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Discussion Starter #1
Been having a recurring intermittent issue I can't wrap my head around. Thought it might be bad gas but need advice. I have a 1994 NAS Def 90 with stock 3.9L V8.

Issue: on occasion the vehicle suddenly stutters and loses power dramatically but does not actually stall. It can happen at highway speeds and lower speeds. Some times just for a minute or less sometimes a few minutes.

The car runs like it is not getting air out the exhaust almost like someone stuck a banana in the tail pipe. Sounds very muffled and runs on limited power even if floored for a while then goes fine again.

I noticed twice that when it did this and I got a look underneath the cat converters seem very hot (more than normal actually one was glowing red) and I smelled a slight burning smell.

When I shut the engine off, it starts right up and is perfectly fine until it happens again (could be weeks before it happens again).

What I have changed recently (within past 8-10 months) as new parts:
Fuel filter
Fuel pump
Fuel pump relay
Fuel lines
spark plugs, distributor, tune up
Throttle position switch
Cat converters and exhaust
Cleaned out MAF
K&N air filter
Snorkle

What I haven't changed:
O2 sensors

Any thoughts? I don't know what "bad gas" symptoms are like but a few said it could be that.

Any advice would be appreciated!
Thanks,
Will
 

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Coil comes to mind to me... the red hot cats mean it's running lean, which runs hotter. How's your engine temps? Does the little code box display anything after one of these episodes?
 

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Discussion Starter #5
Engine temp seems fine nothing unusual there. I hadn't thought to check code usually too focused on the issue. Silly question what is a coil?
 

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Engine temp seems fine nothing unusual there. I hadn't thought to check code usually too focused on the issue. Silly question what is a coil?
It looks like a thin but tell version of a can of soda, with a large ignition style wire coming out of it. This large wire goes to the distributor. It's mounted inside the engine bay on the fender
 

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It looks like a thin but tell version of a can of soda, with a large ignition style wire coming out of it. This large wire goes to the distributor. It's mounted inside the engine bay on the fender
The important part is that the coil provides the high voltage juice to fire your spark plugs. They are also notorious for having issues once they heat up. Poor current production from the coil = less electricity for the spark plug = no cylinders firing correctly.
 
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Discussion Starter #9
I ended up finding a new OEM Bosch Coil on line for a good price and also decided to get two new O2 sensors and replace those as well. I have heard they recommend replacing those between 60k and 90k miles. I bought mine with 117k miles on it and don't know that they have been changed (may be original). So while I'm at it I might as well do those as I understand it could also be causing the problem. Doesn't look like my code reader is working.

I have seen a video online on how to replace a later model with 4 o2 sensors and it looks pretty simple. Any trick to doing this that I'm missing? I don't have stock cats but assume it is the same concept as stock?
 

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I'd also check your cats to see if one is starting to clog
 
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I ended up finding a new OEM Bosch Coil on line for a good price and also decided to get two new O2 sensors and replace those as well. I have heard they recommend replacing those between 60k and 90k miles. I bought mine with 117k miles on it and don't know that they have been changed (may be original). So while I'm at it I might as well do those as I understand it could also be causing the problem. Doesn't look like my code reader is working.

I have seen a video online on how to replace a later model with 4 o2 sensors and it looks pretty simple. Any trick to doing this that I'm missing? I don't have stock cats but assume it is the same concept as stock?
The code reader will only display something if the check engine light is on. And, at least on my truck, occasionally I'll pop a check engine light with no code at all. Go figure :) - Sorry I didn't see this earlier. I have the stock coil from my truck sitting in the basement I took out at ~60k miles.

I like the catalytic convertor thought, but would surprised if that's the issue if they're not rattling at all. As far as replacing the O2 sensors, it's pretty easy (a flare nut wrench makes it easier). Just make sure you spray with PB Blaster or something equivalent. Sometimes they come out easier with the exhaust hot, sometimes not.

------ Follow up post added August 10th, 2015 01:04 PM ------

I do have rovergauge all setup if you want to check out how your O2 trim looks. But I'm not sure how helpful that'll be in this case.
 

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Ignition is cutting out, probably due to a bad amplifier or amplifier wiring. When it cuts out you are dumping raw fuel into those new cats which is what is causing them to glow. Won't take long to damage the cats.
 

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Discussion Starter #13
Ignition is cutting out, probably due to a bad amplifier or amplifier wiring. When it cuts out you are dumping raw fuel into those new cats which is what is causing them to glow. Won't take long to damage the cats.
That's good to know as well. Would you recommend taking the cats out when replacing the O2 sensors? And when looking into the cats is there something in particular I should be looking for? Like burnt fuel?

It has happened in total about 5 times never for very long (a minute or two) so I'm hoping these brand news and expensive cats are okay?!?

Thanks!
 

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Discussion Starter #14
Ignition is cutting out, probably due to a bad amplifier or amplifier wiring. When it cuts out you are dumping raw fuel into those new cats which is what is causing them to glow. Won't take long to damage the cats.
I ordered a new 3pin ignition amplifier module as well (Lucas) so thanks for the advice. Prior owner looks to have done the popular remount of this module to the same location on side wall as the Coil to keep it away from excessive heat. So since I am replacing the coil and this is on the same mount now made sense to replace as well and just cancel that possibility out

Thanks for the advice.
 

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Be careful.... It is very easy to throw parts (and money) at a problem. You got a lot of advice with all possible solutions to a problem. Just make sure you figure out what problem you are addressing. My advice would be: only fix "one problem" and wait for "another problem" to appear before you try to address it. You don't want to have to chase -two- problems at the same time.
 

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Be careful.... It is very easy to throw parts (and money) at a problem. You got a lot of advice with all possible solutions to a problem. Just make sure you figure out what problem you are addressing. My advice would be: only fix "one problem" and wait for "another problem" to appear before you try to address it. You don't want to have to chase -two- problems at the same time.
Ain't that the truth.
Sometimes you just have to wait for stuff to break completely to figure out what's wrong.
Took months of on again off again work starting at WR before I figured the Dizzy and rotor were toast.
 

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Discussion Starter #18
Be careful.... It is very easy to throw parts (and money) at a problem. You got a lot of advice with all possible solutions to a problem. Just make sure you figure out what problem you are addressing. My advice would be: only fix "one problem" and wait for "another problem" to appear before you try to address it. You don't want to have to chase -two- problems at the same time.
Totally understand and great advice. I do plan on changing one item, drive for a while make sure all is okay before moving on.

------ Follow up post added August 10th, 2015 06:00 PM ------

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Mine ended up being really simple, but pretty much the exact same problem.
what was the issue on yours?
 

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Dirty connections on the wiring harness @ the fuse block. Pulled them, cleaned them and put a little grease on them.


For some reason my link did not work, but if you search for threads started by me you can find it. Tons of other good information in the thread.
 

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Discussion Starter #20
Dirty connections on the wiring harness @ the fuse block. Pulled them, cleaned them and put a little grease on them.


For some reason my link did not work, but if you search for threads started by me you can find it. Tons of other good information in the thread.
You know that reminds me. Just yesterday looking at where the coil and amplifier are I was poking around and near the fire wall noticed a block of fuses near the fire wall that was just hanging loose. Looked totally unusual to me and don't recall ever seeing it just hanging like that. I will snap a pic when I have a minute. But is that the fuse block you are referring to? If so I assume it is not just supposed to dangle as a bunch like that and actually be fixed to something like the fire wall/bulk head?
 
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