Smoking Truck

[Mmoore031908]

I have an 1985 k10 with a 305. It has roughly 150000 miles. It has been smoking on startup since I got it. It hasnt been much smoke just a puff but it smokes until the truck has started moving or is at normal operating temperature. And then just stops. I feel like it is the valve seals but am really not sure what it is. Truck has really great power. (Doesnt seem to be losing compression or anything. I have not tested that yet just looking for some insight.) It will smoke at idle or when revved up be before it is being driven or before its warmed up but will stop after I start driving it or its has reached Normal Operating Temps.

 

[highdesertrange]

without seeing the truck and the problem it does sound like the valve guides/seals. has the motor been rebuilt or is that 150,000 total miles. highdesertranger

 

[Mmoore031908]

Its 150000 original miles on the original 305. Thats what i thought. I want to go ahead and replace seals but im wanting to know if there is a way to check piston rings.

 

[highdesertrange]

well that's like just over 5,000 miles per year, not many. did the truck sit for years? you can kind of check the rings by doing a compression check if you are low in one or more cylinders squirt some heavy oil into those cylinders(50w) if the compression comes up it's the rings if it doesn't it's the valves. this test is not 100%. have you done a compression check? highdesertranger

 

[Mmoore031908]

I have not done a compression test yet. I will try to get that done this week. An old man had it and an upholstry company bought it from him and they drove it once or twice a week it may have sat some but I dont know. I am also looking for oil advice. The previous owner said that he used mobil 1 synthetic 10w-30. Im pretty sure it has used conventional until he used synthetic. Would that make it smoke more than normal and can I go back to conventional oil?

 

[chengny]

First, as HDR says, do a compression test.

Once you have a compression test, you can then make an intelligent decision on what your motor needs instead of throwing parts at it.

A better method of determining engine condition is a leakdown test. A leakdown test will give you a more accurate idea of how good your compression is.

And, if compression is found to be low, a leakdown can also help determine whether the it is the rings or valves (or both) that are worn.

But (and these are general guidelines - not always the case), without doing any tests and just going by what you observe during daily operation:

If your engine runs well - the valve seats are still tight

If it burns an excessive amount of oil, the rings are worn

If it runs well (and only needs a quart of oil every few months) but smokes heavily (usually blue) for a short time after a cold start - your valve stem seals are leaking.



That last one - leaking stem seals - is the easiest and cheapest to remedy. They can be replaced without tearing the engine apart - or even pulling the heads. The job can be done with just the valve covers removed.

If it is determined that you only need to do stem seals, come back. There are some very knowledgeable guys here who will help you.

The previous owner said that he used mobil 1 synthetic 10w-30. Im pretty sure it has used conventional until he used synthetic. Would that make it smoke more than normal and can I go back to conventional oil?

The use of synthetic oil (Mobil 1) does not cause excessive smoking. And, yes, you can go back to running conventional oil without any problem.

It is going to be stem seals - trust me.

 

[rich weyand]

It is going to be stem seals - trust me.

Agreed. BTDT.

 

[Mmoore031908]

It runs strong but shakes some at idle but still has sufficient power no hiccups during hard acceleration and no puffs of smoke during hard acceleration. Why would it smoke until the truck was warmed or started moving and not just one puff or so at start up? I have probably put about 750-1000 miles on the truck since I have had it and the oil was about 1/4 of an inch above the full line on the dipstick after the miles it is about 1/4-1/8 inch below the full line. It smokes pretty heavy at start up and revving during idle until at normal operating temp or I start driving it. I also read somewhere that getting the rpms high and then letting off the gas will create a high vacuum situation and it should smoke some it did not do that either.

 

[Driver4r]

If your trucks smoking, atleast try and cut it down to 1 pack a day

 

[rich weyand]

It runs strong but shakes some at idle but still has sufficient power no hiccups during hard acceleration and no puffs of smoke during hard acceleration. Why would it smoke until the truck was warmed or started moving and not just one puff or so at start up? I have probably put about 750-1000 miles on the truck since I have had it and the oil was about 1/4 of an inch above the full line on the dipstick after the miles it is about 1/4-1/8 inch below the full line. It smokes pretty heavy at start up and revving during idle until at normal operating temp or I start driving it. I also read somewhere that getting the rpms high and then letting off the gas will create a high vacuum situation and it should smoke some it did not do that either.

Hot oil slowly leaks down into the intake and exhaust runners when the engine sits overnight. When you start it up, much of the oil sitting in the runners is against the valves and gets sucked in (or blown out) and burned. Lots of smoke. But there is still cold oil in the intake runners that will be washed into the cylinders by the mixture as the engine warms up, and some of the oil that gets blown out the exhaust runners will coat the inside of the exhaust manifold and burn or vaporize as the manifold heats up. High vacuum situations won't cause smoke because the leak is so slow; it only occurs overnight with hot oil draining down by gravity. As it gets worse, the leak will pick up, and then you may get smoke in high vacuum situations.

 

[chengny]

Why would it smoke until the truck was warmed or started moving and not just one puff or so at start up?

It smokes only when cold because the valve stem seals are worn.

Valve stem seals - as the name implies - seal the gap between the valve guides and the valve stems. When they are brand new, they have a tight fit to the OD of the valve stem. This seal effectively prevents engine oil (specifically the oil that lubricates the rocker arms and pivots) from leaking between the valve stems and their associated valve guides.

Any oil around a loose stem seal will be drawn into the combustion chamber by the vacuum created during the intake stroke of the piston.

Smoking at cold start - the reasons that it clears up as the engine warms are mostly because of:

1. Excessive stem seal to valve stem clearance. As the stems heat up and expand, the gap is reduced and slows the flow of oil into the combustion chamber.

2. The viscosity of the engine oil is reduced and flows more easily as the motor heats up. This allows the oil that comes out of the tops of the push rods (where it is used to lubricate the rocker arms) to move quickly away from the valve stems and flow back down into the crankcase. When the engine is first started, and the oil is cold, it has a higher viscosity. It tends to linger around the valve stems rather than flowing away from them. Any puddled oil in the area of a worn valve seal will be pulled into the combustion chamber by the intake stroke vacuum.

 

[rich weyand]

I like my explanation better. I have always had the most smoke when starting after sitting overnight as oil drains down the valve stems into the runners. When I pulled the head on my old 195 six, back in the day, it had oil in the runners.

BTW, the part about the stems heating up and expanding, reducing the gap, doesn't exactly hold water, because the guides heat up too, so the holes expand as well. The gap doesn't change, except perhaps that the exhaust valves heat up more than the guides, so expand more than the guides, closing the leak, and the intake valves heat up less than the guides, and expand less than the guides, opening the leak. Since the intake valves are the ones with vacuum, that will leak oil while running, so I guess there I agree.

In any case, regardless of the details of the mechanism, chengny and I both think it's valve seals. The burst of smoke when starting, then the smoke gradually going away as the engine heats up, is characteristic of the problem.

 

[bucket]

I have an 1985 k10 with a 305... Truck has really great power.

This doesn't make sense to me, lol.

One reason for smoke only on startup is it's just the smoke you notice. It probably smokes all the time, just a much lesser amount that is hard to see. While the engine is sitting, the oil leaks into the runners and just gathers because it ain't running. Then when you start it up, it burns off that oil all at once. It continues to leak oil after that, it's just such small amounts at a time that you don't notice it out the tailpipe.

Also could have the oil drainback holes in the heads plugged up with gunk, causing oil to pool around the seals.