Weird VVT Problem 4AGE 20V Blacktop

C.M Kyo
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Joined: Tue Jul 29, 2014 6:15 am

Weird VVT Problem 4AGE 20V Blacktop

Postby C.M Kyo » Thu Sep 21, 2017 4:37 am

Hello everyone !

I have a weird problem with my Blacktop. I tried to do some research but I found nothing which can solve it.

Let me explain :

I am running a 4AGE 20V Blacktop in my AE86 for 9 years. It runs pretty well.

Last week, I decided to do a test with the VVT. I unplugged the VVT solenoid connector and go for a test drive => No difference !

It turns out that the B+ wire was not connected ! I ran it for almost 10 years without the VVT ! I haven't drove another 20V, so I did not know the VVT was not active.

Anyway, I connected everything right and tested the car again. The VVT is now working. I can cleary see the sound difference and the increase torque.

But here comes my problem :

When the RPM are between 1500 and 2500, if I hit the gas pedal halfway, the car runs like crap. It looks like fuel cut or ignition cut. If I press the pedal gently, there is no problem. If I floor it, the engine hiccup for a second and then runs OK.

If I am higher than 2500 RPM, there is no problem. I can press the gas pedal like I want, the car runs great with VVT.

The weird thing is that if I unplug the VVT, there is absolutely no problem at lower RPM.

I have checked a number of things like the STA wire, ground points etc etc. Nothing wrong just the O2 sensor witch seems to be a little off (The voltage does not change like it should). I have ordered a new 02 sensor at Toyota.

Do you guys have any ideas ?

For information I am running a stock MT Blacktop ECU with Open Stacks and Tomei Poncams camshafts.

In advance, thank you very much for your advices.

And sorry for my English.

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jondee86
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Re: Weird VVT Problem 4AGE 20V Blacktop

Postby jondee86 » Sat Sep 23, 2017 12:32 am

This problem is very common with the 20V blacktop and I have read many posts
similar to yours describing "bogging" or "stumbling" at certain rpm's and light throttle.
Unfortunately, I don't ever remember seeing a thread where a specific cause was found.

As far as I know the BT ECU decides when to switch the VVT ON according to a combination
of engine temperature, load and rpm. VVT can switch on early or late, and with a warmed
up engine, it can switch ON in the engine speed range you describe. When VVT switches
ON the intake cam is advanced by 30 degrees. It is also possible that the ECU makes an
adjustment to ignition timing at the same moment that the VVT switches ON.

I suspect (although I have never seen it proven) that when the VVT switches ON at low rpm
and light throttle, the ignition timing becomes too far advanced for a moment. The stumble
is caused by combustion occurring too early and trying to slow or stop the piston which is
still rising on the compression stroke. Flooring the pedal will retard the spark and very
light pedal will probably delay switching of the VVT until the engine reaches higher rpm.
NB: This is one of jondee's many theories and may or may not have a grain of truth :-P

Replacing the O2 sensor sometimes helps, and checking your ignition timing is always a
good idea. If all else fails, you could always just unplug the VVT solenoid and be happy.

Cheers... jondee86

PS: Nothing wrong with your English :)
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

C.M Kyo
Posts: 9
Joined: Tue Jul 29, 2014 6:15 am

Re: Weird VVT Problem 4AGE 20V Blacktop

Postby C.M Kyo » Sat Sep 23, 2017 11:53 am

Thank you very much for your advices ^^

Today I have installed a no name 02 sensor just to try before I get the OEM one from Toyota. The signal is good but it made no difference.

I have checked everything and it seems to be ok.

I will try to get my ignition timing to 8° (I'm at 11) and see what happens.

I have bought another ECU on Ebay for a swap top.

If it's still not working, I think I'm gonna install a switch near the dashboard to get the VVT on/off depending on how I drive.

Anyway, thanks again for your explainations ;-)

C.M Kyo
Posts: 9
Joined: Tue Jul 29, 2014 6:15 am

Re: Weird VVT Problem 4AGE 20V Blacktop

Postby C.M Kyo » Sun Sep 29, 2019 2:19 am

Hello !

Some news after all this time.

I never killed the issue. Since it was only between 1500 and 2500 rpms, when driving normaly, it was OK.

Fast forward, I took out the engine from my Levin to put it in my JDM Trueno. Same engine, same ECU, same sensors, same VVT problem but not at the same rpms ^^

Now the problem occurs around 3000 to 3500 rpms.

The only differences are the transmissons. The Levin is equiped with TRD gearbox and 4.7 final gear. The Trueno runs stocks gearbox/final gear. And the velocity stacks. I've got 100mm Tec Art's on the Levin. I cannot reuse them on the Trueno because it is an RHD. So for now it is running without velocity stacks at all (Waiting new ones from SQ Engineering).

I tried to disconnect the VVT and the engine runs perfectly.

I found information in Japan that a 6 speed gearbox ECU could resolve this issue.

C.M Kyo
Posts: 9
Joined: Tue Jul 29, 2014 6:15 am

Re: Weird VVT Problem 4AGE 20V Blacktop

Postby C.M Kyo » Sat Oct 05, 2019 11:56 am

Hello !

Just to tell you that the 6 Speed Gearbox ECU solved the issue :D

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jondee86
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Re: Weird VVT Problem 4AGE 20V Blacktop

Postby jondee86 » Sat Oct 05, 2019 1:28 pm

Interesting :) The 6-speed ECU must be mapped to activate at slightly different
load/rpm points due to the different gearing. By some quirk of fate this happens
to match the needs of your conversion... well done and good to know !!!

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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oldeskewltoy
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Re: Weird VVT Problem 4AGE 20V Blacktop

Postby oldeskewltoy » Sun Oct 06, 2019 8:50 am

wondering..... (brainfart)

could your ecu have been bad... ??? not horribly bad, just bad when switching???

Doesn't matter now that you have replaced it.
OST Cyl head porting, - viewtopic.php?f=22&t=300

Building a great engine takes knowing the end... before you begin :ugeek:

Enjoy Life... its the only one you get!

C.M Kyo
Posts: 9
Joined: Tue Jul 29, 2014 6:15 am

Re: Weird VVT Problem 4AGE 20V Blacktop

Postby C.M Kyo » Sun Oct 13, 2019 12:57 am

I tested 3 ECU. One was a Blitz Access.