Low compression 1988 m20 528e

E28 technical advice asked and given! Troubleshooting, modifications and more.
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somethin2do
Posts: 21
Joined: Apr 25, 2022 4:06 PM
Location: Arkansas

Low compression 1988 m20 528e

Post by somethin2do »

Hello - purchased a 528e this last weekend and found an oil leak at the valve cover (screw was missing). I also noticed some light smoke coming from the exhaust / engine bay after the car warms up.

Just ran a compression test and am surprised at my low readings. Car has 170k miles and was sitting for the last 5 years.

Compression is:

1 = 100
2 = 100
3 = 100
4 = 100
5 = 85
6 = 110

I'm debating on just pulling out the head, getting it machine shopped and having valve guide / seals put in. I also noticed oil / bubbling coming from the exhaust manifold bottom screw near cylinder 1... odd as I am not sure how oil is getting there but definitely have an exhaust leak.

Trying to figure out next steps. Push forward and rebuild the head or start with a leakdown to figure out if my exhaust headers are causing the problem ??

Would love to get advice from some of you considering this is my first e28.

Thanks!
BENAllison
Posts: 373
Joined: Jul 31, 2007 4:04 PM
Location: The Queen City.. NC

Re: Low compression 1988 m20 528e

Post by BENAllison »

Hello and Welcome.

First :If it’s been sitting for that long and you don’t have records /confirmation of timing belt replacement I’d recommend doing that because if it breaks it will damage the valves (at best). Also checking valve lash would be a good idea when you replace the valve cover gasket.

Second: assuming you are at or near sea level and your compression gauge is accurate then I agree the readings are low. However given they are somewhat consistent I recommend doing an oil change /timing belt and driving it for a bit then retesting with an another gauge to confirm. Was your battery fully charged , all spark plugs removed and throttle held wide open during your testing?

Third: hard to tell on oil leaks without seeing it but don’t think you should immediately tear it down for a rebuild. M20s are pretty robust and chances are it’s fine and just needs driven and some maintenance/gasket replacement on the valve cover.

(Also leak down is not a bad idea but you should drive it a bit first before diving into that as rings etc could just be gummed up from sitting , and an exhaust leak will have no impact on compression or leak down , just since you mention that. )
somethin2do
Posts: 21
Joined: Apr 25, 2022 4:06 PM
Location: Arkansas

Re: Low compression 1988 m20 528e

Post by somethin2do »

Thank you for the advice! I will do the belts, valve cover and fluid changes then run it for a bit to see if that helps. Also, I did not test compression with the throttle open so maybe that's why my results were low.

Will report back in a week or two with updates. Thanks!
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Low compression 1988 m20 528e

Post by Mike W. »

I have not found compression readings to be particularly useful, and the readings while not all that good, aren't that far apart, which is a good sign. I would wait a bit before diving in due to those readings. Adjust the valves for starters. I don't know M20s, but on M30s some of the exhaust manifold studs go into the valve train area, which means it could leak from there out the threads. I do second the suggestion of replacing the timing belt, perhaps I'm overly cautious, but if it fails it can, not necessarily will, but can trash the engine, not just the head. There have been new owners here who have bought the kit, but not gotten to it yet, who have had failures. Don't be one of them.
garageboy
Posts: 2479
Joined: Feb 12, 2006 12:00 PM
Location: New York City
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Re: Low compression 1988 m20 528e

Post by garageboy »

somethin2do wrote: May 02, 2022 2:41 PMThank you for the advice! I will do the belts, valve cover and fluid changes then run it for a bit to see if that helps. Also, I did not test compression with the throttle open so maybe that's why my results were low.

Will report back in a week or two with updates. Thanks!
Apologies if this was obvious, but as this is your first E28, I'm guessing it might not be totally obvious.

It's not just "the belts". "The belts" refer to the 3 fan belts on the outside of the engine -- alternator, air conditioner, and power steering. What we were recommending was the TIMING belt, a somewhat internal mechanism that synchronizes the cam to the crank. With the timing belt, there is a pulley that should also be replaced when you do the timing belt. And traditionally, since one does the timing belt at the Major II Service (every 60k miles), since you have everything apart, it's a great time to replace the water pump (another 60k part), distributor cap, rotor, spark plugs, O2 sensor. And since you're doing the water pump and have to drain the coolant, it's a great time to replace the coolant with BMW Blue/distilled water... and the three front large coolant hoses... and the thermostat... and sometimes the fan clutch... and sometimes the radiator... it may seem like a lot, but these cars will run forever if maintained according to the car's schedule.

Good luck and enjoy your E28!
somethin2do
Posts: 21
Joined: Apr 25, 2022 4:06 PM
Location: Arkansas

Re: Low compression 1988 m20 528e

Post by somethin2do »

Thank you all! I replaced quite a bit (below) and still have some issues that need to be addressed.

1) My check engine light comes on while driving but disappears when I first turn on the car. (Does not show up during idle) Stomp test not working because check engine clears. I believe I will need to buy a reader and check it while the engine is still on correct ? (1988 super eta 528e)

2) I am getting chatter from the transmission. In neutral and at idle i can feel the chatter and sometimes idle seems a bit rough. Putting the clutch in removes the chatter. Also revving up the idle while in neutral removes the chatter. Shifting from 1st to second gear is tough and i get some rattle / back of the car shake while accelerating into second. 'm going to change the transmission fluid to see if that helps.

3) looks like I solved 90% of my oil leak problem. No more smoke / burning of oil after it warms up. I do still see a minor leak coming onto the exhaust stud. Still trying to diagnose this and not sure yet what the cause is.

Replaced timing belt, cam seal, water pump, thermostat housing, thermostat, radiator hoses, belts, spark plugs, spark plug wires (old ones were falling apart), oil (20w-50), oil filter, valve cover gasket, breather hose, and did a valve job.
jacobthegoat
Posts: 60
Joined: Mar 20, 2020 12:59 PM
Location: Nashville, TN

Re: Low compression 1988 m20 528e

Post by jacobthegoat »

The chatter heard in the gearbox is called “getrag rattle” and is very common. Not an issue.

Some people alleviate this with varying results with transmission gear oil, some people swear by redline ATF. Personally I use ATF in my climate area. Almost all BMWs this age have the rattle.

The rattle should disappear when you push in the clutch.
It’s a non issue.
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