Need advice on dealing with bad compression numbers

[Earendil]

My dad just took his 325ix to a mechanic and received some bad news.

"four cylinders look good, but one of them tests at 113, and another at 83".

The primary question I have is, what all could cause low compression? This is an area I know very little about.
We're trying to figure out if it's worth having the head removed, since that's the mechanic's suggested next move.

A few things we don't know:
If the test was wet or dry. I believe this would point to rings or not?
If the mechanic is component. The first suggestion was O2 sensor, low compression is what they came back with after the O2 sensor didn't do it.

The problem came on rather suddenly. Does that point to one problem or another?

We're going to get a second opinion, hopefully from a shop that is more old BMW savvy, and one that we'd be willing to let work on the head. But unless the first mechanic is crooked, we're looking at bad compression.

Thanks for any information, advice, or kind words.

[jlevie]

A wet compression test will tell if the rings are bad.

[M-technik-3]

You could purchase or rent a leak down testor. This will give you answers as well.

[LJ851]

Compression test is pretty crude as mentioned above.

Do a leakdown test to ID the source of the leakage.

What are the symptoms (why you took it to the shop) ?

Have you checked the valve clearance? You could have no or negative clearance on a couple valves causing the low numbers.

Are the two low cylinders next to each other?

[M-technik-3]

Autozone and Advanced do compression testors as well, Easy to use them just make sure the fuel pump relay is pulled and throttle is wide open.

[jlevie]

Autozone and Advanced do compression testors as well, Easy to use them just make sure the fuel pump relay is pulled and throttle is wide open.

Until the stored pressure in the fuel rail is exhausted that will still allow fuel to be sprayed into the cylinders. A better approach is the use the starter pins in the diag connector or to jump the starter solenoid lead to 12v. You can get an inexpensive switch with attached leads and clips for this at most any auto parts house. Either approach makes testing compression a one person job.

[AndrewBird]

If the cylinders that have low compression are next to each other, it could be a blown head gasket. If the gasket failed between the two cylinders, compression will leak from one cylinder to the other.

Have you replaced the timing belt before?

[M-technik-3]

Until the stored pressure in the fuel rail is exhausted that will still allow fuel to be sprayed into the cylinders. A better approach is the use the starter pins in the diag connector or to jump the starter solenoid lead to 12v. You can get an inexpensive switch with attached leads and clips for this at most any auto parts house. Either approach makes testing compression a one person job.

Very true forgot to mention that.

[RockingM]

How many miles on the engine?

I just tore mine down and found, despite good compression numbers, the rings were worn slam out, the oil rings were shot and full of crud.

This was all prompted by blow by and high oil leakage.

170,000 miles, oil changed every 3k for the last 7 years that we owned it.

[Earendil]

Thank you everyone for your replies! Sorry it has taken so long to get back to this thread.

A wet compression test will tell if the rings are bad.

Great, that's what I sort of remember reading but wasn't sure.

You could purchase or rent a leak down testor. This will give you answers as well.

I'll propose the option to my Dad, but I live 5 hours away from him and can't personally get my hands dirty with this one.

Compression test is pretty crude as mentioned above.

Do a leakdown test to ID the source of the leakage.

I just looked up what a leakdown test is, and this seems like it would be a good next step before ever taking the head off, especially when we don' thave thousands standing by for a complete engine rebuild. Knowing if it's a head gasket or rings would be very important.

What are the symptoms (why you took it to the shop) ?

Have you checked the valve clearance? You could have no or negative clearance on a couple valves causing the low numbers.

Are the two low cylinders next to each other?

The two cylinders, according to the mechanic, are in fact next to each other (#3 and #4 iirc).

The problem is a lack of power and rough running engine. I know, it could be a million things, but my Dad was able to pull a plug wire while the car was running and get zero change in the idle on one cylinder. We suspected spark or fuel for that one cylinder. After testing/replacing a few parts we thought we eliminated the most common issues, and gave up and took it to a mechanic. The car is still driveable, but is down about 25% on power.

I didn't suspect engine failure because the problem came on quickly. By the description, an electrical component failure seemed far more likely. But then I haven't experienced bad rings/valves.

What what it's worth, there is no coolant in the oil, and no oil in the coolant.

If the cylinders that have low compression are next to each other, it could be a blown head gasket. If the gasket failed between the two cylinders, compression will leak from one cylinder to the other. Perhaps it was just wishful thinking on our part.

Have you replaced the timing belt before?

That's a good point with the cylinders being next to each other. The two cylinders are in fact next to each other. The Timing belt was replaced about 2.5 years ago right after he bought the car.

How many miles on the engine?

I just tore mine down and found, despite good compression numbers, the rings were worn slam out, the oil rings were shot and full of crud.

This was all prompted by blow by and high oil leakage.

170,000 miles, oil changed every 3k for the last 7 years that we owned it.

I knew and trusted the previous two owners, and didn't run the VIN when we bought the car. I know, rookie mistake. The car reads 200K on the odo, but I ran the VIN last year and discovered it was emissions checked in like 2002 (3rd owner back) with 220K on it, so the car has up over 300K easy at this point. The current owner and last 2 all used it as a daily driver.

It sounds like the right move at this point is to get a second and more qualified opinion to do a leakdown test and/or a wet compression test to try and determine if we're talking head gasket, rings, or some costly problem between those two extremes.

Thank you everyone for your advice and time!

As an aside, concerning the mechanic that simply did a compression test and then offered to pull the head off and take a look, would that ever be the right thing to do? Sounds like a leakdown test with the engine assembled is a great diagnostic step prior to ever removing the head. If that isn't done, it seems like, baring a visible hole in something, you couldn't be sure exactly what all was causing the lack of compression, and the only option would be to reassemble and test it again. Or are there simple ways of testing rings, HG, and valve components with just the head off?

[603Racing]

I would never pull a head just because of bad comp #'s. Leakdown is a good idea. I would HIGHLY suggest not driving the car if at all possible until you find out what's wrong and fix it.

Did your dad or mechanic check to make sure there is not oil in the coolant or vise versa? That's a telltale sign of a head gasket failure. Also, any oil/coolant leakage from the head gasket on the outside of the block.

I would say try to find a mechanic familiar with e30's, that can make all the difference in the world. Also find a mechanic that will try to diagnose it without throwing parts at it or ripping it apart. Post your dad's location, there's probably someone near there.

[Earendil]

I would never pull a head just because of bad comp #'s. Leakdown is a good idea. I would HIGHLY suggest not driving the car if at all possible until you find out what's wrong and fix it.

Did your dad or mechanic check to make sure there is not oil in the coolant or vise versa? That's a telltale sign of a head gasket failure. Also, any oil/coolant leakage from the head gasket on the outside of the block.

I would say try to find a mechanic familiar with e30's, that can make all the difference in the world. Also find a mechanic that will try to diagnose it without throwing parts at it or ripping it apart. Post your dad's location, there's probably someone near there.

Thanks for the advice. No, there is no coolant in the oil or vice versa. That was the first thing I had him check before he did things like cap and rotor (which is was overdue for). There isn't any additional leakage either, or blowing of coolant/oil out the exhaust in an obvious way.

I have already inquired in a regional forum about possible location of good mechanics, which are all about 60 miles away.

Besides being stranded, what's the failure case for continuing to drive it? He currently thinks it's driveable the 60 miles to a BMW mechanic "in the city", which would of course be preferable to having it flatbed towed (it's an ix) that distance.

Thanks again!

[digger]

what can causes an oily mess in the intake manifold despite the valve cover vent not being fed back into the intake?

[EyExR]

you need to do a leak down test

[Cabriolet]

Earendil, does the engine idle smooth or does it sound like a WRX STI? every time i've had a had HG blow it sounds like a WRX. this sounds different from a surging idle, caused by an air leak on the intake.
mine with a blown HG

http://youtu.be/GAVrPR0T6Rc

if is smokes like this when you open the cap, it could be the rings

http://youtu.be/Z_Dawprq3dc

my rings were 100 blown apart in this video.

however, since you have 300k on a m20b25, it needs to be fully rebuilt.