Idle Speed Variation

[crispy]

I've noticed that while waiting at a red light, my idle increases from ~1000 to 1300 RPM when i put my foot on the brake, and settles back down to ~1000 when i take it off again. I'd appreciate any thoughts on what may be causing this. Thanks!

Ian

 

[GregW]

Vacuum leak would be a thought.

 

[Bob_Spidell]

Does your car have a brake servo?

 

[crispy]

Does your car have a brake servo?

Yes. Is that the culprit?

 

[Bob_Spidell]

Maybe, since the problem occurs when pressing the brake pedal. In normal driving, the servo has manifold vacuum gated to both sides of the big piston then, when you press on the brake pedal ambient air is gated to the 'top side' of the piston creating the 'boost.' It's conceivable the servo is allowing air back into the intake manifold somehow, though I'm not sure how, yet. The servo should have a check valve that allows 'vacuum' into the piston chamber, it should prevent this but these parts are old. How well does your servo work; i.e. how large is the braking effort?

One quick test of the servo is to stand on the brake pedal as you start the car, when the engine fires the pedal should go down an inch or so. Also, you could plug or pinch off the hose from the manifold to the booster, but I've not tried this and there may be issues I haven't considered (don't try it on the road).

 

[crispy]

Thanks guys. I suspect the servo too...I’ll have a go with the tests Bob suggests and report back.

 

[tahoe healey]

Your idle seems very high to me. My BJ8 idles at 600 to 700 rpm. I don't know if that has anything to do with it.

 

[crispy]

Here’s the latest...to test the servo, I started the car w my foot on the brake, the brake pedal didn’t depress any further
Secondarily, I detached the vacuum hose from the manifold. Plugging the hose made no difference in the idle speed. Although, when I plugged the hose receptacle on the manifold...the idle speed reduced significantly.
I’m thinking a booster/servo rebuild is in order...anyone thinking differently???

I appreciate all of your input.

 

[steveg]

Crispy,
When you plugged the vacuum stub from the manifold and the idle reduced, you proved you have an air leak downstream probably in the servo.

 

[vette]

Secondarily, I detached the vacuum hose from the manifold. Plugging the hose made no difference in the idle speed. Although, when I plugged the hose receptacle on the manifold...the idle speed reduced significantly.

When you took the hose off the manifold and the idle didn't change that means the engine was already idling on way too much air. So you have a massive vacuum leak. When you plugged the opening in the manifold and the idle reduced that showed that you finally cut off all that excess air and the engine was closer to what it wanted.

I’m thinking a booster/servo rebuild is in order...anyone thinking differently???

I appreciate all of your input.

 

[John Turney]

....
I’m thinking a booster/servo rebuild is in order...anyone thinking differently???

I appreciate all of your input.

Unless the hose to the booster became disconnected at the other end, it looks like your thinking is correct.

 

[crispy]

Thanks for sharing your insight. Turns out the booster/servo is of the PBR variety. I’m going to source a rebuild kit and tackle it in the fall. I’ll report back on my results.

 

[Jerry]

Let us know how the rebuild works. I was told that rebuilding servos was an art and takes some practice, I have never tried myself.

 

[Bob_Spidell]

Let us know how the rebuild works. I was told that rebuilding servos was an art and takes some practice, I have never tried myself.

There's a couple 'gotchas,' at least with the Girlings, but it's doable. Austin Healey Magazine has a good article in the latest issue.

 

[Keoke]

I really do not think it is worth the risk as a DIY rebuild-?

 

[crispy]

Yes, I saw the article in the Austin Healey mag. Mine is a PBR servo, not a girling. I’ll check it out to see if I think it’s rebuildable... then go from there. If it’s not straight forward I’ll replace it.

 

[Editor_Reid]

I'm slightly disappointed that the definitive treatise on the subject hasn't been referenced in this thread. I speak of course of my own article in a magazine I produced at the time, namely pages 14-15 of the January 1986 issue of Healey Highlights, a scant 35 years ago. Seriously, I'll be surprised if it is of any use at all to you, but it was fun to go dig it out and scan it. By the way, "Carnut" referred to in the article is Bruce Erfer, as a few old-timers here may recall. Good luck.

 

[Keoke]

Reid ! :
Crispy weren't even born Then---N I bet Bruce still in Hawaii---

 

[crispy]

Awesome!!!!!

 

[Bob_Spidell]

I must be an 'old-timer;' I bought a set of wheels for my BJ8 from Bruce Erfer when BWW was his garage, and spoke to Mike Meindorfer on the phone a few times.

I remember being told by someone that the 'locking plate seal' should be trimmed to length--by a SWAG, apparently--because, as supplied it was too long and would cause the lockup described. I did, and have never suffered lockup. The later servos were a bit different, as the seal went behind the leather seal (is part# 35 a carb balance tube?). Later servos also used a dry lubricant in the vacuum canister.