New to the forum so hello everyone. Hope I can get some help. Did some searching before posting but couldn't find my predicament specifically so thought I would post to ask.

I get a random P03003 misfire code on the truck when I first start it. It runs really rough when this occurs so definitely misfiring. Once it idles for a few minutes getting warmer, it smooths out and I very seldom get a code once it is warm and I am running it normally. No issues I can feel for acceleration after warm or hesitation etc. It also doesn't always flip a code on startup either. Runs very rich when it is misbehaving but when smoothed out, I don't get the rich smell. This issue has been going on since I bought the truck from a young fella a few months ago. He used it for some bush playing so it has assaulted the odd solid object and has 260k kms on it.

I have done the following in tracking down the issue:

1. New wires and plugs - No fix
2. Cleaned MAF and IAC - no fix
3. Vacuum test with brake cleaner - no issues
4. New fuel filter - no fix
5. Sea Foam in the tank - No fix. I have not done the sea foam into the brake booster vacuum line as seen on a few youtube vids as I am somewhat skeptical on that practice
6. New coil block - better as it is less frequent now but still comes back

My question is what would be next? Given it is always just the one cylinder:

1. Do I now check into the injector or
2. Other wiring issues?
3. I am leaning away from bad MAF as it smooths out after warming up and only one cylinder so doesn't necessarily scream MAF to me. Could be wrong?
4. Do I look to other senors that are more prevalent in the computer inputs when in open loop (or closed....can't remember) when cold?
5. Could it be a bad O2 sensor on that side?
6. Do a compression test to see if the cylinder is bad? (not sure I want to know....) and I think I would notice more with still not having great running after warm?

Any thoughts or guidance would be appreciated as it is a bit frustrating at this point.

Thanks
Don

 

Welcome to the forum

What YEAR Ranger?
There were two different 4.0l engines that used OBD2 codes(i.e. P0303)
4.0l OHV 1995 to 2000
4.0l SOHC 2001 to 2012

Each had different reasons for misfire codes

 

 

Update

Truck started fine the past couple days without issues when cold and ran very well and then yesterday got back in to the rough start and P0303 code.
It seems a bit random so I am at a bit of a loss on what to look at next without just starting to replace everything one sensor at a time until it goes away...
I have a code reader that does more than just read codes but every time I look up the other reading functions it says nothing there.
Guess I need to read the instructions again to see what I have to do to get that hooked up a reading....

Any help in what direction to take would be very much appreciated
Thanks
Don

 

You have a 4.0l SOHC engine, used in Rangers from 2001 and up

Sensors rarely fail but are often replaced by DIYers and Pro Mechanics, lol.
DIYers do it to try and fix problems, Pro Mechanics do it to pad bills since it was just a vacuum leak and they can't charge much for that, but if they replace 3 or 4 sensors it sounds like they did a better job

Cause of any "code" is usually a mechanical issue unless code specifically says "circuit" issue, computer monitors electrical flow thru and from all the sensors, so if one goes bad it will set a code for "circuit" problem.
Codes report what computer "sees" FROM the sensors, which means........................sensors are WORKING, so don't "shoot the messenger"

You can unplug and plug back in sensors, this cleans their wiring contacts, and this is often why replacing a sensor worked to fix a problem, contacts were dirty inside, replacing sensor means it has to be unplugged and plugged back in, lol, so save your money, just do the free thing first, you can replace it later

P0303 is specific, it means passenger side rear cylinder, #3, is not adding power to the crank when it should, this is monitored by the Cam sensor and some what by the crank sensor.
As an engine is running the computer monitors crank and cam speeds with those sensors, when a cylinder misfires the speed changes slightly, so computer "sees" that, cam sensor can ID the specific cylinder, crank can't but can ID Pairs, i.e. 3/4, 1/5 or 2/6
So if Cam and crank both detect speed change then cam sensor will be used to confirm which cylinder it is, any misfire has to be seen a few times before a code is set

Not that any of that matters, just FYI

3 reasons a cylinder misfire
low compression
no spark
wrong air:fuel mix

I wouldn't suspect compression in your case since misfire comes and goes, compression is either good or bad, it doesn't tend to change, it can, but just way down on the list of possible failures, if you want to do a compression test(all 6 cylinders) to take it off the table that would be good

Spark can be tested with old time timing light, see if #3 wire has steady spark pulse, look at spark plug to make sure gap is correct, maybe swap it with #2 to see if problem changes to #2

Air:fuel mix can be thrown off by small vacuum leak at lower intake, so could cause intermittent misfire, especially while cold, as engine warms up the metal expands and can seal small gasket leaks, this would be harder to test for
You could spray some soapy water on the top of lower intake by #3, then cold start the engine, if it is leaking then you will get constant misfire as it sucks in that soapy water

 

Excellent and thanks very much Ron. I will do the easy stuff first. Would a faulty injector cause a random fuel delivery which might then cause the error as well? I will pull and put the injector circuit back on but I like the vacuum thoughts. That makes a lot of sense to me as a possibility as the misfire usually goes away when warm and expansion etc. I sprayed brake cleaner down there but the engine was warm so might not have had that.

I now have a plan for next steps so very much appreciate the info and help.

More updates to come.
Thanks
Don