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Discussion Starter · #1 ·
I haven't made any mechanical changes recently. It was running fine Friday and I gave her a bath that afternoon. Saturday she starts up fine. After I get about 2 miles down the road I sense something abnormal, turn my radio down and hear she is running rough. With just my ear, it sounds like one of the cylinders may not be firing (perhaps #5? as the roughness sounded most prominent near the rear of the engine)

I've checked the cap, rotor, swapped plug wires (I have an '82 as well), checked the plugs (running rich but otherwise normal). I thought for a moment it may be bad gas, but I drove 70 miles after topping it off with no issues at all. The fuel pump & filter were replaced by the previous owner a few years back so I don't suspect those items.

The car has a 6m w/MSD 6A and SS coil. If it were the MSD equipment, then wouldn't all the cylinders be affected? Perhaps it is not just one cylinder as I'm not the biggest gear-head but it's certainly not a smooth of an idle or acceleration. What other items should I be checking? The TSRM doesn't go into much detail beyond what I've looked at.

[edit]
I haven't yet checked vaccume hoses to see if there may be a leak somewhere but this is a clean car and has been well maintained. Not much under the hood is typical of a 20+ y/o car with brittle rubber and what-not.
 

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Discussion Starter · #2 ·
Checked some of the vaccume hoses; I didn't have time to break out the TSRM and check every one but there was nothing obvious out of order.
 

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does it sound like it misses at idle, or only when you are cruising down the road?

...best way to diagnose a cylinder miss is unplug the sparkplug wires one at a time until one of them does NOT change the sound of the motor. That is your problem cylinder.

--billyM
 

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could be the makings of a bhg thats how mine started out
 

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Discussion Starter · #5 ·
BillyM - it is both at idle and at RPM. Good thought on unplugging the wires one at a time; skipped my mind. I'll try it tonight.

Red83MK2Supra - I'm not going to say your wrong (though I hope you are), but this engine was reworked with a metal head gasket, ARP studs, etc. and I certainly haven't driven it hard.
 

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Discussion Starter · #6 ·
I did as BillyM suggested and it appeared the cylinder #4 was the culprit. However, it is definitly getting a spark cause you could hear the "tick, tick, tick" of the wire grounding against anything it could when I pulled it off.

Perhaps a cracked insulator on a plug that I couldn't see? I'll try swapping a plug from my other supra just as a quick "lets see if it works" kinda thing. Otherwise I'm at the point of just replacing cheap parts (cap, rotor, plugs, wires [since I stole them from my '82] etc.) as a "for sure" eliminator.
 

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swap 3 and 4 plugs. If miss goes with plug then plug is bad. if not u may have a bad ignition wire to 4.
 

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If it is a daily driver, it is recommended to change your fuel filter every year. I've had similar symptoms that were solved with a new FF. I know they are a PITA to change, but it could be the culprit.
 

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swap 3 and 4 plugs. If miss goes with plug then plug is bad. if not u may have a bad ignition wire to 4.

Yep, swap plug and wire from 3 to 4. Sparking is normal, you'll always get that on every plug when the car is running, but you don't know if it's sparking all the way to the ground strap on the plug, that's why you swap plug and wire for the one next to it (don't forget to change the location at the distributor as well).

...everyone else, seriously, this isn't a game of "guess a million things that could be wrong", but helping someone diagnose an issue.

BHG, Fuel Filter, etc...

--billyM
 

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i was having the exact same issue as you are right now but i did replace my wires, plugs, cap and igniter/coil, also did a fuel pump and fuel filter... after it sits does yours blow any blue smoke?
 

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Discussion Starter · #13 ·
So I replaced plugs, plug wires, cap, and rotor to no avail. Had a handy mechanic friend come over and give me a couple suggestions, but most of which I tried. He did listen to the injectors by putting a dowel rod nexto them and they all =appeared= to be working correclty (could feel a strong pulse through the rod on each one)

Doesn't blow any smoke at all and never has since I've owned it.

During this diagnostic process there has been a time or two where I could hear it began running normal but only for a short while (3-6 seconds) but nothing I was doing really made any sense as to what would cause that. I took the top timing belt cover off just to take a look and confirmed the previous owners statement that it is pretty new. Didn't see anything out of the ordinary but I didn't go over the tooth by tooth either.

Arg, I really want to avoid dropping the tank but I'm starting to run out of things to check! I certainly appreciate all the suggestions thus far.
 

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Discussion Starter · #15 ·
#4 seems to be the only one that when you pull a wire, it sounds no different. I'm starting to think injector on #4?
 

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Discussion Starter · #17 ·
So an interesting note; I drove it the past few days to put some miles on it since I dumped some injector cleaner in it (yeah a cheap attempt but what the heck) At one point driving this-morning, voila it began running perfect... ...for about 5-6 seconds, then back to crap again.

Still haven't checked compression but if it were an issue with a cylinder not holding pressure would it be something that appears to "come and go" now and again?

Eitherway, will try checking compression as time permits. What about valves sticking or something along those linse? That is normally accompanied by a ping or a tick sort of sound though, right? No unusual sounds under the hood.
 

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We can rule out compression almost certainly. Like you said, wouldn't be come and go. ...probably not a sticking valve, our valvetrain does some stupid shit when a valve gets stuck, drops rocker arms and such.

You're looking at intermittent spark or sensor issue affecting fuel delivery, most likely.

--billyM
 

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Discussion Starter · #19 ·
An update:

So I start it up this morning and it appears to have the same issue, maybe running a bit better. About halfway on the drive to work, I jump on the highway and immediately feel it running normal again and it stays this way all the way 'till I shut her down at work. This is ~50 miles into a new tank of gas.

Here's to hoping it was simply a plugged injector or bad gas. I suppose we'll see as I jump in it for lunch / when I go home.
 

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Discussion Starter · #20 ·
Looks to be a 50/50. Runs well half the time and you can certainly feel it cutting in and out. If you stomp on it, it will eventually get over itself ~35-4000 and will run strong, then when you shift it's just a matter of luck whether it stays smooth or begins cutting in and out again. I hate to get too focused on one thing, but I'm really thinking it is a fuel delivery issue at this point. May invest in one more bottle of injector cleaner but I think it may be time to simply replace the injectors and/or drop the tank.
 
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