Decel fuel cut vaccum switch

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CurtB

CurtB

Joined
Jan 9, 2016
Threads
98
Messages
346
Location
Florida
I'm trying to diagnose a very elusive mid-range stumble and am checking out the fuel cut stuff. The FSM is vague, to me, about how to test. It says to check for continuity between switch and body with the engine off and on, but it isn't clear (to me) as to the "correct" result. I think, from the pictures, that there should be continuity (a complete circuit) when the engine is off, but the circuit should be open when the engine is running. Right?

I get a closed circuit in both cases, so I'm assuming that indicates a broken switch, right? Anyone have source for a replacement?

Screenshot 2017-03-30 at 8.02.42 AM.webp
 
they left out the part where you verify vacuum is present while the engine is on...don't give up yet. someone might have put a bb in there...and you can prolly find a used on in the classified ads...put a vac gauge on the hose going to your switch and turn on the truck- you should have like 17+" vacuum...
 
Ok. Here's an update. I think the switch should be closed at rest, and open under high vacuum. I got it to open under VERY high vacuum (picture me sucking on a hose until my eyes bulge!) but I did get it to open. I can't image the fuel cut port generating enough suction to get that sucker to budge, tho! I'm trying to loosen it up now...
 
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seems about right...decel fuel cut would kick in under the highest vacuum; ie, you were hot on it for a bit then you let off the gass all of a sudden...revs are high but power is low and your vacuum is HIGH as it will get; this is when you don't want to be SUCKING in fuel from the idle circuit...I don't know if this would contribute to your mid-range throttle response as the DFCO switch is operating the ISC valve which feeds fuel to the idle port when the throttle plate is closed- it'll only affect at idle, or whenever the plate is closed(like during decel). if the plate is open, no fuel is being drawn thru the idle port IIRC, as the vacuum is happening higher up the throat of the carb. you might have a jetting issue or accel pump issue...
 
are you still smogged?
 
LAMBCRUSHER said:
are you still smogged?
Click to expand...
yes, still smogged. I was going down this path with the idea that maybe the decel circuit was kicking in when it shouldn't. Doesn't look like the case.

I'm really bumfuzzled. Engine idles smoothly and performs as expected under load. Getting decent mileage...just back from a trip and averaged 13.5 mpg. No pinging. At 2000-2500 rpm, under minimal load, the engine hesitates. Like dirty plugs or rotor would except it goes away if I give it a little juice.

Carb, distibutor, cap, rotors, wires, plugs, filters, hoses, EGR stuff all fresh and tested. Right now, the only thing that doesn't check according to the FSM is the air injection system. It is always blowing into the bypass and I'm assuming it is a bad valve(ABV?). I just haven't given it much attention because I though it was unrelated.
 
AIR shouldn't affect mid-range. EGR does, as does a possibly leanish slo jet...seems like a mid range lean to me...
 
Testing the operation of the vacuum switch & deceleration fuel cut circuit is very easy.

Car parked in neutral
Rev up the engine to about 2200 rpm & hold it there for a few seconds, then start letting off the gas. Not too fast, not too slow. When the RPMs hit about 1800, the engine (not ignition) should suddenly cut off for a second or two and the rpms should suddenly drop to about 1200 or so, then the engine should spring back to life.

Conversely, you can double check it by removing the vacuum hose that connects to the vacuum switch (and plug the hose) and then do the same procedure described above.
This time, the engine won't suddenly hiccup between 1800 & 1200 rpm and will slowly drop its rpms in accordance to the gas pedal position (like you would expect it to).

---

Even if the EGR system tests functionally ok accordung to the FSM, it can still cause acceleration stumble when the valve is old. The diaphragm and maybe the spring inside it get looser with age and the valve opens up sooner and farther than a new valve.

The EGR valve requires quite a bit of exhaust back pressure to work well during acceleration. That back pressure was created by the OEM catalytic converter. If the cat (or any cat) has less flow resistance than the new oem cat did, the EGR valve can open sooner and farther during acceleration... leading to the classic 2F acceleration stumble.
 
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Hey Curt.. tell us a little more about the carb. If you're not losing vacuum anywhere, you can probably trace your problem back to the carb.
 
CaptClose said:
Hey Curt.. tell us a little more about the carb. If you're not losing vacuum anywhere, you can probably trace your problem back to the carb.
Click to expand...
I don't think I'm loosing vacuum.... Carb is a rebuild from SOR with probably 15,000 miles on it. Bone stock.
 
Output Shaft said:
Testing the operation of the vacuum switch & deceleration fuel cut circuit is very easy.

Even if the EGR system tests functionally ok accordung to the FSM, it can still cause acceleration stumble when the valve is old. The diaphragm and maybe the spring inside it get looser with age and the valve opens up sooner and farther than a new valve.

The EGR valve requires quite a bit of exhaust back pressure to work well during acceleration. That back pressure was created by the OEM catalytic converter. If the cat (or any cat) has less flow resistance than the new oem cat did, the EGR valve can open sooner and farther during acceleration... leading to the classic 2F acceleration stumble.
Click to expand...

So, the EGR valve was replaced by the PO but I have no idea of the mileage at the time. I THINK the cat is stock. The symptom I'm getting is exacerbated on a hot engine.

Right now I'm running midrange fuel (89 octane) and have the timing advanced an additional nudge. I can run 93 and advance the timing a bit more, too, if I'm feeling flush. No ping. When I retard the timing back to 7 BTDC, the hesitation gets much worse. Engine compression tests out at about 132 across 1-4. 5 & 6 are down to about 118.
 
One way to see if the stumble IS in the decel circuit is to bypass it: Disconnect the ICS and wire it directly, with the green to 12V and the white to ground and drive it and see if the stumble is still there.

If it is, it ain't the decel.

These stumbles are difficult to diagnose as the decel is controlled by the emissions computer that gets signals from the Tach, speed, and manifold vacuum. Also, the emissions computers are known to get wonky.

You can bypass all of it by grounding the white wire on the ICS to the carb body.
 
Oh, I forgot to say that the EVAP VCV is inop and I have the hoses on the canister reversed.
 
The VCV can leak internally from the vacuum source connected to pipe "S" in the drawing.

Pull the vacuum hose connected to pipe "S" on the VCV & plug the hose (to rule it out... since you're not using it anyway)

Also... swapping the charcoal canister hoses isn't ideal because the EVAP air injection into the manifold was accounted for in the A/F mixture. It's a fair amount of air. If the charcoal canister vent line is plugged (no EVAP) the engine will run richer than Toyota wanted it to run. Not sure if that's good or bad, but it will lower mpg a bit.

image.webp
 
So this is frustrating. I'm pretty sure I don't have a vacuum leak anywhere...sprayed carb cleaner around and couldn't detect a change.

EGR system tests to spec as does decel circuit. Air pump system is on bypass all the time but I don't think that is a factor. The VCV for the dizzy vent is disconnected and pluged because the valve is inop.

I continue to get hesitation during low load mid-range from 2000-3000 RPM. If I punch it, it smooths out. MPG is ok. Compression is ok for an old engine. Carb, distributor, plugs, etc. are all fresh. I just pulled the distributor cap to check the rotor and cap...everything is clean.
 
The EGR valve can still "test to spec" yet continue to create a stumble every time it opens. Just to rule it out, pull the vac hose leading to the EGR valve & plug the hose. Drive the truck around the block. See if that fixed the stumble.

image.webp
 
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