Running rough at start up

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Hey all -
My 94 is running rough for the first 20-30 seconds at start up. I replaced the cap, wires and plugs about a month ago. All Toyota correct parts. I’m at a dead end with the diagnosis. I have a check engine light on, but the code comes back 21, O2 sensor. I doubt this would be the cause. I drove a car a while back that was missing due to ignition coils going bad, this feels identical. One of the cylinders isn’t firing. Any thoughts?
 
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Temperature is around 70 degrees. It’s running around 1100 - 1200 rpm. If I hit the gas it will run rough for a minute and then run correctly. I’m certain a cylinder isn’t firing
 

zapatius

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But if it's not firing, what condition would make it start firing suddenly? Seems like it wouldn't just suddenly start firing and continue after 30 seconds? Have you pulled the wires individually while running to see if you can narrow down which cylinder?
 
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I’m not sure. The only other time I’ve had an issue like this I had to replace all of the ignition coils on a pathfinder. I like the idea of narrowing it down. I’m going to give that a try and see if it gets me anywhere. Might take awhile, the issue is intermittent, but seems to be getting worse. Good thought
 
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So just to update - I replaced the RT O2 sensor and the check engine went off, pretty easy fix as far as the light is concerned. I’m unsure if it fixed the rough running issue. It hasn’t been an issue for the past week or so, intermittent at best. If the issue comes up again I’ll starting pulling wires to isolate the cylinder. I bought a replacement water temp sensor, but I held off on putting it in. If I get the rough running again and take care of that as well.
 
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It's not necessary, more than likely the other one is going to go sooner or later that's the only reason. Unless you can confirm this one is bad.
 

LandLocked93

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So it's running 11/1200 rpms at startup. That's good.
Does the roughness remain after letting the engine come to temp and rpms go to normal idle speed ~600rpms?
Could be a fuel pump. Those are notoriously intermittent...until they aren't and you're stuck somewhere. (not 80 specific, any pump on any rig)
I mention as it's 50/50 imo that the O2 sensors, tho surely needing attention, are not the culprit.
But replacing them is good nevertheless.
 

ppc

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Why is it necessary to change both sensors?

It's not necessary, more than likely the other one is going to go sooner or later that's the only reason. Unless you can confirm this one is bad.

O2 sensors will degrade over time sending weaker signals. On the 93 & 94 ODB1 engines each O2 sensor, located in front of the side by side catalytic convertors provides feedback for each bank of three cylinders to the ECM. If the O2 sensors are not matched then one set of cylinders could be fueled significantly different from the other which would not be good. On the 95 and up ODB2 engines the O2 sensors and catalytic convertors are not side by side but inline where replacing one sensor at a time would be acceptable.
 
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I looked at the drivers side sensor when I installed the rt side. The nuts looked like they had been pulled off much more recently than the other side. Maybe the previous owner changed it out. I’m just going to go ahead and replace it. I was thinking leave well enough alone because the check engine light went off, but I can’t see a new sensor changing that.
 
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I’m going to replace the other sensor and see where it stands. It’s certainly possible the fuel pump is bad. The sensor alleviated the check engine light, so I guess we will have to wait and see. It’s running great today, no issues.
 
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O2 sensors will degrade over time sending weaker signals. On the 93 & 94 ODB1 engines each O2 sensor, located in front of the side by side catalytic convertors provides feedback for each bank of three cylinders to the ECM. If the O2 sensors are not matched then one set of cylinders could be fueled significantly different from the other which would not be good. On the 95 and up ODB2 engines the O2 sensors and catalytic convertors are not side by side but inline where replacing one sensor at a time would be acceptable.
I agree with O2 sensors degrade, the computer doesn't rely on oxygen sensors to do significant amount of fuel trim. It relies on oxygen sensors to fine-tune fuel trim. Significant fuel trim is provided by air fuel meter and coolant sensor also throttle position sensor.
 
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Also if the sensor is out of range and not switching back and forth within a period of time, it will set a code. No need to be replacing everything you can check with a
DVOM or a fast voltmeter like fluke 88 for example.
 
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I considered putting a meter on it. I agree it will set a code. I have the part, so I think I’ll just put the new one on. The return process is a bigger pain than the install.

I’m going to go ahead and replace the water temp sensor this weekend. Anyone ever replace one of these? Do I need to pull anything off to get to it or can hey I reach it with an extension? I can get my hand on the plug.
 

LandLocked93

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I’m going to go ahead and replace the water temp sensor this weekend. Anyone ever replace one of these? Do I need to pull anything off to get to it or can hey I reach it with an extension? I can get my hand on the plug.
If this is the one closest to the dizzy, I think I may have unbolted the ps reservoir to move it out of the way a little bit. But otherwise it's straight forward.
Be gentle with the plug. Mine disintegrated in my hand. Had to order 2 other parts to restore it back to normal - the plug and the sensor pig tail.
 
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Yes moving the power steering reservoir you can get to it I believe it's a 24 mm. More than likely you're going to break the connector especially the knock sensor connector. You can check temp sensor values at the computer, fairly easy to pull out the computer. And you probably won't break anything at the computer. The way you're describing it I believe your head gasket is failing and sealling up as soon as it warms up. Have you had any overheating issues?
 

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