Evap system air leak in L-Jetronic Spiders

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Evap system air leak in L-Jetronic Spiders

Post by bartigue » Fri Feb 02, 2018 5:34 pm

I found an air leak today in an area I wouldn't have thought to look (I was replacing the gas tank...so I was intimate with the trunk)...

The upper hose on the charcoal canister runs to the FI plenum. The lower hose on the charcoal canister runs to the fuel tank. When the motor is stopped vapors from the engine go into the canister; when it is running vapors are drawn through the charcoal. A small orifice on top (has a little hat on it) draws clean air into the system, preventing a vacuum. Under normal circumstances this is a metered source of air.

I found the line that runs from the bottom of the can to the fuel tank, which routes under the car, to be broken. Therefore a visual inspection of the trunk and engine bay were just fine, but unplugging and plugging the vacuum line resulting in much smoother idle and transition. This shouldn't be the case, so I inspected under the car while messing with the tank, filter, and pump, and sure enough, a broken line.

So....if you have a good but not great idle and the occasional wimpy transition, inspect the canister and its hoses. Besides the little hat (which you need or its just a big unmetered air leak) make sure the hoses are intact. By now the old plastic cavis lines in the back aren't very flexible and if they are like mine, they break.
Brad Artigue
1969, 1969, 1970, 1979, 1980, 1981, 1982, and 1982 124 Spiders
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Re: Evap system air leak in L-Jetronic Spiders

Post by jseabolt » Sun Mar 11, 2018 4:50 pm

On my 1980 FI Spider, it appeared that someone had snipped one of the clear poly hoses running to a red (check?) valve underneath the car with a pair of scissors. My guess was so the fuel tank would vent to the atmosphere. It was a perfect straight cut. I did not bother to repair or track this hose down since I had already removed the canister and capped off the port on the intake plenum.

When the hose going to the plenum was connected I never noticed any idling issues and it took me several years before I even discovered it. I think in this case, the snipped hose simply caused the fuel tank to remain at atmospheric pressure at all times. Still looks like it would be sucking in un-metered air. Unless the fumes off the tank was compensating.

Seems like I read somewhere that if one of these check valves (3 or 4 of them) failed it can cause vacuum to build up inside the tank and cause fuel starvation issues as the fuel drops and develops vacuum inside the tank.

Why so many of these valves and hoses in the trunk? What do they all do? Looks like all that would be necessary to catch gasoline fumes would be a hose coming off the tank filler neck to the charcoal canister and an un-vented fuel cap.

By the way. I cut one of these canisters open on my Yugo and it was nasty. It looked like the stuff had broken down and became sticky and oily. Makes me wonder if these 30+ year canisters are clogged up they would be causing the fuel tanks to build up vacuum and cause drivability issues.

I made good use of the 8mm fuel vapor recovery line on my Yugo. It's now the fuel return line. When using a diesel pusher pump and the Malpassi regulator from a Maserati BI-turbo, the regulator could dump enough fuel back to the tank through the 6mm return line causing fuel pressure spikes.
6mm was too restrictive.
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