To make the idle as smooth and possible your goal is to make all of the cylinders as close to each other as possible. Start with a cylinder pressure test. Check the valve adjustments. If the car hasn't had a tune-up in a long time maybe you are overdue.

[Bitsyncmaster]

I know this is an old thread, but I had a couple of questions. I am attempting to solve a idle lumpiness in my engine. I have already replaced all ignition/fuel components on the car and yet it still stumbles a little. One of the things that I didn't replace was the vacuum advance on the distributer or the solenoid valve coming from the distributer. How can I test for vacuum pressure on the engine first, then each component operation second? what am I looking for?

From what I have been reading, failure of one or both of these component cant create a mild idle stumble.

Thanks in advance. hehe.....see what I did there...

For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.

[Trstno1]

For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.

Ok so at idle if i pull off the hose from the distributer and hook it up to a vacuum gauge it should read 0 at idle and 20 once the throttle is activated thus activating the vacuum advance on the distributer right? Right now with that situation I am getting bobbling round the 15 bar mark and 0bar at idle. Is 15 bar pointing towards a vacuum leak? Should it always be 20 bar or am i testing engine vacuum at the wrong spot? Also is there a way to test the vacuum advance valve that the vacuum hose plugs into on the distributer without tearing things apart?

On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?

[Bitsyncmaster]

Ok so at idle if i pull off the hose from the distributer and hook it up to a vacuum gauge it should read 0 at idle and 20 once the throttle is activated thus activating the vacuum advance on the distributer right? Right now with that situation I am getting bobbling round the 15 bar mark and 0bar at idle. Is 15 bar pointing towards a vacuum leak? Should it always be 20 bar or am i testing engine vacuum at the wrong spot? Also is there a way to test the vacuum advance valve that the vacuum hose plugs into on the distributer without tearing things apart?

On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?

If you open the throttle to much your engine vacuum will probably drop to 15 inches. To test the engine vacuum you attach to a hose on the intake manifold. The vacuum does bounce more so with our engines because of the uneven fire sequence.

Since you have a timing light, you can check the vacuum advance and mechanical advance. Pull the hose off the ignition distributor to check the mechanical advance at the RPMs as shown in the book. Then when the vacuum advance is connected you check a little above idle so no mechanical advance is taking place.

[dmruschell]

For your idle problem, just pull the hose off the distributor and see if that corrects the idle problem. If the vacuum advance is on at idle you will get a crappy idle.

I've messed with vacuum advance at idle on some of my cars that are older than the DeLorean that could be made to use both manifold vacuum advance (vacuum advance all the time) or ported vacuum advance (no vacuum advance at idle, like the DeLorean). Vacuum advance at idle will definitely not give you a crappy idle. In almost every case, it will smooth out the idle because the engine is running more efficiently at idle. A ported vacuum advance setup is only really used for emissions purposes (or with a carburetor on a car that has AC so that the effect on idle RPM on the engine at idle caused by the compressor is minimized. Lower RPM = lower vacuum = less advance = lower RPM, and the cycle continues. Ported advance prevents that cycle).

[dmruschell]

On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?

The stock timing spec is 13 +/- 2 degrees. One degree difference isn't really going to affect the idle quality very much, if at all.

For comparison, I've gotten a huge Buick 455 with a lumpy cam to idle steady at 500rpm with 4 degrees of advance (which is the stock setting for that engine - some Buicks had a stock setting of 0 degrees advanced!). After getting the distributor dialed in, I got the engine to idle steady, but with over 10 degrees of advance. The idle quality should have been vastly different, but when the fuel mixture is dialed in perfectly, the engine will idle just fine. The engine had more midrange power with the higher advance setting though, since it was running more efficiently.

[David T]

The stock timing spec is 13 +/- 2 degrees. One degree difference isn't really going to affect the idle quality very much, if at all.

For comparison, I've gotten a huge Buick 455 with a lumpy cam to idle steady at 500rpm with 4 degrees of advance (which is the stock setting for that engine - some Buicks had a stock setting of 0 degrees advanced!). After getting the distributor dialed in, I got the engine to idle steady, but with over 10 degrees of advance. The idle quality should have been vastly different, but when the fuel mixture is dialed in perfectly, the engine will idle just fine. The engine had more midrange power with the higher advance setting though, since it was running more efficiently.

While the timing will not have a large effect on idle quality, it has a BIG effect on emissions. Back in the day, they only tested for emissions at idle so the object was to get them as low as possible at idle. That meant messing around with the timing. Some of the older cars had more rubber hoses than you could count just to get emissions down at idle and still make the car drive able, especially when cold.

[dmruschell]

While the timing will not have a large effect on idle quality, it has a BIG effect on emissions. Back in the day, they only tested for emissions at idle so the object was to get them as low as possible at idle. That meant messing around with the timing. Some of the older cars had more rubber hoses than you could count just to get emissions down at idle and still make the car drive able, especially when cold.

My 70s cars definitely came from the factory with a few more hoses than my 50s and 60s cars. In researching getting my 79 Corvette closer to stock and restoring (kind of) a 75 Corvette I have, I researched all of the emissions equipment that was used on those cars during the 70s and how it changed over the years. The catalytic converter was first used on the Corvette in 1975, which is also the same year that the Quadrajet was redesigned to use full manifold vacuum for the vacuum advance, like had been done on pre-emissions cars. The only ported vacuum source after 75 was for the EGR valves. I guess the catalytic converter helped emissions enough that a lot of the other emissions "band aids" that were used in the early 70s didn't need to be used anymore. The only emissions equipment left was EGR, a system (like on the DeLorean) that deactivates the vacuum advance until the engine is warmed up, and certain applications used an AIR pump to pump air into the exhaust manifolds. Some of the stuff they used before 1975 for emissions controls still seems odd to me.

[Bitsyncmaster]

What gives our cars crappy idle with advance is the closed loop idle system. The advance brings the RPM up higher (300 to 500 RPM) and that exceeds or brings the idle motor almost fully closed. In most cases you will get excessive idle hunt. Older cars without the idle control don't have that problem. You can test on the D, just unplug the advance solenoid electrical connector on a hot idling engine.

[dmruschell]

What gives our cars crappy idle with advance is the closed loop idle system. The advance brings the RPM up higher (300 to 500 RPM) and that exceeds or brings the idle motor almost fully closed. In most cases you will get excessive idle hunt. Older cars without the idle control don't have that problem. You can test on the D, just unplug the advance solenoid electrical connector on a hot idling engine.

That makes sense. Though, I wonder if adjusting the idle screws so that the butterflies are closed a bit more would resolve that situation.

The advance solenoid electrical connector on my car fell off at some point. I noticed it when looking for reasons my car wasn't running quite right. Reconnecting it seemed to help. It still idled relatively okay, but it did idle better when the connector was in place.

[David T]

The throttle valve plates should be adjusted so that at idle the plates are fully closed. The decel springs should be strong enough to hold the decel valves closed so no air bypasses them. The less air passing through the throttle body the more control the idle motor will have. The throttle stop is a very sensitive adjustment. You want the plates closed when hitting the stop. Otherwise you are bending the throttle shaft and eventually it can break or loosen the plates attached to it. The throttle stop screw adjustment should never need to be adjusted unless it has been tampered with. It was glued so it can't move easily. If you touch that adjustment you may have to readjust the micro switch adjustment too.