Hey, where'd my vacuum go?

[SlowLane]

Up until today my engine has pulled a healthy vacuum at idle of 16-17" Hg. Stock Federal '81 2.0L L-jet with the ignition system from a Cal-spec '83.

Today, however, my idle vacuum has dropped to 13-15", and it wanders significantly. Paradoxically, the AFR, which normally hovers around 14.0-14.2 at idle, is now drifting between 13.3-13.8.

All that I did to the engine today was top up the oil. There's a possibiity I overfilled it by maybe half a pint, as the car was on a slight downward slope. The oil cap went back on nice and tight.

The trend of the AFR getting richer with a drop in vacuum is puzzling. I would have thought that if there were a vacuum leak, the mixture would lean out. I did pump the brake pedal rapidly and got the expected temporary leaning of the mixture.

So tomorrow I'll be out there with my Mity-Vac checking out the static vacuum systems. I'll also adjust the valves and make sure the valve cover gaskets are sealing properly. I'll check the AAR for full closure at operating temperature and make sure the decel valve isn't leaking.

Anything else I should be checking? Ideas from outside the box(er)?

Thanks,
Lyle

[Bleyseng]

oil cap seal is leaking..would be my guess. A small leak but one none the less.

[Amskeptic]

Today, however, my idle vacuum has dropped to 13-15", and it wanders significantly. Paradoxically, the AFR, which normally hovers around 14.0-14.2 at idle, is now drifting between 13.3-13.8.
The oil cap went back on nice and tight.

The trend of the AFR getting richer with a drop in vacuum is puzzling.

There are paradoxes aplenty when an electronic control unit is interpreting engine operating parameters. For example, if you "richen" the wiper at idle, the ECU leans out the mixture due to anti-flood circuitry that reads the ignition signal, and "decides" that you most certainly do not need more fuel.

Do you have Hall Effect or idle stablilizer or oxygen sensor?
Colin

[SlowLane]

Do you have Hall Effect or idle stablilizer or oxygen sensor?
Colin

All three, but the oxygen sensor is simply feeding my AFR gauge. I'm still using the Federal ECU which doesn't have lambda feedback.

After mulling over the symptoms, I'm going to start with the AAR. I'm thinking that if it isn't fully closing when warm, that'll reduce the vacuum in the plenum and also cause the ECU to deliver more fuel than necessary because more air is being drawn in. The idle stabilizer could then be compensating for the tendency to run faster.

I'll know in a bit. I've been slow in getting to work on it today. Life has a habit of getting in the way sometimes...

Update: The AAR was leaking a little when closed. I replaced it with one I had pulled from a junker a couple of weeks ago that doesn't leak as much. No change in symptoms.
All static vacuum systems hold vacuum. The decel valve leaks a little on the working side, but slowly. I started the engine with the decel valve and brake booster disconnected from the air plenum. Same symptoms.
Oil cap is on TIGHT. The cap gasket looks to be just fine. Honestly, I can't picture the mechanism that would lower manifold vacuum if the cranckase had a leak to atmosphere.
Didn't get around to adjusting the valves. I think I'll ask around some of the local shops and see if any of them have a smoke machine. I'm rather interested to see one of those devices in action.

[SlowLane]

Well, this is starting to be quite the puzzler.

Replacing the AAR didn't help. So then I thought "leaking EGR valve". Took that apart and cleaned it. Very sooty, and leaking a little when closed, but not a great deal. Reassembled. Same problem.

Adjusted valves, replaced injector seals, snugged down intake manifold nuts, new throttle-body-to-plenum seal.

No change. Still seeing idle vacuum between 13" and 15". The engine still runs just dandy, but I'm still frettiing about where those couple of inches went.

Tried something I read on the Web: hooked my shop-vac up to the intake with PVC fittings and garden hose and tried to hear vacuum leaks with the engine off and the shop-vac sitting 30' away. Two problems with this idea: 1) the shop-vac only pulls about 3" of vacuum through the garden hose, and 2) the shop-vac doesn't like working on a static vacuum. With no airflow through the canister, the shop-vac motor gets pretty warm. Smelled funny too.

So this being a holiday for me, I thought I'd take the Westy to a local Euro-only mechanic and see if he'd let me watch him pump smoke through my intake. My smog guy said that this mechanic "loves those old cars". Perfect.

So I walk in and ask the guy if he has a smoke machine for diagnosing intake problems. Affirmative. Great, I have a pesky vacuum leak, can I schedule a time where I can come in and get you to hook it up to my van so I can find the leak.
"We don't work on air-cooled vehicles any more."
Well, I don't expect you to work on it. I will be working on it. I just want your help in diagnosing a vacuum leak.
"We don't work on air-cooled vehicles any more. You can't get good parts for them."
I'm not asking you to supply parts. I just want to pump smoke in the intake and see where it comes out.
"We don't work on air-cooled vehicles any more, 'cept maybe some Porsches."
Okay, yeah. I get it. You clearly don't need or want my business, what with all the pretty jelly-bean VWs and Porsches littering your shop. Thanks for nothing.

Okay, venting over. Thanks for listening.
Lyle

[Amskeptic]

Well, this is starting to be quite the puzzler.

Replacing the AAR didn't help. So then I thought "leaking EGR valve". Took that apart and cleaned it. Very sooty, and leaking a little when closed, but not a great deal. Reassembled. Same problem.

Adjusted valves, replaced injector seals, snugged down intake manifold nuts, new throttle-body-to-plenum seal.

No change. Still seeing idle vacuum between 13" and 15". The engine still runs just dandy, but I'm still frettiing about where those couple of inches went.

Tried something I read on the Web: hooked my shop-vac up to the intake with PVC fittings and garden hose and tried to hear vacuum leaks with the engine off and the shop-vac sitting 30' away. Two problems with this idea: 1) the shop-vac only pulls about 3" of vacuum through the garden hose, and 2) the shop-vac doesn't like working on a static vacuum. With no airflow through the canister, the shop-vac motor gets pretty warm. Smelled funny too.

So this being a holiday for me, I thought I'd take the Westy to a local Euro-only mechanic and see if he'd let me watch him pump smoke through my intake. My smog guy said that this mechanic "loves those old cars". Perfect.

So I walk in and ask the guy if he has a smoke machine for diagnosing intake problems. Affirmative. Great, I have a pesky vacuum leak, can I schedule a time where I can come in and get you to hook it up to my van so I can find the leak.
"We don't work on air-cooled vehicles any more."
Well, I don't expect you to work on it. I will be working on it. I just want your help in diagnosing a vacuum leak.
"We don't work on air-cooled vehicles any more. You can't get good parts for them."
I'm not asking you to supply parts. I just want to pump smoke in the intake and see where it comes out.
"We don't work on air-cooled vehicles any more, 'cept maybe some Porsches."
Okay, yeah. I get it. You clearly don't need or want my business, what with all the pretty jelly-bean VWs and Porsches littering your shop. Thanks for nothing.

Okay, venting over. Thanks for listening.
Lyle

Lyle? We are in this together. We fix our own cars. Right here. 13"-15" is normal vacuum for most every bus I have worked on.

Any idle bypass throttle body is sending air around the throttle plate like a vacuum leak (even sounds like one). If you want to see how the engine is doing, rev and release. It should swing to 20-17 on the way back down.
The retard distributor VWs rarely see more than 10" at idle. Think about whjere your timing is. On the BobD, I had to reduce idle timing to keep the upper end at 28* (as the distributor loosens up). Idle timing is now a solid 4-5* more retarded than when new, around 2* BTDC. Vacuum at idle is barely 11-12.
Colin

[SlowLane]

Lyle? We are in this together. We fix our own cars. Right here. 13"-15" is normal vacuum for most every bus I have worked on.

Hmm. Interesting. That may very well be so, but as I said, I've seen a solid 17" of vacuum since I first installed the vacuum gauge, so the sudden drop is perplexing.

The retard distributor VWs rarely see more than 10" at idle. Think about where your timing is.

My timing is at 5 degrees ATDC, or as close as I can get it to that mark. When I disconnect (and plug) the vacuum retard, the timing goes to 7.5 degrees BTDC, as expected. I don't grok how changing the timing advance can affect the vacuum at a given RPM, unless it's through changing the idle speed adjustment screw. Seems to me that the engine is going to pump the same amount of air at 850-900 RPM regardless of when the spark goes off, or am I missing something?

Thanks for the feedback. I'll try taking the vacuum retard out of the equation and see if that changes anything.

[Amskeptic]

Lyle? We are in this together. We fix our own cars. Right here. 13"-15" is normal vacuum for most every bus I have worked on.

Hmm. Interesting. That may very well be so, but as I said, I've seen a solid 17" of vacuum since I first installed the vacuum gauge, so the sudden drop is perplexing.

The retard distributor VWs rarely see more than 10" at idle. Think about where your timing is.

My timing is at 5 degrees ATDC, or as close as I can get it to that mark. When I disconnect (and plug) the vacuum retard, the timing goes to 7.5 degrees BTDC, as expected. I don't grok how changing the timing advance can affect the vacuum at a given RPM, unless it's through changing the idle speed adjustment screw. Seems to me that the engine is going to pump the same amount of air at 850-900 RPM regardless of when the spark goes off, or am I missing something?

Thanks for the feedback. I'll try taking the vacuum retard out of the equation and see if that changes anything.

So you are a retard .......... equipped engine! Aw come on, it was a joke. Come back.

A retarded spark reduces intake vacuum exactly at valve overlap, the exhaust pulse is much stronger.
Colin

[SlowLane]

So you are a retard .......... equipped engine! Aw come on, it was a joke. Come back.

I happen to take pride in being cognizant of my areas of ignorance. One of my better professors pointed out that some of the most dangerous people are those who don't know what they don't know. So yes, I'll admit to being a retard on ignition retard until I understand it better. Or at least until I'm less dangerous...

A retarded spark reduces intake vacuum exactly at valve overlap, the exhaust pulse is much stronger.

While I mulled over this statement and its import (one of Kolin's Koans?), I disconnected the vacuum line to my retard diaphragm, bypassed the idle stabilizer and fired her up. It immediately revved to 1500 RPM and stayed there, so I had to turn in the idle speed screw until it came down to something reasonable. Took it for a warming-up drive, after which I again adjusted the idle speed screw to get a nice 850-900 RPM idle. Voila! 18" of vacuum (give or take. The VDO Cockpit gauge series isn't the world's most accurate).

It's been my intention to run this engine with the better CA-spec ignition system and the Federal L-jet system. I currently have two ignition setups installed in my engine compartment, so that I can quickly swap between the CA-spec distributor and a new Mexican SVDA that I bought a few months ago. The CA-spec ignition delivers a stronger, more consistant spark and has the idle stablizer built in, plus I thought that the retarded igition at idle would help with emissions. Now I'm re-thinking thet idea, as it's possible the Federal ECU is getting fooled by the increased air volume it's seeing with the lower idle vacuum and is thus delivering a richer mixture than it would normally.

I have also found that ambient air temperature seems to have a significant effect on the AFR seen at idle. The Vanagon Bentley stipulates that the idle CO measurement must be taken with the ambient air temperature at 68F or greater. I've been keeping an eye on my AFR as the temperatures go through their seasonal swing, and the mixture definitely leans out as the temperature rises into the 60's and above. I understand that the earlier L-jet systems didn't incorporate a TS1 sensor in the intake stream, but for the ones that did, air temperature definitely has an impact.

I should mention that I'm using an AFM that I had FIC here in Livermore rebuild for me just after Christmas. They were having a slow week at the time, so they were able to rebuild my unit in less than a day. When I picked the AFM up I could scarcely believe it was the same unit. It looked brand-new.

I had the privilege of talking with the owner of FIC, Bob White, who said he used to train mechanics in how to tune old L-jet vehicles like Vanagons and used that experience to teach himself the intricacies of the AFM. It was his assertion that you needed to get the idle vacuum up to 20" or so "and everything follows from there". I was skeptical that these engines could ever develop 20" at idle, but you can't argue with a man whose had a long lifetime of experience in these matters, so I thanked him and took my leave. Since then I've been chasing the grail of high idle vacuum. During my discussion with Mr. White, I said that as far as I could tell, the largest potential source of vacuum leak on my engine was the throttle shaft bushing, and did he know of anyone who was set up to re-bush these old throttle bodies. He said that his son was looking at doing some re-bushing work to the water-boxer bodies for one of the Vanagon aftermarket vendors, and that perhaps the air-cooled bodies could also be worked in the same manner. I need to follow up on that and get in touch with the son to see if it's worthwhile for him.

[dingo]

did you figure it our yet ?

my idle vacuum has dropped to 13-15", and it wanders significantly

the 'wandering' : is it wavering, drifting, ? regularly or irregularly ? is it rhythmic (periodic) ? over what range does it waver ?

[RSorak 71Westy]

Any engine with a smaller factory type camshaft, when running right, should pull close to or even above 20 inches at idle.

[Amskeptic]

Any engine with a smaller factory type camshaft, when running right, should pull close to or even above 20 inches at idle.

Any VW engine with factory retard will barely hit 12. And like it.
Colin

[SlowLane]

did you figure it our yet ?

my idle vacuum has dropped to 13-15", and it wanders significantly

the 'wandering' : is it wavering, drifting, ? regularly or irregularly ? is it rhythmic (periodic) ? over what range does it waver ?

It doesn't wander nearly as much with the vacuum retard and idle stabilizer out of the equation.

I think that what causes it to wander without idle stabilization is the variation in idle RPM, and what causes it to wander with idle stabilization is the variation in timing that the stabilizer enforces to keep the idle speed constant.

Either way, I have learned enough to stop worrying about idle vacuum. On to the next thing to worry, er, learn about...

[Amskeptic]

I think that what causes it to wander without idle stabilization is the variation in idle RPM, and what causes it to wander with idle stabilization is the variation in timing that the stabilizer enforces to keep the idle speed constant....

It is an option to check it out when I get out there . . .
Colin

[SlowLane]

Okay, re-opening this thread.

My experimentations with ignition timing and its effect on idle vacuum were limited to connecting and disconnecting the vacuum retard line on my disttributor can. Initially, I set the timing to 5 degrees ATDC with the retard line connected, as per factory specs. I then disconnected the retard line. The idle speed climbed immediately to about 1400 RPM, which required turning in of the idle speed screw until the idle speed returned to 850-900 RPM. At that speed, the ignition timing was at 7.5 degrees BTDC.

This, I think, is the key difference: the increased flow restriction required at the throttle plate with the more advanced timing. As an air pump, the engine is constrained by its mechanical design (displacement, valve size, timing, etc.) to pump a given volume of air at a given speed.

Here's my hypothesis: when the timing is set to 7.5 degrees BTDC, the combustion impulse is stronger than when it is set to 5 degrees ATDC. The engine is more "eager" at the more advanced timing, and so requires more restraint (via the restriction imposed by the idle speed screw) to stay down at the prescribed idle speed. Since the volume being moved by the air pump is (more or less) equal for the two ignition settings, the increased restriction at the more advanced setting results in a higher vacuum.

I'm sure that this is an oversimplification. I haven't tried bumping up the ignition advance to say, 10 or 12 degrees to see if the trend continues (mostly because I don't have any swing left in my distributor: it's already snug up against the cold start valve, but also because I don't see much value in doing so, beyond idle curiousity).