We had really heavy rains here today. I was in my '99 M5, idling in traffic when I noticed the brake pedal feel became really notchy. When I let go of the brakes the pedal would not travel back up easily and the engine started sputtering until I gave it a bootful of throttle. I also noticed a lot of steam coming out of the exhaust when adding copious amounts of throttle.

This kind of behavior is well-documented on M5 forums - basically the compartment containing the brake booster fills up with water when the drain plug gets clogged, which has the effect of ingesting water into the engine through the brake booster vacuum line (not to mention the booster itself). If enough water is ingested, it can result in hydrolock.

My Concern

I'm not concerned that the engine reached the stage of hydrolock while running, mainly because the engine would only idle poorly or stall whenever I pressed the brake pedal, and would idle well after the exhaust blew out a decent amount of steam via 5-10 seconds of high throttle.

What I'm concerned about is whether it is safe to turn the engine over now that it is cold, since steam may have condensed to form water in the combustion chambers.

I should mention that before turning the car off after reaching home, I pulled up the handbrake and gave the car 5-10 seconds' worth of sustained high throttle to blow out the steam, but have no way of telling if that was enough to make it "safe" for the engine.


Is it possible to determine if the cylinders are too flooded to safely turn over?

In the absence of such a test, I was thinking of pulling the spark plugs, siphoning off any water using a vacuum line and cranking the engine over manually.

I'm open to suggestions.

  • 4
    The simplest way could be to simply remove the spark plugs and turn the engine over. There will be no compression and excess water should squirt safely out of the holes left by the spark plugs. Once you have removed the excess you should be able to put the plugs back in and turn it over to start. Having never done this though, I've not put it as an answer.
    – Mauro
    Nov 25, 2015 at 15:25
  • 1
    @Mauro That is a viable option for (I'd say) the vast majority of cars. However, some cars don't facilitate an easy removal of spark plugs (e.g. H4 Subarus, many transverse V-engines)... in that case, other potential methods would also be useful. I comment because I'm very curious if there's anything else one could think of besides the spark-plug removal route.
    – Shamtam
    Nov 25, 2015 at 15:54
  • @Mauro does the piston typically come all the way to the top of the head? Seems like you wouldn't get enough water out like that. Shop vac with a straw taped to the end is my vote if you're going to remove the plugs.
    – JPhi1618
    Nov 25, 2015 at 16:55
  • @JPhi1618. This is one of the ways you can used de-coke products like redex. You pour some in the plug holes, leave it for a while then put rags over the plug holes and turn the engine over with the starter. It blows out with such pressure that there will only be a thin coating left in the cylinder.
    – HandyHowie
    Nov 25, 2015 at 17:32

3 Answers 3


If it is just condensation you are worried about, then I wouldn't worry. Water is produced normally by the combustion process. I would be more worried about water in the inlet tracts waiting to be sucked in. If it is too difficult to remove the plugs, I would turn the engine over with a wrench a few times, if there is significant water in the cylinders you will feel it.

I would also disconnect any ducting on the inlet manifold to make sure that there is nothing pooled in the pipes that could get sucked in, if I thought it was a possibility.

  • Plug removal isn't difficult, just a tad time-consuming. I like the thought of just cranking it over to see if I hit a proverbial brick wall - do you know if it is doable with the plugs in situ? Also, I'll certainly have to address the possibility of a waterlogged vacuum line, not to mention the brake booster, but for me the engine's health takes priority at this moment in time
    – Zaid
    Nov 25, 2015 at 18:07
  • @Zaid You should be able to slowly turn it over by hand using a wrench with the plugs still in.
    – HandyHowie
    Nov 25, 2015 at 18:13
  • Yes, by cranking I meant hand-cranking of course
    – Zaid
    Nov 25, 2015 at 18:44
  • I am pleased to report that I didn't have to remove the spark plugs and that the engine cranked over manually without resistance. The engine fired up and seems to idle ok. Time to tackle the brake booster now...
    – Zaid
    Nov 27, 2015 at 14:28

Given that there are no sensors inside the cylinders, I can't think of any way of doing so, other than Mauro's suggestion of removing the plugs and turning it over, which would spurt out any water in them.

You could try leaving it to evaporate, but if any cylinders are sitting with both sets of valves shut, any water in those pots won't evaporate off, so you'd have to turn the engine over part of a turn (by hand) to cover all of them...

  • Removing the plugs is pretty standard for something like this.
    – cloudnyn3
    Nov 25, 2015 at 20:43

This particular problem has been a pain for many years in the industry, unfortunately there is no sure shot way of telling if the engine will hydro-lock on cranking.

Standard Solution is to remove the spark plugs and vacuum out the water basically not taking any chance.

That said, there are a few things which can be done to ascertain if its safe to crank the engine or not.

  • If you are in moving traffic with water level below your intake but above the silencer and your car stalls, you can push the car to a dry area and then crank it , it will take a few cranks but it should start in most cases.

  • if your car has been standing and water enters through silencer , same procedure as above, note that it should not be in water for too long probable a day or two with silencer underwater will not affect the car and its safe to crank.

  • HOWEVER , if water enters through the intake then you should Never try to crank the engine as it will definitely hydro-lock, removing the sparks and cranking is the best way to get rid of that water.

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