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Old 07-01-2018, 11:17 AM   #1
Figman
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Default Intermittent vacuum leak

I own a 2000 SL2 automatic with 130,000 miles. A couple of weeks ago the car suddenly began running rough and I had to keep the rpms above 2000 to keep it from stalling. At the same time, I lost all power assist to the brakes. Classic vacuum leak, right? Well I checked all the hoses, the brake booster and the check valve on the booster and could find nothing wrong. I started the car and drove it several miles with no issues. The engine was fully up to temperature. idle was normal and the brakes worked fine. A couple days go by and the same symptoms return. I was barely able to get the car home and I had no power assist to the brakes. Again, checking for vacuum leaks turned up nothing. I am afraid to drive the car now, but it is needed every day. Any assistance would be greatly appreciated.
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Old 07-01-2018, 01:01 PM   #2
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Default Re: Intermittent vacuum leak

The rubber tubes are all still supple and not dried out or cracked?
If you have a bit of tube slack, i'd cut off a chunk and reattach the tubing anyway.
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Old 07-01-2018, 01:19 PM   #3
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Default Re: Intermittent vacuum leak

The hoses all look good. I pulled them off one at a time to be sure of a good connection. I can't understand why it is good one time and bad another.
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Old 07-01-2018, 03:09 PM   #4
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Default Re: Intermittent vacuum leak

Any engine misfire when brake pedal is pressed on? Spongy or a stiff
pedal? Might be the brake booster check valve or the booster itself.
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Old 07-01-2018, 04:02 PM   #5
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Default Re: Intermittent vacuum leak

The check valve on the booster appears to be OK. I don't know how to test the booster itself. The thing is, the problem is intermittent. Sometimes it runs and brakes fine.
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Old 07-01-2018, 04:41 PM   #6
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Default Re: Intermittent vacuum leak

The reason to suspect the vacuum line, check valve and vacuum brake boost unit is your description of sudden brake pedal loss when braking then it returns. Unless you have a massive brake fluid leak (check brake fluid level), loss of braking with the brake pedal mysteriously going to the floor then operating normally suggests something wrong somewhere. Rubber ages and dry rot sets in with visible crack lines. If in doubt, replace the hose. Check valves inline with the brake vacuum hose tends to last but they too can fail. The check valve operates in one direction - vacuum sucks open the valve and when the engine stops, the valve closes. The built up vacuum in the round brake boost unit holds enough vacuum for several power assisted braking emergencies if the engine stopped while driving, enough reserve to brake to a stop. If the check valve or brake boost unit fails, power assisted braking isn't available and braking will require every bit of leg muscles to brake.

While the metal vacuum brake boost unit rarely fails from internal failure or exterior corrosion (rust), they can fail. Severe rusting on the exterior of the round boost unit behind the brake master cylinder may be a hint.
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Old 08-09-2018, 09:54 PM   #7
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Default Re: Intermittent vacuum leak

I have the same issue as well, when cold it will start up fine and run normally, even if I go on a trip and start it up in the next hour.

However if on a hot day or it just sits the sun the engine starts but idles extremely rough @ 500-650RPM and sounds like its sucking on a straw until I tap the throttle and it jumps to 4,000RPM and stays there before slowly coming down to 3,000RPM after ~30sec. If I stop-start the engine it just goes back the 4,000RPM without input (It still sounds like a straw).


The kicker is if I start driving it will go away after about 15mins, but the last time I drove it, it kept running at 4kRPM (out of gear) for an ~1h:30m and then settled down. I tried to get a shop to look at it but the CR-V incident shut down that option with them citing the liability law of Michigan.

So I left it that to decide on A. Fixing the trailing arm body side mount. or B. go to a dealer and ask for low mile '99 which I'm sure they'll charge ~5k for it, the Used car market is stupid inflated in Michigan.

P.S. the check valve on my booster still had its pressure when I attempted to take it off and cut my hand really good when it 'popped' off. So it's generating a lot of vacuum for a vacuum leak. The computer shows nominal values beside the B1S2 and EVAP Purge Code.
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Old 08-09-2018, 10:29 PM   #8
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Default Re: Intermittent vacuum leak

Medtactics, I believe your issues are not the same as Figman. He's describing low rpm and brake pedal loss of operation. Your issues are about a runaway rpm holding at 3k or 4k rpm. No mention of losing brakes.

A vacuum leak tends to increase idle. The throttle position sensor may be at fault and easily tested; once a runaway idle appears, pull off to the side of the road, park and shut off the engine for a restart without touching the gas pedal. A restart with a normal idle suggests the tps is worn out and needs to be replaced. If restarting remains well above idle, something else is failing and its intermittent. A vacuum leak is easily found by spraying water around every vacuum hose and fitting. An immediate change in idle suggests a vacuum leak from cracked vacuum hoses, worn grommets to pcv valve, loose throttle body mounting bolts/nuts, etc.. Another part that's difficult to understand is the idle air control valve (IACV) mounted on the throttle body. If a sticking or worn out iacv occurs, engine rpm may be erratic. The iacv can be easily tested but requires a steady idle before testing it.

At normal idle rpm, the iacv is the pcm's way of fine tuning idle rpm from high idle cold weather startup, ac loads, electrical loads and emissions when coasting from any speed above approximately 15 mph. With the air intake tube removed for access to the throttle plate and normal 650-850 idle rpm, cover the hole in front of the throttle plate with a finger - idle should drop down to around 500-650 rpm. Blocking the supplemental bypass air port blocks off airflow around the throttle plate where air flows around the iacv. The pcm detecting lower than normal idle immediately tries to raise idle by retracting the iacv to allow more air. Your finger blocking off the bypass air port won't allow this. Remove your finger and if the iacv is operating correctly, rpm will suddenly shoot up momentarily until the pcm detects the sudden jump in rpm by extending the iacv to choke off excess bypass air until rpm returns to idle. This jump and drop in rpm tests the pcm controlling the iacv directly. The rpm change should be sudden without hesitation otherwise the iacv is suspect.
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Old 08-10-2018, 11:34 PM   #9
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Default Re: Intermittent vacuum leak

Well IACV would explain rough idle, but the 3k-4k rpm not so much after taping the throttle as the only user input, stop-start dosen't do anything besides skip the rough idle and goes straight to 4krpm without any input. I also already confirmed with the sensor output log before hand that TPS is functional either way.

Can a faulty IACV even do 4k idle at max bypass (completely open)?
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