Misfiring...

[gamellott]

I've recently developed a bad misfire which appears to pop up as a heat dependent issue. When COLD, Cold, cold, it runs perfectly. After it's heated up (Usually), It misfires on the #4 cylinder consistently. There is spark. I have no real reason to believe it's due to a failed injector, in part because it's rebuilt one that I put in a couple of weeks ago. The ECU seems to think there is no problem with anything.

 

When cold, the ECU flags the Knock Sensor, and Coolant Temperature Sensor (Not sure if that's normal, but they are 30 years old...). Once it's heated up, nothing, nada...

 

Aside from the Cap, Rotor, and wires, is there something else that the computer doesn't pick up that could cause something like this?

 

Would it be worth it to go in after the injector on #4 with a NEW one vs rebuilt?

 

Tune up parts are on order, so I should be able to rule those out quickly....  I say they're all relatively new, but it has been a hot minute since I swapped them out.... like nearly 10 years I think... 

[NavyP503]

Good luck! Mine was stumbling on two cylinders and after the cap, rotor, wires, plugs, and coil I was amazed how much better it ran. So hopefully that’s your issue.

[adamzan]

Running well when cold but poor when hot is indicative of a sensor issue. When it goes for closed loop it acts up. On mine this was caused by the o2 sensor. But if she’s due for a tune up start there. I’d also toss a new CTS in there since it throws a code. It should not do that, ever.

 

You can also check the resistance on the injector cold vs hot. Healthy ones are around 12 or 14 ohms. Can’t remember which.

[Strato_54]

My O2 ran lean bias for a long time I switched them and it cleaned up my idle issues. the wd21 only has 1 so its probably a fair chance its on its way out

[adamzan]

14 hours ago, Strato_54 said:

My O2 ran lean bias for a long time I switched them and it cleaned up my idle issues. the wd21 only has 1 so its probably a fair chance its on its way out

Yeah if you don't know how old it is, just replace it, they're not expensive or hard to change.

 

I know years ago the aftermarket ones (even bosch) would cause a slight surging on acceleration after it was warmed up. Irritated the heck out of me.

[gamellott]

On 10/6/2026 at 3:48 AM, adamzan said:

Running well when cold but poor when hot is indicative of a sensor issue. When it goes for closed loop it acts up. On mine this was caused by the o2 sensor. But if she’s due for a tune up start there. I’d also toss a new CTS in there since it throws a code. It should not do that, ever.

 

You can also check the resistance on the injector cold vs hot. Healthy ones are around 12 or 14 ohms. Can’t remember which.

I'll probably tackle it this weekend after all of the parts show up. 

 

Where can you easily check the resistance of the injector?

 

If nothing pans out with the Cap/Rotor/wires/spark plugs, I'll probably be swapping out that injector, along with the rest of the old ones for good measure. I have 4 brand new ones in the tool box. I bought them before they became unobtainium.... And yes, I'll probably be swapping out the CTS since it's super easy to replace. I might as well put in a new Knock sensor if I'm taking off the upper intake... I'm assuming that's possible with that removed....