I have a '95 850 GLT Wagon with 114 K km. I tried to start it 3 days ago and it began to turn over and catch, then there was a soft "bop" noise and the engine began to rotate like it had no compression. I've dealt with broken timing belts (2) on a 740, so suspected that's what I'd find along with up to 20 bent little valves. I took the compression on all 5 cylinders, it varied between 35 PSI to 60 PSI. I checked the timing mark alignment on the camshafts and crank sprocket - they seemed OK and lined up although the crank sprocket mark was difficult to determine. I took the belt off and it is in pristine condition - I replaced it 30 K km ago. The belt is like new, there are no cracks on the bottom of the cogs and no chips. Since the belt was not broken I thought that it may have slipped a few cogs. I took off the belt tensioner, the rod that connects to the tensioner pulley is exposed from the top of tensioner body by 7/16"(measured from the metal top of the tensioner), see attached pic. Questions: Should the tensioner rod be pushed out further by the piston, providing more tension to the belt ie. is the tensionser toast? Are there any other causes that I should be looking into before taking the head off tomorrow? I will take the head off regardless, but wanted to problem solve along the way. Thanks for any ideas or suggestions.

 

Not sure how far that should be out, the important thing is if it was putting the proper tension on the tensioner roller to keep the belt tight. You can look at different timing belt instructions to see if you can tell how other tensioners have been when removed (if they posted pics). The big thing is to test it. You can check my instructions, step 16. Mine looks the same.
Timing belt replacement instructions on a 1997 Volvo 960

Most cars get burnt valves, which causes low compression. I have heard mechanics say that running low octane fuel causes carbon build up and burnt valves. What octane gas/fuel do you use? Another cause could be a blown or leaking head gasket and worn piston rings. If you have a leak down test done it will tell you which problem you have (so I've been told).
Compression Test - Volvo S70 GLT, 1998

 

The tensioner looks normal. I am not sure what the compression should be either but for a turbo its like 160 to 180. I would expect a N/A to be higher. I think pulling the head is a good idea.

 

It seems odd all 5 cylinders would lose compression at the same time. There would have to be a "weak link" someplace between the crankshaft and the camshaft for all of them to drop that low at the same time.

When you go to take the head off, but before you do I'd check the key and key way on the crankshaft and how the gears are held to the cams. If this was a normal running engine before the "bop" and the belt didn't jump I would be looking at a gear moving on the crank or cam as that is the only thing that I can think of that can change the timing of all five at once and drop the compression so radically ??

 

Maybe we should get the specs on an N/A and see what the compression is supposed to be.
Also, there is the possibility that the compression tester was leaking so the measurement was not accurate.

I agree with improbability that all 5 cylinders go out at the same time but we dont know what it was before, could have always had really low compression. Nothing explains that bop though.

 

From the "Book" for a 95 NA

ENGINE COMPRESSION
Warm engine to normal operating temperature. Disconnect
single wire from negative terminal of ignition coil(s). Check
compression with all spark plugs removed, throttle valve wide open and
at normal cranking speed (250-300 RPM).
CAUTION: Failure to disconnect ignition coil negative terminal may
cause damage to ignition system control unit(s).

(That or just disconnect the coil lead at the cap and ground it)

COMPRESSION SPECIFICATIONS TABLE

Application Specification
Compression Ratio ................................. 10.5:1

BUT .... darn thing didn't give a spec for compression test

This is a "rule of thumb" and turbo or not it should apply.

As a rule, most engines should have 140 to 160 lbs. of cranking compression with no more than 10% difference between any of the cylinders.

If you're using you own compression tester and you know it's good .... but if not you might want to try someone else's or get a loaner from a parts store just to double check.

The fact that it doesn't start makes me think your numbers are correct and the way to find the problem is to start to dismantle it. I wish I had some other advice to give you.

 

Off the subject, I recently seen a TV show that said that any motor with a compression ratio of more than 9.5:1 should really use 91+ octane. Looks like the turbo motors only have 8.5:1 ratio, that would allow us to use lower octane gas without the chance of knocking.

 

Let me know how much that used engine is

That would be without a blower or turbo. The reason the pressure is lower is because the turbo makes up for it. Stick to premium or you're counting on your knock sensors to keep from ventilating your pistons!
And if they don't retard soon enough ... you're screwed!

 

FYI:

// from 1995 850 NA Manual:

MECHANICAL INSPECTION
Compression
Check engine mechanical condition using a compression gauge,
vacuum gauge or engine analyzer. See engine analyzer manual for
specific instructions.
WARNING: Do not use ignition switch during compression tests on fuel
injected vehicles. Use a remote starter to crank engine. Fuel
injectors on many models are triggered by ignition switch
during cranking mode, which can create a fire hazard or
contaminate engine oiling system.

COMPRESSION SPECIFICATIONS TABLE
Application Specification (Minimum)
Compression Pressure ....... 189-219 psi (13.2-15.3 kg/cm )


// from 1995 850 Turbo Manual:
Compression Pressure ........... 156-185 psi (11-13 kg/cm )



P.S. Don't believe anything you see on TV and even less of what you find on the web..