At wits end with misfire.

[Ditchbanger]

93 3.0 128k on motor. In the morning I go out and start my truck sometimes I let it warm up other times i just take off. It misfires (bucks and pops) when its under 2k rpms no matter what gear Im in. It doesnt do it nonstop it sometimes does it in spells, and some times it will come out of it for a day and start again. I have replaced MAF, distributor cap, rotor button, plugs, plug wires, coil, cleaned my air filter, everything to possibly do with ignition. My plugs were all white, seemed to be burning fine. The only possible culprits left are maybe my distributor or fuel pump. Since it goes in and out of spells I dont think its my fuel pump. I dont know what to do! Please help

 

[Ditchbanger]

And its not my fuel filter. Replaced that not to long ago

 

[shane96ranger]

Fuel injector?

 

[Ditchbanger]

I was questioning the same thing. What keeps me questioning is the fact that it only does it under a load and that it only does it between 1200 or 1300 to 2000 rpms. Im gonna run some sea foam through it

 

[RonD]

I am thinking distributor shaft wobble.

Did you save the old distributor cap, have a look inside and see if any of the electrodes have marks on them.
Pop off the new cap and look at it and also feel the shaft for movement.

Higher RPM can smooth out a shaft wobble and worn bushings could make it a come and go issue.


TPS(throttle position sensor), these are easy to test with a volt/ohm meter.
The TPS tells the engine computer to open the injectors longer based on your foot position on the gas pedal, opening the throttle plate increases RPM but if fuel doesn't increase then engine will run too lean and misfire.
The TPS is just a rheostat, like a light dimmer, if it has a bad spot then computer cuts fuel to idle level.

Does your engine have an EGR system?
Some 3.0ls did, if so it could be opening when it shouldn't, this will cause missing at lower RPM

Have you had the OBDI codes read, there could be a hint in the codes.
No the CEL(check engine light) doesn't have to be on for there to be codes in the memory

 

[Ditchbanger]

I pulled my bed off tonight to discover that I have a pinhole in my sending unit. Im guessing that its sucking air. Gonna be sundays project.

 

[shane96ranger]

Where is the pinhole exactly? If it's just on the housing, that won't affect much. Is it in a fuel line?

 

[Ditchbanger]

Its in the line of the sending unit

 

[Ditchbanger]

The sending unit stopped my small leak but its still acting up. Ron I'm going to check my distributor. Last thing is what should my TPS be running at?

 

[shane96ranger]

The sending unit stopped my small leak but its still acting up. Ron I'm going to check my distributor. Last thing is what should my TPS be running at?

Idle should be around .9 VDC, and WOT should be around 4.7 VDC. The factory wire is green that you need to measure from. Might be different if it's been replaced though.

Sent from my Quad Core Acer A210

 

[RonD]

yes.

And the TPS is like a light dimmer, so voltage will slowly go up or down as you manually move the throttle plate, there should be no sudden jumps or drops in voltage, that means a dead spot or short.


It is a long shot for your symptom but there a valve on the intake call the IAC(idle air control).
This is a controllable vacuum leak, it is bolted to the intake a small tube with 2 wire connector.
Inside the tube is a "needle valve", this valve is control by the PCM(engine computer).
When it is closed(less air) engine RPM will be low, 700 rpm, when open(more air) engine RPM will be 1,100+, this is the cold engine idle control.
When you first start a newer engine, the RPM will often go up to 1,500 and then drop back down to 1,100 or 700 depending on if the engine is warm or cold, the IAC is doing this.
These can be cleaned and test, just connect it and see if it clicks open with key on/engine cold and closed with key off

 

[Ditchbanger]

Pulled my distributor out and everything was fine. I tried swapping out my TPS cuz I have a spare one but I dont think that it the issue. My IAC is not the issue, it idles fine and that only affects idle, not when the throttle is opened up.

 

[Ditchbanger]

I should edit my symptoms a little and say that it does it over 2k rpms but not nearly as bad. Its the worst at about 1500

 

[99RangerBoss]

Maybe plugged cat? I had one of my precats blow apart and take out the next one and was misfiring on all cylinders on drivers side bank, all i did to test it was grab my trusty unibit and drill a hole close to the collector and done it worked and then eliminated the pre cat bs!

 

[Ditchbanger]

Thanks for the responses but it ended up being my IAC valve!?? It was carboned up pretty bad and was choking it out. Weird part is now that I have a new IAC valve it starts hard?? Oh well, it runs better.