996AE

Well, after a spirited Sunday drive... I rounded the corner into the neighborhood and to my surprise a flashing check engine light appeared. I shut it down immediately and coasted into the driveway.

OBII reader confirmed cylinder 6 misfire. Pulled coil and inspected plug, both new less than 12 months and a 1,000 miles on them. Just for kicks I swapped 5 and 6 coils, cleared ecu and fired her up. Rough idle and check engine light soon appeared with same PO306 code.

Monday had her towed to shop. Today I am told that coil, plug and injectors all tested in spec. Compression down on 6 180lb. Mechanic suspects valves.

Wants to pull motor and replace just 6 intake and exhaust. 16 hours labor plus parts 2,800.00 est. 10hrs for motor R&R.

Background. 2004 996 AE w/ 31k mile car, oil changed 12 months for life of car. Blackstone oil reports all good. Car is 9.5-10 condition. I have three P cars. None of them daily drivers.

Question

Suggestions while motor out? I am not going to do IMS or clutch in less something shows its needed.

Thoughts

1. AOS
2. Coolant
3. RMS (small weep)
4. Oil filler tube
5.
6.

Last, do full right side valves or just 6? Mechanic said just 6 with low mile car in less after inspection something suggests otherwise.

Positive helpful comments appreciated.

rockhouse66

So your mechanic thinks the #6 valves are...??..bent? bad guides? what exactly?

If it were me, engine out would be an opportunity to replace IMSB, RMS and AOS for sure. You seem resistant to this but that is what I would do. The coolant will be replaced, no doubt, since it will be drained. I would also replace the coolant reservoir since it is so easy with engine out and I think age affects these more than miles.

996AE

Shop said until they pull covers they wont know lifter stuck, valve bent broken etc... having ruled our coil, plug and injector, and testing low compression on 6 this is their best opinion.

reservoir great idea.

might do water plump too.

extanker

180 lbs should fire ....look for something else

Schnell Gelb

He is right. What are the other compressions measured at the same time/conditions?
Really should have done a leak down instead.

996AE

I asked for a leak down the shop owner said it wouldnt be helpful or reliable. Not sure what he meant. He stated its a lifter or valve and no way to know more until opened up. He said he ruled all other issues out. All other cylinders measured above 200lb on compression with 180lb on 6.

I am starting to think I need a second shop opinion here.

Schnell Gelb

If the lifter has collapsed it would make noticeable ticking noise from the cam cover .
Lifters can be replaced with the engine in the car

Cosmo Kramer

180 psi of compression will absolutely NOT cause a misfire. There must be something else wrong.

996AE

After 4 plus hours he cant confirm the problem with any certainty. Except to say he ruled our coil, plugs and injectors.

I am going to have a flatbed PU and take it to a recommended Indy for second opinion.

Byprodriver

Sometimes 996 3.6L valve seats are known to come loose.

996AE

I assume a leak down wont tell you if its a bent valve, loose or stuck lifter etc...

Maybe this shop is correct. Needs to be opened up to see what is going on.

Byprodriver

Need to have a borescope inspection thru the sparkplug hole. If the 1st shop didn't recommend this I would go elsewhere. Callas Rennsport in Torrance may be the best at diagnoseing this. You can go to their website & post a question.

996AE

So borescope inspection can determine valve or lifter stuck or bent or?

Macster

Bore scope can't help unless the valve head is really bent and if it was it would probably have broken by now. The only way I know a valve can get bent is if one does a money shift. Do you money shift the car?

If you tell me the misfires occur at low RPMs my WAG is it probably isn't a bent valve or stuck valve or a collapsed lifter.

More than likely one intake valve lifter for the #6 cylinder has had its low/high lift feature fail.

The lifters have to come out but the head can remain on the engine and the engine can remain in the car. The camshaft cover comes off and the cams come out.

The factory manual has a section on trouble shooting / diagnosing misfires due to a bad intake valve lifter which should be used to ensure one doesn't replace lifters for no reason.

Oh, and the factory guidelines are to if even one lifter on a bank is bad to replace all lifters. The problem can arise from "dirt" and this can affect more than one lifter. The concern is one cylinder misfires first and this gets fixed but after the engine is buttoned up then other cylinders can manifest a misfire due to the same problem and one is back under the camshaft cover replacing another pair of lifters.

996AE

No money shifts.

Misfire at idle occurred.

10-4 on right bank replace all lifters if one has failed. This seems more likely than a valve based on opinions I have been reading and your comments.

I spoke with a second shop owner and based on my description he said same thing. More likely a lifter problem. They are a race shop and specialize in 996. He said he rarely sees valve issues but lifters are much more likely after ruling out coil, plug and injector and their connections. The lower compression leds him to think lifter but he wants the car of course to test.

Flatbed PU for second opinion tomorrow.

5 hours from first shop and no firm diagnosis. Bummer.

I will ask the second shop about testing bad intake valve lifter procedure due to misfire before opening up to inspect.

More later in the week.

Thank you all for your comments.