Engine stalls when breaking very hard

oport

oport

Member
Hello fellow Rover drivers,

i recently had the problem that my engine (with a BW35 auto box behind) just stops when breaking very hard and i had to start the engine again. Otherwise there is no visible problem, the engine runs beautifully and idles perfect. But i assume that my idling revs of about 350-400 are a littlebit low and the consequence of this is that the servo pulls the engine below the running level of revs when breaking hard. Normal breaking is no problem.
Would the cure be slightly higher idling revs? At which revs do your cars idle? I am always surprised how low these engines idle perfectly. My other V8 (MGB with manual shifting) idles at around 600.

CU Olaf
 
Hi Olaf,

Sounds like you have answered your own question. The revs at 350 - 400rpm are too low and should be when in Park and at normal operating temperature no less than 600rpm. Mine idles at 650rpm in these circumstances. If a car has air conditioning, then the normal idle speed is from memory 750rpm.

Ron.
 
My car used to do this when braking hard too and my idle speed was around 650rpm. I never did find the cause, but it hasn't done it since I fitted an electric fuel pump so I can only assume it was something to do with fuel pressure or the fuel sloshing around in the float bowls.
 
Hello,

thank you for so far backing up my theory. I will raise the idle revs to approx. 650, that should solve the problem.
I would rule out fuel starvation or the like as a possible problem, since i have an electronic fuel pump and no starvation problems ever since, regardless of the actual driving situation.

CU Olaf
 
Isn't there a valve in most cars thar can go wring on the manifold to servo pipe? If idle speed doesn't fix it that could be the next place to look!
 
Hi Olaf,
I agree with all the other replies i.e. the engine revs are too low, mine idles at around 650/700 with the BW35 auto box, another thing you could check is the hydraulic dampers (dash pots) in the carbs, make sure you can see oil in the carbs when you unscrew the top, I have heard that you should use a "3 in 1" type oil but I use a 20/50 engine oil in mine and it has always been okay
Best Regards Dave
DAV8
 
Hi Olaf

I'd guess that the reason your engine revs are that low is to avoid the clunk when engaging drive on the BW35. Setting the engine revs is a very inexact science. Although, as Ron pointed out, the book advises 650rpm, you are best to set them as low as you can get away with. Normally this is determined by the lowest speed at which the engine will idle smoothly, but in your case it could be the stalling effect. Remeber also that the rev counter is unlikely to be accurate at such low numbers.

Chris
 
Hello everybody,

first of all thank you for your advice so far. I just got around to test the idle revs again, and to be precise: I read 650 when the BW35 is in neutral and approx. 450 when in D. I compared that with a couple of other P6s and these values seem to be correct. It is also not a mixture problem or something like fuel starvation.
One other theory i will pursue is that probably the break servo is drawing unwanted air somewhere which stalls the engine. This is something a friend recommended to check. I will do that when i reanimate the car next spring, currently the P6 is stored in my garage.

I wish everybody a merry X-mas and a very happy new year. You have been a bunch of very helpful people throughout the past two years since i have my P6. Appreciate that!

CU Olaf
 
Hello everybody,

just a quick update if anybody encounters the same problem: The problem was cured by replacing the little hose from the brake servo to the actuator. The hose was not tight anymore and unwanted air was drawn. Replacing it returned everything to normal.

CU Olaf
 
Mine also does this intermittantly and I run a Weber, I had suspected that iw was the Bush on the fire wall throttle arm anchor point as mine has gone missing, engine revs are a precise 650 on idle so maybe there is a chance that it maybe the vacuum hose.

Graeme
 
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