Vacuum sensor hold a vacuum?

Is the ignition system vacuum sensor on my '88 VR supposed to hold a steady vacuum? I checked all the related hoses for vacuum leaks and found none, but the sensor itself will not hold a steady vacuum as the vacuum bleeds off moderately. Is this a problem, or is this the way it was designed?

-Pete, in Tacoma WA USA

The sensor (aka 'Boost sensor') should hold a perfect vacuum. That includes the hoses and the sensor itself. If it leaks, it'll lean that cylinder out!! On my 89, the hose that attaches to the intake port had split (the first section).

Frank D.

This is good to know. I will try that on both of mine.

OK, so I removed the boost sensor from my '88VR for close examination for cracks and/or leaks. Found low pressure air escaping from what appears to be a designed-in vent hole located in the lower edge/corner of the three prong electical connecter blades. No cracks or evidence of damage.

-Pete, in Tacoma WA USA

'83,88 Venture

So I tested the boost sensor (MAP sensor) on my '88VR with a good vacuum pump/guage and digital volt meter, and the results match the service manual specs. This is good. The transition time for the vacuum in the sensor to go down from 21in Hg. to zero is 14-15 seconds. This is such a slow draw down that I believe the vacuum loss is intentional and a way to bleed/balance the system. So I don't think the sensor is supposed to hold an absolutely stable vacuum.

I'm now going to test the sensor on my '83 for comparrison. Seems they both have the same part number.

-Pete, in Tacoma WA

'83,88 Venture

Well I vacuum tested the boost sensor on my '83 and it holds a steady vacuum. The sensor on my '88VR slowly looses vacuum in 14-15 seconds, but it works. Next I'm going to plug the vent hole mentioned in one of my previous posts and see what difference it makes.

-Pete, in Tacoma WA USA

'83,88 Venture

Mine on the 89 holds vacuum perfectly.

Frank D.

OK, so I got tired of fooling with the leaking boost sensor and ordered a replacement from an e-bay seller. I'll test it after it gets here and before I put things back together. Sure is a bother to take so much of the bike part just to get at this little MAP sensor, then put it all back together again, especially during riding weather.

-Pete, in Tacoma WA USA

'83, 88 Venture

The sensor should hold a vacuum. A slow leak is overcome by the constant vacuum of a running engine. There is an opening to ambient but that is used as the reference pressure. The seal of RTV to the actual sensor head deteriorates and you get the leak, the amount of the leak being dependant on the amount of damage.

You can see the relative pressure openings on the back of the sensor head and lifting the board (almost always destroying the integrated electronics) shows the cracked seal.

http://www.bergall.org/temp/venture/boost1.jpg

http://www.bergall.org/temp/venture/boost2.jpg

You MIGHT be able to reestablish a new seal by carefully slicing along the housing side walls and gently pivoting the internals to get to the seal. The stuff in there is like rubber cement that is still very soft and tacky.

Thanks for the info, explanation, and images. Do you suppose the mounting location, being subject to constant engine heat, may contibute to the seal deterioration? It may be possible to relocate inside the left fairing as the lead wires seem long enough and the vacuum line is a non issue. This new location could minimize the heat exposure and resulting slow failure.

-Pete, in Tacoma WA USA

'83,88 Venture

Where this mounted/located out of curiosity ? Would 84 MKI's fit MKII's ?

OKAY.... ( I love this forum!)

I just went out and checked my 89. Held a vacuum no problem.

Checked the 87 and found it lost vacuum fast ! This might explain why the previous owner disconnected it and ran the cruise control vacuum line to the cylinder port. ( I know ...go figure ) could be part of my ongoing carb issues.

So, if this vacuum line is losing vacuum that fast, how will that affect the running and the carbs ? At start up ? when warm ?

It looks like Mk I & MK II use the same boost sensor. Yammie wants $177 (including 20% off) for one of these!!!!

Did you check it at the hose, or did you go to the sensor itself? Mine had the first section of hose split. The hose that goes to the intake port is what I call the first section, then there is a restricter, and then the 2nd section of hose that goes to the sensor. On my 83, the sensor was mounted right next to the ignition module (left side).

Frank

With the boost sensor (vacuum/MAP sensor) disconnected, the voltage signal will not arrive at the Spark Control Unit such that proper engine ignition timing can be achieved. A slow boost sensor response due to rapid vaccum loss will result in 4 things: 1) retarded ignition timing, and 2) lean fuel mix at cyl #2. and 3) delayed or slow acceleration, and 4) susceptability to preignition (pinging) or the sound of marbles in the cranckcase, which is very bad.

-Pete, in Tacoma WA USA

'83,88 Venture

I was able to pull one off of my parts bike. Will put it on my 87. Thanks to this thread, I would never have known to check that puppy. :happy65:

Although the sensor is rubber isolated to it's mountings (vibration), this doesn't allieviate it's mount location from engine heat, mine was relocated to up and to the left of the gas filler neck shroud (vertically) and it appears to be a happy camper there. You can see in the picture that there was some heat involved and a LOT of time, but I think that long brass interconnect tube was a culprit in conducting engine heat to the sensor. Relocation of the boost AND the TCI (for different reasons) is a very desireable mod.