Secondary throttle failure

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mrdance123
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Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Secondary throttle failure

Post by mrdance123 »

My T4 runs perfectly while in primary throttle (less than about 2/3 throttle).
It balks, loses most of it's power when opening the secondary, beyond about 2/3, to full throttle.
I have 2 T4's, and swapped the carbs between them.
The bad car has identical secondary failure using either carb.
The good car gets good afterburner kick from secondary with either carb.
It's not a bad carb causing the problem.
Ideas? Maybe it feels fat and started a low carb diet?

For fuel economy it's good idea to avoid the secondary when not needed,
I put a stiffer return spring on it, so I can clearly feel in the foot where secondary starts,
makes it easy to stay out of the secondary while getting full primary.
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
xirdneh
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My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: Secondary throttle failure

Post by xirdneh »

starving for gas? thats where i would start looking
then swap the distributors
Love those Tercell 4x4 wagons but they sure suffer from road noise.
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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
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Re: Secondary throttle failure

Post by Petros »

partially blocked fuel filter will cause fuel starvation at full throttle.

The problem is not in the carb obviously, so consider what did not change that can cause that kind of behavior.

EGR valve perhaps, some other device that only is engaged at full throttle, not related to the carb?

if your centrifugal advance in the distributor is stuck, it will cause it to loose power at higher rpms. not so much a cut out, but just weak acceleration.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
mrdance123
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Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

Swapped distributors (a spare part, not from my other running car) - no change in behavior.
The failure of secondary throttle is not RPM related,
it fails, balks, at any engine speed from about 1300 RPM on up (less RPM will lug excessively).

Checked power to both primary and secondary fuel cutoff solenoids.
They both have power, can hear the click.
Checked both solenoids before and after the test drives.
This test done with inline switches to manually cut the solenoids,
with ignition on, engine not running, and listening for the clicks.
The first solenoid cut kills running engine at idle.

My next test is to drive the good car with the secondary power cut,
to see if it behaves the same way. Later.

Fuel filter is new-ish, was replaced last summer.
Symptoms of the fuel starvation were much different then.
Failure now is 100% related to and responds instantly to throttle position,
it is not helped by going down a long hill at 0% throttle,
it still fails and balks instantly after such a hill, only in the secondary.

Someone mentioned EGR issues.
Who knows how to disable it for testing,
to force it to stay closed, or open?
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
mrdance123
Top Notch Member
Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

Has anybody had luck cleaning the EGR valve.... any tips to share?
I found mine completely plugged with hardened carbon.
Soaking it for days with penetration spray used to release nuts bolts, had no effect on the carbon.
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
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ARCHINSTL
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My tercel:: Goldie is a 1986 SR5 attualmente con Weber/also owned the first T4WD in STL in late '82
Location: Kirkwood, a 'burb of St. Louis

Re: Secondary throttle failure

Post by ARCHINSTL »

@ mrdance123 - did you look in the Repair Guides Forum?
viewtopic.php?f=7&t=1496&hilit=egr
Also look for takza's earlier treatise on the subject.

I will add, however, when I switched to the Weber 32/36 in 2008, I removed the emissions stuff (MO no longer required it on pre-'96 cars) and simply blocked the manifold hole with a plate.
Obviously, I also removed the pipe-from-manifold-to-EGR valve and blocked the manifold hole for it with a bolt.
Tom M.
T4WD augury?
"Oh, do not ask, 'What is it?' Let us go and make our visit."
T.S. Eliot - "The Love Song of J. Alfred Prufrock"
"Now and then we had a hope that, if we lived and were good, God would permit us to be pirates."
Mark Twain
xirdneh
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Joined: Thu Aug 04, 2005 11:38 am
My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: Secondary throttle failure

Post by xirdneh »

i have cleaned quite a few EGR valves
the first 4 or 5 i spent quite a bit of time trying to get the carbon out with screwdrivers and drills
now i drill two 3/16 dia holes in the bends so i can poke the carbon out
then tap the holes for 1/4-20 screws and plug with short screws
the small stainless steel tube for the hose connection can be hard to get loose
for that i use a vise grip on the screw. when tube is off heat it up with propane torch and carbon will come out when you tap tube against something hard.
much easier and faster doing it this way
Love those Tercell 4x4 wagons but they sure suffer from road noise.
mrdance123
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Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

Replaced the carbon clogged EGR valve.
Installed brand new EGR valve, and it passes the Service Manual test,
engine dies when idling, when vacuum is applied to EGR valve.

New EGR had zero effect on secondary throttle problem,
no noticeable difference in driving overall.

Severe balking, loss of most power using secondary at or near full throttle,
almost as bad as if ignition was cut off.
Backing throttle out of secondary, the car instantly takes off.
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
mrdance123
Top Notch Member
Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

Replaced the carbon clogged EGR valve.
Installed brand new EGR valve, and it passes the Service Manual test,
engine dies when idling, when vacuum is applied to EGR valve.

New EGR had zero effect on secondary throttle problem,
no noticeable difference in driving overall.

Severe balking, loss of most power using secondary at or near full throttle,
almost as bad as if ignition was cut off.
Backing throttle out of secondary, the car instantly takes off.
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: Secondary throttle failure

Post by Petros »

I do not think the EGR would cause that kind of behavior, you can completely remove the egr and it will drive normally.

there is a device on the right fender called an EBCV that allows bleed air under the carb during certain running conditions to adjust airfuel mix. I have seen these go bad and cause the car to be undrivable above certain throttle positions. also causes poor fuel economy if not working. Not sure if you can by-pass it or if it must be replaced, but you might look there.

Consider that you tired swaping the carb out, but it made no difference, so it has to be caused by some other system that is allowing too much air into the system which causes it to lean out and than bulk or stall out. there are only a few devices that can do that, the EVCV can do that and should be tested and reither replaced or by=passes if it does not pass the test.

good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Re: Secondary throttle failure

Post by larry mcgrath »

Have you checked the fuel pump pressure at the carb? Should be around 3psi.
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ARCHINSTL
Goldie Forever
Posts: 6369
Joined: Sat Apr 30, 2005 1:52 pm
My tercel:: Goldie is a 1986 SR5 attualmente con Weber/also owned the first T4WD in STL in late '82
Location: Kirkwood, a 'burb of St. Louis

Re: Secondary throttle failure

Post by ARCHINSTL »

My car exhibited some similar probs when I got in back in '05.
There is a very long thread (65 posts!) on this helpful:
viewtopic.php?f=4&t=1372&hilit=EBCV
Also - check out the various posts on this link (from Search using "EBCV"):
search.php?keywords=%22EBCV%22&terms=al ... mit=Search

Tom M.
T4WD augury?
"Oh, do not ask, 'What is it?' Let us go and make our visit."
T.S. Eliot - "The Love Song of J. Alfred Prufrock"
"Now and then we had a hope that, if we lived and were good, God would permit us to be pirates."
Mark Twain
mrdance123
Top Notch Member
Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

Thanks guys for all the tips and post threads you are sharing!
I removed the two EBCV vacuum lines from the carburetor,
connected them to each other to seal them off,
and capped shut the 2 carb ports they were connected to.
I'm sure this also disabled the HAC, but i'm at sea level, so it's not important to me.

This cured the secondary throttle problem....
now getting a good power boost from secondary rather than the balking and severe hesitation.
Seems to drive just fine, exactly like before, at lower throttle levels.

I am now wondering if it's better to just unplug the EBCV green electrical connector,
leaving the vacuum lines connected normal?
Will try this at later time and see what happens.

I will be monitoring MPG to see if it gets better or worse after doing this.
I'm at 34-35 mpg using any of 3 carburetors I have swapped in.

I have 4 EBCV valves, 2 of them in running cars, they all test the same 13-14 ohms,
with no voltage applied, one vacuum line holds tight,
the other bleeds fairly quickly in about 2 seconds when testing with a vacuum pump gauge.

Has anyone experimented to see if these vacuum tests change with 12v applied?
And both wires are identical black, which one is 'positive' ??
I believe that the ECU sends varying voltages based on O2 sensor and 2 vacuum switch readings.
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
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Petros
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Posts: 11930
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: Secondary throttle failure

Post by Petros »

the EBCV adjusts the fuel air mixture to reduce emissios by allowing air to bleed in under the base of the carb during certain driving conditions. it is far better to make these fuel air adjustments with the modern electronic fuel injection, but this is how it was adapted to the old carburetor before EFI was common.

I am not sure what driving conditions it is activated, researching the FSM should tell you how it works. It seems to me to leave it in place but disabling would do the same as just by passing it, if it is not functioning properly it will not run properly either. the car was designed to run with it in place and working properly. If you got the car to run poperly with it by-passed, I see no advantage to connect it up but have it disabled.

BTW, the high altitude compensater (HIC) also works by leaning the fuel air mixture when you get above about 7000 ft (as I recall), it did not come on all models, only the "mountain state" Tercels where there are roads above that altitudes (in many states in the east their highest point is 1000 ft or less). No harm should come by by passing it, even if you should drive above the altitude it will still run okay, my own tercel never had a HIC but I have had it on high altitude roads a few times. When it works properly, it leans the fuel air ratio and advances the spark timing (a leaner fuel air mixture burns slower so they advance the spark to allow a more optium peak pressure to occur at TDC).
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
mrdance123
Top Notch Member
Posts: 152
Joined: Sat Feb 21, 2015 9:03 pm
My tercel:: 84 and 85 4wd wagons

Re: Secondary throttle failure

Post by mrdance123 »

I found one of my 2nd throttle diaphragms to be full of old gas (smells like gas,
brown and thicker like rotten gas). A lot dripped out when I removed diaphragm from carb.
Not sure how this can happen even if the flange gasket was bad.
Is this an o ring that gets squished flat?
There was barely any passage left in the middle of 'gasket' o ring for vacuum air flow to move the diaphragm.
Have any of you replaced this gasket, and know what size generic o ring to get?
85 T4 Speedy Blue, SR5, 270-312k, operational R12 A/C, in use.
84 T4 Curmudgeon Copper, 188-294k, my first T4 in 1990, now Dad's daily driver.
RIP - 84 T4 Foxy Blue, 256-288k, engine died 2021, sold to PNP
Sold: 86 T2 Silver, VIN ending 5847, 194k, cherry condition, now in a good home.
RIP - 83 T4 Smokin' Gold, 270k, rusted out body, parted & scrapped
RIP - 84 T4 Quicksilver SR5, 125k, blown engine no oil, had sat 10+ years, parted & scrapped.
RIP - 80 Corolla (purchased new), 0-473k, 358k original engine, rear-ended totaled in 2003.
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