Weird VVT Problem 4AGE 20V Blacktop

C.M Kyo
Posts: 5
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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Posts: 2114
Joined: Fri Jan 11, 2013 10:21 pm
Location: Wellington, New Zealand

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 :)
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

C.M Kyo
Posts: 5
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 ;-)

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