When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
With a emission delete (and possibly a change to parralled turbos) a good deal of the "Emission Harness" (often referred to as the "Engine Harness') would likely have been deleted as well. So replacing the harness with a custom harness may be a more cost effective route than buying an entirely new EM harness and chopping it up.
I am assuming your Power FC is the proper spec one for your year and model.
There is a remote chance someone swapped in a newer set of coils. Even with the plugs hooked up wrong the motor will run, but as you get into higher revs (above 6000) trouble appears....in a big way.
i just unplugged all the emissions related connectors and left them in when i did the emissions delete. Power FC is model fd3s4-020-5400 which should be the newer version for the 92-95 fd
I have double checked coil to plug leads, at least the leads are correct. I dont know if its possible to get the connectors wrong and have the car still run but ill check that when i get back under the UIM
After taking a few laps around the car and tidying up a few loose ends, (trim bits and clips and whatnot) i came to the realization that ive made the dumbest of dumb mistakes.
Im my eagerness to install my pfc and start playing with fc tweak i had forgotten to reattach the ecu ground in the footwell where the ecu is mounted.
Not sure if its responsible for all or any of my issues but surely it couldnt have helped the situation much.
Too busy today to test the car and check the coolant sensor situation but i will post updates once i have a chance to turn it on and pull logs
Now that the seeds of doubt have been planted in my head I decided to take off my UIM to check my wiring and the ground located beneath the UIM, and also check the plug leads & coil packs while I'm at it.
Everything here appears to be in order. L1 to front leading (lower) plug, l2 to rear leading plug, with black harness connector to the leading coilpack. T1 to front trailing (top) plug with white connector. T2 to rear trailing plug with blue connector.
Ground strap on the UIM was also connected before I took it off.
I also went ahead and continuity tested all my grounds in the engine bay with the car cold. Everything checked out. Had no problem finding continuity to the frame from anywhere on the engine. And also found continuity from the exterior grounds to the interior ground by the ECU.
I guess I'll run this same continuity test and a resistance test with the car fully warmed up and I'll have my answer as to if my harness has temperature sensitive intermittent breaks.
Got the UIM off and grounds tested before work. Will go ahead with the post warm-up tests after work.
Genuine question, couldn't failing coil packs also be temperature sensitive?
For example if my leading coilpack is completely failing to ignite once the car is up to temp, could it potentially cause this behavior?
But I guess it generally wouldn't cause my sensors to go ballistic-- unless that's a side effect of the car trying to compensate for the misfire somehow?
Just theorizing. Clearly I'm not very knowledgable on the subject but hoping to learn..
Genuine question, couldn't failing coil packs also be temperature sensitive?
For example if my leading coilpack is completely failing to ignite once the car is up to temp, could it potentially cause this behavior?
But I guess it generally wouldn't cause my sensors to go ballistic-- unless that's a side effect of the car trying to compensate for the misfire somehow?
Just theorizing. Clearly I'm not very knowledgable on the subject but hoping to learn..
Yes coil packs are very heat sensitive, especially when failing. I've had plenty where they work fine when cool then lose spark or weak spark as heat increases. I had a splitfire coil that was doing this and eventually got hot enough to bubble the case before it totally died. Wasn't easy to track down because it was still working well enough not to misfire at idle and light load but not strong enough to ignite the mixture over 5 lbs of boost.
I don't think the wiring diagram mentions the colours of the plugs, although that may be covered elsewhere. Of note that for early cars the coil harness had a secondary grounding arrangement that was eliminated in later cars. Speculations is that that extra grounding arrangement helped eliminate excess electronic noise. Why Mazda removed it for later cars we did not find an explanation..
As I could not know which colour plug goes where, we took the extra step to check the actual wire colour codes matched the wiring diagram.
I believe the version 1 JDM and USDM wiring in that area is substantially the same, but I would have to check later.
For my flooding problem we checked the coils as per the USDM for continuity and measured current.
But in the end we suspected the coils may have been somehow weak and swapped in another set.
As this was one of the last "trys" before we got the car running, we continue to suspect weak coils were part of the problem.
I don't think the wiring diagram mentions the colours of the plugs, although that may be covered elsewhere. Of note that for early cars the coil harness had a secondary grounding arrangement that was eliminated in later cars. Speculations is that that extra grounding arrangement helped eliminate excess electronic noise. Why Mazda removed it for later cars we did not find an explanation..
As I could not know which colour plug goes where, we took the extra step to check the actual wire colour codes matched the wiring diagram.
I believe the version 1 JDM and USDM wiring in that area is substantially the same, but I would have to check later.
For my flooding problem we checked the coils as per the USDM for continuity and measured current.
But in the end we suspected the coils may have been somehow weak and swapped in another set.
As this was one of the last "trys" before we got the car running, we continue to suspect weak coils were part of the problem.
Thanks for the input. I am becoming highly suspicious of the coil packs as mine appear to be original and the car has just hit 90,000km (55,000 miles).
I'm not aware of a recommended service interval for coil packs on these cars but if I had to use some kentucky windage to make a guess it'd sound something like "50,000 miles or 30 years whichever comes first"
I'm still planning to do concrete testing on every failure point so I'm not just playing a guessing game of which part to swap/upgrade next indefinitely until eventually the whole car is brand spanking new or I blow another motor along the way and call it quits.
Below (left) is the JDM wiring schematic for version one and (right) USDM Version 2 (1994) for the coils. Unfortunately I don't have the USDM Version 1 wiring schematic.
For JDM wiring colors are written in Chinese script (Kanji), in some instances the JDM and USDM (V2) wire colours are different. However, that might be because of changes Mazda made between Version 1 and Version 2.
In the USDM chart you can see the "Shield" wiring. This is the extra grounding I referred to before. This appears in the JDM (V1) diagram as well. Check to see if you have such ground wiring and if it is in good shape.
If it is missing, someone may have subbed in a newer version of the coil harness.
Unfortunately the wiring diagrams don't match the physical order of the coils, which might cause some confusion.
@Akagi'sWhiteComet Akagi might be able to confirm if USDM Version 1 is the same as JDM version 1.
If it matches you can use the USDM Version diagram to check the wire colours.
Akagi is translating the JDM wiring diagram for us all.
( fyou google "Japanese Kanji for Colours" you might get a table that will help you read the colours on the JDM schematic.
Here is an article from a Japanese tuner magazine discussing the problem. It shows how the newer coil harness hooks up to the new placement of the coils. Unfortunately it does not show the harness hooked up to the older version.
Somewhere I read you should replace the spark plug wires every year and the coil harness every five years.
Below (left) is the JDM wiring schematic for version one and (right) USDM Version 2 (1994) for the coils. Unfortunately I don't have the USDM Version 1 wiring schematic.
For JDM wiring colors are written in Chinese script (Kanji), in some instances the JDM and USDM (V2) wire colours are different. However, that might be because of changes Mazda made between Version 1 and Version 2.
In the USDM chart you can see the "Shield" wiring. This is the extra grounding I referred to before. This appears in the JDM (V1) diagram as well. Check to see if you have such ground wiring and if it is in good shape.
If it is missing, someone may have subbed in a newer version of the coil harness.
Unfortunately the wiring diagrams don't match the physical order of the coils, which might cause some confusion.
@Akagi'sWhiteComet Akagi might be able to confirm if USDM Version 1 is the same as JDM version 1.
If it matches you can use the USDM Version diagram to check the wire colours.
Akagi is translating the JDM wiring diagram for us all.
( fyou google "Japanese Kanji for Colours" you might get a table that will help you read the colours on the JDM schematic.
Thanks so much for this!
I can read japanese so I just went ahead and tried to make out those pixelated kanji.
From bottom to top: "Black > L1,L2, white > T1, blue > T2"
Looks like I have my connectors plugged in correctly.
(It'd be pretty hard to mix them up anyway with the way the harness is baked rigid.)
Also, I do have the grounding wire that attaches to the vacuum/coil rack and it is in good shape and was a part of my ground wire continuity tests with the car cold.
Had no trouble getting continuity from the coil ground all the way to my strut tower nuts
Here is an article from a Japanese tuner magazine discussing the problem. It shows how the newer coil harness hooks up to the new placement of the coils. Unfortunately it does not show the harness hooked up to the older version.
Somewhere I read you should replace the spark plug wires every year and the coil harness every five years.
My spark plug wires were replaced in 2022 and have about 1000 miles on them. I'll stretch them out just a tiny bit longer LOL. The harness, on the other hand, is probably overdue.
Coils new are very expensive, but on the other hand, used sets show up for about $100 on YAJ/Buyee quite often. Of course you'd want to test before installing.
Every time your coolant temperature bottoms out, your throttle position sensor voltage also goes haywire at the same time.
One or both of these is causing your idle speed control valve to spike, and your ignition timing to shift significantly.
You should at least see if bolting the ECU ground down affected your sensor readings before you start trying to diagnose the ignition system.
Also:....
"but i cant see how it could be a fuel system issue when ive been through the entire fuel system already."
It could still be the fuel system. The more you work on something, the more likely you are to mess something up that was working fine. This goes doubly if you modify something!
Every time your coolant temperature bottoms out, your throttle position sensor voltage also goes haywire at the same time.
One or both of these is causing your idle speed control valve to spike, and your ignition timing to shift significantly.
You should at least see if bolting the ECU ground down affected your sensor readings before you start trying to diagnose the ignition system.
Also:....
"but i cant see how it could be a fuel system issue when ive been through the entire fuel system already."
It could still be the fuel system. The more you work on something, the more likely you are to mess something up that was working fine. This goes doubly if you modify something!
+1
The ignition coils are a rabbit hole. As long as other things are acting funky your coils are not the problem.
Thanks for the input guys.
The current battle plan:
Install an inline fuel pressure tester
Install a new coolant temp sensor
Put the UIM back on and run the car on jack stands.
Check ground continuity and resistance with the car warm
Pull logs and check sensors
Pull coils and bench test with a multimeter
Every time your coolant temperature bottoms out, your throttle position sensor voltage also goes haywire at the same time.
One or both of these is causing your idle speed control valve to spike, and your ignition timing to shift significantly.
You should at least see if bolting the ECU ground down affected your sensor readings before you start trying to diagnose the ignition system.
Also:....
"but i cant see how it could be a fuel system issue when ive been through the entire fuel system already."
It could still be the fuel system. The more you work on something, the more likely you are to mess something up that was working fine. This goes doubly if you modify something!
I changed the fuel pump and rewired it because the car was exhibiting the same issue of dying out prior to the fuel system work.Also changed a bunch of the factory relays to be on the safe side.
Couldn't hear the fuel pump prime anymore, so I assumed it was a bad fuel pump or fuel pump wiring.
After replacing and rewiring the fuel pump, the car is still exhibiting the same issues.
When I was running those ECU's prior, the ECU ground that I forgot to install on PFC was connected correctly, so that's why I have my doubts that ground was the cause of my issues.
Every time your coolant temperature bottoms out, your throttle position sensor voltage also goes haywire at the same time.
One or both of these is causing your idle speed control valve to spike, and your ignition timing to shift significantly.
^DING DING DING - this is a telling clue! As I mentioned regarding the wiring harness, a bunch of mission critical sensors (i.e., MAP, TPS, IAT, CLT etc.) your ECU rely on for proper functioning share the same +5VDC & sensor ground circuit - ANY interruption in the +5VDC power or the sensor ground returns can cause very wonky things to happen.
^DING DING DING - this is a telling clue! As I mentioned regarding the wiring harness, a bunch of mission critical sensors (i.e., MAP, TPS, IAT, CLT etc.) your ECU rely on for proper functioning share the same +5VDC & sensor ground circuit - ANY interruption in the +5VDC power or the sensor ground returns can cause very wonky things to happen.
It's less likely but it could be an issue with the PFC as well, depending on the age of the unit.
It's less likely but it could be an issue with the PFC as well, depending on the age of the unit.
Probably not because this behavior persists on other ECU's I tried running--stock ECU & eco-cpu ecu (Jdm tuner thing)
Not in exactly the same way but the car also sputtered out and died when running on those ECU's. Without the IACV weirdness making the car lurch though.
08-26-24, 05:16 PM
