erratic misfire

[will2045]

Okay I decided to clean up my other thread from multiple codes and do this one alone because all others have been cleaned out. I have a 1996 4.0l ohv that keeps throwing a code P0306


cyl 6 erratic misfire
new plugs and wires
switched plug wires as well as visually verified spark
switched 2 and 6 on coil and cleared code. code came back at cyl 6 still.
listened to injector with stethoscope and verified visually on a wet plug
compression test at 112psi
starts a little hard but always has
not using coolant or oil. both are clean when examined.
misfire seems to happen more when warmed up and ONLY at an idle... any ideas?

need help ASAP and I'm completely out of ideas.

 

[adsm08]

How is the compression on the other cylinders? The computer determines a misfire by comparing the cylinders to each other, so if you don't have the numbers for the other 5 then knowing that you have 112PSI on 6 isn't that helpful.

You are looking for less than a 10% loss from high to low.

 

[RonD]

Yes, good point.
Computer sets misfire code based on a cylinder not adding the same power as the others.
Computer gets crank rotation data from CKP sensor, so it knows when each cylinder should be adding power(firing) to rotation of the crank, a millisecond delay means cylinder didn't add enough to rotation.

Compression equals power, higher compression is more power, lower compression less power.
My '94 4.0l runs 160-170psi cold test

 

[will2045]

Yes, good point.
Computer sets misfire code based on a cylinder not adding the same power as the others.
Computer gets crank rotation data from CKP sensor, so it knows when each cylinder should be adding power(firing) to rotation of the crank, a millisecond delay means cylinder didn't add enough to rotation.

Compression equals power, higher compression is more power, lower compression less power.
My '94 4.0l runs 160-170psi cold test

I'm pulling a compression test on the other cylinders today after work but over the weekend I did replace that injector and it made zero difference you can still hear the truck at an idle occasionally miss

 

[pgman]

Hello everyone. I'm a long-time member but I haven't been here in a very long time. The truth is, I have been so busy and my truck has been so remarkably trouble-free for so long I kinda forgot about TRS - please forgive me. I am having a nearly identical issue - except mine is randomly (and intermittently) misfiring on cylinder 3. It will go away for a long period of time but return if I accelerate rapidly, as if to overtake a slower vehicle. I've also replaced plugs, wires and even the coil. I was thinking next that it might be a fuel injector (the engine has 211,000 miles on it) but I didn't want to get into that before doing more homework. A friend of mine who is a mechanic told me that I should go over all the grounds and check the connector on the ECU for any corrosion. This misfire seems worse at part throttle when cruising, but that may be because the TC is locked during those times (mine is an automatic).

I am going to check the wiring / connections today but would appreciate any other input or suggestions.

 

[RonD]

Hello everyone. I'm a long-time member but I haven't been here in a very long time. The truth is, I have been so busy and my truck has been so remarkably trouble-free for so long I kinda forgot about TRS - please forgive me. I am having a nearly identical issue - except mine is randomly (and intermittently) misfiring on cylinder 3. It will go away for a long period of time but return if I accelerate rapidly, as if to overtake a slower vehicle. I've also replaced plugs, wires and even the coil. I was thinking next that it might be a fuel injector (the engine has 211,000 miles on it) but I didn't want to get into that before doing more homework. A friend of mine who is a mechanic told me that I should go over all the grounds and check the connector on the ECU for any corrosion. This misfire seems worse at part throttle when cruising, but that may be because the TC is locked during those times (mine is an automatic).
I am going to check the wiring / connections today but would appreciate any other input or suggestions.

What year and what engine?

My '94 4.0l(300k) needs a can of Seafoam in the gas tank once a year now or it will start running rough, I think it is addicted, lol.
Factory injectors.

Yes, under load a partially clogged injector would show itself as a miss because the air/fuel would be too lean to add power to the crank.
A dead injector would cause miss all the time.
Ford injectors should test between 10 to 16ohms if you want to test #3, but testing injectors is like a compression test, you really should test all the injectors and write down the OHMs for each, they should all be within 1 or 2 OHMs of each other.

If you look at a diagram of an injector you will see it is just a needle valve that is pulled open by an electro-magnet, and pushed closed again by a spring(so a small solenoid).
At the tip of the injector is a pintle, which causes fuel to spray/atomize, if the pintle gets dirty the fuel drips instead of sprays, this causes a leaner fuel mix which is often not noticed until you need extra power.
Gas treatments like Seafoam clean the pintles and other fuel related parts.

 

[pgman]

Sorry - it's a 1998 4.0L V6. I think that information used to be in my signature but I see that it isn't.

I was able to go look at the truck late this morning and discovered that the #3 spark plug was deformed. The tip was bent and the gap was excessively large. I am not sure how this would have happened but a new spark plug made it run like new again. Obviously the damaged spark plug would cause the erratic misfire but I can't figure how it got that way. These are NGK Iridium plugs, by the way - possibly not recommended but I've always had very good luck with NGK and my engine ran noticeably better after installing them (before this most recent issue).

I read here about a test to see if there was compression leakage into the cooling system and I tried that this morning too. Basically, the post I read said to stretch a latex mechanic's glove (or balloon, etc.) over the radiator fill neck and then cap off the overflow spout before cranking the engine (with the coil disconnected). Doing so caused a very small pulsing of the latex membrane. I don't know (from reading the other post) if ANY deflection is acceptable, or if it should be completely still. I would think that the water pump, alone, would create some turbulence and perhaps cause what I saw (which was very slight movement). This engine does not leak and there's no smoke (white or any other color) coming from the exhaust. It has been reliable as the sunrise for many, many years. If I DO have a combustion chamber leak it must be VERY small.

Thanks for the suggestions.

 

[RonD]

Water pump just circulates, it isn't really a pump.
And if there was some initial defection it shouldn't pulse repeatedly like a cylinder would, water pump impeller would create a steady push.

If in doubt just loosen the serpentine belt so water pump isn't turning.

I had an NGK just go dead one day, driving along the highway, just got a steady miss, no warning or stumbling just stopped working.
Plugs were about a year old.
IDed it, pulled it, and it looked perfect, would have put it back in if I didn't test it, oddist thing that, just died.