2 cylinders @ 160 psi, others @190... how long will it last?

[Rickey_Bobby]

First off, what is the minium acceptable compression for a stock 1999 4.6L motor (PI heads)?
And second, Currently at 95K miles, any guesses how long I have to save for a new motor before my engine decides for me?

BTW, I'm running 11#'s of boost @ 10.9:1 A/F (average)

 

[aleborjas]

you can run 15 pounds with the stock engine and good tune, good fuel pumps and the most important intercooler or boostcooler...

 

[Rickey_Bobby]

I am refering to the compression, like when you do a compression test.

 

[triggz]

What cylinders are losing comp? If they're are neighboring cylinders, you're probably about to blow a headgasket and can save yourself some trouble by replacing it BEFORE it blows.

The compression numbers arent so much on a specific scale, as there can be a lot of tolerance differences, but they should be within 10% of each other.

 

[Mike97gt]

According to the ford compression chart it's well within spec.

 

[Stan Weiss]

The first thing you need to do is see why the 2 cylinders are lower than the other 6. Is it rings? Are starting to burn and exhaust valve? Did you over rev it and bend an intake or exhaust valve? Once we know what is wrong than we can say how long you got.

 

[Mike97gt]

it's down 16 percent on those two cylinders. while it is in spec as I said earlier I would rather see under 10 percent difference..

try doing another compresion test, with the engine at normal operating temp, all plugs removed and the throttle blade wide open...

 

[the98stang]

I'm with SVT ^

If the numbers turn out the same, your engine probably isn't doing so well. If I were you, I would run that b1tch intil it dies! Might as well get some use out of her, right?

Then when it dies, build the motor, and it'll hand more boost.

11 pounds is too much boost for a stock block IMO....

 

[Gauger]

Have you tried letting it run for 10 minutes and retesting? I've been told that this should be done since rings can move around and if they line up it can cause low compression readings like this. It's just hearsay, but it's not hard to do.

 

[Rickey_Bobby]

Well, I did another test and tried an old trick I remembered. I put 1/2 an ounce of oil in each cylinder before testing and got readings over 220psi on all cylinders. And since the oil settles on the piston, and not the valves, this shows that my rings are bad i guess.

What got me worried enough to check this out in the first place was this....

On the interstate @70mph, the car would pull 12#'s of vacuum.
I had 2 coils die and I drove around for a few weeks with intermittent spark.
After replacing coils, car ran fine, but not only pulls 9-10#'s of vacuum @ 70mph.

..This shows the engine is working harder to keep the car moving.

My theory, which could be WAY off, is that while those coils were misfiring, it allowed the fuel to wash the cylinder walls of oil and damage those cylinders. (The 2 low cylinders are the ones that had bad coils)

Now I do want to rebuild the motor with forged crap, but not now, or any time soon. Is there anything I can do to squeeze 5-7K more miles out of this motor?
And has anyone else run 11# of boost on a motor with 95K miles?

 

[RoadconeTuning]

go back to NA....

 

[LaserRed01GT]

Well, I did another test and tried an old trick I remembered. I put 1/2 an ounce of oil in each cylinder before testing and got readings over 220psi on all cylinders. And since the oil settles on the piston, and not the valves, this shows that my rings are bad i guess.

What got me worried enough to check this out in the first place was this....

On the interstate @70mph, the car would pull 12#'s of vacuum.
I had 2 coils die and I drove around for a few weeks with intermittent spark.
After replacing coils, car ran fine, but not only pulls 9-10#'s of vacuum @ 70mph.

..This shows the engine is working harder to keep the car moving.

My theory, which could be WAY off, is that while those coils were misfiring, it allowed the fuel to wash the cylinder walls of oil and damage those cylinders. (The 2 low cylinders are the ones that had bad coils)

Now I do want to rebuild the motor with forged crap, but not now, or any time soon. Is there anything I can do to squeeze 5-7K more miles out of this motor?
And has anyone else run 11# of boost on a motor with 95K miles?

It is quite possible that you could have washed out those two cylinders, BUT... I would do a few more tests to just attempt to rule out coincidence. Coolant system pressure test, and cylinder leakdown test. A static compression check can be helpful but will not always tell the whole story. Worn valve guides, burnt exhaust valves, and damaged or distorted valve seats can also be a culprit of decreased compression but they won't always show up with just a static compression check. A leakdown test will tell you precisely at what percentage the cylinder is leaking down.

But since you mentioned that the compression jumped up so high once you poured the oil into the cylinder would suggest a washed out cyilnder. The actual PSI that each cylinder shows is not as significant as how close they are all to each other. That 15.8% difference in PSI on those two cylinders is significant enough to look into.

By any chance, are the two cylinders in question adjacent to one another? Could be a head gasket leak in between those two cylinders.

 

[Rickey_Bobby]

The cylinders are the middle 2 on the pass side.
Just don't want to tear down for a HG until I'm sure thats what it is.

 

[the98stang]

Well, I did another test and tried an old trick I remembered. I put 1/2 an ounce of oil in each cylinder before testing and got readings over 220psi on all cylinders. And since the oil settles on the piston, and not the valves, this shows that my rings are bad i guess.

What got me worried enough to check this out in the first place was this....

On the interstate @70mph, the car would pull 12#'s of vacuum.
I had 2 coils die and I drove around for a few weeks with intermittent spark.
After replacing coils, car ran fine, but not only pulls 9-10#'s of vacuum @ 70mph.

..This shows the engine is working harder to keep the car moving.

My theory, which could be WAY off, is that while those coils were misfiring, it allowed the fuel to wash the cylinder walls of oil and damage those cylinders. (The 2 low cylinders are the ones that had bad coils)

Now I do want to rebuild the motor with forged crap, but not now, or any time soon. Is there anything I can do to squeeze 5-7K more miles out of this motor?
And has anyone else run 11# of boost on a motor with 95K miles?

If you want your motor to last another 7k miles miles, you'll need to back off the boost, or remove the SC completely (that would be the better choice).

Like I said earlier, 11 pounds of boost on a stock block is a lot of boost, especially since the motor has almost 100k miles. Most people run 6-10 pounds on a stock motor, with a conservative tune. How many WHP do you have? What is the AFR? Who tuned the car?

 

[Rickey_Bobby]

well, here's my problem... If I had a supercharger, I would remove it... But i currently have a T-70. I'm running on average 10.9:1 A/F and a local shop called modular madness did the tuning. At the last dyno pull It was making 468/555 at the rear. I tried lowering the boost, but I got a CEL because I was running rich due to less boost. I would have to get a re-tune to drop boost and thats $450.
...I guess I could drive normal, but I'm sure everyone on here knows how hard that is...

Is there any way to verify if the problem is the head gaskets?
What places do leakdown tests and dont charge out the a$$ for it?

 

[LaserRed01GT]

well, here's my problem... If I had a supercharger, I would remove it... But i currently have a T-70. I'm running on average 10.9:1 A/F and a local shop called modular madness did the tuning. At the last dyno pull It was making 468/555 at the rear. I tried lowering the boost, but I got a CEL because I was running rich due to less boost. I would have to get a re-tune to drop boost and thats $450.
...I guess I could drive normal, but I'm sure everyone on here knows how hard that is...

Is there any way to verify if the problem is the head gaskets?
What places do leakdown tests and dont charge out the a$$ for it?

You can pick up your own leakdown tester at just about any parts store. The inexpensive ones usually run around $50-$80. All you need is access to a compressor.

Another advantage to doing a leak down test is you can almost always hear where the leak is coming from. If it is getting by the intake valve, you should be able to hear it reversing back into the intake manfold. You could open the throttle blase to listen even better.

If it is going past the exhaust valve, you should be able to hear it going through the exhaust. I actually put my ear to the tailpipe. You would be surprised how well you can hear a significant leak through an exhaust valve.

If it is getting by the rings or valve guides, you will hear the escaping air through the PCV valve opening or the oil filler neck.

Here is a decent writeup:
http://www.moddedmustangs.com/forums/cylinder-leak-down-testing-vt3705.html

 

[dartvader]

Hmm My brother broke 2 pistons with only 9 psi of a little powerdyne
either way if its a headgasket or piston Your going to have to take it apart, or wait for it to die .................

 

[Rickey_Bobby]

You can pick up your own leakdown tester at just about any parts store. The inexpensive ones usually run around $50-$80. All you need is access to a compressor.

Another advantage to doing a leak down test is you can almost always hear where the leak is coming from. If it is getting by the intake valve, you should be able to hear it reversing back into the intake manfold. You could open the throttle blase to listen even better.

If it is going past the exhaust valve, you should be able to hear it going through the exhaust. I actually put my ear to the tailpipe. You would be surprised how well you can hear a significant leak through an exhaust valve.

If it is getting by the rings or valve guides, you will hear the escaping air through the PCV valve opening or the oil filler neck.

Here is a decent writeup:
http://www.moddedmustangs.com/forums/cylinder-leak-down-testing-vt3705.html

VERY good information!! Thanks a lot!!!

 

[Rickey_Bobby]

one more thing... What is the "standard/acceptable" leak down percentage?
What is normal? And what is the limit?

 

[LaserRed01GT]

one more thing... What is the "standard/acceptable" leak down percentage?
What is normal? And what is the limit?

On a lower mileage motor (less than 70k) they will normally show between 5-10% leakdown. With your mileage it could be anywhere between 5-20%. So far from what I have seen on these 2V motors, I would say you should be under 15%.

What you want to look for is consistancy in the leakdown percentage amongst all 8 cylinders. And keep an eye on #2 and #3 since you already suspect them. Also when you are testing #2 and #3, listen closely for leaks.