Lawnmower syndrome experience, BMW

[abscate]

Ive read about Lawnmower syndrome, which is a temporary no-start condition due to loss of compression, root cause being excess fuel washing oil of the cylinder walls, leaving poor sealing of piston rings.

The syndrome is named for the phenomonon of backing your car out of the garage to access the lawn mower - then being unable to start.

Caitlin moved Moms BMW X3 to the back of the driveway, so she could get out the turbo Beetle convertible, and this morning, the BMW (a high compression X3 3.0 liter inline 6) would not start. It cranked fine, but it sounded funny - like a sewing machine. I actually thought the timing chain might have jumped. It was firing, and fuel pressure was present at the rail, but it would not catch and run.

I opened up all the spark plugs and cranked it for 30 seconds, replaced the plugs, and then it started and ran poorly for about 15 seconds, white smoke from exhaust, then all of a sudden settled down and idled normally over about 5 seconds of stumbling. I then took it out on the highway and it had great power, acceleration, etc.

Im pretty sure the short trip pushed too much fuel and killed the compression

No error codes were thrown at all during this. Car has 132k miles on it.

[cvergi01]

Germans...

[dosbricks]

Interesting. If I cold start my wagon to back it out of the garage and immediately shut it off--on restart, it will run slightly rough for about a minute. It has finally trained me to let it idle for a minute or two before shutting it off--then no problem. Maybe my injectors are dirty, or is this just normal cold engine running rich?

[jimmy57]

This gets described as ring seal on cylinder issue but I have several instances of techs not killing injection system and the compression will go up progressively as the succession of cylinders tested gets wetted by the fuel delivered by injectors.

The usual cause is the exh valve stem coked oil from porous guides lubing the stems and it gets heated and forms deposits. The water and fuel fog from a cold engine exhaust port will partly dissolve the coked oil and make a viscous sludge that dampens valve motion in the very tight cold valve guides. The hydraulic tappets get the opportunity overextend a bit as the valves close slowly and do not keep the usual pressure on valve tappets that keeps the hydraulic lash correction function working properly.
There is also valve seat deposits to contend with on engines in vehicles driven very easily that can be flaked off and hold exhaust valves open.
In either situation a leakdown test performed on an engine that will not start will show leakage out of exhaust side.

The plugs will get fuel fouled usually but prolonged cranking, one minute of starter operation before a rest, with thottle held wide open will usually get one started before the second full minute of starter has elapsed. With the compression low a minute of starter use is not detrimental like it would be while cranking an engine with normal compression and the higher starter load. Wide open throttle on most fuel systems cancels the initial few engine revolution cold enrichment as it is a programmed flooded condition clearing override.

Volvo's TSB fix was get it running by flood clearing, do a several minute high rev test drive to clean valve stems and seats (higher revs makes valves rotate), and fit new spark plugs with electrode designs more resistant to misfire under low compression. If the plugs don't foul and the engine ever starts and gets over 600 RPM the problem will not occur.

[abscate]

I was getting phutting about every third rev, and the revs climbed while I cranked but it would not catch and run for over 15 attempts. I did try commanding clearing by flooring the pedal but I don't know the X3 software to know if that shuts off the fuel. I think that did help get closer to starting if I remember the sequence correctly.

Pulling plugs and cranking worked like a charm though. SWMBO has to take the car to NYc Monday morning so she is still a bit nervous about starting, but I'm pretty confident on the diagnosis. Never had a driveability problem with this car

[rspi]

I have seen this with Jaguar's and other makes as well. I run into this a lot doing head replacement jobs and dealing with cars that have been sitting for several weeks without being started. I also make it a point to allow my cars to run for at least 1 minute before shutting them down.

[rspi]

There is also valve seat deposits to contend with on engines in vehicles driven very easily that can be flaked off and hold exhaust valves open.

Volvo's TSB fix was get it running by flood clearing, do a several minute high rev test drive to clean valve stems and seats (higher revs makes valves rotate),..

A little off topic, I believe this is the main cause of burnt valves. In several cases recently I've seen people burn valves on cars that were not driven aggressively by previous owners. Then a new owner gets that vehicle and will usually experience a burnt valve within 5,000 miles.

[bmdubya1198]

Ask around on a BMW forum like Bimmerfest.com. Not a common issue I've heard about on BMWs. Maybe another BMW owner on here like cn90 can offer some insight...?

[abscate]

I've posted it on bimmerfest but put it here as I've seen a few skeptics here. Thanks to MVS I was on this in about 2 minutes.

[jimmy57]

It was a common issue 10+ years back. Different spark plugs, valve guides, valve stem clearance getting bigger, and material changes all helped.

The ultimate fix is to not EVER run the engine briefly twice in a row when temps are below 60F.