Hi everyone, this is my first post, but I've been visiting the site for a while!
I have a 2003 4.0 SOHC 4x4 5sp manual that has been having an intermittent severe misfire issue. The only codes I've gotten are misfire cylinder 3, misfire detected on startup, mutliple cylinder misfire, and occasionaly another random cylinder misfire code. At times the truck runs mint, at other times it sounds like its only running on two cylinders. It first started doing this last fall. Just one day all of a sudden it started misfiring on the road. It sounded like it was only running on two cylinders. Compression test showed good compression. I replaced the plugs, wires, coil, cam sensor, then finally the intake manifold gaskets. At the time, the intake manifold gaskets seemed to do it, and I drove it all winter without issue. Just a week or two ago it started intermittently severly misfiriring again, just like it was in the fall. It was totally undrivable, could hardly move around the driveway. Some days I start it up and it idles perfect, drives down the road perfect, get home and its sputtering a little. Some days on start up and hardly runs, again sounding like its only running on two cylinders. Some times it misfires for a while, then one second it decides to run perfect. Sometimes the other way, it runs perfectly, then just falls on its face. The misfire ranges from just a little stumbling almost undectible all the way to hardly running, sounding like only a few cylinders are firing. Checked vacuum, very steady 21 Hg. I though maybe the knock sensor was messed up and severly retarding the timing. I replaced it, and the truck seemed to run fine! Drove it for 10 miles then it acted up again like nothing had changed.

Im out of ideas and dont want to spend any more on this truck, doesnt seem worth it.

any ideas!?

 

Welcome to the forum

What were the old spark plug tips like?
Any indication of why there was a misfire the first time around?

I put in a can of Seafoam in the gas tank once a year to keep injectors clear
Seem to stop my slight idle misfires

There are only two reasons for misfires, spark or fuel, assuming compression is still OK

Computer controls both of these
If you have an old fashion timing light you can test each spark plug wire to make sure it has a steady pulse from coil pack with engine running at various RPMs

To test if fuel injector is getting correct pulse you need a NOID light, these connect between injector and computer and show if Ground pulse from computer is steady


The cam sensor on the driver side valve cover should set a code it its intermittent, try unplugging it and starting the engine to see if it sets a code, or if engine runs any better


And then there is the chance of water in the gas tank?

Or air is getting in to fuel line inside the gas tank, if possible test fuel pressure, see if its steady with engine running 50-55psi could be fluctuating which causes misfires

 

I dont recall what the original plugs looked like when I took them out, but dont remember anything strange. I replaced them with motorcraft 500, and gapped them to .065. There was nothing indicating what caused the misfire the first time around. Less than a mile from my house in the morning it started flashing the CEL and stumbling at a stop light. Less than another mile later it didn't have enough power even to move. I left it on the side of the road and went back a couple hours later and it was running well enough to get it home. It was one of the first colder mornings and I though maybe something contracted and there was a severe vacuum leak. I dinked around with it for a couple weeks and finally replaced the intake manifold gaskets and it seemed to run fine so I attributed it to that. All winter long, in cold weather it was great, then just one day it came back exactly the way it was.

I seafoamed the gas and added a bottle of dry gas, seemed to help some but maybe not.

The coil seems to be sparking fine. While it was misfiring I removed the #3 plug wire and it didint affect the way it was running (obviously nothing was happeing on that cylinder). The coil was sending a good regular spark through the air towards the #3 wire when I held it close.

I dont have a noid light. When I put a long screw driver from the injector to my ear I can hear a fast clicking, the same as the other injectors. I believe its getting gas. I have a cheap engine light tool with live data and short term fuel trims are around -1 to 5ish. They never seem different between when its running fine and not. The fist time it happened I thought I'd replace the inector adaptors and orings while I had the manifold off. I took the left side injectors off (the side with cylinder 3), but I couldn't figure out how to remove the adaptors. They all seemed fine so I just replaced the orings on that side (it was a cold day and I didn't feel like doing the other side). I had the three injectors off together and didn't remember which on was where so the chances of putting the #3 back on #3 were only 1/3 and the misfire didn't change cylinders. I wish I had knowingly put it on a different cylinder. I did mess up the orings the first time though and it was just pouring gas out all over the engine and I think down the cylinders. there was gas in the oil. So I pulled the intake off and replaced them.

I havent tried uplugging the cam sensor, but it is new

Before the knock sensor I added 5 gallons of 93 to help out.

From what i've read its a real pain to test fuel pressure in the SOHC?

Also, it does it on cold start up, which I've read is 'closed loop' and not relying on MAF and O2 sensors and just running on preset values. This rules out a lot of the sensors, correct?

What else can I test/ look at with the live data on my scan tool? I'm pretty new to it so I'm not sure what I'm looking at.

Thanks again!

 

no idea how this got in this thread. Had to go looking for it.

 

Why do my posts keep going into this thread? They don't show up in mine, so I go looking...second time now