Did a Compression Test

[Tidybuoy]

In previous posts, I mentioned that my car is making a loud knocking noise coming from the cam area.

I believe this may be cause by a stuck oil pressure relief valve as the oil pressure is pegged at 4.5 to 5 bar.

To rule out Valve issues, I did a compression test (the dry type) and all cylinders were virtually identical with all coming in at 210 psi (one was at 211).

My question: is this pressure normal - that seems high but I have no expertise in this area.

I used a craftsman compression tester. I checked with the engine at normal operating temperature. I removed all 4 plugs (they all looked normal). I cranked the engine with the throttle wide open, and cranked about 12 times per cylinder. Each cylinder behaved the same building up pressure in about 4 compression strokes.

I'm going to take the car in and have the belts tightened - it's been 15k miles since they were replaced - of course, I may be having the entire engine torn apart if the relief valve dosen't cure the problem - I may need to get a night job by the time this is over.

Vern

I ordered a new oil pressure relief valve and it should be here tomorrow. I hope this solves the problem but I am still concerned as to why this would have happened.

[Razorback1980]

Hey Vern,

The good thing is that all cylinders are about the same. That is more important than what the actual number is. The good news is that probably means the valves are opening far enough to allow the compression levels where they belong. If a lifter were collapsing on the intake side, the compression number would be lower on that cylinder. If one cylinder were higher than the other three, then you might see a difference on that cylinder but they are all even so I'm thinking the OPRV is the problem.

[Tidybuoy]

I can only hope. I ordered the oil pressure relief valve. I probably could have just removed and cleaned the old one but I ordered a new one anyway.

I'm still a little worried since there is a definate rattling sound as I coast to a stop - this noise does not sound the same as the knocking but sounds like something is loose. After I replace the oil pressure valve, I'm going to take the car into the shop that did the belts (a very reputable shop) and have them retension but also discuss this noise.

I looked on the PCA website to find out what a normal compression reading is and the posts that I found stated that most compression testors are not accurate and will vary from brand-to-brand. The key was comparability from cylinder to cylinder. This is a little comforting as the 210 read on my craftsman seems much higher than any other posts I have read (usually 150-175).

[Dean924s]

Are you sure you dont have high comp pistons or that the head was skimmed at some point?

[Tidybuoy]

My car is completely stock. It has 32k miles on it and I know the orig owner very well. I bought the car when it had 17k miles. The only work ever done on this car is having the belts changed and basic maintenance

[Dean924s]

Hummmm The reason I ask is 210 is getting up around 10:1

[Makis]

210 on all 4 is pretty good and rules out head gasket as well. Try to make sure the noise is not an exhaust noise as I remeber you saying it only happens under load.

[Tidybuoy]

Originally Posted by Dean924s

Hummmm The reason I ask is 210 is getting up around 10:1

I thought the factory up'd the compression in 1988 to 10.1 which is how they squeezed a few more hp out of the engine. Maybe that's why the 210psi readings.

I thought that seemed high when I compare to previous compression postings on this board but I never considered differences in compression ratio. I'm just glad that all cylinders were the same.

The new oil pressure relief valve arrived this morning so I'm going to swap that out tonight. I find it hard to believe that this could be my problem but according to a service bulletin (thanks John_AZ) that could be the culprit. We'll see.

Vern

[Tidybuoy]

Originally Posted by Makis

210 on all 4 is pretty good and rules out head gasket as well. Try to make sure the noise is not an exhaust noise as I remember you saying it only happens under load.

It's definitely valve train noise (and remember the oil pressure is really high at 4.5-5 at all times).

But, it also sounds like exhaust - when the car is cold.

Last night, I listened & listened. It's valve train noise but after the car is warm and returned to the garage - I cannot get the sound to re-occur.

When coasting to a stop, it sounds like a rattle and sounds like a completely different sound - almost like a pulley ready to fall off (I checked em all - their tight) or a clanking exhaust pipe. However, after the engine is fully warm and back in the garage - the engine is smooth as can be, even when revving to 3000 or 4000 rpm.

So, last night I poked around and cannot hear any exhaust leak. I then plugged the exhaust pipe with my hand and I can completely stop all flow of gas from the tail pipe and the car keeps running - doe this imply an exhaust leak.

I really need a helper to listen while I do certain things...............Vern

[looneybin]

Vern, send me a PM if you need any help.
I'd be glad to do what i can
What part of town are you in?
I'm near Van Ness & Shields

[Tidybuoy]

I may give you a call (funny, I'm at Van Ness & Shields too - on Fedora). Don't tell me you're my next door neighbor. What color is your 944...I've seen a gold one, similar to mine, driving around.

I'm going to start with changing out the oil pressure relief valve & changing the oil. We'll see what effect that has.

Vern

[Makis]

Originally Posted by Tidybuoy

So, last night I poked around and cannot hear any exhaust leak. I then plugged the exhaust pipe with my hand and I can completely stop all flow of gas from the tail pipe and the car keeps running - doe this imply an exhaust leak.

You should feel some back pressure in your hand when you do this. If it is very easy to stop the flow then you probably have a leak which could be the cause of the noise. Also if the noise happens when cold and goes away when hot it could be a exhaust sealing issue that gets better once the metal heats and expands. A cracked exhaust manifold can do this as well.

If the noise is oil related then it is likely a sticking lifter doing this. As the noise disappears most of the time I believe valve gear lubrication is OK.

[looneybin]

Ohh... So you're the one making all the noise
An old friend that i used to officiate football with lived on Fedora between Van Ness & the tracks - but he has since passed away.
I'm south of shields on Michigan & my '44 is alpine white.
I've also seen the other gold one, i think it has gold BBS rims on it.
Let me know if you need any help or moral support.

[Tidybuoy]

I'm on Fedora between Van Ness & Wishon.

I'm gonna switch out the oil pressure relief valve tonight. If that dosen't work, I'll call you to maybe just listen for the noise - but, I don't think I will be able to to that for another week as I have to go to LA this weekend to visit with my boss (it's a pain but sombody's got to do it).

I appreciate the offer - either way, I will post the results of the switch tomorrow.

Vern

[Razorback1980]

The compression on my 87 was in the 190 range so if they up'ed in 88, then 210 may be right. Keep us posted on what you find as this is interesting. Never seen or heard of a sticking OPRV. Blocking the exhaust is something you can do for a minute or two but you can't do it too long. Even if your headers are cracked, that wouldn't let enough exhaust gases out to keep the car running for an extended period of time.

[Tidybuoy]

Originally Posted by Razorback1980

Blocking the exhaust is something you can do for a minute or two but you can't do it too long. Even if your headers are cracked, that wouldn't let enough exhaust gases out to keep the car running for an extended period of time.

I only did it for about 15-30 seconds but it surprized me that I could do it at all. This is where I need a second person to listen upfront while I do do that.

We'll see how the OPRV switch effects things.

[Tidybuoy]

Originally Posted by Dean924s

Hummmm The reason I ask is 210 is getting up around 10:1

According to Clark's 944 FAQ:

1986 & Earlier 944 9.5:1
1987 & Later 944 10.2:1
this is for regular n/a 944's the s/s2/&turbo are different

sorry, you were off by .2 (lol). Very good observation

[phoenix_iii]

Originally Posted by Dean924s

Hummmm The reason I ask is 210 is getting up around 10:1

Carbon buildup++ ?

[924Sman]

"I used a craftsman compression tester. I checked with the engine at normal operating temperature. I removed all 4 plugs (they all looked normal). I cranked the engine with the throttle wide open, and cranked about 12 times per cylinder. Each cylinder behaved the same building up pressure in about 4 compression strokes."

.................should be done on a cold engine. Warm will cause the pressure to read higher. Aluminum expands when hot thus creating a higher compression range/reading.

Dal

[Techno Duck]

Remember that your altitude will also play some effect on the #'s. Your #'s at sea level will be higher than that at Mount Everest.

Everytime i have done a compression test it was on a warm engine. Usually i drive for about 20 minutes, then let it cool off for about 5 - 10 minutes. Throttle must be held open also. I think you can expect 15psi lower on a cold engine.

Also not all gauges are made the same. Unfortunetley a good portion of Craftsman tools now days are not always made in USA.

Also as Tom mentioned, compression test really dosent tell you much. Whats most important is all the #'s are within 5% of each other. A leak down test will tell you much more, though you need access to an air compressor.

And i should also mention, the information about compresion ratio's is incorrect on ClarksGarage. '87 had 9.5:1 . The owners manual actually states 9.7:1, but there is no change in pistons or cylinder head design between the early 944's and the '87 to make up for that extra .2 points of compression. Only the '88 received higher compression pistons, at 10.2:1 . Euro spec cars all had 10.6:1