tj4fa

Hi I'm a new member - I tried to post this during my probation with photos but it never made it through to the real world here.

BLUF (Bottom Line Up Front) - Idle Surge - Vacuum Leak

This weekend, I pulled the master cylinder off the brake booster out of my wife's Civic as the brakes were still feeling weak and low in the pedal after bleeding the system.
Immediately after removing the master cylinder, I needed to reposition the car and started it up and the car started getting the typical high revs and then surging up and down of the RPMs.

Thinking something ELSE went wrong that day, I researched for a few hours of people trying to fix or replace this or that, then decided to start with the last thing I did.
I never did anything with the brake booster vacuum line but thought it might somehow be leaking vacuum.

So I disconnected the vacuum line from the intake manifold, plugged the intake port with a short piece of hose plugged with a bolt and cranked up the engine.
Immediately, the engine was running back to normal, rock steady idling at normal rpm and no more surging up and down.

Maybe this might help someone in the future track down a phantom idle surge.




Vacuum line on Intake Capped with Hose and Bolt and Isolated from Power Brake Booster - No More Surging RPMs

Oh Yeah - Wife's 93 Civic EX Below - That's why I joined here.


Wife's '93 Civic Stock EX AT - 127K Miles

mk378

The booster holds vacuum with the rubber seal around the master cylinder rod. It's a huge vacuum leak to take the cylinder off.

tj4fa

I didn't hear any vacuum air leaking when depressing/not depressing the brake pedal but didn't know that and that is Really Good To Know - Thanks!!

Now that's known, it could be a quick place and easy method to check for an unknown brake system vacuum leak (leaking vacuum line, check valve, leaking brake booster
or master cylinder booster seal etc., rather than someone jumping into tougher areas to tear into and check or just going out and throwing new parts at the problem (keeps the economy running).

Now you got me thinking dammit...(not something that I'm used to - and now my head is starting to hurt).

Since now the car is running back to "normal", If one take's a vacuum gauge reading at that intake vacuum port at standard idle with the engine warmed-up without any accessory systems running that pull vacuum,
we could get a normal idle benchmark reading of our engine's "normal at idle" Vacuum.

And again, while the engine is running normal, one takes another benchmark reading beyond normal idle and the point of idle surging, let's say 3000 rpms, w/throttle plate about half open, we would have a second
vacuum benchmark for future reference.

Later on, when we run into a low idle rpm surge problem, we could easily check at that intake port at those two benchmark rpm points to see if somewhere there is a vacuum leak difference that could cause the dreaded idle surge.
If the vacuum benchmarks are good, then the idle surge problem is likely somewhere else. If the vacuum is out of whack, then a leak in a vacuum driven accessory may be the problem and can be troubleshot individually.

Am I "off" or "way out" there in my thinking?
If so, then let's hear it or flame on...

NVturbo

You have a vacuum leak in the brake booster. Plugging up the port (for the BB) eliminates the idle surge but doesn't fix your problem until you do something about the BB.

tj4fa

Yes thanks I understand that.

Right now, I'm waiting for the new master cylinder to show up and once I install it, the vacuum leak issue (apparently caused by the master cylinder removal causing a massive vacuum leak as pointed out previously by member mk378) may be resolved.
The thing is, is that someone else with an idle surge problem may have a vacuum leak in the brake vacuum system and never even consider troubleshooting this area.

A lot of people jump straight to FITV valves, TPS and MAF sensors etc. but may be something as simple as a cracked or disconnected vacuum line or something else with a blown vacuum diaphragm.

NVturbo

thing is....you removed that "portion" out of the equation and idle went to normal. That was a simple troubleshooting there...

tj4fa

Yes it was simple except for the couple hours of researching the internet for other people's attempted causes and fixes for the dreaded idle surge.
And right before isolating the vacuum line because it happened right after pulling the master cylinder, I had already dug into the spare parts box and
was thinking about changing out the TPS with a new one I had ordered a ways back that I hadn't installed yet...

I probably ordered that to fix something that turned out I didn't need it at the time...

mk378

A general rule is to have all major parts installed and hooked up before concluding that there is a problem.

The MAP controlled engine such as these will idle too fast if there is a vacuum leak. This is in contrast to the MAF controlled engine which tends to stall out when it gets unmetered air.