The Bentley manual says 15 to 22 in-Hg of vacuum is normal but it doesn't say whether this is at idle or with the engine running faster. I had a steady 15 at idle and a steady 23 at around 2000 rpm. The 15 seems borderline low, but I don't know what's typical. Are these normal seeming numbers for these cars?

I also did a compression test. Bentley says 146 to 152 psi. I measured 155 to 170 across all cylinders after about 5 cranks. I was pretty happy about these numbers but then on another forum a guy said he was getting 200psi. So, same question -- what is a typical healthy compression figure for a 6er?

Thanks!

The vacuum sounds ok, you could always run a temporary gauge and ck it as you drive around under different loads. As for the compression #'s every engine is different based on age and wear. The importsnt thing is, as a general rule, the compression #'s should be roughly between 10% of each other. So that if you have a low of 150lb. the highest should be around 165lb. You also want to see them fairly even, like if you have 165lb. across 5 cylinders and then there's a sharp drop, that's no good. Could be an indication of a badly adjusted or chipped valve, broken or worn compression ring.. There are many motors that run great right down to 120lb. or so, though they are usually using oil at that point. When you do your compression test, to get a truly accurate reading first pull the relays on the fuel pump so no fuel flows. When cranking the mtr make sure the throttle is on the floor so you draw maximum air into the cylinder.At 5-6 cranks the gauge will stabilize w/ your reading. If you have one that is truly low compared to the rest, try squirting oil into that cylinder to see if you get an upward bump (rings).

Thanks Joe. I think I'm good on the <10% variation in compression, which actually was a big relief. I've been trying to get this over-rich/underpowered problem tracked down and I was a little worried I might have a bum cylinder or low compression all around. The car does burn some oil but you only see it occasionally on startup, which together with the apparently good compression makes me think it's the valve stem seals (BTW, I was surprised to read in Bentley that BMW considers up to 1 quart every 380 miles to be within spec -- that's a lot of oil between changes). 15psi of vacuum still seems lowish but maybe not. What kind of vacuum does your car have at idle (was it you that had the vacuum gauge in your dash)?

My next test is going to be fuel pressure. I've suspected high fuel pressure for a while now but haven't been motivated enough to splice in a pressure gauge. It occurred to me the other day that a blocked return line from the FPR could cause high pressure so I tried blowing through it to see if it was blocked and sure enough, it was hard to pass any air through it. So I'm cautiously optimistic that might be it.

To tell the truth, I haven't paid much attention to the vacuum gauge other than to see it swinging around. I'll watch it though for you and see what it tells me. As for the oil consumption figures from BMW, you have to remember that back in the 60's-80's valve stem seals were always an issue, especially in the late '70's and '80's w/ the onset of unleaded gas. Old style leaded gas was the lubricant for the valve stems and when unleaded came on the scene valves stems started running hotter and ruining the bronze guides. That's why new engines nowadays use totally different materials. Some guys (and manufacturers) went to total seal valve stem seals to try and stop the leaking. All they did was starve the guides of lubrication..It wouldn't smoke, but it would only last a little while. I used to use 400mi as a cut-off on my old audi 100. I think I put seals in that head every 15k mi up to 140k and finally got tired of it...even brand new Ford 302's and Chevy 350's used a qt every 1k mi brand new!...as for your fuel pressure, it'd be ok to check it, but remember to check it where it leaves the rail and goes to the regulator, not on the pump end. You'll get a more accurate reading....Just a thought. If you we're high on pressure you'd have gas leaking, reason I say this is that you're injectors will only pass a certain volume based on the pulse signal received. There's only so much that will pass thru the nozzle. So saying that, perhaps the injectors are weak and/or you're getting excessive leakdown during the "off" pulse? or is there a temp sensor up front out of wack telling the ecu to dump gas? You can also check for a weak cold start valve that's leaking down while it's off. A really rich condition will play havoc with the o2 also. So now there's plenty to check, eh?

To tell the truth, I haven't paid much attention to the vacuum gauge other than to see it swinging around.

Do you have an aftermarket vacuum gauge installed?

If you are referring to the "MPG" or fuel economy gauge under the tach, it is not a vacuum gauge but rather uses inputs from the ECU and Speedo. The white nylon vacuum line that goes from the bottom of the intake manifold into the cabin is not for the vacuum gauge but goes to the interior temperature sensor to continuously draw a small amount of air through it to help maintain an even cabin temperature.

...as for your fuel pressure, it'd be ok to check it, but remember to check it where it leaves the rail and goes to the regulator, not on the pump end. You'll get a more accurate reading.... Unfortunately the regulator is bolted to the fuel rail in the six so you have to remove the hose from the fuel rail by the firewall and use a "Tee" to insert a gauge. There is also a fuel delivery test where you collect output from the regulator for 1 minute. You will need to make a jumper wire with a switch to jump the fuel pump relay to make these tests. The Bentley shows all of this and gives the specs., the chapters on fuel injection and "Driveability" are worth the price alone.

-I'm gonna have to get my glasses checked! Is that what that thing is for, mileage? My bad and I apologize for passing bum info. I had a gauge like that in an old Pontiac and it was definately vacuum. Sorry.

Hey, that's what I thought it was when I first bought my car too. :)

I had a old car with a vacuum gauge marked in "Mileage" also.

The white nylon vacuum line that goes from the bottom of the intake manifold into the cabin is not for the vacuum gauge but goes to the interior temperature sensor to continuously draw a small amount of air through it to help maintain an even cabin temperature.

Ah ha! I've been wondering about that thing. Thats a very odd use for a vacuum line, my 740 has a tiny fan in the console for that purpose.

Oh, yeah, a bit of trivia ...... don't replace that short piece of vacuum hose that connects the white nylon line to the manifold like I did. It turns out that there is a restricter orifice in it! Or so the parts catalog indicates. I guess I've got everything else tight enough that it doesn't seem to make any difference.

--Go figure! I did an oil change this morning and used the reset tool...Now that mileage gauge is dead. The inspection light has stayed on since last week, but the yellows and reds are out...and of course the speedo's been dead since I got the car. So I oredered a remanufactured SI Board this morning, and then when the cluster is out I'll finally figure out whats up with the speedo. Maybe a gear, maybe mort? Maybe the SI board is the culprit? Slow and steady...

As for the oil consumption figures from BMW, you have to remember that back in the 60's-80's valve stem seals were always an issue, especially in the late '70's and '80's w/ the onset of unleaded gas. Old style leaded gas was the lubricant for the valve stems and when unleaded came on the scene valves stems started running hotter and ruining the bronze guides. That's why new engines nowadays use totally different materials. Some guys (and manufacturers) went to total seal valve stem seals to try and stop the leaking. All they did was starve the guides of lubrication..It wouldn't smoke, but it would only last a little while. I used to use 400mi as a cut-off on my old audi 100. I think I put seals in that head every 15k mi up to 140k and finally got tired of it...even brand new Ford 302's and Chevy 350's used a qt every 1k mi brand new!...

Very interesting. Is replacing the seals a big job on the Six? That's something I've never done before on a car. You can do it without removing the head, correct?

If you we're high on pressure you'd have gas leaking, reason I say this is that you're injectors will only pass a certain volume based on the pulse signal received.

I do have telltale signs of gas seepage around a couple of the injectors. I never see them wet but I can tell by the stains around where they fit into the rail that at some point there's been gas dribbling. Still, it's nothing really bad, like nothing flowing out. I wonder how high the pressure has to go to get actual dripping leakage? Maybe my pressure is high but not that high.

So saying that, perhaps the injectors are weak and/or you're getting excessive leakdown during the "off" pulse? or is there a temp sensor up front out of wack telling the ecu to dump gas? You can also check for a weak cold start valve that's leaking down while it's off. A really rich condition will play havoc with the o2 also. So now there's plenty to check, eh?

I checked all the sensor inputs to the Motronic with the engine hot and they seemed correct. Not sure about injector leak down or leaky cold start valve though. Yes, plenty to check! But first I'll see what the pressure looks like. The fact that the FPR return line seemed restricted makes me think that might be it.

Very interesting. Is replacing the seals a big job on the Six? That's something I've never done before on a car. You can do it without removing the head, correct? There are 2 ways of doing it. One is by threading a thin rope thru the sparkplug hole into the combustion chamber, packing it in there and slowly easing the piston up to compress the rope up against the valves keeping them in place so you can release the valve spring and get at the seals...The other way is to be tied into a strong air compressor (150-200lb) and keep the valves pushed up that way. Either way and/or pulling the head and having a valve job done. So, 3 choices, all time consuming. It depends on how far you want to go ($) and what results you're after.



I do have telltale signs of gas seepage around a couple of the injectors. I never see them wet but I can tell by the stains around where they fit into the rail that at some point there's been gas dribbling. Still, it's nothing really bad, like nothing flowing out. I wonder how high the pressure has to go to get actual dripping leakage? Maybe my pressure is high but not that high.
Have you tried spraying around the injectors to see if there's a change in running. I figure if an injector can push gas out it can suck air in right?


I checked all the sensor inputs to the Motronic with the engine hot and they seemed correct. Not sure about injector leak down or leaky cold start valve though. Yes, plenty to check! But first I'll see what the pressure looks like. The fact that the FPR return line seemed restricted makes me think that might be it.I haven't looked in the prints but is there a one way valve somewhere?, maybe near the tank?

There is a check valve in the fuel pump to keep the pressure up in the fuel rail after the engine is shut off to help prevent vapor lock on a hot restart. Normal pressure is about 43 psi.

Well, I checked my fuel pressure and it's perfectly normal, which means another theory goes out the window. That resistance in the FPR return line must've just been the normal resistance you get with a long narrow hose (from FPR back to the gas tank). I think I'll check the cold start valve next.

What are you're plugs saying? What's the tailpipe look like? I was able to determine a lot by these two. Prolonged idling in my car was a rich affair, going down the road and clearing it out, then pulling the plugs and they were clean. So that told me it was an idle issue. If they're black all the time then that tells you something else. You never did say what you're compression numbers were. And how long would the combustion chamber hold that pressure (leakdown), Joe ps a cold start injector should give a certain amount into a jar when cold starting, and then stop. Many have been known to leak away after they cycle off. The procedure is in the book.

When I did the compression test the plugs looked moderately sooty, but the car had been idling. My exhaust tips get sooty pretty soon after a cleaning and the rear bumper around the exhaust tends to get a light film of soot too. The exhaust smells rich to me (idling) and someone else recently commented that it smelled rich. I guess none of this says much about whether it's rich when going down the road versus idling though. I probably ought to check the plugs right after going down the road. One thing I do know is my gas mileage is bad (12 - 15 mpg), which makes me think it's more than just at idle. Another tidbit is that my OBC usually says my mileage is around 19.2 mpg which doesn't agree with my actual mileage. My economy gauge in the dash is dead.

I did the compression test twice and the numbers were a little different each time. But both times all six cylinders were clustered around ~160 psi. Maybe the highest one would be 170 and the lowest 155 or so. It was right on the edge of 10% difference between hi and lo. My plan was to have a leakdown test done if the regular compression numbers were unusual, but they seemed okay so I figured I'd skip it. What could a leakdown test reveal, and is it possible to do it with just a regular compression tester?

Also, how hard is it to get to the cold start valve? I've got the book but I'm curious about difficulty.

Thanks...

-getting at the cold start valve can be a bit of a PITA. From what you're saying it sounds like you're running rich. Did you ever what your oil consumption is if any?

Yeah, seems like I've seen others say it's a PITA too. Do you think the valves being out of adjustment could cause it to run rich? I've been meaning to do the valves but haven't gotten around to it yet.

Oilwise, it's using about a quart every 500 miles. Not terrible but not exactly good either.

Do you have a first name? just wondering who I'm talking too...anyway...go back about 3 pages in this forum and about mid page you'll see a thread of mine called "making progress". You might find it interesting what ended up curing my overly rich condition. I must apologize, cause I don't know how to do the cut and paste thing. I'm a bit ignorant when it comes to computers.

Okay, so your richness problem was a combo of valve adjustment and a hi-flo radiator causing a temp sensor to send a rich signal. I'm pretty sure my ECU's getting the right signals, but my valves -- I had planned on doing them anyways - maybe I'll up their priority on my "to do" list.

First name is Hayek. Last name is Fan.:cool

cool, cause the way I was reading it I thought you were a fan of the actress Selma Hayek! And I was thinking "RIGHT ON !!" For me she is one of the sexiest woman breathing right now. haha OK Hayek, nice meeting you, Joe