Just got a Code PO306 on my 350 engine. This code is a misfire on #6 cylinder. I changed the plugs, wires, cap & rotor and did a compression check on all cylinders. They were all reading right around 150 psi which may have been a little higher had I thought to warm up the engine first. I pulled the #6 spark plug back out after starting up and found it is as clean as when I put it in. No evidence of combustion. The compression check tells me my valves are closing but the shiny spark plug tells me my intake valve must be stuck shut. I poured 8 oz of Marvel Mystery oil in both the gas tank and the crankcase and let her run for 15 minutes to no avail. Same miss, same code. I would appreciate any and all thoughts. Thanks in advance.

I'm a little confused on the PO306 code. This is a OBDII code. What model are you working on?

Joel

I am with Joel...

But have you pulled the valve cover? Is there any sign of a bent valve or a collapsed spring?

Have you tested the injectors?

Quote:

Yep, do the above but first, pull the plug, pit the wire on and see if there's spark.

You have to know this before you can do anything else.

Did the spark test and it was good. This is actually for my GMC Sierra truck. Pulling valve cover today. Bad news, truck is down, good news is I have to drive the Vette while its down. Thanks for your inputs.

year? Engine? Mileage?

I will move this to "Other Vehicles".

1997 350 80k. Thanks, didn't know there was a forum for other vehicles.

Ah, ok... old school SBC... that's nice to know.

Pull the valve cover and look for valve damage.

This system uses that damned spider, I think, for fuel delivery. They are prone to failure (I know they used it on the V6 truck engines).

What a ride.Rocker arms are rockin, pushrods are going up and down smoothly soooo its not a stuck intake or exhaust valve. I started t-shooting the vacuum system and much to my surprise when I pull off a vacuum hose I do not have vacuum. I pulled the brake booster hose and there was plenty of vacuum. There is a Evap Cannister Purge Solenoid valve that sits right on top of the intake. I checked the hose coming right off of that solenoid and no vacuum.

update: I removed the ECP Solenoid valve (it has an o-ring seal)and found PLENTY of vacuum. Hmmm let's see, lots of vacuum on one side of the valve and NO vacuum on the other side. Sounds defective to me. Thoughts/experiences? Also the mystery is why only #6 cylinder is coding as a misfire.

Appreciate your help.

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Another fail. Replaced the Evap Cannister Purge Solenoid valve with same results. Only thing left is intake manifold gasket. Real close to taking a trip to my mechanic. $$$$

Try this: spray some carb cleaner around the intake base.

If the engine picks up, you found your leak.

Have the lower intake almost off (work is getting in the way. What a hassle. Hope it's a vacuum leak like suspected.

Since you've pulled the intake, I'd like to suggest a few things.
Inspect those fuel delivery "tubes" very carefully for any carbon/fuel deposit blockage.
The injectors really need to be cleaned/inspected for proper funtion. Possibly call Jon at FIC for tech advise on this. I suspect from your earlier threads this might be the problem.
Possiby swap the #6 injector with another one just to see if mis-fire goes with the injector.

Good luck
Joel

Thanks Joel but I have a throttle body and not individual injectors. I thought I was going to get away with not having to remove the a/c compressor and bracket but have since discovered that the a/c bracket and power steering bracket have to be removed to get to that one last bolt.

Well sir, I was wrong. There is individual injectors I had to remove the upper intake to see them. So this is what they mean by a "Spider".

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Yea, that be it.

Joel

Maybe this is worth your time:

http://www.instructables.com/id/Poor- ... C-Vortec-Fuel-I/?ALLSTEPS

Thanks for the info. I'm going to try it as soon as I get off of these Graveyard shifts.

Saga is over and truck is running GREAT. Problem turned out to be fuel injectors. Had I put some thought into it before tearing it all the way down to the lower intake (suspecting a vacuum leak)I would have saved myself a ton of work.

A few things I learned from this evolution. I only had to remove one valve cover, after you re-install the distributor even though it was marked and matched as it was put in the timing still has to be set with a machine (rpm's over 1000 in order the set the retard), you Have to unbolt and push the power steering and air pump forward and finally don't forget to re-install the bracket (which is located on top of the lower intake) and holds the spider injector module up into the upper intake.

Glad it is resolved!

yay!

I am more then likely going to be dealing with that one myself soon enough...

Glad you got it fixed!

Joel

Thanks to all who have helped. You guys are the best.

There is another twist however! I drove my truck to work (30 miles) and when I got out I was standing in a cloud of smoke. I have an oil leak and it took me a while to find it. It is on the connection to the intake for the oil pressure sending unit. Not the unit itself but the standpipe. Unbelievable. Now I'm waiting for the dealership to open to get that piece.

Nothing is permanent.......

UNbelieveable. I still have an oil leak. My mechanic first told me it was coming from the distributor so I turned it over to them to fix. No luck. NOW they say it is a combination of the oil pan leaking and the rear main seal. I put in some dye and it looks to me to be coming from the back of the lower intake. Anybody ever experience a leak here?

oh, yea... the rear of the intake is common leak point. The RTV blows out on the China wall.

Thanks Bogus. Word for the day: d-i-s-a-s-s-e-m-b-l-e.

sadly... yes...

when you reassemble, use "the Right Stuff."

Works GREAT!!!!!!!!!

Did you remove and reinstall the intake base just now before the leak?

Pretty much valve covers, intake base, or oil pressure switch/senders back there are all that will leak.

If it is the china wall, then get a punch and put some divots in the cast iron along the top of it. Supposedly it helps keep the RTV in place. Especially if you use synthetic oil. The Right Stuff with the nice pressurized can is bitchin for applying a very even bead of RTV. 3/16" bead plus a little more in the corners and 1/2" up the cylinder heads should do it.

Yes, I removed the intake to replace the Fuel "Spider" assembly. Didn't realize until after it was off that I didn't have to remove the lower intake, just the upper intake. I'll try the Right Stuff. Can't wait for this nightmare to be over.Thanks for your inputs!

Update. All to no avail. Still getting misfire codes AND losing fuel pressure after engine is shutdown. Just bought a new fuel press reg and am installing that this weekend. Put in a new fuel pump last year......

Oh my gaggin heart. Just replaced the fuel pressure reg and the trucks runs GREAT except at idle or especially when I brake. It stalls when I brake. I removed the vacuum hose that goes from the intake to the Booster and lubed up the o-rings. Didn't help. Any ideas?

Check all the vacuum lines, you might have pinched or broken one. Also make sure the booster is still good as well as the check valve.

Checked out the booster (its fine) and narrowed it down to a vacuum leak in one of two places. The Throttle Body upper gasket or the o-ring where the spider assembly fits up into the upper intake.

If its the upper TB gasket then its a two minute fix, if its the spider assembly o-ring gasket then its another tear down.

Thanks for your help!

Its not the spider assembly o-ring. Turns out the Throttle Body is sucking air in thru the throttle return spring seal is defective. Go figure.

After all this work my reward is a smooth running engine and I went from 13 mpg to 14.4 mpg.

Wow what a strange resolution! Glad it's fixed.