Brake Booster Rebuild

[Jan Gullett]

Not to long ago my brake booster bladder failed and I was driving for a while on old fashioned manual brakes. As part of an engine compartment rebuild (AC, Power Steering, Power Brakes and Repaint) I pulled the brake booster and sent it off for rebuild by a well known rebuilder.

Since then I have been troubleshooting a problematic idle on my L Jetronic system. When cold, idle is slow. As it warms up and 02 sensor kicks in idle speed increases significantly and some time it reaches the ECU cut out rpm and oscillates. This is normally a vacuum leak problem.

After thoroughly testing everything and swapping aux air valve, I confirmed a vacuum leak. The engine pulls 9 in HG at cold idle yielding about 650 RPM. After warming up the 02 sensor, idle grows to 1300 RPM at 14 in HG or oscillates between 1100 and the ECU cutout at roughly 1450 RPM with vacuum oscillating from 11 to 17 in HG. I believe normal operating condition for L Jet is 16 to 22 in HG with 20 in HG considered normal. So I think this confirms the vacuum leak as cause.

I have been using the Engine Start spray method to track down vacuum leaks. But the biggest issue seems to be my newly rebuilt Behr late coupe brake booster. I sealed the area around the vacuum hose fitting and also the master cylinder to the brake booster with very modest improvement. Testing further, the leak seems to be around the rim where the two halves of the booster mate. Spraying cold start there specifically increases engine idle speed into oscillation mode.

I have tested pretty thoroughly for other vacuum leaks by removing each hose from the throttle body or intake plenum and plugging entrance to detect any change and well as spraying Engine Start. I can't find anything. With O2 sensor disconnected or oil filler cap removed the engine stays at the low idle mode. Oil filter cap test seems to rule out an engine problem.

My question is whether this is a common brake booster problem and what I should do. I guess I could seal around the rim with caulk of some sort, but that is not my ideal solution from a visual standpoint and I would probably have to pull the brake booster to get a decent look. I could pull the brake booster and send it out again for a second rebuild, but if there is rust or some other internal issue I might end up in the same place. I could find another core and have it rebuilt but I have heard there are wear issues to be concerned about. Or I could try to find a new booster. I know that the 2002tii booster is available new but requires some fitting. Any suggestions?

 

[halboyles]

I know that the 2002tii booster is available new but requires some fitting.

It does sound like your rebuilder didn't do a proper job of clamping the two clam halves back together or that the condition of the mating surfaces wasn't optimum. We've have properly rebuilt and tested 2002tii brake boosters specifically modified to be a bolt-on replacement for the original booster. The vacuum boost is the same or better than the original and, because it is a smaller diameter booster, you gain a couple of inches or working room around it. Also, no core is required!

You can find these boosters at:

M30/2000CS tii Brake Booster | 2002s Only

That’s right! We have adapted the ATE T51 brake booster used on the 2000CS and 2002tii for use with the E9 brake system. You gain about 1 1/4" of clearance from the intake system. And, because the tii booster is a dual diaphragm version, you loose nothing in vacuum boost. This is the same ATE...

www.2002sonly.com

 

[skk]

1) consult with the vendor that performed the rebuild.
2) just for laughs, wrap the interfaces in plastic wrap to contain the leakage and see if it runs manageably.

 

[Jan Gullett]

Hal, thanks for the note. In retrospect I probably should have gone with your solution in the first place. After all my coupe is not particularly original.

I can of course consult the vendor. But I think I would like to settle on a direction first. I am sure they did the work in good faith and I know the difficulty with working with old parts. I would want to pay them again for another rebuild even though I recognize something was probably missed in quality control.

Maybe I should just seal the halves and go from there. But I am fairly certain this is the problem so I doubt this accomplishes anything but to make me more confident. I guess I might learn this sealing is helpful but not sufficient to solve the problem. Plastic wrap might do this. Thanks for the idea.

 

[Stevehose]

Pinch the booster hose or cap the manifold connection to see if that solves your problem

 

[teahead]

Not to long ago my brake booster bladder failed and I was driving for a while on old fashioned manual brakes. As part of an engine compartment rebuild (AC, Power Steering, Power Brakes and Repaint) I pulled the brake booster and sent it off for rebuild by a well known rebuilder.

Who did this rebuild?

They should have tested it to see if it holds vacuum. Pretty basic thing to do for a rebuild I would think.

 

[Jan Gullett]

I am happy to support ANYONE who helps with parts availability for our cars. New parts are great if available and used parts are valuable when new no longer exist. But those who rebuild and restore old parts get especially high marks in my book surpassed only by those that reproduce new parts.

Certainly it should have been tested before I received it. But I most definitely should have thought to test it before I went to the trouble to install it. And I didn’t. Who knows, maybe it worked fine when it left the rebuilder and was dropped in shipping or something.

I admit I don’t know much about brake boosters and rebuilding them. But I do like driving my car. So I guess I will just try sealing the clamshell rim and if it turns out problematic I will just do another rebuild driving with the current booster in manual mode until I get another booster.

Meanwhile I can see if there are any other issues with my L Jetronic. I love the system. It has been very reliable for me for over 125k miles and 25+ years.

But I have to admit I am not sure the tuning has been optimum for my modified engine (increased displacement to 3.5, sport cam, b35 manifolds, 320i intake runners, ported heads and oversize exhaust).

It seems a bit rich at start (smell). It leans out as it warms and rpms increase. Interestingly, gas analyzer indicates it is actually on the lean side in both situations. But I will post over at senior six on that since it is a 528i injection system. I know Motronic is better and there is megasquirt even, but given my very positive experience with L Jet, I am just going to tackle this for the next 125k or at least until the next engine or tranny rebuild.

 

[Jan Gullett]

Brief update.

Just for confirmation, I detached the brake booster and plugged the plenum booster vacuum inlet. After readjusting idle valve in the throttle body, the vacuum level jumped (from ~14 in HG) to 22 in HG with no oscillation. Cold start idle is still slow as low as ~300 RPM initially gradually stabilizing at 800-900 RPM as the engine reaches full operating temperature. I can see O2 sensor dropping out after a throttle blip when engine is partially warmed but it stays operating when engine is near full operating temperature with no drop out. So I can see how 3 or 4 wire O2 sensor would help a bit in this time interval.

I also double checked the vacuum booster hose fitting & seal and master brake cylinder seal. No leaks there. No change in engine speed with Cold Start sprayed toward plunger side either, only along the rim. I boned up on booster rebuilds, and noted that FDCengineers on youtube says many rebuilders pull apart the clamshell by force without relieving the tabs along the rim. According to him this can bend parts and result in rim leaks. He recommends drilling out these tabs (not the best for appearance). My guess is that this is what caused my rim leak.

My guess is that O2 meter adds air to the air/fuel ratio when it comes on line with heat resulting in the idle rpm increase. But my exhaust gas measurements don't seem to confirm rich idle (though it is richer than hot engine or 3000 rpm levels). I will do more testing on that later.