Hi - I'm new to the forum. Please help. I have a 1999 E36 318is South African spec with 126000km. It drives great and has good high-end power. However, it has quite a rough idle (as if it's misfiring every few seconds). The engine also seems a bit noisy. My fuel consumption doesn't seem too bad. I'm new to the car and do not know if the above is all normal but the rough idle really irritates me so I took it to a BMW dealer. Replaced camshaft sensor, O2 sensor and air-flow sensor but these did not solve the problem. The dealer checked everything and then resorted to a compression test. They found that 2 of my cylinders were low (one almost half normal). They said they could not tell me more without opening up the engine! This sounds a bit drastic to me considering that the car really runs quite nicely.

I'd like to know what the low compression could mean and whether it's possible that the test results were inaccurate. Also, if I just leave it will the problem likely get worse? I've driven it like this since I got it (about 10 000km) with no noticeable difference.
Any advice or comments would be appreciated!

Low compression means the valves or rings aren't sealing properly. Yes, that would account for your rough idle. Have them do a leakdown test in addition to the compression test. That'll tell you if it's rings or valves.

Since the repair shop replaced a whole list of parts that didn't solve the problem, anything they tell you should be suspect, including compression test results. Like Hal said, the compression test should have been followed by a leakdown test, which will tell you a lot about your engine without having to tear it down.

If I was diagnosing that car, I'd check for intake air leaks, then check the crankcase vent valve using a slack tube manometer. I fixed a 318 Ti with a similar complaint a couple of weeks ago. Good power, rough idle, check engine light with misfire faults stored. The crankcase vent valve was a little out of spec, and replacing that fixed the car. That's not to say that's the fault here, just something that a competent BMW mechanic should know about and check.

If you have low compression on adjacent cylinders there is a possibility of a leaking/blown head gasket. Check your coolant level, and check your oil for water.

Thanks for the above comments and sorry for the delay -off line for a while. I chatted to one of the BMW mechanics at the dealer. He suggested if it's going fine I should just ignore it rather than open up the engine to find the fault. What I can't understand is if the compression is that bad how could it still be driving fine? Surely I would feel a major difference in driveability?
I'm not losing any water or oil and the oil and water are both clean.

First off, your dealer has hired a bunch of idiots for mechanics and you should stay away from them. You ought to consider sueing them in small claims court for all the "easter-egg" parts swapping that they did, they appear to be totally incompetent. Did they suggest that you just trade the car in on a new one as well? The idiot you chatted with is not a Mechanic, he's utterly unworthy of that august title, he's a parts-swapping pretender who is not worthy to sweep the floor of a proper garage. :mad

Perhaps they did the compression test correctly, but with the incompetence they have shown I would try to re-test it. If you have low compression on 2 adjacent cylinders (1 and 2, 2 and 3, or 3 and 4) then it very well could be a head gasket blown between them. If you really do have low compression on 2 adjacent cyls, this would be my best guess.

A leakdown test would confirm this, probably - you would be able to hear/feel air coming out the adjacent spark plug hole (probably at least some, depending on the position of the intake and exhaust valves probably one of them will be at least off it's seat so you may or may not be able to feel a lot of air flow - although if you put a length of vacuum hose right in the sparkplug hole and listen to the other end you should be able to hear the hissing leak, if it's there)

318's of your vintage are known to have head gasket problems.

Second most likely (but not that likely) would be 2 burnt valves - sort of goes against Occam's Razor - that the simplest explanation for your observation is the correct one -but you have to take the head off either way.

Replacing the head gasket is not that hard, take a look in a good shop manual like the Bentley and see if you think you are up for it. Alternatively find a good independent shop and have the job done there. :buttrock

A good shop will probably suggest lapping in your valves and replacing the valve guide seals. And of course checking that the head is not warped, that may be your problem right there. These will add a little to the parts and labor bills but if you plan to keep the car would be wise to do them.

If you pull the head yourself you can take it to a machine shop and they can check for flatness, I'd suggest also checking for cracks/pressure test, and do the valve guide seals at least - if your valves are sealing perfectly you can certainly leave them alone.

There is a school of thought to the effect that the exhaust valves should be replaced if the head comes off. If the valves in the low-compression cylinders look a little burnt, you might want to replace them and if I did 2 I would likely do all 4 (or if your car is a 16 valve, all 8 - )

The car runs OK at higher RPM because the leak is relatively small, so you get more dynamic compression at say 3000 than you can develop at cranking speed.

But it's not a good idea to ignore this. If you have the "bad" head gasket they tend to let go all at once with possible "hydralicking" of the engine and/or overheating...

Thanks for all the info! Will have a leakdown test performed as suggested. Hope it's not too serious!

I took my car to an independent Bosch service station and had them re-do the compression test. Just as I expected - they say it's excellent and that they honestly cannot find anything wrong with my engine! They even gave me the printout and assured me that their tests have never been wrong in the past.

This raises a few new questions:
1) How competent were the BMW agents in performing their diagnostics?
2) Should I get a 3rd opinion?
3) I'm still stuck with an irritating, lumpy idle. What now?

Is it true that the 318is is known for a rougher idle? It smoothes out completely at slightly higher revs so maybe it's just idleing at too low rpm (around 600 to 700)?

Incidently, I think South African's have a different name for a "leak-down" test because no one seems to know what that is here.

Any new comments would be appreciated.

Please refer to my previous post.

rough idle??? haha... do a kero flush. that fixed it on my american piece of shit... but then again, this is a bmw so if you wanna keep it, flush it w/ atx fluid or something.... this will clean the gunk... but then again, this is probably not your problem, just throwing ideas at ya,

if its misfiring try pulling one of the spark plugs and see if it has oil? that's what happened to my friends bimmer, again, throwing ideas at ya

You can always do the compression test yourself. I think the first shop that worked on your car was incompetent. There is no reason to replace those items if you're not throwing codes (check engine light or CEL).

Compression testing is disconnect power through distributor or whatever provides electricity to your spark plugs (just unplug a wire, basically). Remove spark plug, screw-in compression tester, turn engine over 3-4 times (quickly). Compression tester will show reading. They are relatively inexpensive.

Rough idle could be attributable to a vacuum leak somewhere, you have an older car. Could be a cracked hose, or crack in a connection somewhere.

Again, hard to diagnose over the internet, but just throwing some ideas at you.

A "Leak-Down" test is the same as a "Differential Compression" test that is done on piston engined aircraft. Basically....you have a regulated compressed air source that passes through a small orifice (.040" on general aviation aircraft) and is screwed into the sparkplug hole on a cylinder. There is a pressure gauge on either side of the orifice. The piston is placed at TDC of the compression stroke and compressed air is applied, 80 psi on planes. If there was NO leakage in that cylinder (rarely happens!) then the two gauges would read the same. With SOME leakage in that cylinder, the gauge on the cylinder side of the orifice will read lower than the supply-side gauge. Usually, if you have a reading on the cylinder side that is 90% or so of the supply side, that cylinder is OK. If a cylinder reads really low, then you need to find out what's leaking! Air coming out of the breather indicates rings leaking. Air coming out the exhaust indicates exhaust valve leaking. Air coming out of the intake manifold indicates intake valve leaking. Air coming out of the adjacent cylinder or radiator indicates head gasket leaking.

Good Luck!

Er, how about HansundFranz's suggestion? Check for all sorts of intake leaks, and look at the crankcase vent valve. If you can't test the crankcase vent valve, I don't think they are $$$$, you might just replace it. I would think if your car runs OK and has normal compression, there is no need to do a leakdown - I 'd only do it if I had bad compression...

How old are your spark plugs, ignition wires, etc?