Hissing Sound Near Plenum

[joelittel]

Yesterday I started the car (1980 fi) for the first time in close to two months. The coolant hose that connects to the head had sprung a leak and it took me all winter to find the time to fix it, so the car sat through the polar vortex.

It started up just fine, nothing seemed out of the ordinary until it suddenly died. No warning, just as if a switch had been flipped and the car was suddenly off.

When I went to restart it would't fire up. Cranked over just fine, all necessary dash lights illuminated but would not fire up.

I left the keys in the "On" position while I was snooping around for anything obvious and near the plenum I could hear a hissing sound that I don't remember being there before. It's similar to the fuel pump sound but a hiss not a hum and most certainly coming from the injection side of the engine bay.

Is this sound normal or should I be looking into this as a potential clue to my no start problems?

[narfire]

Not normal. With the ignition on but not running there would not be any noise. If you can push the AFM flap with the ignition on, the fuel pump should kick on. Is the flap stuck open a bit?
Check for fuel leaks. The injector soft hoses get brittle with time and will leak, not a good thing with alternator and starter in the area. I put a dry shop towel under the injectors and crank and let sit for a bit. Drips are easier to see on the dry towel.
All the hose connections tight? Any un-metered air getting in after the AFM will give you issues. A torn diaphragm in the brake booster would hiss a bit when the engine is running but don't see it making any noise when it is not running. It will give you issues as well with un metered air getting in.

[joelittel]

Thanks for the lightening fast response!

The AFM flat does not make a tight seal when "closed." There is a sliver of gap that I'll have to check out, but it does seem like it's closing as far as it's supposed to. Obviously pictures would help to describe the condition of my AFM flap, so I'll make sure to post some... after I caffinate the wife, walk the hounds and clean up all the beer bottles from last night.

Will also check for fuel leaks. The towel trick sounds like a great plan and I'll make sure to try it before pulling the AMF for photos.

The injector hoses are suspect. They are original and did just sit through a series of 9 degree weeks.

I recently refreshed all vacuum hoses and all was running very well for a few months so I don't think I have air leaks, but I'll check that they are all secured tightly.

[bradartigue]

Is your brake booster leaking? A slow leak can hiss and cause a big enough air leak to make the engine die. There isn't anything except fuel pressurized in the FI system, and the only other device under pressure hooked to the intake system is the brake booster.

The good thing - if there is a good thing - about a gas leak is it stinks and drips.

[joelittel]

I put a shop towel under my injectors and cranked the ignition a few times, but saw no drips or any signs of leaking.

The AFM looks to be ok, but in need of a cleaning.


When I got into the car I could smell a faint fuel smell, which is not the norm for my car. The engine bay smelled normal (smells like an old car) and the trunk smelled normal (faintest hint of fuel but nothing more than usual). Never before have I smelled fuel in the cabin.

I do have a leaking master cylinder which I intended on addressing in the next couple weeks, but my booster looks to be dirty but newish, and the previous owner has documentation saying that it was replaced a year before I bought it, so it's been in the car about 5 years.

Master cylinder


The hiss I described earlier has a gurgle to it now, and because my car loves to cause problems on my weekends off my new temperature sensor is reading approximately 200 even though I know the car is ice cold. As soon as I turned the ignition on my gauge jumped to 200 and danced around between 100 and 200. The car is not warm, in fact it's very cold, the temperature outside right now is only 17 and it's been 24 hours since the car was last running. This will be replaced, and the wire checked for damage.

What would cause a brake booster to go bad? It's newish, but is there something about my car that could be hard on the booster? Seeing as how the previous owner thought to replace it before selling it, I'm wondering if something about my car has caused it to go bad again.

[azruss]

you may have cracked the vacuum hose to the booster. might not be evident without looking closely. Also if you have any air leaks in the rubber plenum, you will get the same results. Your MC looks like you have a hose or reservoir or hose fitting leak.I know the plastic inserts in the top of the MC get really brittle. They can be purchased in either a straight or angled format.

[majicwrench]

Not sure why people are so obsessed with their AFM all the time. Don't worry about some little hiss. Hiss is not keeping car from starting. vacuum hose is not keeping car from starting. Before yanking afm, playing with vacuum lines, do the BASICS, check for spark, if you have spark, loosen fitting at CSI and see if you have fuel. Then go from there.

[RRoller123]

That fuel smell in the cabin is worrisome, the fuel lines run inside the cabin along the left side rail, under the carpet, and may have a conversion from steel to rubber along there somewhere. Take a look, as that can be very dangerous. Hopefully nobody smokes....

[joelittel]

No smokers in the car, ever.

After work I went out and sniffed around the car. I couldn't smell any fuel in the cabin or engine bay which made me feel better about my current problem. Maybe I was smelling fuel on my clothing or from some other source Saturday, but tonight the cabin smelled like a nice blend of vinyl and leather just like it usually does.

The hissing sound is still there, and seems like it could be coming from the brake booster but I can't be sure.

I didn't start the car, only turned the key to on, and the hissing was immediately noticeable.

Can anyone recommend a post or two I can study before I attempt to go any further down the rabbit hole?

Earlier someone recommended checking the CSI, which I'll do. I haven't found one yet but I'm sure someone has posted a thread about the procedure for checking the brake booster.

[bradartigue]

If it isn't running and wasn't running then it isn't the booster. The booster is pressurized - for lack of a better term - by the diaphragm being sucked in by the manifold. If you just turned on the key and didn't start it and you hear a hiss, it isn't the booster. To be sure turn the key ON with the engine OFF and stomp on the brake pedal, if its the booster it will stop hissing. But it isn't the booster...

With the key ON and the engine OFF your fuel pump isn't running either, or it shouldn't be. Turn the key ON, unplug the fuel pump from under the car. If the hiss is still there then it isn't fuel. If the hiss stops then it is fuel.

You sure the radio isn't on and tuned to white noise? There just isn't that much on a FIAT that can hiss. And if it only does it with the key ON and engine OFF then it is an electrical device.

[majicwrench]

Worrying about your hiss is wild goose chase big time. Fix your problem.
You need to check SPARK and FUEL
I did not recommend checking the CSI (maybe somebody else did), I mentioned that is the easiest place to quickly establish if you have fuel.

[joelittel]

Definitely not the radio.

It's a very faint noise, like the hum of the fuel pump, but faint enough that you need to have the hood open and be outside of the car to hear it. You can not hear it while sitting in the car. In the past when I would turn the key to on I could always hear a faint hum from the fuel pump while I was seated in the car. Does that mean the fuel pump is/was always running or is a slight hum/buzz normal?

I'll test the fuel pump by unplugging it, and the booster by stepping on the brake per the above recommendations.

I'll also start with the basics, check for fuel, spark etc. as soon as practical.

The car was running just fine before it died so whatever caused it to die happened quickly. Could my fuel pressure regulator (don't know what it's really called but it's the small item with a vacuum hose on the top end and a fuel line on the other) have given up and now it's not holding any pressure? I did replace that vacuum hose over the winter.

[joelittel]

Majicwrench you wrote, "do the BASICS, check for spark, if you have spark, loosen fitting at CSI and see if you have fuel. Then go from there."

When you say "loosen fitting at CSI" what fitting are you referring to? Should I take the injector out or take the fuel line off the injector?

Sorry if it's a silly question, this is my first "hobby car" and I literally knew nothing about working on it when I bought it. Over the years I've learned a lot but I'm still reluctant to dive in and take items I'm not familiar with apart.

[ORFORD2004]

What about connection that are not tight at the alternator or starter. And The pump was not supppose to prime the system when you turn the switch on? If so, maybe the fuel pressure regulator is releasing the pressure slowly.

[majicwrench]

Did you check for spark ?? I do that first, tis easy, grab an old plug, pull a plug wire off, stick plug in lay on head and have someone crank engine while watching for zap zap.
CSi has that little fuel line that goes to it, leave valve in, just loosen the line, then crank engine, fuel should come spraying out of loosened fitting. Fire ext handy right??