To Do or Not To Do?

[Will308]

I'm at a crossroad with my 91 525i. I've been chasing a rough idle/ miss problem for a while now and so far no-one has been able to diagnose the source.
I have it at the shop currently. ( not the shop I normally used) to check it out. I felt a fresh set of eyes and ears might help. They went through the ignition system( coil packs, boots, plugs) and the injection/intake systems and came up with nothing. Everything is working properly.
The tech is recommending a freshening up of the head. Remove it, machine it, change the valve seals, clean and lap the valves, new head gasket, etc.
He's telling me the cost to do this would be approx. $ 1600.00. He's 95% sure this would cure the problem, but not 100% sure. He thinks the valves may have some carbon buildup causing them to not fully seat.
Right now the car does burn some oil, about 1 quart every 2000 miles. But I don't have any white smoke comming out of the exhaust system or oil in the coolant. No real problems other than the idle. It's more of an annoyance than anything.

My question. Given the value of the car and the cost of the repairs do I just drive it until it really needs the head work, or pony up the money for the repairs hoping it corrects the problem?

Thanks in advance!

[tim s]

there is several sensors, crank, cam, throttle position, air flow meter, temperature. you need to thoroughly test their resistence & see if it meets spec. it could also be vaccuum leak, what is the compression test results? i just spent $800.00 on my 540i/a trying to find why it has a slight mis & runs rough up to 1300rpm's & smooths out around 2000rpm's. mech. is now telling me he thinks it is the torque convertor, but the dealer says the idle is almost perfect.
tim s.

[Will308]

there is several sensors, crank, cam, throttle position, air flow meter, temperature. you need to thoroughly test their resistence & see if it meets spec. it could also be vaccuum leak, what is the compression test results? i just spent $800.00 on my 540i/a trying to find why it has a slight mis & runs rough up to 1300rpm's & smooths out around 2000rpm's. mech. is now telling me he thinks it is the torque convertor, but the dealer says the idle is almost perfect.
tim s.

I don't know what the results of the comp test will reveal. Tech thinks the valves are coated with carbon buildup or some other problem in the head. I though I read somewhere you could use crushed walnut shells to remove carbon buildup while the engine is running, and that it won't hurt the motor?

I'm more inclined to think like you, I think it's a sensor that's not funtioning properly. The rough idle is somewhat erratic. Sometimes it's worse sometimes better. I'm not sure I want to open up the motor ( at a cost of $ 1600.00) only to find it wasn't the valves in the first place.

As far as your problem is concerned how do they equate your problem with the converter?

[tim s]

they get extremly dirty & stop working properly.
get the compression test done. you can do it yourself. the tools are not expensive & it is easy to do.

as for my car they think the torque converter is out of balance. i did read that the valve body can fail in a way that the reverse has pressure continually even in neutral.
thanks
tim s.

[Bill R.]

this somewhere they had it all wrong. When carbon buildup on the backside of the intake valves first became a problem back in the 1980's bmw came up with a machine to remove this carbon, it was similar to a sand blaster but used walnut hulls instead of sand or glass bead.. walnut hulls won't remove aluminum like sand will...This process required removing the intake manifold and then this blaster would go up against the port in the head. You had to make sure that each cylinder had its valves closed and then you blasted the backside of the valve stem with the hulls to remove the carbon. Then you vacumed out the remaining hulls and carbon and went to the next cylinder port and did the same.... Nowdays there are enough intake valve cleaning chemicals like techron that the hulls are not longer needed. If your valves actually have enough buildup on them to prevent them from seating correctly then a compression test will show this. I doubt that this is the problem. I would try another mechanic myself.









I'm at a crossroad with my 91 525i. I've been chasing a rough idle/ miss problem for a while now and so far no-one has been able to diagnose the source.
I have it at the shop currently. ( not the shop I normally used) to check it out. I felt a fresh set of eyes and ears might help. They went through the ignition system( coil packs, boots, plugs) and the injection/intake systems and came up with nothing. Everything is working properly.
The tech is recommending a freshening up of the head. Remove it, machine it, change the valve seals, clean and lap the valves, new head gasket, etc.
He's telling me the cost to do this would be approx. $ 1600.00. He's 95% sure this would cure the problem, but not 100% sure. He thinks the valves may have some carbon buildup causing them to not fully seat.
Right now the car does burn some oil, about 1 quart every 2000 miles. But I don't have any white smoke comming out of the exhaust system or oil in the coolant. No real problems other than the idle. It's more of an annoyance than anything.

My question. Given the value of the car and the cost of the repairs do I just drive it until it really needs the head work, or pony up the money for the repairs hoping it corrects the problem?

Thanks in advance!

[winfred]

the walnut shell thing is like sand blasting, they remove the intake and rotate the motor till the valves are closed on a given cylinder and blast away, being soft if any are left behind they don't hurt anything,


I though I read somewhere you could use crushed walnut shells to remove carbon buildup while the engine is running, and that it won't hurt the motor?

[Bill R.]

difference.









[





QUOTE=Will308]I'm at a crossroad with my 91 525i. I've been chasing a rough idle/ miss problem for a while now and so far no-one has been able to diagnose the source.
I have it at the shop currently. ( not the shop I normally used) to check it out. I felt a fresh set of eyes and ears might help. They went through the ignition system( coil packs, boots, plugs) and the injection/intake systems and came up with nothing. Everything is working properly.
The tech is recommending a freshening up of the head. Remove it, machine it, change the valve seals, clean and lap the valves, new head gasket, etc.
He's telling me the cost to do this would be approx. $ 1600.00. He's 95% sure this would cure the problem, but not 100% sure. He thinks the valves may have some carbon buildup causing them to not fully seat.
Right now the car does burn some oil, about 1 quart every 2000 miles. But I don't have any white smoke comming out of the exhaust system or oil in the coolant. No real problems other than the idle. It's more of an annoyance than anything.

My question. Given the value of the car and the cost of the repairs do I just drive it until it really needs the head work, or pony up the money for the repairs hoping it corrects the problem?

Thanks in advance![/QUOTE]

[ocampg]

I had the same problems on my 89 525i, it got to the point that the car was stumbling/missing so bad that the car would not drive. Anyway, it turned out to be the MAF sensor, purchase a new/rebuilt one from autozone, its only $100 plus $50 core charge. I bought a used one from someone on this board, and it also was defective. I still have a rough idle, which i believe may be the ICV, but changing the MAF sensor definitely solved my engine stumble/misfire problem.

[MicahO]

That's how they clean motors on T37 trainers.....

Of course, on those motors, they come shooting out the backside ;)

[Will308]

they get extremly dirty & stop working properly.
get the compression test done. you can do it yourself. the tools are not expensive & it is easy to do.

as for my car they think the torque converter is out of balance. i did read that the valve body can fail in a way that the reverse has pressure continually even in neutral.
thanks
tim s.

Replaced the idle control valve myself ,along with the modified rubber pipe that connects it to the intake manifold, a while ago. No improvement.
Thanks for the reply!

[Will308]

the walnut shell thing is like sand blasting, they remove the intake and rotate the motor till the valves are closed on a given cylinder and blast away, being soft if any are left behind they don't hurt anything,


Ah! That makes sense. I was wondering how the walnutshells were going to get through the combustion chamber, catalytic conv. etc. of a running engine!

[Will308]

this somewhere they had it all wrong. When carbon buildup on the backside of the intake valves first became a problem back in the 1980's bmw came up with a machine to remove this carbon, it was similar to a sand blaster but used walnut hulls instead of sand or glass bead.. walnut hulls won't remove aluminum like sand will...This process required removing the intake manifold and then this blaster would go up against the port in the head. You had to make sure that each cylinder had its valves closed and then you blasted the backside of the valve stem with the hulls to remove the carbon. Then you vacumed out the remaining hulls and carbon and went to the next cylinder port and did the same.... Nowdays there are enough intake valve cleaning chemicals like techron that the hulls are not longer needed. If your valves actually have enough buildup on them to prevent them from seating correctly then a compression test will show this. I doubt that this is the problem. I would try another mechanic myself.


Thanks Bill. I thought it sounded like the headwork wasn't going to solve the problem, too. The car runs well at speed. No smoke,no misses etc. It's just the erratic idle that has me bugged.

[Will308]

That's how they clean motors on T37 trainers.....

Of course, on those motors, they come shooting out the backside ;)

Saw a short video of a guy get sucked into intake of a A-6E Intruder.
Bet he won't do that again!

[Will308]

Bill,
I don't think it's mechanical in nature. I really think it's some sensor. I just don't know which one would be the culprit.
When I come to a stop at a light,the engine will idle fine for about 3 to 5 seconds.
Then the rough idle will rear it's ugly head. Sometimes it's not as bad as other times. It's not a consistant/predictable occurence.

[Will308]

I had the same problems on my 89 525i, it got to the point that the car was stumbling/missing so bad that the car would not drive. Anyway, it turned out to be the MAF sensor, purchase a new/rebuilt one from autozone, its only $100 plus $50 core charge. I bought a used one from someone on this board, and it also was defective. I still have a rough idle, which i believe may be the ICV, but changing the MAF sensor definitely solved my engine stumble/misfire problem.


Mine has the M-50 motor. I never experienced the stumbling/missfire problem. Car runs great at anything but idle. I replaced the Idle Control Valve myself. I obtained a T.S.B. from BMW that gave a part number for a modified control valve and the rubber tube that connects it to the intake manifold. No change in behavior. You might be right. For the money( $100.00) It's worth a try.
Thanks!

[tim s]

it might clear any bad learned numbers.
tim s.

[Will308]

it might clear any bad learned numbers.
tim s.


Thanks Tim,
What are you refering to by Bad LEARNED Numbers?

[tim s]

reacts to changes in fuel mixture & it also learns sensor readings. sometimes it can learn bad numbers from wrong or bad fuel, a failing sensor, or for some other unknown reason. at least this is what different mechanics have told me. it is not common, but not impossible either.
hope this helps
have you read the engine codes???
tim s.

[Will308]

reacts to changes in fuel mixture & it also learns sensor readings. sometimes it can learn bad numbers from wrong or bad fuel, a failing sensor, or for some other unknown reason. at least this is what different mechanics have told me. it is not common, but not impossible either.
hope this helps
have you read the engine codes???
tim s.


Had a " check engine " light come on for about 2 days. As mysteriously as it came on it went out. First time it ever came on since the car was new. The mechanic said the code read a fault in the ox sensor and cleared the code.
The mechanic called today. The results of the comp. test indicated about 200lbs for cyl 1,3,5. He said 2,4,6 had about 120lbs, he's 95% sure there's a problem with carbon buildup on these cylinders and a complete rebuilding of the head would straighten things out. I was thinking it might be a collapsed lifter,but there's no clicking/tapping noise from the upper end. No smoke/steam from the exhaust. No severe loss of power going uphill. I told him to check the Ox sensor, I think it's the original part, and to replace it if necessary.
I think I'll pass on the head rebuild until I really need it. Then maybe I'll get a set of Schrick cams, do a Comforti chip, maybe headers etc.
Do you think the numbers for the comp test reveal anything odd?

[tim s]

most cars need 140-160lbs to run properly. if the mech. did the test right a head gaskets sounds like a good thing to do. just seams odd how it's every other cylinder.
tim s.

[winfred]

the numbers and the diagnosis sound fishy, in all my 7+ years of official auto mechanical employment i've never seen a motor skip every other cylinder on a comp test. generally if it runs and starts good it's ok, i deal with carbon a couple ways, 1st i get the motor hot then let it suck a pint of water into the intake through about a 1/8" vaccum hose with the engine running around 2k rpm, then get it warm again because the water cools it good. then do it again, then a italian tune up and let it fart the rest out

[winfred]

i've seen turds run ok on 100lbs across the board, a head gasket is not going to effect every other hole, one or two joining cylinders, or a outright blown gasket with all of the milkshake puking and running on a couple cylinders that goes with it


most cars need 140-160lbs to run properly. if the mech. did the test right a head gaskets sounds like a good thing to do. just seams odd how it's every other cylinder.
tim s.

[Bill R.]

m50 motor call for a compression on 143 to 156...his numbers seem awfully strange.. and carbon does not build up directly on the sealing surface of the valve, the valves have a rotating action that self cleans the seating surface. The carbon builds up on the backside of the valve affecting the distribution and atomization of the gas. Usually the first valve that you lose seating on is the exhaust valve from heat..If an intake valve isn't seating then the mixture pops back into the manifold and you get those muffled backfires in the manifold...Whereas a leaking exhaust valve just goes phut phut out the tailpipe.. I think i would hunt up another mechanic to check it out..







i've seen turds run ok on 100lbs across the board, a head gasket is not going to effect every other hole, one or two joining cylinders, or a outright blown gasket with all of the milkshake puking and running on a couple cylinders that goes with it

[Jon K]

m50 motor call for a compression on 143 to 156...his numbers seem awfully strange.. and carbon does not build up directly on the sealing surface of the valve, the valves have a rotating action that self cleans the seating surface. The carbon builds up on the backside of the valve affecting the distribution and atomization of the gas. Usually the first valve that you lose seating on is the exhaust valve from heat..If an intake valve isn't seating then the mixture pops back into the manifold and you get those muffled backfires in the manifold...Whereas a leaking exhaust valve just goes phut phut out the tailpipe.. I think i would hunt up another mechanic to check it out..


Bill! These muffled backfire you speak of... every now and then, I blip my throttle ( i don't mean to hijack this thread...) to rev match and i hear a "POP" maybe 1/10 times i blip the throttle. it comes from the motor, not the tail pipe. its like someone took a large paper brown bag full of air and popped it under the hood. its muffled in the cabin, and noticeable with the windows down for sure. Is my problem potentially what you listed above? 1992 M50, non vanos... 145k...

[Bill R.]

can be caused by an ignition misfire where you hear it pop in the manifold or an excessively lean mixture can also cause this... more likely what you would hear when revmatching.. I would take your mass airflow sensor off and check it for dirt buildup on the sensor itself on the side that faces the airflow coming into the engine.. If you notice a hesitation or flat spot when first accelerating quickly this would also indicate that its going lean








Bill! These muffled backfire you speak of... every now and then, I blip my throttle ( i don't mean to hijack this thread...) to rev match and i hear a "POP" maybe 1/10 times i blip the throttle. it comes from the motor, not the tail pipe. its like someone took a large paper brown bag full of air and popped it under the hood. its muffled in the cabin, and noticeable with the windows down for sure. Is my problem potentially what you listed above? 1992 M50, non vanos... 145k...

[Jon K]

can be caused by an ignition misfire where you hear it pop in the manifold or an excessively lean mixture can also cause this... more likely what you would hear when revmatching.. I would take your mass airflow sensor off and check it for dirt buildup on the sensor itself on the side that faces the airflow coming into the engine.. If you notice a hesitation or flat spot when first accelerating quickly this would also indicate that its going lean

Bill, it's funny you mention. My car feels VERY pokey from idle. much more than it usually is (being an M50). So you're recommending checking the maf for dirt? Is there a way to clean it without damaging it? Can I obtain another one for testing somehow?

[Bill R.]

hotwire mass air flow sensor.. I'm not sure if you have big snap rings holding the screens in place in front and back of yours or not..If you do you can take the maf (mass air flow sensor) out and then remove the snap rings holding the screens in place in front and back of it with the screens out you can see the hot wire that actually is the sensor, its fragile so don't touch. Look at the front side of it that the air hits first and see if it looks black or brown colored then look at the back of the wire, if it looks more like bare metal dull... then you have a fine layer of dirt built up on the front of the wire...With sensor safe throttle body spray cleaner you can clean this... not carb cleaner, not brake spray cleaner but throttle body cleaner. after spraying it it should come out the same color and appearance as on the backside of the wire.... What actually happens is this wire has a current running through it when the engine is running.. This current causes the wire to heat up.. the engineers know that X amount of current will make this wire heat up to X number of degrees, changing the resistance of the wire.. so by feeding this voltage and current into the wire and measuring the resistance they know how hot the wire should be.. Now when the engine is running and air is passing over this hot wire it cools it down changing the resistance.
So when the wire should be a certain temp and resistance based on the voltage and current going into it but its a different resistance and temp they can accurately determine how much air is flowing into the engine based on this resistance change.. and the DME knows how much fuel to inject based on this airflow number...Now a layer of dirt on the front side of this wire will act as an insulator so the airflow doesn't cool this wire down as much.. the computer sees this and determines that less air is going into the engine so less fuel is needed and it injects less. Which causes the engine to run lean and on rapid throttle openings it goes lean enough that you hear that pop back in the manifold.... So needless to say after you clean the wire the airflow has more of an effect on resistance and the computer gets more accurate readings to set the fuel injection by..















Bill, it's funny you mention. My car feels VERY pokey from idle. much more than it usually is (being an M50). So you're recommending checking the maf for dirt? Is there a way to clean it without damaging it? Can I obtain another one for testing somehow?

[Will308]

the numbers and the diagnosis sound fishy, in all my 7+ years of official auto mechanical employment i've never seen a motor skip every other cylinder on a comp test. generally if it runs and starts good it's ok, i deal with carbon a couple ways, 1st i get the motor hot then let it suck a pint of water into the intake through about a 1/8" vaccum hose with the engine running around 2k rpm, then get it warm again because the water cools it good. then do it again, then a italian tune up and let it fart the rest out


What's an Italian tune up? Be careful, Uncle Guido reads my e-mails!

[KenB]

What's an Italian tune up? Be careful, Uncle Guido reads my e-mails!


finally something I know. (I just read it here 2 days ago and it works!)

run the engine in 3rd gear @5,500rpm for 5 minutes. It blows out the carbon buildup.

I tried this yesterday, caution: 5500rpm in 3rd gear = 75mph. I wasn't able to hold it for 5 minutes, (maybe about 2) but it helped smooth out the engine and got rid of my slight ping. I think this procedure is actually mentioned in the owner's manual; after driving at low rpm's as in city traffic, I don't think the manual refers to it as an Italian tune up, though.

Just take it out on an open road wind it up and blow it out!

[winfred]

flog the **** out of it


What's an Italian tune up? Be careful, Uncle Guido reads my e-mails!

[Will308]

flog the **** out of it


Funny, I've given every car I've ever owned, an Italian tune-up and didn't even know it. If it ran badly, I must not have been going fast enough. More pedal!!!!!!!!

Seriously,
Thanks to everyone for your help. I get the car back tomorrow with a new Ox sensor. I'll keep you posted as to the results.
Will

Did I mention my last name ends in a vowel?