Its been awhile since I have been on the site! Last time was when I changed my timing chain / adjuster kit that did in fact solve my knock issue....

Now I have a new issue! po308 cyl 8 misfire, here is what I have done!

1. I started with spark...I swapped #8 and #5 coil packs to see if misfire followed...It did not..Still po308. tested coil with spark tester and she is getting fire...So moved on to fuel..

2. I swapped the #8 and #5 fuel injector to see if code moved to cyl #5?
it did not! still po308.

3. So I reluctantly pulled out my compression tester and sure enough...like only 20-35 psi vrs 180 like the others...Well that sucks I thought, but at least I know what is up now! So I pulled the head on that side and sure enough, the exhaust valve on #8 was burnt and had a chip missing from it....

4. I ordered a new exhaust valve and engine gasket kit. when the new valve came in, I replaced it and lapped all the valves in ( to seat them ) and tested them by poring gas into the exhaust manifold and letting it sit for over an hour and verifying no leaks past valves.... I put everything back together and.....guess what...po308 again?? So Im like wtf...pulled out my compression tester again thinking maybe I did something wrong! But infact have 180 psi in the #8 cylinder, I checked 5,6,7,8 cylinders and they where #5=175, #6=184, #7=181, #8=180psi....?

5. I went back to square one starting with spark testing...after ohm testing coils and verifying spark on cylinders 5,6,7,8 I did notice it seemed like the spark on #8 coil was a little weaker than the rest...so I did a BUNCH of internet research! And although VERY uncommon, I found a couple of instances where a faulty PCM had issues causing a weak spark....So I purchased a rebuilt pcm, had it programmed, new key's and all that and still no change!! still the dreaded po308???

6. I did notice that ever since I have had this thing that it has always had long crank times...I did not think that this could be related as it has always fired up and ran with no apparent starvation of fuel ...just always had the long crank time before starting....But, as I was out of any other ideas, I rented a fuel pressure tester from auto-zone and sure enough it is hardly registering pressure on the gauge ( like 12-16 psi.) and the pressure instantly bleeds off to 0 psi when I kill it...So I know I have a week fuel pump and a bad return fuel check ball as the fuel instantly bleeds/drains out of the fuel rail back to the tank after killing the car....but even with that I find it hard to believe that would cause a repeated po308 code? as I would think a week fuel pump would not set a po308 code ( more like a po300 at best if any code at all. right?)

Im just at a loss now? short of maybe a short in the wiring harness from the pcm to coil is about the only other thing I can think of....

If you guys have no good suggestions for me from here I might have to just spend the money and have it hooked up to a oscilloscope or some other high end snap-on scan tool for further diagnostics???

Any suggestions from sabe fellow members would be great. have I missed something here??? mess up something in head even though I have good compression now?? wiring short in harness?? week fuel pump cause something like this?? I have put to much money in this thing now to give up....just that even with my 25 years of wrenching/experience working on cars this thing is kicking my ***!!!

Help! Danny

 

Rings? Any blow by?


Got to ask. sound like you covered everything on top. Do a new compression check.


The other question? Is it running right?

 

Hey outlaw,

no blow by on rings. great cross hatching still in cylinder bore ( looked when head was off.) I did a dry compression test as well as a wet one. both numbers were very similar...

It has a prominent miss, most noticeable at idle. (engine has a shake to it.) not as noticeable at higher rpm.

Thanks.

 

Good job Danny! . You have really tried to beat this thing down!

Just my $0.02 worth, but ohming the coil doesn't prove much as far as I'm concerned. I mean, it can tell you if a coil is bad, but it can't tell you if it's good.

Get a scope on the ignition (and on the injector). The current flow waveform will let you examine the current ramp, the peak and hold, and will indicate not only the condition of the coil, but the complete circuit, including the coil driver,s transistors ability to quickly release the ground.

I'd lean more to the injector circuit causing the issue, as I wouldn't think a weak spark would have any impact on the #8 valve, but a marginal injector CIRCUIT might take out an exhaust valve.

 

Have you tried comparing voltage reading from the #8 COP and injector to others while the engine is running? A lower reading can be indicative of a wire or connector issue. As to the fuel pump, I suppose an argument could be made as to the relationship of the location of the #8 cylinder and where the fuel first enters the fuel rail.

 

Hey Alloro,

I actually have not compared them with a volt meter ( good point!) I just used a noid light on the injector connector to confirm signal. I will definitely do that and report back...

I just looked at a diagram of the fuel rail online, and the fuel both enters and returns on the passenger side ( same side regulator is on. and opposite side of #8 cyl) and loops around to the passenger side. And it sure looks like ( at least from diagram ) #8 is infact the last cyl in the loop to get fuel...So it just might be there is not enough fuel pressure by the time it gets there....thanks for pointing that out!
I will check voltage numbers and I will also probably just go ahead and drop the tank and do the fuel pump also because it needs to be done anyway...
I might not get to the fuel pump til next week on the navi because I'm working on my daughters car today..

Thanks for the pointers...now I have a few more things I can try before paying to hook it up to a scope somewhere!

Thank you sir!

 

Hey pdgford,
Thanks for the kuddos man I have been trying everything I could think of!!

Im going to go ahead and do the fuel pump ( needs done anyhoo ) and at least I can rule one more thing out that way...

But if that fuel pump does not do the job, after a few more voltage checks, I will get this thing on a scope to hopefully put an end to this pita....

Thanks for the pointers sir!

 

You sure? It could be a bad regulator or a clogged fuel filter and not a bad fuel pump.

 

Yea, I'm sure. When I did a fuel pressure test on it, it was only reading like 7-10 PSI vrs 35-45 psi that its supposed to be, plus the return check ball on the pump is toast anyway because the system drains the fuel back to the tank instead of staying pressurized which is why I have the long crank time issue, the fuel pump has to re-pressurize the lines and fuel rail from the tank every time its started because the return check valve quit working on the pump...And I'm really hoping that with a new pump, it might even cure the #8 misfire issue? maybe not, but since it needs a new pump anyhow I wouldn't just be throwing that $ away on the pump if it dont fix my #8 misfire issue...So I will cross my fingers I kill 2 birds with one stone on that deal!!

Oh yea, I tested the fuel regulator and it checked out ok.

 

Just an FYI, those same symptom will be there if the fuel pressure regulator leaks and allows fuel to bleed off back into the tank. But since you've tested the FPR and you know it to be good then you're on the right track.

 

Hello. Reviving this old thread as I'm having the exact same issue. Did replacing fuel pump fixed the issue?

 

Unknown....

 

Hi original poster here. The fuel pump did not fix my issue! I ended up doing a compression test and cylinder #8 was like 130-140 I did a wet compression test on that cylinder and number did not come up. So I ended up pulling the head again.... I was hoping it was something simple like coil, fuel pump, etc. but not the case. I got that exhaust valve seat reground putting everything back together and she runs great now...So for anyone having a similar misfire issue start with the easy first... #1 once you determine which cylinder is misfiring (example #8 ) swap a coils with an adjacent hole, drive and see if misfire follows! If so, you have a bad coil. If not, I would suggest a compression test at that point. If you find a hole low on compression to find out if its piston rings or valves do a wet cylinder test ( put a couple of tea spoons worth of oil in that spark plug hole and redo test, if the compression numbers come up then you have a valvetrain issue, if number stays the same then you have a piston ring issue.

My issue ended up being I had lapped the new valve in but I should have taken the head to a machine shop to have that valve seat and new valve reground! ( live and learn) I hope this helps someone chasing a misfire....Good luck!!

 

Thank You for reporting back what the solutions was.
It should help others with the same issues.