996 rebuild with poor compression

Ok so I have a fairly daunting issue here, and need some moral support. I�m trying to remain positive so no scare mongering please. I need constructive, creative ideas or experiences I can pull on to climb out of this hole. So here�s the story, the engine rolled a bearing 10 months ago and I have been slowly rebuilding it bit by bit. I have used the forum a million times to answer my questions, but on this occasion I can't did a similar issue to read up on. While the engine was apart I put new pistons (JE), rings and cylinders installed, as well as all the other bits like shells IMS, stem seals etc etc. so a fairly comprehensive rebuild. Before I installed the engine back into the car I ran a compression test on the engine and all looked well. I now have an issue. Number 2 is not well!!! I have low compression! Around 50 PSI. So here�s what I have done and what I�m thinking so far. If the combustion was blowing past the rings the crank case would be pressurised, but I actually have a vacuum. So it can�t be that can it? The engine oil pressure is 5 bar cold on start up. When I Unplug the coil pack this does not change. Again I thought it would if I was pressurising the case?? If the valves where leaking I would hear the ignition firing right?? Stuck tapped? Again wouldn�t I hear this?? Head gasket?? Again wouldn�t there be another sign of this issue?? If the timing was a tiny bit out could this cause the issue? I can�t see how�. So what do you think, top or bottom end? And you got any further tests I can run?? I�m considering a 2mile soft run out to see if it seats down and settles��just incase u do have a sticky tapper or valve. Thanks for your support Sam

I would do a leak down test before I ran it again .With that low a compression you will quickly find what is not right .

Have you driven the car at all? What about a ring seal issue? Are you running a break in oil?

Thanks for the replies fellas. I dot think it's a break in issue purely because the others are ok. Will check out the leak down test as I am bit up to speed on these but have heard of them. They expensive? What I don't want to do is strip anything till I know what I'm attacking. Can't run it once it's removed so the diagnostics now is critical.

Hi Sam I'm rebuilding like you but slightly behind, I'm just doing the heads, was reading about the hydraulic lifters , some are saying it is best to empty any residual oil out of the lifters by applying pressure to bleed out old oil, and also if this is not done it is possible for them to get over extended, especially if you soak them in oil before installation or try and pump them up, effectively putting too much oil inside which gets locked in the system not allowing the lifter to depress enough in the engine causing a valve to stay unseated when it should be closed?? I suppose if you do a leak down test you would hear air escaping through the exhaust or throttle body if this is your problem?? Maybe a borescope check to look inside the cylinder might help also?? Hope it's something simple for you !

Sam-Harbor Freight sells the tester for $40.00 .A google search will provide all you need to know and more .

Thanks mate. As it happens I wondered about that as I did soak the tappets. I think the leak down test is a good idea so I will start there and give you a shout. Won't sleep a wink till I know the severity of the issue.

Leak down test is basically using a compressor connected to the spark plug hole to pressurise the cylinder to a set amount then measure how much it drops over a set time attachments are usually not expensive but you need a compressor to do it with which obviously is!

I feel for you mate!! had a few stressful moments myself, it's Deffo better if its the top than the bottom I would think!! just out of interest did you use new head bolts and what tightening sequence did you do ?? I got the Porsche manual on disk it says to use the following tightening technique for head bolts. 30nm of torque in sequence inside to outside to flatten Fully loosen bolts 20 nm of torque in sequence then.. 60 degrees of turn in sequence then.. A second 60 degrees of turn Phew!!!

Seems that most on this forum would have you pretend there is no problem and just drive the s*** out of it. Problem solved!

What did you do for cylinders? I would do a leak down test with the engine warm. Do it at both TDC and BDC and record data. I'd also redo the compression test and pay attention to the value thats given on the first "beat" of the tester gauge. I can tell a long by this alone. What did you gap the rings to? What ring compressor was used?

Just looking at your advice jake , what pressure are you looking at putting in the cylinder and for what time period, what percentage loss is acceptable??? Also what values are we looking for to be good on this first beat with the compression test I'm also going to be running these tests soon , any info would be greatly appreciated, I'm sure the OP needs these too!!

with both valves closed put shop air in offending cylinder......if ya feel air coming out exhaust/intake or oil fill.........there ya go.my wag is valve not seated for whatever reason.

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I do leak down tests at 100 PSI, where percentages of leakage are 1:1. The rest of what you asked is another thread, it will take this one way off topic.

Thanks for the responses fellas. Ok so I tried the compressed airline input into the plug and could hear leakage thought the oil filler! A mechanic friend then said its worth a run out to see if it just needs a shake down...... Scary idea i know! I ran a slow 4 miles and then retested the compression pressure. This time it bounced to 100 psi, granted this was on a warm engine not cold. So I guess my next question is could this be a run In issue that I need to keep an eye on over the next few short trips. During the run out the car behaved great with no warning lights or issues. The alternative is stripping and starting again so I guess I prefer the run out idea at this moment I time. Only major concern I do have is hot air passing the rings and damaging the big ends. The cylinders were installed by hartech, who are a well known uk company, they also used the pistons to get it all snug, so that side of things should be perfect. Therefore if there's an issue it's my assembly or install of the rings I guess. Used a standard wrap around tool, not bodging. So in a nutshell, compression better but not right....... What to do???

Sam I also had my blocks done at Hartech, Grant is a top bloke and will help you if you give them a call ! Also was just wondering, did you stagger the ring gaps on the two compression rings to stop blow by? It's not the big end you need to worry about, if you have a lot of gasses passing the rings into the crankcase it can reduce the oil coating on the piston skirts and damage your cylinders so be careful!! I would definitely bore scope that cylinder to see what is going on in there, I also know of another Porsche Independent in Standish near Wigan called Cath Burrows who is also really good if your in the area??

In that case refer to Hartech for all future directives. They know what's normal and it's their baby to support.

ok so I have a bit of an update. Have been travelling so not had much time to work on the old girl. After reading your replies I ran the air compressor test and could not hear much apart from a small weeze via the oil filler cap......... at this point I jumped to the conclusion it was the rings. To be honest a bit of hysteria may have snuck up on me at that point alone in the garage and I was up against it with time due to work! Not a great combination. I began pulling the cams to get the head off, at which point my mate turned up (better late than never) with a leak down tester, he insisted it was worth reading even without the cams installed. Just to know for future reference. The rather confusing result was 10% leakage across all three pots on a cold engine (April in the UK). This was weird as I expected number two to be shocking. After all it was at 50psi when first built! Last thing I want to do it tear it down to find it was something stupid so my plan (at present) is to swap the lifters around and see if that makes any changes........ and then maybe re-time it though I cant see that was wrong as all three would be off if I had the wrong correct?? Speaking with Grant at Hartech, he said they usually identify compression issues earlier so cant comment if the crank case should be blowing out or not.... anyone else know and answer to this? As when it was running the crank case was a vacuum not compressed. Only other idea I had was a leaking valve guide but again I cant see how this would work once the valve was closed anyhow.... In answer to how the compression tester looked, the first beat had 140ish PSI, then as the starter kept cranking the PSI settle lower, and when compression tested at operating temp that cylinder was 100psi so doubled from when cold. Not sure if that tells anyone anything?? I am up for the tear down, but only if I know what I'm aiming for. Don't want to get it stripped and be faced with a faultless engine Any ideas, suggestions welcome no matter how mad!

Install new lifters after pumping them full of new oil.

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First unfortunately I'm pressed for time. Next, IIRC you have run the engine, driven the car? If so and if you heard no scary noises and the CEL remained off and the engine otherwise behaved itself there is probably (probably) no problem. However... I can't seem to get a sense of what the leak down numbers are for all 6 cylinders tested under the same conditions. I have never leak down or compression tested a new/rebuilt engine. I have been around when they have been first fired up and while the engine can generally sound a bit rougher than one would like, or expect, this is because the engine is burning away a good amount of lube/assembly oil that is on all surfaces exposed to the combustion. But this burns off in just moments and then the engine soon smooths out. I dare say it doesn't take minutes either. This is the rings "seating". Now I have never been along side a modern engine when first fired up. I do not know if the CEL comes on, possibly due to misfires due to less the optimum combustion arising from of course the fresh cylinders/rings. The rebuilt engines I was around when they were first fired up did not have any engine controllers or misfire detectors, etc. I do not know how long it takes a modern engine, one of these engines, to settle down after it is first fired up either. I know the engine controller is sensitive enough to detect misfires from an otherwise healthy engine arising from just a bit of dampness on marginal coils. The engine in this case was running a bit rougher than usual. Not much but it was a bit noticeable. So my thinking if there was something amiss ( no pun ) the engine controller would have detected this and turned on the CEL and logged one or more error codes. If you tell me the CEL remained dark then I have to believe the engine controller detected nothing untoward. Which is a good sign. So, where does this leave you? Well, probably not where you would like to be. My advice and advice I have followed myself is if you suspect the engine has an internal problem, is dare I say not assembled correctly, you really should tear the engine down and double check it as you reassemble the engine to know it is right. When I assembled an engine I'd double check the piston and cylinder clearances. I'd check/set the ring end gaps. I'd even check the valve to piston crown clearance just to make sure. Even so, I got bit one time when I failed to ensure all the oil passages were clear. When I had the block in for cleaning the oil galley plugs were punched and in one case the old plug was shoved into its oil galley. When I started the engine after just a few moments there was a ticking noise -- I won't delay the suspense, it was a hydraulic lifter. Oil was not flowing to this lifter. I had to remove the intake manifold, the front engine cover -- it was a Chrysler 318CI V8 -- and fish around and luckily I found and was able to remove the old oil galley plug. After I buttoned the engine up it fired right up and was a quiet and smooth as anything. So smooth and quiet that more than once I tried to start the engine when the engine was already running.

Is this a 3.4 or 3.6 ?

Thanks for the replies fellas. The engine is a 3.4, 2001. The engine warning light did stay off when it ran. There is no way that its going to get to the desired pressure in that cylinder, so I agree it needs identifying (my ***** up) and fixing. I'm just going around in circles now the leak test show even across them all with the cylinders set at TDC. I guess the simple answer is that if the head checks out the only thing it can be is the pistons/rings/bores......

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No that's not how a leakdown test is performed. Nothing to do with time.

When you do a compression test, the needs should always go up, never down! Is your gauge broken? 10% leakdown, if performed properly, is not good but should keep the engine running fairly smoothly. You should do another leakdown when the camshafts are in and timed properly and see if the percentage goes up.

Did you do any valve work when it was apart ?If not and the leakdown test is in the 10 % range as indicated I would lean toward toward the valves not seating correctly (carbon buildup) which might explain the wide fluctuations you experienced when warmed up . I don't think a broken ring or rings would fluctuate that much .