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Solved - Hesitation On Acceleration, Loss Of Power, Clicking/knocking


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#1
valkoles

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Would like to share my discovery (with the help of this forum and mighty google).

I have been experiencing some minor but annoying symptoms, i.e. hesitation on acceleration, intermittent loss of power, engine "misses" at idle etc - pointing to a vacuum leak at or near intake manifold. After replacement of intake manifold gasket, cleaning of injectors and other minor tweaks symptoms became less pronounced - I could finally live with them.

Quite a few months later, a new symptom appeared: loud metal knocking in the engine bay on the passenger side. Would start after the car warmed up, stay on for ~10-20 seconds, then disappear. In addition, a much quieter clicking could be heard when accelerating near a concrete wall (which worked as a sound reflector). Could not reproduce this one when not in gear - only under load.

When I got fed up and started researching, a purge valve became a suspect. I disconnected the harness and - voila! - all clicking/knocking is GONE! Not only that - my car seems cured of all the hesitation/stumbling/loss of power!!! It feels like the engine has grown an extra cylinder :-)

Have been driving like this for over a week (> 600 km), no problems except a Check Engine light on (which is understandable and does not bother me). The new purge solenoid is ~$80-$90 - I am too cheap.

Hope this can help someone fix similar symptoms.

#2
djdevon3

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Congratulations on figuring out your own issue! Always great to hear when someone can fix their own issue with a forum search. :)

Just for anyone else in the future I'll detail what the factory service manual has to say about the purge solenoid valve.


Operation of Purge Solenoid Valve
The purge solenoid valve is controlled by duty signals from the PCM to perform purging of the charcoal canister. Purging is done when these conditions are met:
(1) After warm-up
(2) Driving in gear
(3) Accelerator pedal depressed
(4) Heated oxygen sensor functioning normally

Well my understanding of that was completely wrong in a previous topic. I thought it only worked mostly at idle. So the purge solenoid is in fact supposed to work mostly just the opposite, when driving after warm-up. This could definitely affect performance. Good job and thanks for getting me to take a 2nd look at my own understanding of it.

#3
valkoles

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I am happy as punch! :-)

The only question that I still have is, would my disconnecting the harness have any detrimental impact shot/long-term? I can't say I am clear as to how the canister operates...

#4
djdevon3

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Meet the Purge Solenoid

Maybe I'll make a video out of this one. Great questions.

#5
valkoles

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Thanks for a great explanation! (you do sound a bit like a prof in a classroom :-))))) - authoritative and well-paced!)

#6
cpl

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Yes. Great write-up.

But can you explain this statement: "A blown intake manifold gasket and old charcoal canister are the worst enemies to the purge solenoid valve"? As far as I know, my intake manifold gasket is fine, as is the purge solenoid valve, but in a few months the charcoal canister in my '93 626 will be 20 years old (though only 125k miles).

The car runs fine, but the mpgs could be better. I get about 21-22 mpg with the FS engine in mostly city driving. Does the charcoal wear out? Could replacing the charcoal cannister improve my mileage?

Thanks.

#7
djdevon3

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Sure thing. The purge solenoid relies on the manifold vacuum. If you have a vacuum leak then the solenoid will have less vacuum pressure to remove fuel vapor with. It only takes a very small leak to mess with vacuum pressure. I'm talking the size of a pen tip or more. The bigger the leak the less effective the purge solenoid becomes.

The job of the charcoal canister is to scrub fumes before they enter the intake manifold. Like any charcoal media it's effectiveness is reduced over time. Same concept as charcoal filter in a fish tank. It needs to be changed every so often because the charcoal isn't effective forever. Charcoal has a shelf life.

I would only recommend replacing the charcoal canister when needed. How can you tell if it needs to be replaced? You'll start smelling rotten eggs in your engine bay. Not to be confused by the smell of a bad catalytic converter which will produce a very similar smell out of the exhaust. If you have leaks in your exhaust and it's a windy day the two might be confused so check for exhaust leaks and vice versa if you smell rotten eggs.... or your back seat in case someone left eggs in your car.

#8
snailman153624

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It serves only to reduce the amount of fuel vapors that escape from your tank into the atmosphere. The amount is beyond negligible from a fuel efficiency standpoint.

#9
djdevon3

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Yeah I should have said negligible MPG reduction because it's only vapor. The Mazda 626 doesn't meter the vapor line in any way whatsoever. If any MPG decline can be measured it will only be about 1-5 MPG less (guessing) for a full tank of gas. It's the same thing as the EGR system. It's just another way to make sure every molecule of fuel is burnt so you end up with the best possible MPG that the car can provide. You can more than makeup for it by slightly altering your driving habits like not going WOT as much to get on a highway or use slower acceleration in city traffic. You can actually alter your driving habits to improve your MPG even with a busted purge solenoid. In the grand scheme of things the vapor doesn't count for much but it does have the possibility of reducing your MPG when not working.

#10
cpl

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Thanks. I'll sniff around in the engine compartment--thought I've never noticed any smell of gas or rotten eggs there.

Where I do get a gas smell sometimes is near the gas cap. It's as if the system is getting over-pressured and venting through the cap. And the inside of the fuel filler door and around the cap gets all dirty suggesting the same thing.

#11
djdevon3

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There are 2 vapor lines. One goes up to the charcoal canister and in the event that something in that vapor line is faulty (purge solenoid) then the other vapor line must do double duty. The other goes up the filler neck and is plugged in right before the gas cap. You can loosen the gas cap so the fuel tank can vent the vapor. Like I said that is less than ideal and comes with some dangers. I recommend anyone with a faulty purge solenoid get it replaced asap.

The gas tank will be over pressurized (more than normal pressure) however the gas tank is rated to hold that pressure in case of a faulty purge solenoid. You don't have to worry about the gas tank cracking or leaking unless it's integrity has been otherwise compromised. If you have a bad purge solenoid I would also recommend getting under the car to do a visual inspection of the gas tank for integrity.




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