Need help: rough engine diagnosis

[jbh425]

I've been trolling the boards and going back and forth with my 1998 3.0 OHC trying to diagnose and fix a misfire. I have new plugs, wires, ignition coil, fuel pump and filter.

Codes read by mechanic
P0305
P0102
P0351
P0174

Symptoms are intermittent misfire/rough idle, worse at idle and under power. Occasionally bucks. I've tried unplugging the MAF, and the car *almost* dies, but then idles back up and continues to run rough.

Next steps?

[Breeves002]

Start here: How to Understand and Diagnose Misfires

Sounds like this is pretty complicated.

Let's go through the basics:

- Make sure your wires are routed correctly. There is a diagram on the thread I linked above. Also make sure your wires are just plugged in all the way.

- Make sure your Mass Airflow (MAF) is plugged in. Clean it if you have not already, it may need to be replaced. There are pinpoint test you can do to check. EDIT: After re-reading your post, it looks like this code could be caused from you unplugging your MAF to do tests. So we'll come back to this if we have to.

- Looks like you have a vacuum leak somewhere, you can do a cigar smoke test. Video on that in the linked thread above. Check PCV assembly (hose and valve, location on above thread).

- One thing you can do now: preform a power balance test to make sure only #5 is misfiring. Just remove the wires from the coil and replace one by one. If the idle of the engine does not change, that cylinder is misfiring.

[sheila]

Check pcv u-shaped hose under the throttle body.

[behlinla]

P0305 = Cylinder #5 misfire
P0102 = Mass Air Flow (MAF) Circuit Low Input
P0351 = Ignition Coil A Primary/ Secondary Circuit Malfunction
P0174 = System to Lean (Bank 2)

You might have caused the P0102 code by unplugging the MAF if you checked the codes after you did that test.

Check the spark on all cylinders, especially #5. You have a coil A circuit code, which is the coil for cylinders #1 and #5. If you replaced the coil already, you might have gotten a bad one, or you have some other issue with the wires going to the ignition coil (or possible a PCM driver failure). It's important to check for spark to make sure the coil is working properly.

Bank #2 lean is probably related to the misfire on #5.

I would also pull the #5 spark plug and have a look. It's the middle one on the radiator side of the engine.

P.S. Your engine is either OHV or DOHC. Which one?

[Breeves002]

^
He says OHC, but he may have meant OHV... Took me a while to figure out the difference.

P.S. How often do you see that the PCM is actually bad?

[jbh425]

OHV (sorry, sticky fingers)

I cleaned the MAF tonight and the engine is running smoother, but I can still feel a misfire.

I'll try to find a vacuum leak, check the pcv, and check spark on #5 and go from there.

Thank you all for the suggestions so far.

[behlinla]

Originally Posted by Breeves002

P.S. How often do you see that the PCM is actually bad?

Not very often. Somewhere about 1-5% of the time. GM products seem to have a much higher PCM failure rate though.

[Breeves002]

I usually see people replace the PCM when they really don't need to. I read online that 60% of the time the original isn't faulty. Every one is so quick to blame the computer...

I notice in alldata with all of the pinpoint tests, there are a lot that lead to the option "replace PCM". I feel like they are wasting your time with a lot of those tests.

[sheila]

If you have the Vulcan ohv eng., your pcv is on the rear pass. valve cover. On gen3 Duratec dohc engs., it's under the throttle body.

[jbh425]

Used carb cleaner to search for a vacuum leak, but couldn't find anything. I sprayed all around the manifold, hoses, connectors, etc. Nothing caused an idle change.
Checked spark at the ignition coil and everything there looks fine. It is definitely #5 that was missing, but now that the MAF is clean the problem is less noticeable.
I pulled plugs 4, 5 and 6 and 5 looked substantially worse than 4 and 6. Replaced it. When I bought the plugs the O'reilly staff told me that the OEM plugs were platinum, whereas the plugs I currently have are copper. I bought the platinum plugs but decided to replace just the #5 for now (with copper) until the source of the miss has been fixed.
The SEL is on and solid, not flashing anymore. Should I clear the codes now and see what pops up next, or does the computer automatically reset when problems are fixed?