I have a 1991 d150 p/u and it has blue smoke from exhaust when started

Blue smoke is oil being burned in the cylinders with the gas.

You need to determine:
How bad is it?
Where is it coming from?

It could be anything from not too serious to your engine is on its last legs.

On the "not too serious side" is a chance that it is bad valve seals. If this is the case, your oil consumption will likely not be high and the blue smoke will likely be mostly right when you start the engine.

On the "really bad" side, it could be the piston rings, or a cracked block or head. This would mean the fix is either an overhaull or an engine replacement. On the positive side, this is a relatively easy engine to overhaull and good used engines are readily available. 88-91 1.5L Honda Engines are interchangeable. If it is one of these, you will probably see blue smoke all the time and maybe more when accellerating. Oil consumption will likely be high (you will have to add oil often.)

Start with a compression test. Are all the cylinders about the same or is there a big variation? Do it with the engine warmed up and at normal operating temperature.

When you pull the spark plugs, for the compression test, look at each one. Do they all look about the same or is one or more fouled with oil?
You should see nothing liquid on the "business end" of the plugs and if you see oil, you have a problem..

If one cylinder has low compression, then you may want to charge the cylinder with it set to Top Dead Center (TDC) on the compression cycle with compressed air or nitrogen and listen to where it is leaking.

If it is hissing out the exhaust, it is probably an exhaust valve or seal.
If it is hissing in the block, listen where you put the oil in, then it is probably rings. If it is hissing out the tube that connects to the air filter, it is probably an intake valve or seal.

If all cylinders are low, it is likely worn out piston rings.

Another check that will help identify the problem is to squirt some oil into the spark plug hole of all 4 cylinders and recheck the compression. If it comes up significantly, it is likely rings.

Good luck. Blue smoke in the exhaust is one of the things you should always look for when buying a used car. Another simple thing is look at the air cleaner - if you see any oil in the air cleaner housing - walk away. (it is from what is called "blow-by" and means the engine is worn out). Take a look at the dipstick at see if the oil looks like oil and has no milky or foamy look and does not have a strong burned smell to it. When the engine is running and warmed up, look at the coolant container. Are there air bubbles coming up? Yes means blown head gasket or cracked block. Also look for oil mixed with the coolant (same thing). There are a number of easy checks you can make that can save you from buying someone else's expensive problems. (or at least know what you are getting)

Blue smoke in the exhaust is always a bad thing, but not always a catastrophic thing. If it is a small leak from a valve seal, and gives you a little blue smoke at startup, you may be able to ignore it for many tens of thousands of miles. If you see a lot of blue smoke and all the time, expect the spark plugs to foul out soon, and the car may not even start in cold weather. Also expect to add a lot of oil or else have the engine run out and self destruct. If the oil light comes on (and the temperature starts to go up quickly) the engine bearings are starved for oil and it is about to become a boat anchor.

Black smoke is excess unburnt fuel. Blue smoke is oil being burned, if blue the valve stem seals may be allowing oil to seep down over night.

Old spark plugs were likely fouled with oil and not burning fuel (and oil) completely so minimal smoke before change out. New plugs are clean and burning everything in cylinders well including oil (therefore more smoke). If smoke from exhaust is blue, then engine oil is getting into cylinders either through worn out piston rings, or a leaking gasket. If black smoke, then this is a sign of too much fuel getting into cylinders and not all the fuel is getting burnt during combustion (a fuel system problem).

starting problem ,takes 4-5 times turning over engine then blue smoke comes out of exhaust ,is very reliable when got started though -
ive replaced battery/glowplugs-&-relay/
a recovery man says no power to glowplugs ?
can you advise ?

You may notice when fired up blue smoke that fades when warm, oilrings worn..

Most likely valve seals or piston rings. Best doing a compression test to find out.

I question the thinking of your mehcanic, the exhaust has odorless cumulatively POISIONOUS Carbon monoxide, smell testing went out in the 50s.

The questions are: is it consuming oil, is it running rich, any error codes? How is fuel mileage? What are the exhaust readings? The answers to these dicate the next step.

You mean lots of smoke? White, blue, black? Or just smelly stuff? Rotten eggs or shotgun shells, usually an oxygen sensor. I will give you the rundown, white smoke when first starts up is gas, should clear up in a minute.No big deal, normal. Black smoke, carbon, burning rich, settings are way off, computer is not right, oxygen sensors or air flow sensors are bad. Blue smoke is oil, you have lost comp and oil is getting into the cyclinders. Steam, comes out after it warms up and is a blown head gasket. You can look for oil in the radiator. A radiator shop can tell you if it is blown or not. Hope this helps.

my engine check light on 4age engine dim before start, will not go off after started and slump throttle black smoke from exhaust

blue smoke out of 1.8 turbo its allways bad oil seal inside turbo.need new turbo or rebuild OEM. with 24 years experience working with vw never seen single headhasket go bad on 1.8 turbo engines.blue smoke its 100% turbo every time