electric circuit question

[Igovert500]

OK, long story short...I have an injector circuit issue which I have though I've nailed down, only to be wrong...repeatedly. I've learned alot since then and was wondering if somebody with knowledge of electrical circuits could read my assumptions and tell me if they are correct assumptions or not. Thanks in advance.

So I'm guessing you guys know, but quick outline on the injector circuit in my car. The injector has 2 wires, one is the 12V constant source as soon as the ignition is on. This wire runs from the MFI relay through the resistor pack and splits to 6 individual wires...each running to the injector. The 2nd wire is the ground wire this runs from the injector to the ECU. The ECU uses an internal switch or whatever to ground the injector when it wants the injector to open and spray.

So Originally I got P0204 Injector Circuit Open Cylinder 4.
I swapped the ECU and the #4 injector with spare ones. Problem remained. I drove through a full tank of gas, pulled the plugs, and #4 plug showed signs of running excessively rich.

I gather at this point that the open part of the circuit is a short to ground, causing the injector to stay open whenever the ignition is on (Is this first assumption correct?).

So my reasoning at this point is that the open in the circuit is on the side of injector -> ECU. Not Injector -> MFI. If it were a short on the MFI -> injector (power) side of the injector, it wouldn't cause the injector to stay open all the time...it would just interrupt the circuit, causing the injector to stay shut (is this 2nd assumption correct?)

If it is, being that I have swapped the ECU and injector, I can assume the issue lies in teh wiring between the two. Since it is a short to ground, I would think the wire is disturbed somewhere in between, and not that the connections at the harnesses are bad. Because if it were the harness connections...it wouldn't have any metal to short to ground on. (is this 3rd assumption correct?)

I just wanted to have somebody who knows what they are talking about, doublecheck my assumptions. Mainly the 2nd one...because it is my understanding that only a short to ground on the (ground) wire would cause the fuel injector to open. A short to ground on the (power) wire wouldn't.

Somebody please tell me this makes sense or it doesn't. Thanks

[abaird]

Disconnect the harness from the ecu and the #4 injector and test resistance of the ground wire if you have an ohm meter that is. Anywhere between 0 and .2 ohms whould be acceptable. If you get an out of limits "OL" or a high resistance value than replace the wire. That should take care of any short or open in the ground circuit for that injector. If you have access to a stethoscope you can actually listen to the injector while the engine is running and you should hear the injector pintle opening and closing if it isn't then it could be stuck open or closed. If it is a short to ground on the ground side then, like you said, it would be remaining open, which will eventually smoke the injector.

[UncleBob]

very common problem on those cars, its the injector harness on the engine. They get internal breaks from heat.

[Igovert500]

Thanks for the replies.

I should have mentioned a few things prior. I have a cheap analog multimeter which I used to determine a few things. 1) I'm an idiot when it comes to using aforementioned multimeter. 2) Somebody tried leading me through testing out each section of the circuit using the multimeter and measuring voltage, resistance, continuity, etc. Not sure I did any of it right, BUT what I am sure of, was that the variation between injector 4 and injectors 2 and 6 (3 injectors on the rear bank, all share same electrical layout up to about 6"s from actual injectors).

I disconnected the 3 pt. plug that attaches to the plenum and which all 3 of these injectors share, and the variation was apparent on the ECU/wiring/everything but injector side of the plug. I replaced hte injector just as a precaution, and inspected the wiring harness at the injector, both were good.


I've literally been working like a madman on this, and I have read every single method of testing, and tried everything I could. Basically, I'm pretty sure I have narrowed down the problem to a foot or so of the ground wire that runs through the firewall and up under the dash. I have inspected every inch of the wiring except this section, and I'm assuming it became frayed in this area and is shorting to ground on the firewall. I just wanted to hear somebody who knows what they are talking about tell me all my assumptions are logical, and they see no obvious flaws. I'm 99% sure they are, and I've already run a new wire, I'm just awaiting a replacement part for a small clip I broke, then I'll fire it up and see how she runs. Just wanted to hear something encouraging while I'm waiting. Thanks.

[UncleBob]

what you are asking is for someone to blindly follow your conclusions that you've found up to this point.

I say this in the most humblest way: electrical diagnostics isn't easy. I couldn't even count the number of times I have led myself astray following a serious of beliefs that I thought were true, that later turned out were not.

That is how you diagnose electrical problems, you sift through what IS good, and what is unknown. You narrow down the unknowns until you are either left with only one last unknown (usually the computer) or you have narrowed a failure to a particular point....of course thats assuming the failure is repeatable!!

But to take a step back (and I'm speaking in very general diagnostic strategy speak here)....the first rule you really should remember about diagnosing electric issues is "Its most likely a simple problem".

That seems overly simplistic, doesn't it? But its true. 99.9% of the time, the cause is very simple. A bad ground, a failed component, or break or short in a wire, that is usually obvious visually.

But the human brain doesn't LIKE keeping things simple, it likes to make things complicated. It likes the "what if" game. "What if from engine torque, the harness was pulled and here at the firewall, only one single wire out of the entire harness was pulled just enough to cause a break...." Seems plausible? Definitely. Likely? No.

Not saying that you aren't right. You very well could be. But I am doing this long winded explanation in hopes that it removes the sting when I say this: I doubt your conclusion is correct. Its just not likely.

[Igovert500]

no offense taken, but perhaps saying I was an 'idiot' with the multimeter cast the wrong tone. I feel like I've replaced/checked the simplest things. I've literally read over 300 threads on this problem, on multiple forums. Wasn't asking anyone to say my conclusion was right, I understand it is near impossible diagnosing anything with 100% accuracy on the internet...even more so when it comes to electrical issues. I was just asking somebody to say, yes you assumptions are logical, or no they are not. I think they are, I guess I'll just let you know when I'm sure or not. I don't mean to come off the wrong way, I just am the type of person who likes to think aloud/bounce his ideas off other people, and was just looking for that. I'll know soon enough. Thanks.

[UncleBob]

the quick test is to bypass that portion of the harness...just don't use scotchlocks to do it....solder a wire from near the computer to near the injector plug. That will give you a firm answer quickly on the problem.

[Blue Bowtie]

Are these peak-and-hold injectors or saturated injectors?

[Igovert500]

low-resistance, voltage-controlled, external resistor

was never sure which that technically closer resembled

[UncleBob]

normally when you have an external resistor block, they are peak and hold (low impediance)

[Igovert500]

Was wondering if I could bring this topic up again. I did run a new ground wire directly from the ECU to the injector. I even replaced the harness as well. Still have hte problem. Now I've replaced the injector, the harness, the entire grounding wire, and the ECU...where to go from here?

My next though is the external resistor. I've been reading alot about the NA guys who do TT conversions. They have high resistance injectors in their cars stock. When they do the conversion and swap in the larger TT low-resistance injectors, they need to wire in external resistors. Generally 6 10Watt 5.6ohm non-inductive resistors. If they don't they run extremely rich, as the ECU cannot tell the difference.

So I'm thinking next step is to swap in a different resistor pack and see if that changes anything.

Thanks for the help so far.