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Discussion Starter · #1 ·
Yesterday the car was idling at 750RPM, last night I drove it and the idle suddenly went up to around 2,200RPM and stayed there. Today I drove to work and it's still at 2,200RPM, then, one stoplight away from work it suddenly drops back down to 750RPM. There is no rhyme or reason to when it does this. And it has been doing this since I rebuit it. It is not affected by the heat of the engine. It's NOT a vaccum leak, It's NOT the idle adjust screw, it's NOT the air flow regulator.

WHAT IN THE **** IS IT???? Could it be a bad Throttle Position Sensor?

Steve G
 

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Discussion Starter · #2 ·
Have you checked your water temp sens and CSV?

Maybe it's getting an intermittant "engine cold" signal from the water temp sensor and opening the cold start valve. Just a wild guess.

What about the BCDD?

It has to be a sensor because you've eliminated a vacuum leak, idle adjust, etc. and nothing else would affect your idle speed like that. (i.e. it wouldn't be an ignition problem, etc.)

Steve G 78VR wrote:
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Yesterday the car was idling at 750RPM, last night I drove it and the idle suddenly went up to around 2,200RPM and stayed there. Today I drove to work and it's still at 2,200RPM, then, one stoplight away from work it suddenly drops back down to 750RPM. There is no rhyme or reason to when it does this. And it has been doing this since I rebuit it. It is not affected by the heat of the engine. It's NOT a vaccum leak, It's NOT the idle adjust screw, it's NOT the air flow regulator.

WHAT IN THE **** IS IT???? Could it be a bad Throttle Position Sensor?

Steve G
 

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Discussion Starter · #4 ·
Here's what the 76 service manual says...

BCCD circuit with function test connector (located by front of AFM, circular 6 pin connector)
1. check continuity between A and B when car is brought to a complete stop (I'm assuming the engine is running, manual does not say). BCCD circuit is functioning properly if continuity exists and voltage meter reading is 0 d.c.
2. check for voltage between A and B at a spped of more than 10 mph...need to be on jack stands or a dyno.

locate the notch on the test connector at the 12 o-clock position...pin to the left is pin A and pin directly across is pin B.

Solenoid valve test
1 turn key to on position
2. ensure solenoid clicks by making / breaking power lead on BCCD. if it clicks...its ok.

Amplifier test (at rear of speedometer)
1. set ciruit tester to dc amp range (1 amp full scale), connect test probes as shown in figure...I can't explain this one...you need to look at it :)
2. turn key on
3. ensure tester deflects when key turned on.
4. if tester does not deflect when solenoid valve and speed detector switch circuits are functioning properly, amplifier is faulty.

there is also a "how to adjust" section...too involved to post. You're welcome to come over and look / make a copy of the pages if you want Steve...e-mail me.
Hope this helps.
 

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Discussion Starter · #5 ·
<b>Whoa, are you talking about the same thing? --&

BCDD= Boost Controlled Deceleration Device, right? I've got one on my '82 and it's mechanical and mounted under the throttle position sensor. It has a vacuum line and does something to reduce oil consumption under deceleration if memory serves. How can this be tested with a circuit? There's nothing electrical going to it or its vacuum source.

Jon
 

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Discussion Starter · #6 ·
RE: Whoa, are you talking about the same thing? --

ummmm...probably changed in later year cars, mines a 76 and has both electrical and vacuum to it...mounted under the throttle body. I have not had to "work" on mine...thats why I titled my resonse as "here's what the nissan manual says", listed under emission control section...its purpose is to reduce HC emissions while coasting...by supplying additional air to the intake manifold during coasting. I would think they would all do similar functions, regardless of year...perhaps differences in hookups??
 

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Discussion Starter · #8 ·
Could I be CRAZY???!

It could be my imagination, but every time I put the car in reverse and back up about 20' the idle drops to normal!!!! I MUST be crazy, there is NO connection between reverse and idle! So is it just coincidence?
 

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Discussion Starter · #9 ·
Probably Are...but not because of THIS! :)

Steve, do you have the vacuum switch on the thermostat housing??? don't know if it would interact...but if you do, it is tied in with a switch on the trans...at least my 4 speed...ahhh the fun we have...checking and checking for gremlins!
 

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Discussion Starter · #10 ·
RE: Probably Are...but not because of THIS! :)

Crazy is just a state of mind over matter, if you don't mind, it doesn't matter! (or something like that!)

Nope, no vaccum swith on Tstat housing, but there is a switch on the tranny, I think its the reverse lights switch.
 
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