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Moderator, l337 M0d3r4t0r
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Discussion Starter · #1 · (Edited)
Hey All,

I'm helping out a local supra owner whos 90 and his car has me stumped.
83 mk2 supra with 7m-GTE

While accelerating, the car randomly bucks. If you drive very low throttle and rev up it's smooth as butter but giving it anything other than 5-15% throttle you get random bucking/missing. Originally the car came to me due to a lack of power. In my quest to find out what was going on, I changed the TPS since it was getting a code 51. However, I think that the 51, which can be AC/NSW or IDL from TPS was actually from the NSW since he does not have a clutch switch on the car. Initially, I thought it was a CPS / Wiring issue because it just felt like there was no timing. I dug the ECU out and wiggled the harness wires, trying to figure out if there was a loose wire. Between that and changing the CPS, it developed this miss/bucking at these specific RPMs.

Here's a video of the issue. At 18 seconds. It happens again at 25 seconds near 3500 RPM but it's a very light miss/sputter. The one at 18 seconds is so significant it sounds like I hit a bump, but it's the whole car bucking. Sometimes it happens so quickly that you don't hear, but you certainly feel it, and it's bucking pretty hard randomly many times in a 1000 rpm span.

Here's what I've changed and fixed.

-New plugs (the old plugs 1-2 had a bunch of deposits on them) Kinda like what NGK shows here
-Pressure tested from turbo inlet to 3000 pipes. (Fixed a huge leak in the IC, which was no doubt the cause of the original issue and why the car was brought to me)
-Used new wire from CPS to ECU bypassing existing wiring, no change
-Swapped TPS (this was when issues started, but I also yanked on the wiring and dug out/disturbed all the wiring under the glove box to test for continuity) - no change
-Fixed loose CPS wire connection to ECU (wiggling while driving would cause the car to shut off) fixed by bending harness pin, so it makes better contact with ECU. I can wiggle the harness wires while driving and have no issues with the engine shutting off or throwing codes.
-Swapped AFM's - no change
-Replaced some of the wires on the ignition harness wires that were exposed and the old cracked clips.
-Replaced the ground from coilpacks to the head.
-Replaced ECU with known good ECU
-Swapped Igniters - no change
-Swapped CPS - no change
-Swapped Coil 1, which tested out of spec - no change
-Fixed a bad main fuseable link
-Replaced the coil pack harness

The only things on my list right now are

-Injector ground

Any other suggestions?
 

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-Pressure tested from turbo inlet to 3000 pipes. (Fixed a huge leak in the IC which was no doubt the cause of the original issue and why the car was brought to me)
Sounds like an IC pipe blown off. As soon as you hit boost, the air blows out the hole and the car stutters. Not sure what 3000 pipes are, but I would be looking at other IC pipe connections.

Al
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #4 ·
That's what I was thinking, but I pressure tested up to the throttle body basically ( only one coupler) (3000 pipe is the one that attaches to the throttle body that crosses the valve covers). I was thinking it could be that, but even at part throttle with no boost, it happens.

I didn't check the actual coils, let me do that... That would be a good test.

FPR and rail should be good holds pressure when the engine is off well, and rail showing good pressure during all times idle/boost.
 

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Light tip in and constant light throttle accel being smooth is a clue to me that you might have a fuel flow problem. Sounds like you checked fuel pressure already, but I wonder if you are getting all the flow you need. Not too often but I have seen a badly clogged fuel filter cause bucking at higher loads before.
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #6 ·
So it seems that the miss is pretty constant around 3000-3500 and 4500-5000 it may or may not happen but its around that point. However, it seems to happen at any TP %. There are no codes, and the only thing I can think of is maybe bad coils or wiring to the coils from the harness. The plug wires are pretty good and look new. Or the CPS is just simply bad.
 

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Buddy thought I had a NA
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fuel issue since the bucking issue seam's so small stick a sniffer in the de pipe and go for a rip
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #8 · (Edited)
Yeah, but I ran a pressure gauge from the rail to the windshield wiper so I could watch it, and it was perfect no drop in the rail... So I ruled that out. I think it must be a coil pack or the CPS. I measured them last night, and all the CPS tests were 240 ohms where the TSRM calls for 180, and the #1 coil was off as well. I'm not sure what else it could be and why it suddenly started to happen.

Here's another video of the issue
 

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I experiencing something similar after my tuner had me set my plug (BKR7E) gaps down to 0.022". It's like you said in the OP, it would be fine under 15% but intermittently buck/misfire if I ask for power. Would happen very intermittently, like every couple hundred miles. Put it back to 0.030" gap and it's been fine for 500 mi so far. So I would double check your gap and ignition system. I am on a standalone tho so it could have been completely different cause
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #10 ·
The gap was .34 i believe by default which is a little large, but the Irridiums that were in there before where gapped correctly and it was happening with that. So strange.
 

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Seems you are chasing something very small by the video. You have a good runner otherwise.

Correct me if I'm wrong but I thought I observed a hesitation as the tach crossed 3000 and then right before 4000 I saw and heard a misfire in video #2. Seemed in your first video you ran up well then things got bad after the decel, could be a clue. I really hope you aren't dealing with an intermittent harness issue you haven't found yet.

I would think you already checked for vacuum leaks. Maybe driving light throttle with a vacuum/boost gauge installed would reveal something.

Did the O2 sensor follow the 7MGTE and is it in the prescribed location (ie no aftermarket mods that moved it)? Any leaks around it or upstream (could make the tip in after a decel bad like I saw in first video)? Is it lazy, not switching fast enough? These old feedback systems can cause performance issues without setting codes.
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #12 ·
It's not a good runner, it's really really bad when it happens times it's a small miss sometimes, it's horrible. like 4-5 times bucking in one gear. It feels a little softer than fuel cut but pretty much the same.

yeah, the harness, the thing was my first thought but I wired a new wire outside of the main harness from ECU/CPS and it was still happening. It's annoying AF... Yes vac is near -20 so it's doing good around -18 actually, can't find any leaks, when it's out of the issue it pulls REAL strong for 10 PSI. since it's higher compression with wisecos and 210PSI across the board from my compression test.

O2 is stock location, but it shouldn't affect WOT, since it goes open loop.

I will change the CPS and one coil that was out of spec, but we'll see from there.
 

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Hopefully it is just a bad coil. Look again on the fuel/mixture side if it still doesn't work. I still think the bug is something small and simple somewhere. That's usually what causes that type of problem and can drive you crazy looking for it.
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #14 ·
Dude, it's crazy, I'm telling you I'm at my wit's end... Changing all this stuff, and it's still happening. Nothing is worse than wire gremlins.
 

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Have you checked the injector grounds? I know it sound unlikely but i forgot to tighten the bolt and i had similar issues. When the engine would start to move with torque i would loose the gound it it would stumble as the injectors would stop.
 

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To me if comes across as a fuel delivery issue that I experienced with my car, but mine was far from stock. Another thing I haven't seen mentioned, how is the alternator output wire? On Megan's Mk4, when we got the car it wasn't tightened down like it should have been, and it caused some goofy behavior that I couldn't figure out until it left us nearly stranded.

Also, is this Derek's car, by chance?
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter · #18 ·
@te72 Yes Derek's car!

So I've replaced the CPS, Igniter, 1 Coilpack. (will replace the other 2 tomorrow), the CPS wiring from CPS to ECU, the coil pack mini harness, checked for boost leaks, fuel pressure and replaced a bad terminal on the main fuseable link. None of the is has resolved the problem. I will double-check the injector ground tomorrow on the intake manifold to make sure that is tight other than that, I'm fresh out of ideas the only thing it could be is the actual harness. Not a job I intend to do, as it's too time-consuming.

I did notice the noise filter by the igniter was broken and will try to replace that if I can find one.
 

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Buddy thought I had a NA
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Yeah, but I ran a pressure gauge from the rail to the windshield wiper so I could watch it, and it was perfect no drop in the rail... So I ruled that out. I think it must be a coil pack or the CPS. I measured them last night, and all the CPS tests were 240 ohms where the TSRM calls for 180, and the #1 coil was off as well. I'm not sure what else it could be and why it suddenly started to happen.

Here's another video of the issue
you really need to hook up a remote sniffer and hook up a 2 channle scope ecu side to the air flow meter signal and cps that is signal lost buck. remmber the cps has a power ground as well it's not just all signal same with the air flow meter
 

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Have you tried asking it nicely to sort itself out? :p

Also, glad to hear Derek's still enjoying the car. Hope he's well. :)
 
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