Cylinder Head Rebuild Problem

[megapod]

I have a '71 02 and just had the head rebuilt. Now when I am on 4th gear, reving at about 4000rpm, and let off the accelerator and then press on again, a big cloud of blue smoke blows out the tailpipe. I read that this is usually caused by worn valve stem seals or worn valve guides or shafts. The things is that all 4 exhaust valves were replaced and the seals were also new. Any suggestions before I call up the shop that rebuilt the head? There is no smoke on acceleration.

[PSloan]

Thats the tight head pushing oil through the worn rings.

[megapod]

thanks for the reply. wouldn't worn rings create smoke at all times, if not on accelaration at least?

[Weasel]

yes. oil is burnt through the valve stem seals during periods of vaccuum, aka decel. a quick compression test will tell you whats really going on though. do a compression check the proper way, than add a little oil to each cylinder before testing it again. if you get a higher reading, its the rings as the oil is helping it seal a little better.

[Biscuit3]

If you smell gas most of the time, that is another indication of worn piston rings. My Karl is about to go through a engine rebuild him self. There are lots of indicators of bad valves or rings. Blue smoke is one of them.

[Guest Anonymous]

Hey, Ash; gasoline smell usually means a fuel line leak or that the carb is also very worn out. a short blast of Blue smoke on accelleration indicated new valve guides are needed (use the guides and stem seals from a 318i) and a relatively steady trail of blue smoke indicates worn out rings and cylinders. A puffy exhaust and loss of smooth idle and power usually is caused by a burned or sticking valve - rev the engine up to ablut 3000 rpms and pour either Marvell Mystery oil or kerosene/paraffin oil slowly through the intake for about 10 minutes.

Hope this helps

[ifayeoh]

and now you need to do the bottom half. classic issue when only half the engine is refreshed.

[Biscuit3]

That's right. I have no idea what I'm talking about...

[Brandon]

Smoke on deacceleration is generally always worn valve guides. Did you have the guides changed during the rebuild? You don't say that you did in your post..you only mention the valve stem seals. If the shop who rebuilt your head did not replace the guides, then they do not know what they are doing and I would go elsewhere....

[Mike Self]

157k to repair a leaking coolant passage, so went ahead and did guides, seals and ground the valves. Prior to that it was using less than a quart of oil between 3k changes; afterwards oil mileage dropped to about a quart per 800 miles--all going past the new weak spot--the rings.

But, at 218k it's still getting about 400 mi/qt and runs great. I'm just careful not to get on it too hard unless I'm fogging for mosquitos...

mike

[winstontj]

I have a '71 02 and just had the head rebuilt. Now when I am on 4th gear, reving at about 4000rpm, and let off the accelerator and then press on again, a big cloud of blue smoke blows out the tailpipe. I read that this is usually caused by worn valve stem seals or worn valve guides or shafts. The things is that all 4 exhaust valves were replaced and the seals were also new. Any suggestions before I call up the shop that rebuilt the head? There is no smoke on acceleration.

It's been covered many times and in this thread there's conflicting info so i'll give it a shot. Smoke on accel is rings, smoke on decel is valves. Smoke at idle or neutral throttle (like when you let off then get back on) could be both or some combination of everything.

If valves are bad it's usualy seals but can be guides too. On intake stroke or when decel creates a vacuum inside the cylinder, oil is pulled past the valve seals down the guides and into the cylinder. It usually takes a few seconds to pull oil through the motor, exhaust and out to where you can see it - try a long decel down a hill or on an off ramp. When you hit the gas you'll see a puff behind you. This is Valves - either seals or guides. Bad guides and good seals usually don't happen it's either just seals or both seals and guides but not usually just guides.

Rings - smoke on accel because as the piston moves up and down oil slips by the rings into the CC and creates smoke. If it's really bad you'll just smoke at idle and all the time. This happens when the rings are worn enough that oil is not scraped off the side wall of the cylinder because the rings are so worn.

When a motor is "bad" and a top end is rebuilt the bottom end's problems now come to light. Remember a few things, first, you've just created an unbalanced motor having a strong new top end but old tired bottom end. Second, there is a break in time. What you describe sounds like valves not rings. It's completely possible that things will wear in and in a few hundred/thousand miles things will get better. It's also possible that your mechanic did a poor valve job, did not replace the guides or seals. Look at your bill. Do you see a list of valves, guides or seals? Seats?

I'd give it a little time, use proper oil... I forget where you are but if it's not too cold stick with a heavy weight oil because it will burn/smoke less. Don't be so quick to jump down your mechanic's throat. Look to make sure new parts were used and then look for proper oil. Drive it for a change of oil and see if things get better.

As a precaution you may want to let the mechanic know that the motor is smoking and you are going to try a few things - this way it won't be a shock in a month or two when you come back.

HTH,

TJW

[megapod]

thanks again for all the replies. I checked the compression dry and wet and while the pressure increased wet, it increased only about 10-15psi in each cylinder. I have been suspecting that this has something to do with the valves. the exhaust valves and seals are new, but guides are old so that might be the problem. could it be sucking oil from the intake valves??? Those were left as is, with only the seals changed out. I'm located in SoCal, so I will try a heavier weight oil and see if it wears in. Right now i have 5w-30 and have not burned any noticeable amount (again leading me to doubt the rings are the problem). Did call the machinist up and he seemed to doubt it was his work, but we'll see how that turns out. For future reference anyone know a good outfit to rebuild the bottom end of my engine in the LA area (at a decent price...)?

[Beaner7102]

When you replace the valve seals/guides you MUST replace BOTH the exhaust side AND the intake side. You paid for half a job, and have the result of a smokey car to show for it. Why was only one side done? If it was under the recomendation of the head builder I would ask for it to be done properly for $0, as his 'recomendation' was wrong. Beaner7102

[megapod]

When you replace the valve seals/guides you MUST replace BOTH the exhaust side AND the intake side. You paid for half a job, and have the result of a smokey car to show for it. Why was only one side done? If it was under the recomendation of the head builder I would ask for it to be done properly for $0, as his 'recomendation' was wrong. Beaner7102

actually, the seals were replaced on both sides, but none of the guides were replaced. only new valves on exhaust side. this guy rebuilds plenty of bimmer heads and has an 02 that he restored. thought he knew what he was doing.

[Lee]

When rebuilding a head, change all 8 valve guides with the new style ones and the smaller stems, allowing to install the M20 (I believe) valve seals. Also change all 8 valves, unless you have a bin full of valves and spend an hour or two measuring the best ones.

If the head is done properly, it will show you how weak the bottom end is. The bottom end will probably let go soon.

A head cannot be done on a cheap budget, or surprises will always pop up...