HVAC vacuum leak

[200 hp]

As I've brought up before, my 1974 350SE develops a hissing sound from behind the dash when engine is running. Determined to be a vacuum leak and likely linked to my engine's hot stalling at low RPM problem, and perhaps the poor central locking action too.

Careful listening has made me determine the hissing is coming from behind the HVAC controls.
How do I get this panel off? Please don't tell me I need to remove the entire centre console! And what sort of vacuum line back there is likely the culprit?

[wbrian63]

"I believe" you remove the radio to get the front cover off. Pull the knobs off the radio and remove the face plate (assuming it's a factory radio). This also assumes you manual HVAC controls. Then remove the retaining bars and pull the radio out. There are 2 screws that hold the front cover in place, accessible once the radio is out. Pull the knobs off the controls, and the 4 lever knobs as well. Cover pivots up from the bottom and unhooks from the top.

Other's may have more accurate instructions. I'm going from memory here...

[flutes]

Nathan wrote up a guide which helped me immensely many moons ago... linl

[Mforcer]

Does the hissing stop if the heater is on?

I had a hissing sound as you describe which stopped when the heater was on. There are some bellows as part of the heater system which needed replacing and my hissing sound was resolved.

[Big_Richard]

ive a hissing sound from my heater lever valves, but thats because ive got the hoses connected incorrecly 

[200 hp]

Does the hissing stop if the heater is on?

I had a hissing sound as you describe which stopped when the heater was on. There are some bellows as part of the heater system which needed replacing and my hissing sound was resolved.

Hissing stays even with the heater on.

Anyway, thanks a lot guy with the instructions! Unfortunately I'm a bit hamfisted when it comes to removing something so delicate as the plastic surrounds for the controls. I had decided to just observe it for now, making sure it is the main problem that's causing the engine to stall.

After a while now, I've determined the stalling issue must be electrical! Heavy rainy nights is when the engine is most likely to cut out, with lights, wipers, and the fan on. Adjusting the idle screw does nothing but make it worse.
Because I have no tachometer I have to rely on the oil pressure gauge to determine the engine rpm. Applying the brakes affects the needle and how smoothly the engine is running.

I don't think it's the alternator as the charge light only illuminates before kicking in the starter, and has never been observed to glow faintly. My biggest suspicion is the battery because it's rather small compared to the size of the battery tray. Perhaps it's the wrong type for the car? Maybe before I bought the car the previous battery had gone flat, making the used car dealership simply swap in some random battery they had lying around (the sketchy battery has no manufacturer labels or anything)? Didn't notice any problems in the summer, but in the winter with colder temperature and lots of rain, the car is struggling to idle.

Also, in case it may be related, the starter sometimes doesn't engage when I got to start the engine. Just spins in its housing. A second try and it works perfectly, oddly.

[200 hp]

Edit: accidental double post!

[WGB]

The simple way to see if you have a vacuum problem affecting the engine is to disconnect and  plug the main vacuum feed line from the rear of the engine where it feeds into the multiple takeoffs to Door locks and HVAC near the brake booster.

If sealing the vacuum helps the engine perform better and stops the hissing you have made a start.

The schematics for the vacuum system as well as the original colours of the tubing ( there were three generations of coloured tubing during the life of the 116) are well covered in the Workshop Manual and the HVAC part cane be found here - HVAC Vacuum system Link.

Just scroll through the PDF - the unconnected line in the diagram is what is connected to the central locking system.

There are only two vacuum systems that are used by the HVAC system itself.

1) A vacuum controlled valve which controls fresh or recirculated air and a picture of this can be seen in this post - Heater problem Link

2) Vacuum is supplied to the two control valves at the base of the heat levers which in turn control the heater valve by applying a vacuum when both heat levers are bottomed in their slot.

You most likely either have a disconnected vacuum tube or a split diaphragm but careful diagnosis will find and fix it.

Bill

[wbrian63]

A starter that just spins rather than engaging the starter ring is either a sticking bendix (the mechanism controls the path of the gear on the starter motor out to and back from the starter ring), or a failing solenoid, which is the device that actually forces the gear outward.

If a second attempt usually makes it work, then it's probably the bendix. Either way, a rebuilt unit is your best bet, as eventually, a 3rd, then 4th then 99th attempt will no longer provide a running engine.

A low-voltage battery will most certainly cause the car to falter and possibly die. However, if the voltage gets that bad, the car will not re-start when it quits.

To check the charging system, first fully charge the battery with a good quality charger. You should see 13.2 or so volts out of a fully charged 12v battery, immediately after the charging cycle is complete.

Start the engine and allow it to idle and reach operating temperature. Measure voltage at the battery. You should see about 14v, maybe a little less.

Add electrical load one accessory at at time, measuring voltage drop at the battery after each item is added. Add headlights - then HVAC blower on high, then wipers, and if you really want to stress it - the rear window defogger. If the alternator is functioning properly, voltage should stay in the vacinity of 12v. If it drops into the 11v range, you've got a problem.