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Discussion Starter · #1 · (Edited)
Bought my car from a single owner 4 months ago. Bone Stock from what I could tell. Would shut off at idle.

I change spark plugs, plug wires, and coil pack. no improvement. I change the MAF/AMF (whichever it is), no improvement. I hear hissing so I try to fix any vacuum leaks, in the process breaking the BVSV plastic nipple.

Not having much mechanic experience aside from regular oil changes and similar tasks, I brought it in to multiple places. First guy tells me he wants to raise the idle and said he needed to drill into the engine block to do so... too fishy so I left. Next guy shorts the vacuum lines between the power steering because it was leaking. no real change other than the hissing stops. After these, I decided to find a Japanese mechanic place to see if the skill is any better.

Next guy is at Nishikawa Auto Service and helped a lot, noticed the CPS had an oil leak so he takes it out and gives it to me to send for a rebuild (thanks @Piratetip for the assist on this). Car now was able to stay on at idle but still stumbled. In the meantime, I had Nishikawa take out the power steering pump so I could go get that rebuilt at CJ Automotive on Firestone in South Gate, CA (cleaned everything, new seals, fresh bead blast, thing looks like they gave me a new one, no leaks, highly recommend.) They put in the rebuilt PS Pump and get rid of the vacuum short. Stumble still exists so I am convinced its a timing issue with the notches on the CPS potentially not matching at TDC. Give Nishikawa a new timing belt, tensioner, and water pump to put in. STILL rough after probably 3k in repair costs (parts and labor). Now I'm pretty broke lol.

Take it home, save and research more. TPS maybe? I go and check it, while I am there i get brave and take the fuel rail out.
  • TPS is super finicky. Like a hair of a turn and it goes from infinite to 6 ohms. Seemed like it might be it, so I buy a new one on rock auto and calibrate. New one isn't as finicky. Get all the requirements down perfect. and tighten.
  • Fuel rail is out and covered in carbon. Carb cleaner and take the injectors off. Replaced all seals and filters. Also noticed this little guy.



Its all dinged up. Look for a new 440cc injector after asking a rebuild shop. (They tell me this type of injector can get rebuilt but not the part that is dinged up. precision angle holes and spherical surface means need a new on and cant resurface.) Find a new one for 80 bucks shipped from eBay Germany (not trying to buy a new set that don't have solid pictures of the surfaces). Put everything back and see if I might need to buy. STILL rough.

Today 20220625 new injector comes and swap it out. Check for vacuum leaks, check all connections.... STILL ROUGH. Still my favorite car I have ever owned. Just don't think I can get this going stock, I just wish I knew what this was. I have been holding off putting on my APEXi Power intake, buy some 550cc injectors, and want to get my ct26 rebuilt with 57mm impeller but I cant justify any of it with a rough idle. I am very frustrated. On the bright side, I can swap out injectors within 2 hours with all of the EGR crap in the way. Anyone thinks I missed something please bring it to my attention. I don't want to fuck my car up by modding before its ready.
 

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Discussion Starter · #3 ·
Missing at a random rate from what I can tell. Nishikawa called it a lean misfire (also said there was nothing I could do to fix it which seems wrong). Only happens at idle after the engine has warmed up (when the rpm drops from 1100 down to 700rpm). The miss can bring the needle down to 500 but only after 30 min of freeway driving.

I cannot tell if its missing under load (in gear) I don't think it is.
 

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Ok, so a key piece here is the engine temperature! On FD3S the coolant thermosensor and fuel thermosensor are the same part, and can fail. The engine will idle high normally when cold, but once the engine warms up past 177°F if one of these sensors is bad or has failed, the engine will idle roughly like it’s blown an apex seal. You may be looking in the wrong place for your diagnosis. I’d look into cold start/warm up vs warm idle conditions. My next investigation would be checking the temperature sensor values. Just a thought….
 

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Moderator, l337 M0d3r4t0r
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Have you inspected the ISC check valve (Idle speed control) the orange membrane in there if broken will cause idle problems? Also, ECU capacitors could be leaking so you can inspect the ECU for bad caps. When you say rough idle is it normal then stumbles or just rough altogether? Also, try to borrow an igniter from another car to see if that helps.

Normal idle on my 91 with 70k miles was around 600-650 silky smooth many 7m's have a stumble and I think it's from the igniter tbh. but it's a slight stumble, not something you see the RPM move just you hear it's not smooth.
 

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Discussion Starter · #6 ·
When you say rough idle is it normal then stumbles or just rough altogether? Also, try to borrow an igniter from another car to see if that helps.
Its normal with stumbles, I tired swapping out the igniter as well and saw no change. But @rts3gen has brought up the temp difference how its smooth at cold and the stumbles once warm. So I might need to look in that direction.

Normal idle on my 91 with 70k miles was around 600-650 silky smooth many 7m's have a stumble and I think it's from the igniter tbh. but it's a slight stumble, not something you see the RPM move just you hear it's not smooth.
I cant say that I dont see the RPM move sometimes, but I definitely have seen most commonly that it is a slight stumble with no rpm change. Not really sure if its a combination of both igniters being so old and the temp sensors potentially going bad or the ISC valve.

Someone on Supramania brought up wiring harness for injectors, which I really dont want to do. I am looking at this wiring diagram and its way out of my capabilities. Anyone know where the EFI Resistor is physically in the vehicle? I got new connectors because the originals are old plastic and tabs are breaking, so I thought it would be simple to run some cables and repin the connector but this is more complex than my simple mind can handle.
Rectangle Slope Font Schematic Parallel
 

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Actually, I really think it's the old igniters, my car has 70k miles and did NOT have the stumble but it's one of the only 7m's I've ever seen that with. EFI resistor for injectors is on the right side it is a box about 3 inches by 1 inch, and has a big round connector with 4 wires, black, orange and blue.

I don't believe it's that, but if you have 550's they should be low imp, to use the resistor box. Also using any after-market injectors other than stock will SKU your idle since the dead times are hard coded to the ECU and none but the same stock size keep the same timings.

The only way I can see you fixing this is going standalone, so you can enter dead times and maybe have an upgraded ignition system if it's just a nit pick on the idle.

Record the idle so we have an idea what we are talking about here.
 

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Discussion Starter · #9 ·
Actually, I really think it's the old igniters, my car has 70k miles and did NOT have the stumble but it's one of the only 7m's I've ever seen that with.
Igniter rebuilders dont exist do they? or at least means of getting a "good" one somehow?

I don't believe it's that, but if you have 550's they should be low imp, to use the resistor box. Also using any after-market injectors other than stock will SKU your idle since the dead times are hard coded to the ECU and none but the same stock size keep the same timings.
Running the Stock 440cc injectors at the moment. I was going to try and just buzz out the ecu connector to check the injectors for similar resistance values but if you dont think that its worth doing or that I might damage some component along the injector wire circuit with the voltage output on a multimeter, then I will just refrain from doing it. Thermosensors and ISC should be easy to inspect/replace hopefully.
 

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If the idle speed is uniform and all you are worried about is a perceived audible misfire this is likely just the ECU idle stabilization algorithm at work. The ECU adjusts timing cylinder to cylinder at idle to maintain steady rpm at 650 rpm. This was revolutionary in 1986 (no one idled that slow) but completely common today. This can results in some lumpy sounds occasionally.

Top trace is idle timing

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Discussion Starter · #12 ·
@ZaZZn @3p141592654 sounds like moving away from stock is more within my grasp than I had thought since this is starting to just sound "normal". I thank you all for your assistance, I am not going to worry about it anymore. I can now start saving for upgrades. I have been doing little things like targa seal replacements and printing interior parts (quality of life stuff). Cant thank this site enough for the relief of stress on my mind. Thanks everyone.
 

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Discussion Starter · #14 ·
Seems mine is different. Just took this cold since I haven't been driving this weekend working on some fiberglass stuff. Don't really know if it's a temp thing if it's rough close to the beginning.
 
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