Mattman928

I was testing the vacuum system tonight, and wondering if these results make sense. On the attached photo, I plugged the end of the large line that runs from the brake booster to the engine intake, and attached the vacuum gauge to the thin hose that runs from the brake booster connection to the blue check valve (so that I could pull a vacuum on the brake booster).

The pressure dropped quickly with only a couple of pulls on the hand vacuum pump. it felt like it pulled a vacuum on the rubber hose to the intake only. From reading on the forums I got the impression that pulled a vacuum on this hose took a while since it would evacuate the brake booster also.

Question: Should the white connector that connects the intake hose to the brake booster be a check valve? If it is a check valve, does it allow airflow into or out of the brake booster?

Thanks

dr bob

That fitting in the booster is indeed a check valve. It allows air to be pulled out of the booster can towards the engine intake. Depending on when you tested vs the last time the engine ran, there's a varying likelihood that the booster can for the brakes was already under vacuum. If so, your hand pump effort was indeed just what was needed to evacuate the hose you plugged.

If you really want to manually pull a vacuum on that booster can, cycle the brake pedal a few times with the engine off. By the third pedal application you'll notice the harder pedal. By the fourth of fifth the pedal will be rock hard and the booster will be full of air. Then start hand-pumping if that's your goal.

Mattman928

Thanks Dr. Bob, I will give that a shot tomorrow!

Do you happen to know how that circuit works? I am curious why the hose from the intake boot to the ISV would also be connected to the brake booster? Wouldn't that interfere with the ISV trying to stabilize the engine idle? Or is the concept that when the engine is first started that the line from the boot to the ISV pulls the initial vacuum on the brake booster and the 4 way connector, then after the vacuum is pulled the pressure in the ISV line is stable?

To pull the thread on this topic, when we read on the forums about a vacuum leak causing an irregular idle, is the issue, that if air can get into those hoses and flow into the ISV it would be air that bypasses the MAF?

dr bob

Hi Matt --

It appears to allow a small amount of air to bypass the throttle, but that air looks like it comes from the intake after the MAF so it isn't un-metered.

Take a look at the drawing and description on pages 24-216 and 217. It's labeled "Fuel path" but shows the hose connections for the venturi flow booster nicely. Also shows the routing of the oil vapor hoses, all nestled in that nice clean and cool space under the intake manifold.

davek9

Just an observation, from your Pic #7 plug wire doesn't look seated all the way down into the cam cover and onto the spark plug, could be the pic but it looks level and not recessed, if so this will cause a poor running engine under load (ask me how I know

Dave K

dr bob

Irregular idle from vacuum leaks is typically un-metered (post-MAF) air that gets into the intake. The MAF itself is pretty much at atmospheric pressure most of the time, and for sure at idle. That means vacuum leaks that mess with idle are on the engine side of the throttle plate.

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Follow up on the suggestion on the plug connection that isn't right. A vacuum leak causes an irregular 'miss' as random cylinders suffer from lean misfire. An ignition problem offers a very regular miss at idle, where the same failed-ignition cylinders are to blame.


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Note that if you have working catalytic converters, the extra fuel pouring through from failed ignition will cause the cats to get pretty hot. More than a few cars have been lost to fire as an overheating cat converter ignited undercoating, then carpet and electrical stuff by your feet. Don't drive it if it isn't running right.

Mattman928

Thanks for the suggestions. I checked the plugs and reseated them all. The one you noticed wasn't seated but the engine still seems to be working great. I reseated them all but there wasn't much of a different.

I think it is an issue with the IDLE stabilization valve (the car sat for about 6 years in the garage so it may be gummed up). I tried the WD40 trick, didn't make much of a difference.

The car seems to idle well, but will randomly stall. it also stalls when I am decelerating the car and push in the clutch. As soon as the car hits idle it stalls.

When I rev the engine on the driveway, when I let off the gas, you can hear the engine RPMs dropping below idle, sounds like it's bogged down for a second and then will either stall or come back up to normal idle speed. (Ive tried a differnt MAF, check the vaccuum lines, seated all the plugs

I am going to try make it to winter before I do the top end refresh

dr bob

Matt, verify that the idle switch on the throttle is working. In digi-dash cars, you can do a quick test by driving it to speed, then let off the throttle with engine RPM's above about 1200. Your instant MPG display will go to 90 MPG if the throttle switch is working. The LH and EZK use the switch to turn fuel off on trailing throttle when you are out of idle-speed range. Once the switch is true and RPM's drop into idle range (less than 1200 or so) the fuel flow is re-enabled and the idle stabilizer valve is enabled. Absent the switch, idle will be pretty erratic since the ISV is staying closed.

Mattman928

I'll check that tomorrow. I've manually pulled on the linkage in the engine bay and can hear the click of the switch when pulled off rest.

the interesting thing is when I am driving the instant mpg shows 6mpg or 8mpg. Clearly number that don't reflect actual mileage. No idea why. Off idle the throttle response is great and the car is responsive.