My truck has had this misfire at light throttle while maintaining speed for so long now its driving me to the point of no return trying to locate the cause.

I have no codes coming up but decided to check and regap the plugs and check the condition of my wires, along with checking the IAC,EGR,TPS,VPC and all my Vac lines. No change other than better trottle response due to 2 plugs being 6 thousandths under gap. Not sure how that happened but glad i discovered it. The darn misfiring at steady light throttle still haunting me.

I posted it here on the forum before and found that I wasnt alone, that others shared the exact same problem, But no one ever came back to mention if they "corrected thiers and how they did it".

If anyone here can shine alittle light on the subject. I sure would appreciate it!

 

Possible clogged injector? 6 thousands gap should not make it misfire, you can also take some flammable spray such as carb cleaner and check for vacuum leaks, just start lightly spraying and see if the idle changes.

 

I replaced my injectors months ago with the same theory of possible solution and no change to the misfire. Its doesnt misfire at idle or firm with the throttle, only when its steady light throttle.
I also tried using starter spray around my vacumm lines, only to find that I off set my EGR gasket while replacing it. Still no result, even after correcting the gasket. I tried so many things and still up against the wall.
I appreciate the suggestions though!

 

Wow. No one here has even a clue of what the cause could be.

I was told today that my thermostat is opening to early for the engine to be at the normal operating temp. May have a incorrect thermostat and to purchase one with higher temp setting. When my engine is completely warmed up, the guage needle doesnt even reach the N in Normal. While driving it falls back down to to just about the first line of the guage. He said that the computer thinks that the engine is still cold and may be adjusting the air/fuel mixture to acommidate for it. Causing my truck to run lean, which it seems to do. And cause the misfire.

Is this possible?

 

sure it is. anytime your starving the motor for fuel it wont run right.... also if you have a O2 sensorin the rich poss, it'll make the comp run to the lean side too. It'lll toss a code, but unplug the o2 sensors and wait a few min and see how it runs... when you unplug them the computer goes into neutral mode...it's not getting a signal to go rich or lean...you can clear the code later. see how it runs then. You should have a 192 t-stat in there.

 

That sounds easy enough. I know there is one just before the cat, Im gonna have to search for the other. Thanks for the Tip!

I was also told this evening to disconnect the vacuum line from the EGR and take it for a drive to see if the misfire will also stop. Thus relating it to the EGR. I did Vacuum and Electric test on it but am gonna give it a shot anyways. Nothing to loose.

Hell, If someone said it was the rear view mirror causing it, i would remove it also. Every time I drive my truck I give it throttle then coast-throttle then coast. I try anymore to avoid steady throttle due to knowing what will happen.

I would take it to the shop but Im afraid thier also gonna do the guessing game with my wallet at stake.

 

OK, does the truck try to die at times when your like at a light, or coming to a stop?

You need to change out the TPS (throttle postition sensor) it has a bunch of lil wire traces that circle the position of the butterfly on the throttle body.... when one of the traces burn out it loses signal and acts like the gas isn't being bushed, causing a jerk/missfire feeling. It's only two screws to remove from the throttle body... but it's a bit of a pain to get to.

 

Unfortunately the 4.9 engine was as dirty as the 360/390. It could not be cleaned up and was discontinued. I owned a 300 CID engine before emissions, a great engine untill they had to clean it up and found it wasn't possible. Basic engine theory. In an engine when the throttle is closed vacuum leaks will affect the performance, any part throttle operation the vacuum is so nill there is no sucking in of air, hence no problem. Forget vacuum leaks as being your culprit. EGR is a different story for your condition, it is open during part throttle operation. And if stays open during closed throttle well its a huge vacuum leak (another source of air). I don't have the answer you are after, Ford didn't either. The computors could not respond accordingly. If you know of another 4.9L that performs better, suggest swapping the PCM to see if its parrameters are better.

 

I have never had it stall out on me at all as of yet. I replaced the TPS due to dropping my original while in the process of removing and cleaning my throttle body.

My EGR has been also removed and cleaned, then reinstalled and tested and the VPS indicated the correct electric reading but you may be on to something as it may not be closing all the way. If not mistaken, I had codes pop up on me in the pass concerning this, but no codes now.

Ill be spending this weekend going through every possibility thats mentioned here on the forum. I got to get this fixed, or up my Zoloft. This has been a great truck for many a years. Still strong as hell. Just a pain to figure out.

What is the purpose of the vacuum piece that is connected to the drivers side, just below the valve cover with the hard plastic vacuum line running to the EGR?

 

Did youever solve this problem? I have been fighting the same problem for about 3 weeks and I have little hair left. I have replaced everythng you did and tried everything in almost mirror fashion before looking for help online. Please help, I love this truck and hate that it annoys me to drive it now.

Thanks