Rover V8 issue

[Ralphypants]

Hi, I wonder if anyone can shed some light on an issue I’m having with my Dax Cobra.
The car is powered by a Rover 4l V8 at 182 bhp. The issue I have is when I drive and depress the clutch and give it a rev (as you do with a V8) the car slows as if the brakes are applied. Another example is when waiting at the lights the car was rolling slowly forward, a little blip of the accelerator and it stops dead. The only idea I have is that a vacuum is being created and working against the crank or operating the brake servo. I’m no mechanic but I’m sure this should not be happening. I don’t have anything binding or rubbing, the engine was rebuilt last year by the previous owner.
Any ideas?

Colin.

[Boosted LS1]

It could be high engine vacuum or the brakes are dragging once warmed up. You can check vacuum by temporarily blocking the pipe to the servo. Obviously you'll have to press the brake pedal harder and do it somewhere safe. Does the car roll freely when you push it? Are any of the wheels very warm after braking?

In the lights situation as the car rolls forward etc do you feel the brake pedal drop down on it's own?

Edited by Boosted LS1 on Friday 19th June 17:07

[gazza285]

I would be checking the non return valve on the vacuum hose.

[Dogwatch]

gazza285 said:

I would be checking the non return valve on the vacuum hose.

Sounds a good place to start. Or the previous owner's rebuild may have left out a few minor items on re-assembly.

[Ralphypants]

The car rolls freely when pushed, wheels aren’t excessively hot, I haven’t notice the brake pedal move on it’s own. The slowing issue I have does seem to differ at different speeds and gear choice. In all honesty it doesn’t feel like the brakes are being operated, it’s more like engine braking.

[Getragdogleg]

If you depress the clutch fully and then rev the engine then the engine will having no effect on the drive train at all, its been separated from the gearbox.

So it stands to reason its not engine braking.

I'm suspicious that the vacuum being made in the servo when revving is applying the brakes a little.

The spring in the servo ought to be strong enough to prevent this happening but with a v8 there is quite a vacuum.

What servo is fitted? Is it off a smaller engined car?

Has the V8 got an interesting Cam which makes good vacuum?

Rovers are easy to trouble shoot, someone somewhere has done it so info is out there.

I rebuilding one at the moment and have just vernier timed the cam and been looking at servos so this is fairly fresh to me!

Eta : is the bulkhead flexing?

Edited by Getragdogleg on Friday 19th June 18:40

[Ralphypants]

I shall look at the servo tomorrow. I have no idea what car it’s from, I think the cam is a mild one rc87 I think and compression is around 150 .

[gazza285]

The only thing connecting the engine to the servo is a bit of pipe. The only thing that does anything in that bit of pipe is the non return valve.

I would be checking the non return valve on the bit of pipe.

[GreenV8S]

I've never experienced the brakes getting applied more by revving the engine and this doesn't feel like a very likely explanation unless the effects happen after you have revved the engine and then closed the throttle again.

If it's related to the servo, you might be able to feel the effects if you put light pressure on the brake pedal and then rev the engine. For the brakes to be being applied, it would have to be pulling the master cylinder down and you would feel the pedal following it down.

[Getragdogleg]

gazza285 said:

The only thing connecting the engine to the servo is a bit of pipe. The only thing that does anything in that bit of pipe is the non return valve.

I would be checking the non return valve on the bit of pipe.

If that valve was faulty then the only thing that would happen is the vacuum would vanish as soon as the engine was switched off.

It has no bearing on making the brakes work, it just to preserve the vacuum for a few pedal presses if the engine is off.

[gazza285]

Getragdogleg said:

I'm suspicious that the vacuum being made in the servo when revving is applying the brakes a little.

There is less vacuum when the engine is revving, as the throttle is not throttling, the vacuum is produced on the overrun after you close the throttles.

Also the vacuum does not pull on the servo, both sides of the diaphragm are subject to the vacuum, it is when you apply the brakes that the vacuum behind the diaphragm is reduced, and the diapragm moves to assist the master cylinder actuator. When you release the pedal, the pressure difference between the sides are equalised, so the vacuum alone will not actuate the brakes.

[Getragdogleg]

gazza285 said:

There is less vacuum when the engine is revving, as the throttle is not throttling, the vacuum is produced on the overrun after you close the throttles.

Also the vacuum does not pull on the servo, both sides of the diaphragm are subject to the vacuum, it is when you apply the brakes that the vacuum behind the diaphragm is reduced, and the diapragm moves to assist the master cylinder actuator. When you release the pedal, the pressure difference between the sides are equalised, so the vacuum alone will not actuate the brakes.

Yes, I hadn't thought it right through.

I'm blaming beer.

[Getragdogleg]

So following that logic the valve between the two servo halves is possibly faulty and is allowing the diaphragm to move when the vacuum drops on the engine side when being revved.

I'm too tired. I'm off to bed.

I will return tomorrow and be educated further I am sure.

[Ralphypants]

Update. I’ve had a very good look at this now, yes the pedals are mounted on a common shaft and yes the brake pedal does move and it moves more the harder you press the accelerator. It’s very slight so I never noticed it. Odd thing is that there is a spring connected to the above bulkhead that was hooked onto the brake pedal in order to pull it up. Therefore this was an existing problem that was kind of bodged. With the spring reconnected, well it helps a bit but it is no cure. Come to think of it there was always a bit of a whistle from the rear inboard brakes when I was on throttle and vanished when not. Therefore I have concluded that when I’m pressing the accelerator the brakes have been operating all the time :?
So, what is the cure? New pedal box or tougher spring?

Colin.

[gazza285]

All the pedals should rotate on the pivot independently, so the first thing to look at would be why the pivot is moving with the accelerator pedal, then why is the pivot moving the brake pedal?

It might not have been lubricated when assembled, and it isn't the easiest place to get to to luricate further, so if you don't use it much, then it might be a build up of dried grease, crap and rust binding them whole lot together.

[Boosted LS1]

I would test the car out without servo operation. Blank the servo line to the engine. I suspect you have to much vacuum especially if the throttle shuts hard after a dose of revs. A smaller diameter servo may be the answer or in line restrictor.

[gazza285]

You cannot have too much vacuum.

[Boosted LS1]

That would depend on if it's applying your brakes surely and sucking on a large diameter diaphragm? In which case you'd be better with a bit less vacuum.

Edited by Boosted LS1 on Saturday 20th June 17:34

[gazza285]

Until the pedal is pressed, the pressure is equal on both sides of the diaphragm, so the amount of it is not important. The valve that controls the pressure difference is attached to the pedal, the vacuum alone cannot apply the brake.

Anyway, it sounds like the OP has found the problem, after stating that there was no binding or rubbing, it now would seem that the pedal box pivot is binding or rubbing on both the accelerator and brake pedals, rather than them pivoting freely.

[Ralphypants]

Morning all, here’s a quick update. The pedal box does have a bit of character but I will deal with that later. I decided to go for the servo. I’ve blank off the vacuum hose from the engine and went for a test drive. I still have great brakes, more feel to be honest as my vw buggy has no servo either and the problem has vanished. The car pulls better, sounds better I’m happier. I shall look into what the cause was in time as it’s always good to know.
Thanks for everyone’s input on this problem of mine, sorry if my details weren’t as accurate as they could have been at the start.

Colin