zekgb

Per my other thread, after replacing pretty much the entire fuel, intake and timing systems (5k+ worth of parts) over the last 8 months it is now clear that I have an exhaust valve issue that's causing the loss of most of the compression in #8. The cylinder is completely dead at idle but seems to fire at 1500 rpm and above based on the results of a tailpipe sniffer. Speedtoys and I ran some seafoam through it today after a few days of running Techron through a half tank or so. We scoped the cylinder afterwards and we saw a huge carbon mess but no obvious damage to the piston. I put air on it tonight and it's definitely leaking into the exhaust.

Needless to say this is discouraging to say the least but now I need to make a decision on how to proceed. For background I'm very much a beginner at this stuff and my confidence level for anything that's not straightforward is pretty low. I know we have some folks in NorCal who do have a lot of experience but I'm quite a ways off the beaten track and can't expect too much in person help. I've never pulled an engine or removed a head but I did manage to get the transmission and torque tube dropped three separate times by myself with the car on stands so I'm not afraid to jump in if I feel like I have a good set of instructions to follow. As far as time pressure I'd like to have this job wrapped up by the end of February if possible as my work schedule will get pretty crazy after that.

For what it's worth, the rest of the car is in good shape mechanically after torque tube/TC bearings, rear axles, motor mounts, rack bushings and tie rods have all been replaced in the last 15k miles. Exterior paint is in decent/good shape other than the bumper cover and the interior is in good/very good shape, mostly leather with no issues other than the dash beginning to pull up around the vents.

Option 1: Pull engine, remove heads and send them out for repair/seals, clean up carbon mess in cylinder(s). Reinstall.
Pros: Uses my own parts, visible lubricated parts of valve train are in very good shape so at least that part is a known quantity. Best case the cheapest option, worst case the most expensive.
Cons: Not sure of bottom end (especially in #8) or the extent of the head work required. Might get pricey and beyond my capabilities if any bottom end work is required.

Option 2: Buy a used engine from Mark ($1k if available via half price sale.) Remove/inspect heads, replace headgasket, send heads for seals and guides. Pull engine then move all recently replaced/refinished parts over to replacement and replace all gaskets/seals.
Pros: Probably the lowest risk option as I'll have two of everything and will be able to proceed accordingly. Can sell/part the current engine and leftover parts, recouping some of the costs.
Cons: More money up front, replacement engine with no real history (though Mark warrants reasonable compression haha). I've got to do something with the leftovers and the wife won't be happy until that process is complete. Possible both bottom ends require work or that replacement engine valve train needs more than just seals/guides.

Option 3: Retreat, run away, don't look back. Sell the car as is, drivable currently with lots of brand new/powdercoated parts or part out. Can potentially acquire a pair of heads from Mark ($500) to go with the car that would allow that work to proceed prior to pulling engine.
Pros: I'm pretty bummed right now and the idea of launching into another long process with my rudimentary skills is not one that is currently exciting me. Pouring more money into a car that on a good day might be worth 10-12k is at best questionable, so don't throw good money after bad. Might be perfectly happy with a 986 for the price I could get for the car/parts + the money I am about to spend.
Cons: Nobody likes a quitter. The car still makes me happy to look at. I don't feel like I ever had it running perfectly and it would be satisfying to experience it.

What say you all? Any options/analysis I'm missing? Truthfully I'm really thinking hard about Option 3, but that just may be my frustration boiling over.

FredR

Kevin,

Did you carry out a proper leakdown test on each cylinder with some measurements to back up what you believe has happened? Do you know the difference between an acceptable amount of leak down and that which is not acceptable? Not saying you do not have a problem but have you exhausted the investigation thoroughly?

If the problem is in the heads why would you be concerned about the bottom end- do you have reason to suspect it?

If you have not done so already do a proper compression test on each cylinder [cylinders warm/throttle bore fully opened]. Do a leak down test on each cylinder and come up with some hard numbers.

If the numbers are bad then we can advise what to do next. Some folks have removed the heads in situ but not sure whether your year model permits this [studs versus head bolts- not possible with studs].

In my opinion if you have a leaky valve the question is why? With the leakdown test, if the numbers are suspect you listen to the inlet tract, you listen to the exhaust and you listen to the crankcase to determine where the leakage lies [hopefully one - not all three of the options to support your current conclusions]. Did you test other cylinders with the method you used to detect "a difference".

If you can remove the transaxle single handed and get it back in three times you can do the engine- just a question of the kit needed to do it with.

Apologies if I am missing anything here but it seems to me you have plenty to play with with what you currently have and I would not go down the second engine route unless and until you know for sure your current motor is totally goosed as such "replacement" motors are now very old and no reason to believe any better than what you currently have unless you know for sure it has a credible history.

Plenty to be positive about here - not an ideal situation but think of it as an opportunity. I can assure you that when you have seen the inside of your motor and know that everything is in good nick it really helps appreciation of what you have.

Regards

Fred

Lizard928

You can pull the head without pulling the engine.....

zekgb

Compression numbers are 145 155 150 150 140 150 145 35 and the result for 8 doesn't change run wet.

Idle doesn't change when pulling the wire on 8 (it does for all other cylinders) and a smog test tailpipe sniffer registers HC as off the scale at idle but within passing range at 2500 rpm (car passed smog easily in May.) Spark is present and the injector is clicking. I don't have a leakdown tester but I was able to hook up my compression tester with the check valve removed while at the top of the compression stroke for that cylinder and it's definitely leaking into the exhaust.

Silver79

I am going to vote for option #1. Take lots of pictures and make notes when taking things apart. The people on this forum are great and always willing to help. Remember, there is no such thing as a STUPID question!!! We are all here to learn. Joe

gbgastowers

Could also be a bad piston ring so you may have to replace rings

GlenL

I'm going to suggest #3. Sell and do something else with your $ and time.

If you want to develop mechanic skills go #1. The 928 isn't good for learning, though, as parts are super expensive and hands-on help, professional or volunteer, is spotty. Maybe you've got a mechanic or friendly locals. Perhaps a "1.5" would work where you pull the engine and have someone fix it. Going #2 has risks or failure that are the same as #1.

Speedtoys

When helping, I thought option #2 gets him on the road sooner and more coordinated.

A fresh motor on a stand to freshen up, and once ready swap motors, and see whats up with the old one.

You'll spend -far- more on WYAIT stuff once you pull a head, then pull TWO heads..and then...and then...

Get the car running on a new motor, then play with the old one see what can be made of it.

FredR

Kevin,

Obviously that compression on number 8 does not look good at all. Still at a loss as to how you can ascertain it is the exhaust valve that is the problem with out doing a leak down test albeit you may well be right.

If I understand your situation correctly you had a motor that was running correctly and then at some stage it was not? If there is a valve problem of this kind it would logically be a bent valve. was there some time or event when suddenly things seemed to go south? It does not take much to bend a valve once it hits something albeit hitting some coke seems unlikely unless the motor was suddenly over revved or the valve timing changed somehow.

Certainly sounds as though you are going to have to pull the head on bank 5/8 as a bear minimum, if so I would do both, new valve stem seals and engine seals in general whilst at it.

Rgds

Fred

Apologies if I have missed something in another thread.

jcorenman

Kevin,

When did this dead-cylinder problem start? From your other thread it sounds as if it was a new problem after the intake refresh. Is that correct? Or was the intake refresh done, in part, to hopefully correct the missing cylinder?

Did you rotate the engine so that both intake and exhaust valves for #8 were closed? Firing order is 1-3-7-2-6-5-4-8, so if you start from #1 TDC (both ignition rotors pointed to the right, roughly at "3:00", when facing the engine) then rotate clockwise 1-3/4 rotations of the crank. (The rotors will wind up 45-deg short of where they started at #1 TDC).

Also, when applying air pressure, make sure the engine does not rotate. If the cylinder does seal then the engine will rotate with air pressure until a valve opens...

If the valves were closed then air leaking into the exhaust is pretty definitive. Which means something wrong with one of the exhaust valves-- which is pretty unusual I think (barring timing-belt issues).

You mentioned lots of carbon: Could this be something as simple as a piece of carbon stuck on a valve seat? Hard to believe that it wouldn't get blown out by running the engine though.

Does your scope have the ability to look backwards and have a look at the valves? I am thinking of something like this:

Speedtoys

When we put a scope down the plug hole, it was all very black and wet.

Were thinking it's fuel deposits from the dead cyl.

I wanted to do a leakdown at the airport, but we couldnt find a 14mm adaptor, only 18mm fittings.

zekgb

hEngine has always run ok'ish (but never as smooth at idle or with throttle applied as Jeff's car for example.) When Bill Ball drove it about 20k miles ago he thought it felt down on power but couldn't find anything obvious. Spark plug seals were leaking heavily on 7 and 8 and both plugs on those cylinders were pretty carboned when pulled prior to the intake job, all others looked normal. I have always had a puff of blue smoke on startup (worse when sitting for a few days) which when I asked about was attributed to either oil in the intake (always a lot of that) or worn stem seals. People that followed me on club runs and such observed no unusual exhaust activity when driving. It always smogged easily, most recently in May of this year. The intake refresh was done for general maintenance purposes and revealed many issues that justified the work.
I didn't remove the caps to see the rotor position, I put a piece tissue in the spark plug hole and rotated the engine until it started to move, then used a long screwdriver to find the top of the compression stroke (well actually my wife told me when it stopped moving up.) She held the crank bolt steady while I applied air. It's obviously possible we messed this up so I can re-do it.

No the scope didn't have the ability to look backwards unfortunately. Have run through a half tank of gas with bottle of Techron and several 20 mile runs to let the cleaner do its job, and then yesterday followed with a full can of seafoam to no obvious improvement. I would have thought that if it were a simple carbon issue it would have resolved by now.

AO

Before you go further, I'd go to a harbor freight (or online) and get a leak-down tester. You need to know where it's leaking. If you're not willing or able to do that, I like Option 2. Put in a "good" used motor. Get it running, and then take your time stripping the current one. It could be valves, head, rings, or block causing the problem, but until you take it apart (or do a proper diagnosis) you'll never know.

Good luck.

jcorenman

That all sounds fine, just as good as what I suggested and simpler.

So back to the original question: There are good arguments for all three choices. For #2 the question is what Mark has available. If you went that route then Jeff's suggestion, to just drop it in and run it, makes more sense than starting by dismantling the new (used) motor. At that point you can enjoy the car while leisurely disassembling your motor to find out what went wrong.

Speedtoys

I -have- one KB, I need a day to get hands on it, someone has it right now, and it will, yes, have 14mm adapters.