1996 850 NA Compression Test - Looks Good!

[PeteB]

Last time I did this was more than 20 years ago. Followed Robert's Youtube video.
Pulled out fuel pump relay, and disconnected the positive wire to the coil.
Pulled the plugs which were NKG Platinums and they looked perfect, white with a
light tan coating, all exactly the same.
Sprayed some WD40 around the seat and threads just to clean it up a bit.
Can't believe the the first 4 cylinders were exactly the same at 155 PSI, last one
was 160 PSI. Cranked it for 15 "pumps" which is more than the usual recommendation
and had a charger hooked up since the car had sat for a week.
Very surprised, the car is running rough and thought it might be a valve. I would have
called anything more than 120 PSI good so this is very good news.

I used the $25 8 piece tester from Harbor Freight - just turned it in hand tight.

[erikv11]

The readings are all very close together so that's very good news.

However those readings would be also super low for an NA, way too low for a well running motor. They should be up in the 190 - 210 psi range. Anything less than 170 would be big trouble for your car's engine. However obviously the HF gauge is not accurate, they wouldn't all be down so much. So just accept that this gauge can't tell you the actual compression and know that the grouping tells you the engine is uniformly healthy. So I agree: looks good!

Edit: I didn't realize the engine is running poorly, are you sure the timing is set properly on this car? It is unusual to have a brand new gauge so far off. Maybe borrow one from Autozone or any auto parts store (they "rent" them for free) and see what numbers it gives you.

[101]

If the compression test was done with the engine cold it could explain the consistent lower readings.

I do agree with erikv11 - definitely not a burnt valve in your case.

[PeteB]

It was good and hot but it took me 10 minutes to take the plugs out, pull
the relay etc.
It has 177K miles, did the timing belt and am quite sure that it is correct.
Doesn't compression drop with age, that's why there is more blow by?
It might be worn but I'm going to call this good enough. The valve stem
seals even seem good as I don't see smoke under any conditions. My old
Alfa didn't have valve stem seals and it really smoked on deceleration.

I did it dry, no oil in the cylinders.

I tried to "rent" one, Advance doesn't have one for rent, I don't think NAPA
rents at all. Suppose that I could try Auto Zone tomorrow.

[rspi]

Yes, try AutoZone. I met a guy last spring that had one of those gauges, it was reading at least 40% lower than mine. He was getting readings between 90 - 125 psi, my gauge was reading around 180 psi.

[PeteB]

I have an air compressor and am going to just compare it to that gauge.

[erikv11]

You can call it good, no one is arguing that. Sounds like healthy internals which is not surprising at only 177k.

Compression will drop with age, but nowhere near the amounts that gauge its showing. My guess is your engine is actually in the 190s.

[PeteB]

Right, there's nothing we are going to do about low compression since we are not going
to rebuild the motor, at least none are zero or below 100 - that is good enough for me.

I didn't release the fuel pressure so some did hit the cylinders and might wash down the
oil that is there - who knows? An engine leaking oil will get a value closer to the wet
figure. I've not done a lot of this, as I mentioned, but I don't put a lot of faith in absolute
numbers - I could be wrong.

[rspi]

Very surprised, the car is running rough and thought it might be a valve...

Are there any codes? Most cars run rough due to vacuum leaks or failing sensor (MAF or ECT).

[PeteB]

No, no codes, I decide to dig into the PCV job today in the garage, got the
manifold off, took about 2 hours with breaks. The 12mm bolts were the
hardest part, everything else was easy.
The hose at the end of the manifold was cracked, that was probably the
big vac leak. Wouldn't this throw off the FPR vac and as a result the
mixture?