Mr. Perrott,

You write. "... too rich of a fuel burn ..."

If the fuel to air ratio is too rich, you may have an engine control issue. Why I suggest you check the OBD-II for fault codes on all control systems. Also, if the engine is burning a rich fuel to air ratio, you'd see black smoke out the tail pipe. Most any cause of even a mild fuel to air ratio imbalance would set one or more engine control system fault codes, and very likely the check engine light (CEL) illuminates and remains on.

Such a fuel to air ratio imbalance at engine combustion, either too rich or too lean, shall ruin the catalytic converter. You may also not be able to pass MO-state emissions.

Yes, please review all of KlausC' suggestions. A compression or, more thoroughly, an engine leak down test can reveal the cause of engine performance issues.

If the engine is burning a rich fuel to air ratio, it would not effect engine oil consumption unless really excessive, like a leaking fuel injector (doubtful). If the if the engine is burning a rich fuel to air ratio, you'd smell the excess fuel in the oil at the oil fill cap or oil on the end of the dipstick tube. (Do this on a cold engine. Engine oil should not smell of gasoline. Unless a faulty control system or component or through motoring use as in how you use the vehicle.)

Yet such concern may call into question how you drive your 1990 Volvo S 70. If you regularly commute, so the engine and engine oil fully heats, combustion by products, like water, unburnt fuel, and other hydrocarbons products, boil out of the engine oil, and get consumed through the PCV system. On engine intake vacuum, as it rises, when you maintain a light and even pressure on the accelerator pedal, or (if fitted) employ cruise control, at a sustained freeway speed, the throttle is about closed, raising engine air intake vacuum, so combustion vapors from the engine oil sump are consumed by the engine through the PCV. Though somewhat unrelated, the EVAP cannister check valve opens from both the raised engine vacuum and / or the positive pressure in the fuel tank, while driving, fuel sloshes around in the fuel tank, and fuel tank temps rise.

If you perform short trip motoring only, on the streets of St. Louis, that does not allow engine oil to get fully heated, and you drive this way exclusively, your 1990 Volvo S 70 can suffer, the PCV will clog, and sooner, and you'll form other emissions issues. Using dino (not synthetic) oil exacerbates the issue.

You can also form clogs in the throttle body, too, at the vacuum inlet pipes for PCV.

The engine cover? The engine cowl with the Volvo logo and engine model name on it, or it says something like 20-Valve in white lettering? So you can see the five spark plugs wires, and ignition power stage on each wire at the spark plug.

Road grime can naturally collect in the depression accommodating the spark plug.

If the spark plugs were not properly torqued (not too tight, that's aluminum thread in that cylinder head!), the interface between the sealing washer at the spark plug body, at the top end of the thread on the spark plug (opposite the bottom end with the spark plug electrode), may not be wholly sealed. Too low a torque value, or the spark plugs have been removed and replaced too many times, flattening or distorting the sealing metal washer on the spark plug, or, someone installed spark plugs too tightly, stripping the aluminum thread in the cylinder head to cause blow by, but not ejection of the spark plug from the spark plug hole with stripped thread.

You don't mention these problems, tough. If you've removed the spark plugs, how did they appear and smell? Normal? Dry black carbon is too rich a fuel to air ratio. Shiny, oily black stuff on the spark plug electrode end could indicate, as KlausC states here, a frozen oil control and / or one or both compression rings. So, please try KlausC' suggestions before cracking that engine open!

And try the OBD scan check.

You may be able to remove the PCV pipe from the engine air intake side. At the PCV sieve, flame trap, or check valve, just down stream, are one or two pipes .... well, i found a diagram using the not so useless Bing image search ...



This diagram may be for Turbo-equipped S 70. I'm not sure.

Looks like a Volvo diagram. On a cold engine .... Verify the dip stick tube is properly installed into the oil dip stick tube. You may want to remove the hoses where they connect to the engine air intake and, after cleaning, blow through these to check for clogs that are partial or complete. Maybe stop one end of one hose and blow through the other hose you removed from the air intake. Remove the oil fill cap. You should be able to blow easily through the large vapor PCV line that connects to the air intake at or just before the throttle body. (Use something to stop of the other line that come out the top of the PVC breather box / oil trap. Conversely, replace the oil fill cap and blow again. You should encounter great resistance as you exhale forcefully through the PCV vapor line pipe at the throttle body end. With the engine oil level dipstick properly sealed in the dipstick tube any air you hear moving should be inside the engine, usually through the piston rings. Remove the oil dipstick tube and exhale again, it should be easier, though oil can come out if your 1998 Volvo S 70 is not resting on a flat surface.

I then put a small plastic bag over oil crankcase opening and it did not have a vacuum. And the oil dipstick was in the dip stick tube when you did this?

You may want to inspect the air intake system, from the air filter, down stream, for vacuum leaks. Again, a fault code may be set of an air intake leak is more than the Bosch Motronic control system can compensate. It only costs a little time to check for OBD-II fault codes.

Do you have engine oil leaks at the lowest seals around the front and rear of the engine? Another symptom of a clogged PCV system is the engine oil level dip stick will lift up after spirited, or hard driving. Though your engine may have a catch to secure the engine oil level dipstick in place. I forget right now.

I hope you have a trust-worthy independent mechanic service. Some bad actors in St. Louis that repair Volvo autos. Check with the BBB, and with one giant salt grain, the opinion web sites.
- https://www.brickboard.com/SHOPS/
- http://www.volvomechanics.com/
- http://www.mechanicsfiles.com
- http://www.cartalk.com/mechanics-files

If you feel you are able, you could perform an engine compression test yourself, unless the quote you receive from your chosen independent Volvo auto repair service is acceptable to your wallet. An hour and no more than two hours of labor. They can check spark plug condition and reset spark plug gap, if you use such spark plug types.

I think that does it. I hope not to confuse you. Please see KlausC' suggestions.

Questions?

Thank you,

John D. McGurk's Irish Pub, 1200 Russell, Soulard, St. Louis MO.
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