A friend is looking at an 850 turbo. We did a compression test, with no idea of what the numbers should be but simply to see if they were all even. They were, but they were low.

I knew a turbo would have lower compression, but all cylinders were between 105 and 115. My first thought was that the rings are shot, but it runs well and doesn't smoke at all. Is this a normal range? If these numbers are low, but there is no smoke to indicate bad rings, what should I be looking for as the cause?

Thanks

 

Hmmmm, that sounds a bit low. Can you do a wet test and see what you get?

Here are the specs for a Turbo engine.

 

Yes, below 140 psi is too low; should be in 150-165 range. Is this a known good compression gauge? I've seen the schrader valve get gunked up and then show low readings. Test the gauge on known good engine and see what you get.

If gauge is good, then yeah, that's low.

 

I'll check the gauge, and do a wet test tomorrow.
Thanks

 

Yeah I did mine before i bought my car. I was between 155 and 160 on all 5 cylinders.

 

My first engine had low compression. Resulted in oil leaks everywhere do to high pressure in the crank case.

 

I performed this tonight, once dry and 2 times wet.
here are my numbers, not good as chart says but car isn't having problems

140, 120, 120, 130, 140__ looks like bad head gasket between cylinders 2,3 maybe 4

130, 149, 150, 149, 150__ looks like rings aren't too much better off

149, 150, 130, 150, 149__ now i feel like we're playing the lottery

tried a 4th time but i put in too much oil squirted out the gauge everywhere. took 15minutes to burn off


What the hell is the variation? My bro was watchin the gauge and recorded everything but how long should you be cranking for? it wasn't clear so i went as long as 20 seconds or till he said it peaked. Rings, and HG need replaced?

 

I don't know; how much do you trust your gauge?

If you do, oil in the cyl seemed to improve the readings (except for #1) which would point to rings like you said.

But if it were me, I would get a 2nd opinion and ck with another (preferably known good) gauge. Or you could ck that gauge on a known good engine.

And yeah, just crank until the pressre peaks.

BTW: I hope you are using a remote starter button, or if using ignition key, have disabled the ignition? And please don't tell me you just pulled the secondary wire out of the coil...

 

its a harbor freight gauge, soooo. yeah.

I followed a writeup on MVS, pulled the fuel pump fuse, and ignition coil. cranked the ignition from the key... did i fubar something?

 

Hey, dumb question... when you crank the engine? You just turn the key? (Of course, once you pull the fuel pump fuse and ignition coil wire.)

 

you could even go overkill and disconnect the fuel injectors, but yea, it'll turn over really slowly one revolution at a time. And cover the other open plug holes when you do it. like i had bugs trying to fly down into my cyclinders, little f***ers

 

Everything I've ever bought at HF is junk and i end up throwing it out within 6 months after buying it. I would be very suspect of your readings now!!

Do you have a link for this writeup? MVS is a great resource but I like this site better; more user friendly (and better advice..).

When using the ignition switch to do a compression test you need to disable the primary side of the ignition coil; easiest way to do that is disconnect one or both of the primary (small diameter) wires to the coil. The heavily insulated secondary coil wire (that goes from coil to the center of the dist cap) is carrying upwards of 20 KV!

If the primary circuit is still active when you're cranking via the ign switch and the secondary coil wire is pulled out of the coil, then that high voltage inside the coil has no path to ground. The secondary voltage can build to such extremes it will eventually break down the coils internal insulation and arc to the nearest ground. If this happens your coil is toast; need new one.

 

Yea, i wanted someone else to pressure test it when i change the plugs. Figures HalFass

Thanks for the tip on the ignition. I'll be replacing it for the ipd one soon but it still works fine for now.

Could the valves have anything to do with getting bad compression readings? I ask because i have several valves tapping but oil is near full. If i decided to do the a HG job, would damage be noticeable if they needed replaced?
You seem almost too knowledgeable of the information gdog. thanks

 

valves wouldnt have any bearing on compression unless they were bent, and if they were bent, youd notice that IMMEDIATELY on driving. When you put compression into a cylinder, the tendency is going to be for the compression to press the valves harder into the seats. If you did a head gasket, i'd send the head off for inspection. Shops have the equipment to detect things that you or me could not. You're going to want to do that anyway just to make sure that it is still square.

 

Hmmm..... I wonder why people do valve jobs?

Gee.... I must have spent a whole lot of wasted time (hours) hand lapping all 20 valves, once with course grit and then with fine grit lapping compound, to get a perfect seal between the valves and the seats.

Hmmm.... maybe carbon deposits after 15 years of use.

 

How much extra power did you see from all your labor, bobec?

 

No offense dude, but you're out on thin branch on this one...

Anyone that has done any engine OH work has seen many a non-bent but carboned up and pitted valve and mating seats leak like sieves, and yes, that will certainly cost you HP!

To do it right, R&R all the valves, clean them up and inspect for pitting; if not too bad, can be cleaned with valve grinding machine (assuming these valves can be cut; not sure). If not, just replace w/new.

Then same with the valve seats; you really need to know what you're doing here (i.e. machine shop that's familiar with the proper angle specs, etc.) Then you can lap the valves into their specific seats. I always test the seal w/fluid to make sure you got a nice tight seal; this is what gives you optimum power...

 

Some of your hydraulic valve lifters could be shot and need replacement.

 

wouldnt argue with you there gdog, just saying that it's not going to be the most likely cause of bad compression

 

lots, it runs real strong.

but I also honed the cylinders, bought .010 over rings and cut the gap myself, rebuilt the turbo, put in new bearings... so how much did the valve work contribute?

My main objective was not so much power related but to refresh the engine so something like this wouldn't happen (see post #18)

https://volvoforums.com/forum/showth...chigara&page=2

I depend on the car and don't want it to have issues when I don't have time for them.