Misfire under load below 2000rpm

[gamellott]

My 92 is the first VG30, and I have had this nuance ever since I've owned it and I've learned to just not lug the engine and accelerate. Other than that, it seems to drive fine.

 

I've had it for 2 1/2 years. It's had a tune up and routine oil changes. Nothing significant has been done to the ignition or fuel system.

 

Is this something that everyone else seems to experience or is there something that could be failing??

[Citron]

When you say tune up, you are referring to plugs, wires, cap, rotor, fuel and air filter? Just trying to get an idea of all that was replaced. If you pull the plugs and one is black, you know that is the cylinder that is not firing.

 

Sometimes misfires are there all the time but at higher rpm it is hard to tell because everything is happening so fast. I believe these trucks have a knock sensor so a hard misfire should throw a code.

[gamellott]

Citron, Tune up was plugs, fuel, air filter and timing belt replaced immediately after I bought it. Engine timing was also adjusted. The Misfire is only at RPMs lower than 2000 and accelerating, in 3rd, 4th or 5th gear. It's normally not running that low of an rpm, but it is on occasion. That's the only consistency. I can sit and idle forward in traffic on level ground and it runs fine. I'm really wondering if this is a normal thing with these engines and Just Maybe I need to use a higher octane fuel to prevent it, it's NOT a knock like my motorcycle does running on low octane fuel. When I had the cap and rotor off, It appeared firm and It doesn't appear to have a centrifugal advance like older vehicles do which would throw the timing off. The misfire definitely goes away at higher RPM and gets good power once over 2000.

 

If I'm light on the pedal to accelerate, it doesn't misfire. If I'm heavy on it, it will. Makes me wonder if one of the injectors could have an issue....

[Citron]

I have a 92 and I can't remember it having the misfire, however it never sees pavement so I haven't had it past third in a long time. It is gutless at low rpm though.

 

Just to rule it out, you could ohm the spark plug wires. I seem to remember that they are supposed to ohm between 6k-8k ohms. I have seen one time were a plug wire had voltage leakage due to bad insulation.

 

I think you are on the right track with the injector.

[adamzan]

Here's something to try. Unplug the o2 sensor connection, and try driving it. It is on the tree of connectors near the fuel filter. It has 3 wires, 2 white, 1 black. Or the opposite. But it will be obvious as it isn't part of the transmission harness.

[gamellott]

Update to this old topic:

 

So, I had never put the ECM into diagnostic mode before and figured out 2+2 and the coolant temperature sensor was faulted. Checked the connection and, oh hhey..., It's not connected. I connected it and it seems to be running smoother and I'm not seeing the misfire at low RPM anymore, or maybe it's less noticeable.