Bad Compression - Skipped time? Help Needed in Raleigh
 

So I spent New Years Eve trying to diagnose wht has happened to my reliable flat 6. Symptom was that the car lost power the other day and I suspected either it jumped time or ignition issues. It sounded as though I was only on 4 or 5 cylinders.

I was hoping for the ignition issue. I pulled the plug wires until I found the bad cylinder. It was cylinder #6. I did a compression test and got 25 psig. Condition was hardly warm motor with throttle open, and other plugs in. I thought this can't be right, so I tested #5. I only got 90 psi. Then I tested #4 and got 115 psi. This motor burns very little oil and I am certain the rings are not that worn. And before this happened, it ran really well.

Now I am pretty familiar with a compression test and I am pretty sure my numbers are within 15 psi from a hot motor, so I am thinking that the valve is open on the compression stroke in #6, and maybe partially closed on 4 and 5. Is this a must for me to pull the motor or can I check it with the motor in.

The first thing is to be sure the valves have some lash, it's not important that you adjust them accurately, only that they aren't being held open. Hopefully, you find a tight valve or two, if not there are more tests, a leak down for example, but you may need to pull the motor. Good luck, Rob

Cam timing can be checked with the engine in the car. You only need to remove the intake valve covers. It can also be adjusted with the engine in the car but you have to remove the muffler, front engine sheet metal and cam box covers.

Like I said in your other thread, a valve tagging a piston is VERY bad. If you suspect your car is out of time, spend an hour to verify the cam timing. Adjust if necessary. If it were me I wouldn't even start the car until I was sure.

I would also do a leakdown test. That will tell you exactly where the loss of compression is coming from.

BK,

Since I drove the car home limping (5 minutes + 10 minutes of diagnosing at idle), I figured most damage was done if it skipped a link. Or do I still have hope?

I am pretty certain that the piston is hitting the valve after the compression test. I heard it when I turned the car over before putting the spark plug back in. I will still dive in to see the root cause and start planning to drop the engine.

If the the cam is out of time and I fix that and the tensioners, is it worth testing the cylinder, or should I go ahead and rebuild the head? Is there a thought that maybe the test will give a false positive?

Judging from some of the catastrophic pictures I have seen of broken cam towers and rockers, I am hoping for the best.

Cylinder compression:
1, ?
2, ?
3, ?
4, 115 psi
5, 90 psi
6, 25 psi

What are the other compression numbers?

Before doing anything drastic, remove the right side valve covers and take a look. If a piston tagged a valve, the rocker arm usually breaks. Even then, that wouldn't affect the adjacent cylinders unless you pulled the trifecta and managed to bend valves in each of those cylinders (your compression would then be zero or almost). Try wiggling the end of each valve stem. Any discernible movement? Broken valve spring(s)? Look hard at cylinder #6. Is there lots of blowby from the oil filler tube at idle? Let us know.

Sherwood

Let me know if you need some help. I have a endoscope you can send through the spark plug hole to see if there is any damage to the piston. I am also in Raleigh (near RBC Center), and work at Leith Porsche. Have done a rebuild on my Turbo, so I am pretty familiar with these engines.

Olin,

Jump fast and high and take up Pelican AngM018 (John A.) on his offer.

You need all six cylinders cranking compression and cylinder leakage tests. You also need both side cam timing check. Carefully inspect the sump screen (looking for pieces of chain ramp) and cut apart the oil filter for inspection. With this information you can start to make informed decisions.

If you need internal engine repair (not yet certain), with help from the many on this Forum you can easily do-it-yourself. Your goal should be to do any repair correctly and enjoy your 911 for the next decades.

Best,
Grady

911pcars - I was a little concerned to carry on with cranking revolutions to get 1-3.

AngM018 - You name the time, and I will be available. I am just inside the beltline below crabtree valley. I hope I jumped fast and high enough.

Grady - I have read so many of your posts that I am honored for you to have advised me on mine. I have complete confidence I can rebuild with all of your help, but being self employed, I am going through a bit of a SLUMP with extra spending $$$. The other reason I am focusing on not rebuilding entirely (if I don't have to) is because when I do rebuild, I want to do a 2.7 mfi RS build. I have been accumulating parts but I just don't have enough yet.

I would add to that "and generations" ;) since I have 4 kids and 2 of them have already claimed it.

you need to perform a leakdown to find out where your compression loss is...sounds like a bent valve...

Olin, just gave you a call, and sent you a PM. Sounds like you are only about a mile from me. Give me a call. I will put the endoscope in my car so I have it with me, and I can drop it off.

Update:
John was very kind to come by after work and bring his borescope to look inside. He was very helpful in lending his time and an extra hand. We got to see the inside of the engine and everything appears to be ok. There was no signs of metal debris on the piston and what I could see of the valve looked ok, but I could not see the seat.

When we attempted to run it down the intake stake, the butterfly on the TB was not opening. John looked at the TBs and realized that the linkage had popped off. Good eye John. I guess this would make the car sputter through all rpm.

I had pulled the intake cover off and John looked at the rockers as I turned the crank by hand. All things were in sync with the timing marks and dizzy - I know this is very general, but there was no obvious difference. We did not pull the timing covers off for any detailed cam timing.

John discussed what we saw and felt like there was no concern of the chain jumping time. He added that if I was really searching, he suggested as an additional check I could inspect the exhaust for "loose parts" or bits that could have gone through the engine. I do have to put on my other muffler, so "while I'm in there" I think I will remove the heat exchanger and check. But then again, if it is out, its out.

That leaves the question about the compression being so low. Well, I did another check and the compression was as high as the others, but to be honest, this gauge used to be my grandfathers and is at least 40 years old. Needless to say, I shouldn't have a whole lot of confidence in its accuracy. The numbers of 90 (5) and 115 (4) earlier were guesses based on the fact that those readings were just below and above 100. Cylinder 6 was just above 100. I do realize that 100 psi would not be a really good number for compression. I also looked at the rest point and the needle stays at 15 psi. Time to get a new test gauge, but I will still hold on to this one.

I have learned a lot of new things thanks to John and others that provided input. I have to consider now was this just a sign that I need to be more active in the chain boxes. I will be draining the oil for the new year, so I will be on the lookout for metal debris.

Could it be the TB linkage plus bad gage = no real problem?
Just a some what tired motor with a couple of cylinders not firing because of the disconnected linkage?

that would be nice.

Having TBs closed makes a big difference in compression numbers. Using an old gauge shouldn't matter. You are really looking for variance across the cylinders, not an exact number. If they were all 100 on an old guage, no biggie. Some 100 and some 150? Problem.

You don't need to pull the cam box covers to check cam timing, just to adjust it. You only need to remove the intake valve covers and measure the intake valve lift. You were right there! But without the right tool it is kinda difficult.

Hope it all works out ok.

You could have some carbon lodge in the valve seat and now it's gone.

I am putting it back together tonight after work. I had to close up the garage for the storm we had was blowing water in. No room to work if it is closed. I will see if it is back to its old self or not and report back.

Yes, all the compression numbers were around 100 on the old gauge, so I felt much better last night getting the new data point for #6. Still don't know what happened with the really low reading, unless I didn't tighten it enough or the chance of lodged carbon.

After seeing the carbon on the piston and plugs, I don't think a can of sea foam would be a bad thing before I change the oil.

The suspense is mounting.....

Could it be that your only problem was the popped off linkage while the subsquent compresion readings were just flawed? Once the borescope confirmed no internal issues I would have buttoned it up and gone for a test ride :D

In any case I will stay tuned.
Ingo

Well folks she is running fine once again. I just buttoned her up and cranked it up and all was back to near original state. It looks like the root cause of all of this was the slipping off of the linkage. This was a happy ending that I was glad to see.

I did run some seafoam through the intakes to purge some of the carbon from the carboned up internals. I am not one of those that believes everything that is claimed about a product, so I was curious enough to try. Breathing around that exhaust probably causes cancer in 98 out of 100 people. I took it out for a spin afterwards and I kid you not, that motor reved like I have never felt. I don't know if it was cleaning the points, cap and plugs or the seafoam, but it is smoother. With the dwell a little high, I did change the timing a very slight amount, that could have been it. The intakes are popping a little more at lower RPM, but the upper end is great. Does this mean that the mixture is lean/rich now that I have seafoamed it?

Since the points and plugs are due for maintenance, I am going to switch to the pertronix and see how that works for it.

Happy Motoring!

Great news Olin!! Sorry I have been away from this thread for a few days. All my free time has been spent under my car. I am just glad to hear it was nothing major! Feel free to let me know if you need any help in the future. We will arrange a time for me to get the borescope back in the future.

P.S. I have a Mac tools compression tester if you ever need to borrow one in the future.

John,

I went to the coffee meet this Saturday, and did not see you. I will be happy to come over and drop it off, and if you need a hand, I can make a trip for that too.

Olin,

I had to catch up on some much needed sleep on Sat. Its a shame too b/c I had a beautiful white 997 C2S with 20inch Champion rims. You can drop off the scope any time either at the dealership or at my home. I will be out early today (wed), and thursday around 4-5, and after 8 the rest of the days. Just give me a ring on the cell.