Engine miss but no CEL (1 Viewer)

[rowdyrabbit]

I have a 93 that suddenly started missing hard yesterday morning. Ran fine Sunday evening, ran rough Monday morning. It has always had a bit of a miss until warm since I've had it. Warms up and runs fine. A while back it started missing hard like this and I found a vacuum line unhooked. Fixed that and it went away.
This time it feels the same, but I can't see a line unhooked or broken. Haven't had a chance to check with propane or spray yet. The CEL has been on with an O2 sensor code. Checked again today and no new codes. Pulled fuse to reset codes and it hasn't came back on.
Seems to be most pronounced at around 1500 rpm. Say 1200-1800, but it's there at all rpm. Truck has always idles a little high to me. 800 idle in drive, 1100 or so with AC on. This with the miss would make me think vacuum leak, but I figured that would throw a code.
Could use all the tuneup parts, but I just really don't need to spend the money right now. I'm afraid that's the problem, but I want to check anything else I can first. And seems odd that plugs, wires, etc would get that bad overnight.
So my question is what will cause engine to run rough, but not throw any codes? What are the simple things to check first.

 

[LandCruisinMy93]

Funny, I just started having the same issue but I have all new O2's and tune up parts. Still throws a CEL of 26 and still idles rough. It had all new tune up parts a month or 2 prior to rough idling. When I get some time to figure it out I will post, meanwhile I will be looking for your solution to see if it solves my issues

 

[120mm]

I had a similar issue and it was the high tension lead for the distributor.

 

[LINUS]

While you can run just about any grade gas here in the States, it's not always going to give you that "sewing machine" perfection.

You ever drop Techron or some similar in the tank?

-Not to say this is the fix, just helps if you buy gas at the same place, the same octane, etc. Rules out the variable of gas.

When's the last fuel filter change, and how many miles on your 80?

 

[rowdyrabbit]

Not sure on the fuel filter. I bought it in February and have not changed it. I put some Techron in it the day I bought it. That was about 3,500 miles ago. It has 193-194,000 on it now.

I didn't notice any loose wires on the distributer, but I'll give them a better look.

 

[rowdyrabbit]

Still running rough this morning. I got a bottle of Techron and added it to about 1/4 tank. After driving about 25-30 miles it was running much better. Then this afternoon it was shuddering so bad it had my side mirror shaking. That was on the gas at about 1800 rpm. The rest of the afternoon it would run rough, clear up, then rough again.

That puts me back to my earlier question...what can cause the engine to run this rough but not throw a code? Admittedly, I don't really know what the triggers are for each code. It's running rough enough that I know I need to check the engine, but apparently the engine doesn't realize it.

 

[LandCruisinMy93]

The problem with the OBD1 is it's really a crap shoot. Non-OBD pushrod engines...easy to diagnose. OBD II easier...OBD 1 crap shoot. I was an owner operator of a repair shop for a few years and I still haven't found my issue...I have access to a $3500 MATCO scan tool that does every make, model OBD I and II you can think of along with mimicking sensor input/output etc... Still not a solution to be had. I had a mild stumble and stutter when I bought the cruiser. Did Full PM, cap, rotor, leads, plugs fuel filter, PCV air filter, oil change PHH even replaced the filter sock on the fuel pump, timing chain and tensioner,oil pump cover seal, distributor o-ring, headgasket with ARP studs, radiator with all heater hoses except for the rear heater. New high amp alternator and battery, new fusible link, new ECM,Checked every piece of vacuum line for cracks, leaks with a fogger and propane as well as the intake hose, intake manifold at the head, the upper to lower mating surfaces, the throttle body and everything plugged into it. As far as I can determine, the possible causes for my issue is either in the wiring, The EGR system, fuel pressure regulator, fuel injectors themselves or valve clearance. Because those are the only things I haven't messed with. And before anyone goes running off at the mouth about throwing parts and money at a problem. None of the above mentioned maintenance was done to solve this problem...I just kind of hoped maybe by solving other issues it would also solve that problem. Honestly if I had to put all my eggs in one basket, if blame it on valve adjustment. I have started noticing that as the problem worsens I notice more audible intermittent ticking from the engine, like its slightly low on oil or something, but it's not. And yes, I've run 92 octane chevron gas through it on several occasions and it didn't do a thing, only thing it helped was lowering my bank account balance a few more dollars.

 

[rowdyrabbit]

Update time. Plugs, wires, cap and rotor solved my problem. I really wanted to wait until I could afford all OEM, but that didn't happen. I pulled the #1 plug early last week to see what I was dealing with. After seeing it, I knew it was time for a tune up OEM or not. I'm surprised it was even running at all. I used NGK V power plugs, NGK wires and an O'Reilly's cap and rotor. Swapped it all out and it runs like a top. Hopefully it will stay that way.
The throttle body is filthy, but I didn't have time or any spray to clean it at the time.

What is the correct idle speed for this engine? When warm, in park, it idles around 800-900. Cold around 1000-1100. A/C on about 1100. Is this about right? Sometimes I see 650 is correct, other times 850.

Thanks for all the earlier replies.