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Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMARE!

LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMARE!

Old 04-27-2018, 10:58 AM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

I have traced the circuits from connector "A" to the 8 transistors in the PCM that are the injector drivers and there is nothing unique about the back 4 cylinder injector grounds. Furthermore there is not a separate path for any set of drivers. The grounds for all 8 transistor/drivers go to the same place on the circuit board.
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Last edited by GaryDoug; 04-27-2018 at 11:29 AM.
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Old 04-27-2018, 11:43 AM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Thank you! Your posts never cease to amaze me.
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Old 04-29-2018, 10:41 AM
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Post Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Yes. Thank you very much for the information. So I'm back at square one it appears. What on Earth do the rear four cylinders have in common?!?!? He did a compression test on at least the rear four and they checked out fine. That and the fact that it runs well during start up for the first few seconds tells me it's not an infamous "cam snapped in half" scenario.
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Old 04-29-2018, 11:33 AM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Has the mechanic ever pulled the rails/injectors to verify that all the injectors are not leaking, are pulsing, flowing, and have a decent spray pattern?

Has he verified spark on ALL cylinders (testing all eight, one at a time)?

Has he done a compression check on all eight cylinders?
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Old 04-30-2018, 09:00 AM
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Post Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Originally Posted by Injuneer View Post
Has the mechanic ever pulled the rails/injectors to verify that all the injectors are not leaking, are pulsing, flowing, and have a decent spray pattern?

Has he verified spark on ALL cylinders (testing all eight, one at a time)?

Has he done a compression check on all eight cylinders?
Pulling the rail/injector assembly as you're describing is one of the things I'd like him to do (he says he'll be working on it again soon).

He says he verified spark, I believe in all eight. At the very least he checked the rear four, the problem area.

Same with compression check. I know at least the rear four were checked for compression and said to be good. I do not believe a cylinder leak down test was performed however.

Now that I've experienced the problem in person (rear four primaries cold while running) I'm beginning to think it's unlikely there's an injector or PCM problem. With the rear four being the issue it seems more logical that there's a ground or circuit/wiring problem.

As in, it's hard to imagine only the rear four injectors going bad. Also, after looking at GaryDoug's pics it's hard to imagine why the PCM would stop driving only the rear four.

One observation that probably isn't relevant that I'll mention none the less: The "ignition" fuse (wired one) in the dash seemed "loose" in it's cavity, but when I pulled it I found no break.

For anyone looking at this thread; what in the heck do the rear four cylinders share in common?

What about the Opti HARNESS? Could it do something like this?

With all the detail and length of this, I just want everyone to remember the original topic (because I start to discount it myself sometimes) ...we do know the noid light did not light up reliably, particularly on the rear four.
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Old 04-30-2018, 12:00 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

There is so much contradictory info in this thread it’s unbelievable. You don’t seem to want to accept/believe the input of the people who are putting a serious effort into helping you solve the problem. I'm at the point of giving up.

The rear four cylinders need fuel, spark, and compression. Simply visually checking the injectors for flow/spray pattern would indicate whether there are issues with the PCM, grounds, low fuel pressure, injector drivers, etc. Until you, or the so-called “mechanic” does this, you are tilting at windmills. Personally, I wouldn't trust any input from the mechanic that you have not PERSONALLY witnessed or hands-on verified.

Assuming it really does have spark at all eight cylinders - and I really think it would be in your best interest to PERSONALLY verify this - and assuming you have satisfactory compression on all eight cylinders, all you are missing is fuel.
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Old 04-30-2018, 11:00 PM
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Post Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Originally Posted by Injuneer View Post
There is so much contradictory info in this thread it’s unbelievable. You don’t seem to want to accept/believe the input of the people who are putting a serious effort into helping you solve the problem. I'm at the point of giving up.

The rear four cylinders need fuel, spark, and compression. Simply visually checking the injectors for flow/spray pattern would indicate whether there are issues with the PCM, grounds, low fuel pressure, injector drivers, etc. Until you, or the so-called “mechanic” does this, you are tilting at windmills. Personally, I wouldn't trust any input from the mechanic that you have not PERSONALLY witnessed or hands-on verified.

Assuming it really does have spark at all eight cylinders - and I really think it would be in your best interest to PERSONALLY verify this - and assuming you have satisfactory compression on all eight cylinders, all you are missing is fuel.
Well Fred, I am just a layperson doing my very best, so I'll ask you to be patient with me if you'll not give up. I'm not sure what you're referring to when you say "You don’t seem to want to accept/believe the input of the people who are putting a serious effort into helping you solve the problem. I'm at the point of giving up." because you don't explain why you feel that way. I am following what is being explained to me (GaryDoug said there is nothing unique about the back 4 cylinder injector grounds and that there is not a separate path for any set of drivers). You said the injector / rail assembly should be pulled and examined for flow pattern and other variables and I agreed with that.

YES, I have thought the same thing; that I want to be hands-on at his place next time he works on it. You cannot imagine the frustration I have endured. He has been a very good mechanic for me for many years and is trustworthy, but sometimes overlooks things. I may be taking it to someone else soon, as he just doesn't have the time or focus that he used to. I have been doing my homework on who has good diagnostic ability in this area, but that is sparse I have explained to you.

Ironic that you say "Assuming it really does have spark at all eight cylinders - and I really think it would be in your best interest to PERSONALLY verify this - and assuming you have satisfactory compression on all eight cylinders, all you are missing is fuel" when the only thing that has been diagnosed so far is low fuel pressure. Wouldn't it be hilarious if the only thing wrong is the failed inline fuel pump (which is currently bypassed) ...argggh.

I don't think there's anything illogical about questioning why only the rear four cylinders are failing to fire (and it's very consistent), after all that's one of the key things we've been discussing and one of the only known issues. Since code 18 has been thrown it is logical to think it's a circuit / injector / ground / PCM / whatever issue. Ask yourself why four injectors would fail and four would remain ok (especially the rear four that share no circuit or ground in common unique to themselves)? By questioning that I am agreeing with what you said earlier (that low pressure isn't necessarily likely to damage an injector). You also explained to me that low pressure wouldn't necessarily allow the front four to live and kill the back four, which I am also following and understanding.

I'm sorry for whatever the misunderstanding is; because I am in complete agreement/understanding with the input from you, GaryDoug and others who have contributed in this thread. Again, I am not "convinced" of anything and my mind is open and flexible. The problem is my mechanic is difficult to communicate with and not willing or able to commit to this to my satisfaction in a timely or detailed manner.

Please qualify your statement about me seemingly not accepting/believing the input of those who have posted in this thread so I can understand just what it is that I've apparently misunderstood.

Sorry for whatever the confusion is, I am doing my best to be the middle man between my mechanic and the experts (you all IMO). If you give up here I still appreciate all of the input and I'll report back later once something has actually been tried. I am so frustrated.

Thanks for any and all input.

Last edited by canbaufo; 04-30-2018 at 11:21 PM.
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Old 05-01-2018, 12:12 AM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

I'm really at the point where I don’t feel I have to justify (qualify?) my analysis, my frustration or my reluctance to continue. Post #19 put my recommended path forward in the most simple, concise terms. I can’t make it any simpler than that. YOU have to get your hands dirty.

My apologies if this does not meet your expectations. Maybe your mechanic is the best person to solve this.
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Old 05-05-2018, 03:03 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Point taken and I agree on observing injector operation (as described in post 19). I have finally decided to give up on him. It is tough and emotional because he's more of a "mechanic-friend" and has been for years...and it sucks after being patient for 5+ months to finally have to give up and essentially start over. Within minutes after posting this I'm going to message him to not do any troubleshooting (as if he's going to get around to it anyway) and that I'll be picking the car up soon.

I'm not afraid to get my hands dirty, but I won't kid myself. I'm mechanically inclined but don't know electronics well. I have put a new fuel rail on myself before. Since I'm unsure of mechanics around here I may start by taking it back home and trying the lifting of the rail/injector assembly. I'm not the most diagnostically inclined, don't even have a fuel pressure gauge (used to have one permanently mounted in A-pillar).

I know I can handle the rail-pull, and also the swapping of injectors and/or connectors as you suggested as an idea. Once it gets too technical beyond that I fear I'll get in over my head though, but at least a starting point might become established.

Until I'm actually hands on (maybe soon) if possible I'd just like some "hypothesis" to possibly guide me in the right direction. So the questions below do not in any way imply that I'm not accepting the info that you all have presented to me, it's more of a "what's likely or what makes sense in the meantime?" analysis:

Can anyone in here tell me what the rear four cylinders have in common; whether fuel, spark, or otherwise? GaryDoug made it clear to me that with regard to the PCM there's nothing common about the rear four cylinders/drivers (other than the fact that all 8 drivers share the same ground).

Therefore is it logical that a PCM injector driver failure is more likely when you have either ALL EIGHT or ONLY ONE injector out (or maybe two injector drivers that are right next to each other)?

I realize the questions may be irrelevant until I've pulled the rail/injector assembly and observe what happens with key on / engine off, engine running, etc.

I'll also note that it's possible that the only reason code 18 was thrown is that the injector connectors had been unplugged while running when he did the noid light test, or correct me if that's wrong please.
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Old 01-18-2019, 01:38 PM
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Update and question on pulling the fuel rail to check for leaking injectors

This is a recap, for anyone new to this or not fresh on it. I really need help with the simple question with some bold facing near the end please.

I fired my mechanic back in May and had the car hauled to storage across town. I changed the oil to find it severely overfilled and flooded with gasoline (about 50% gas). Blackstone lab report indicated accelerated wear and a maxed out scale of fuel contamination ("over 10%"). Life hasn't allowed me to work on the car since then so I have only maintained it with brief monthly starts, moving it in and out to shake up fluids and prevent flat spotting of tires. I put a whole tank of fresh fuel in with stabilizer as well.

The mechanic admitted to running an entire tank of fuel through the engine during "spark troubleshooting" (because the rear four cylinders weren't firing; the plugs were drenched in fuel and the header primaries were cold) only to find no issues with spark. Weeks later he finally checked fuel pressure and found it at 20 PSI due to a bad inline pump. Fuel pressure should have been the first thing he checked. After bypassing the bad inline pump, fuel pressure was restored but the miss in the rear four cylinders persisted. Noid test light indicated issues but I haven't confirmed PCM drivers or injectors/wiring short yet. Need to start by lifting the rail to check for injector leaks / observe flow patterns / swap injectors around to see if issue follows them, etc (see question at end). I'm still morbidly curious as to what if anything the rear four cylinders have in common, if anyone knows. My hope is that the injectors were ruined during the excessive run time the engine saw at low fuel pressure while the former mechanic was chasing spark issues. I'm also concerned my rings may be shot of course. I wonder what will happen if the rear four cylinders fire up after a fix ...now that they've been washed out by fuel so much. I have to change the plugs and confirm spark too of course.

EASY QUESTION THAT I NEED SOME GUIDANCE ON HERE:

I actually put a new rail in myself back in 2011 and I can't remember if I reused the injector retainer clips. I'd like to lift the rail / injectors as an assembly and do the "key on test" by simply priming the fuel system and observing the bottom of the injectors for leaks. Once the injectors are no longer sandwiched between the rail and engine by the four bolts that hold the rail down, will they stay in the rail under fuel pressure without becoming projectiles or leaking at the top where they mate with the rail? The relevance of the question pertains to clips or no clips on the injectors, as I can't remember if I used them in 2011 and the car is across town now. I'm hoping they are "pressed in" well enough to stay in the rail leak-free at the top o-ring even without clips and without the rail being bolted down?

I'd like to start with the leak test, then if I see nothing I'll swap the rear four for the front four injectors and see if the problem follows them upon start up. I'm hoping I can re-use the injector O-rings with a dab of white lithium grease.

If anyone can help with the main question or anything else here it would be sincerely appreciated! I am out 1.5 years, hundreds of hours and $2,500 just trying to prepare this car to sell so I don't have to sell it so cheaply with this unresolved rear four cylinder miss and a broken supercharger (I rebuilt it with success, so at least that part is addressed). It has been a horrible ordeal but I'm not quite ready to throw in the towel.

Last edited by canbaufo; 01-18-2019 at 01:42 PM.
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Old 01-18-2019, 02:07 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

When the bottoms of the injectors are free of the intake manifold, you MUST have the retainer clips in place on the rails to assure the injectors do not blow out of the rails under pressure. Trying to pressure the system without them would be very risky.

I really feel bad for you..... the mechanic was the ultimate hack. A shame people like that are even in business.
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Old 01-18-2019, 05:34 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Thank you for the sympathy, I'm just very thankful I have another vehicle. Sadly, he was one of the best around here. He also misdiagnosed a clicking #7 exhaust valve as a need for an entire engine rebuild many years ago, the reason I have a built 355. It took 1.5 years and the cam manufacturer ground the cam (Reed Cams) wrong with way too much lift and duration, causing a couple valves to hit a piston. He only checked the ICL on installation and not the whole profile of the cam as I had requested. Luckily, I met a very kind member in here who was very gifted. I hauled the car to MD to this kind person and he solved the issue and got it back together. I showed him some of the OEM rod bearings from the 46K mile engine that my mechanic had claimed to be "shot" and he said he was sorry to break it to me, but that the slightly wiped bearings really looked more like normal wear. It was an honest mistake but his blunder cost me 10K+ and 1.5 years of down time. I stuck with him because he'd been honest throughout the process and did other work well for me over the years. He lost a lot of time on it too and didn't charge me much, but still...just saying, it was awful. This time things went really bad again and his attitude did as well, so I ended it. The older "Mark" of Absolute Horsepower LLC" ....Not my first, but definitely my LAST "Absolute" nightmare. There's much more to just how bad it went ... really.

Back on topic, I think I'll try the leak test if the clips are still there. If there are no clips I think I'll swap them front to rear and reinstall, see if anything changes upon startup. I do have a multimeter ...maybe I should learn to do an Ohms test first. Very hard to work on it across town and I can't have it in my way at home.

Last edited by canbaufo; 01-18-2019 at 07:11 PM.
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Old 01-23-2019, 05:20 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

I should probably check FP before fiddling with the injectors. I assume the best place is the valve at the rear of the fuel rail on the passenger side. There is a weird adapter there with a few extensions threaded into it, maybe it's leftover from previously running an FMU, no idea. Any advice on the FP check or a good cheap gauge? I'll watch some vids. First I guess I'd check for bleed down rate with key on. Then see what it is running with vacuum line on and off of the FPR. My main concern is accidentally ruining the fuel rail trying to uncap and connect to the Schrader valve.
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Old 01-23-2019, 06:41 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

If you can, post a picture of the device screwed onto the Schrader valve. Maybe I can tell you what it is. That is a common place for people to pick up fuel for a wet nitrous system. Could have been used for an electronic fuel pressure sensor, or a Hobbs switch to protect the engine on loss of fuel pressure. In both cases, the valve core may have been removed from the Schrader valve, and would need to be replaced. Wouldn’t be a location for an FMU. That would go in the return line.

My concern is that if it’s screwed on the valve tight, you have to be extremely careful removing it so you don't break the valve off the fuel hard line.
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Old 01-23-2019, 11:23 PM
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Re: Bad PCM or Bad Injectors throwing off noid light??? ...opinions wanted ..NIGHTMAR

Yes, that's my concern too, as years ago a leak developed there and I cracked the rail where whatever that is attaches to the Schrader as I tightened it. So where the Schrader meets the rail it twisted as i tightened it and cracked. I had it welded and it failed, so I replaced the whole rail with a polished LT1 rail. This is the best I can recall anyway (2011), maybe I just thought it was the Schrader and it was something else. I cannot recall whether I left the Schrader as is or retrofitted that contraption onto it (or maybe it was already on the replacement rail). The car used to have a fuel pressure gauge with an electronic sending unit, maybe that was used for it. I've never hooked up a FP gauge. I assume the presence of a valve core is imperative from what you said. Thanks for the replies.
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