AE86 Silvertop Misfiring Issue

chaossilvr
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Joined: Mon Sep 29, 2014 3:30 pm

AE86 Silvertop Misfiring Issue

Postby chaossilvr » Wed Oct 25, 2017 1:48 pm

Little bit of history:
-Bought a 1985 AE86 GT-S with a Silvertop swap (BT wiring, ecu, itbs)
-200 miles later engine spun a bearing.
-Had the engine rebuilt by a mechanic because I didn't feel experienced enough at the time to handle a job that big.
-Car returned but would no longer idle. Mechanic basically had a meltdown and said he was done working on my "crap nonsense engine."
-I start digging into the car myself and discover a slew of issues. He removed the vacuum line from the MAP Check Valve and capped it off so he could just put a T in another random line and connect it to the MAP sensor. He also removed the IACV for reasons still unknown to me. After spending a good amount of time sourcing a vacuum diagram I think I have it all hooked up right. That brings us to the title issue:

The engine now idles very rough. Very very rough. However, if you pull on the throttle it doesn't miss on the surge. It's only when its being held at a steady rpm. I've tried adjusting the timing (has a distributor relocation kit from Battlegarage/SQ), haven't been able to get it completely dialed in. Doesn't help the previous owner completely deleted the check connector so I can't put it into diagnostic mode. I discovered I can unplug the connector to Injector 1 and it makes no difference to how the engine runs. All four cylinders have spark, compression, and are getting fuel. The plugs all come out caked in a black soot almost but still dry. I swapped the injector to other cylinders and it seem the problem is locked into the first cylinder. I then put a pulse checker (name?) into the connector and it is in fact getting power and pulsing. At this point I have no clue what the issue is and I feel my only solution is to try and find an aftermarket ECU and wiring harness for large quantities of dollars.

Any help or insight into this matter would be much appreciated

Thanks!
Evan

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Sprite
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Location: Twin cities, MN

Re: AE86 Silvertop Misfiring Issue

Postby Sprite » Wed Oct 25, 2017 7:48 pm

Evan sorry to hear that your mechanic would rather work on the same old crap over and over than learn something new. Anyway, it might be a long shot and too simple but open your ECU and check for leaking capacitors before you go any further.
Rick

chaossilvr
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Re: AE86 Silvertop Misfiring Issue

Postby chaossilvr » Wed Oct 25, 2017 9:22 pm

Yeahhhhh, very quickly made the decision to never use him again and in fact stopped sending our company mechanic work to him as well.

I'll definitely do that! Hopefully it's something like that so I can at least have a positive idea of what the problem is and how to address it.

Thanks!

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jondee86
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Re: AE86 Silvertop Misfiring Issue

Postby jondee86 » Wed Oct 25, 2017 9:57 pm

A few fundamentals...
1. The BT 20V is MAP sensored and the ST 20V has a MAF.
2. The BT has a dedicated vacuum line for the MAP sensor, but the ST does not.
Therefore, if you have a ST with a BT ECU you need to make sure that the MAP sensor
is getting full manifold vacuum by teeing into one of the lines that connects directly
to the round part on the end of the common vacuum rail. If the MAP sensor does not
get full manifold vacuum at idle it will run rich.
3. On the 20V ALL the idle air passes thru the ISCV. If you have the ISCV you should
get it re-installed, as not only does the ISCV control the idle, it also lets in extra air
to compensate for extra electrical or mechanical loads on the engine.
4. Without an ISCV it is common practice to crack the throttles open a fraction to
let idle air into the engine. This can be enough to stop the IDL switch in the TPS from
operating to tell the ECU it should be idling (not driving). If the engine still thinks it
is driving because the throttle is slightly open, the ECU will not adjust the timing and
fuel to promote a smooth idle.

Maybe some of this helps :)

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

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chaossilvr
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Re: AE86 Silvertop Misfiring Issue

Postby chaossilvr » Thu Oct 26, 2017 7:22 am

I do have the IACV and I hooked it back up right away. It brought the idle to a nice 700rpm.
I also fixed the vacuum lines in the sense that the mechanic did just t into a random line, so I blocked that and hooked the MAP to its own line.
I'll hopefully be able to crack open the ECU and take a look at that. In the case that it is in fact a leaky capacitor is that a problem thats fixable or is it better to just get a new ECU off eBay or something? I hope I don't sound TOO much like an amateur, I'm just trying to learn as much as I can so the next time I have a big problem I can work on it myself as opposed to go through what I just went through again.

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jondee86
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Re: AE86 Silvertop Misfiring Issue

Postby jondee86 » Thu Oct 26, 2017 1:33 pm

chaossilvr wrote: In the case that it is in fact a leaky capacitor is that a problem thats fixable?

I've never tried fixing one myself, but I have read several threads that say it is a simple
task for someone that knows what they are doing. Basically replace the faulty capacitor and
repair or bypass any damaged tracks. This pic shows you what to look for...

Image

If I had this problem I would find a shop specialising in TV/electronics repairs, show
them the job and ask if they can fix it and how much :) It should be an easy job for anyone
familiar with repairing printed circuit boards.

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

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CloudStrife
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Re: AE86 Silvertop Misfiring Issue

Postby CloudStrife » Fri Oct 27, 2017 6:07 am

**** PCB repair all the way. I am a qualified technician, and pretty proficient with soldering components, but I don't bother retracing lines or doing board repairs. It's just too much work for the weekend do-it-yourselfer.

I second what jondee says, and go to a qualified repair specialist. The good news is, it doesn't have to be some MADTYTEJDMREAPIR guy, it can be TV/Radio shop as all PCBs are pretty identical in repairs. Especially an older board such as 90s ECUs.

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s24a
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Re: AE86 Silvertop Misfiring Issue

Postby s24a » Sat Oct 28, 2017 8:57 am

BT ECUs are really not known for capacitor failures. I think Toyota/Denso wised up about the NICHICON electrolytics in the Silvertop ECUs.
Not to say it cannot happen, but it is a rarer event.

The BT ECU suffers from blowing the VVT transistor as a higher failure mode.

As for doing either ECU and/or PCB track repairs, I am set up to do that if you need it.
For capacitors, I only use the best Panasonic electrolytics for replacements. I also heat sink the VVT transistors for both ST and BT ECUs just to be safe.
The ST uses a MIL-spec transistor for its VVT control. The BT is a little more common bipolar.

Hank Dozier
"When the going gets, wierd, the Wierd turn Pro" -- Hunter S. Thompson

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