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Weird hanging RPM issue


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1978 280z stock EFI 

I'm having a weird issue that when the engine is revved, the rpms do not go down once the gas pedal is released, the RPM's kind of just stay where they are, around 3k. I am posting because I have gone through the forums and checked off a lot of boxes, the BCCD has been unplugged, could it still be leaking air? after testing with a vacuum gauge I think that there isn't a vacuum leak, that would need to be an obvious leak also. The throttle linkage is working smoothly and the throttle plate is closing. Im just trying to think of what else could have this effect? I have an AFR gauge hooked up and it is running at about 15 when at idle and stays around there when the rpms are hanging, which I guess is quite lean. Is it possibly an EFI thing? But it needs the extra air to Rev higher? Im just trying to figure out where that is coming from if the throttle plate is closed. Anyway, I would love some ideas on this issue. Thanks.

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24 minutes ago, Pochie45566 said:

1978 280z stock EFI 

 the BCCD has been unplugged, could it still be leaking air?

Yes, it's possible.  

How long does it take for the idle speed to drop back down?  Or, what are you doing to make it drop?

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16 minutes ago, Zed Head said:

Yes, it's possible.  

How long does it take for the idle speed to drop back down?  Or, what are you doing to make it drop?

since I was driving home when it happened, I just had to keep the car in gear and put my foot on the brake. It was pretty horrible but luckily I was about a quarter mile from home. the idle doesn't drop down by itself at all

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So, what you're saying is that the engine immediately revs to 3000 RPM when you start it?  Your first post makes it sound like something that happens often, but your last one says it only happened once.  Not sure what's going on.  How do you get the idle to drop down?

Look for a split vacuum hose.

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4 minutes ago, Zed Head said:

So, what you're saying is that the engine immediately revs to 3000 RPM when you start it?  Your first post makes it sound like something that happens often, but your last one says it only happened once.  Not sure what's going on.  How do you get the idle to drop down?

Look for a split vacuum hose.

Sorry for the confusing language, im not very good at describing these issues lol. When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something. I dont see any split vacuum hoses and they have all been replaced in the last couple years. 

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When you disconnect the BCDD, it becomes enabled. That means yours is currently enabled full time, even when the car is sitting still. So it's doubtful that is what's causing the issue.

It really sounds like the throttle is hanging up mechanically. You said that the throttle linkage is working smoothly and the throttle plate is closing. How do you know? Have you manually grabbed the throttle linkage right at the throttle body and pulled it up to make double dog sure the plate is closed?  I've had issues with that in the past.

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1 minute ago, Captain Obvious said:

When you disconnect the BCDD, it becomes enabled. That means yours is currently enabled full time, even when the car is sitting still. So it's doubtful that is what's causing the issue.

It really sounds like the throttle is hanging up mechanically. You said that the throttle linkage is working smoothly and the throttle plate is closing. How do you know? Have you manually grabbed the throttle linkage right at the throttle body and pulled it up to make double dog sure the plate is closed?  I've had issues with that in the past.

yeah, I made sure that the whole linkage was working, I could take the throttle body out to make sure but the linkage has full range of motion and the spring is working well. It sounds like the linkage is sticking to me also but I don't see how that could be happening. I have tried pushing the throttle linkage back down while the rpm's are hanging and it does nothing so it seems like it isn't throttle related. BCDD developed an internal vacuum leak maybe?

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CO, didn't you do some thought-work in the past about the pressure differential across the throttle blade and how it tends to open the throttle blade if there's no spring force holding it closed?

Might just be that the main throttle spring broke.  The one in the throttle body itself will close it with no air flow, but it's weak.

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2 minutes ago, Zed Head said:

CO, didn't you do some thought-work in the past about the pressure differential across the throttle blade and how it tends to open the throttle blade if there's no spring force holding it closed?

Might just be that the main throttle spring broke.  The one in the throttle body itself will close it with no air flow, but it's weak.

The spring has been replaced in the past and while examining the linkage that is attached to the throttle body I can see the whole assembly moving and closing, when the RPM is hanging the throttle plate is shut. 

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ZH, Yes I did some of that thought work because that's the only thing I could come up with for my sticking throttle plate. I had all the springs in place, but the theory was the worn throttle shaft was sticky and a high pressure differential would pull the plate in tight which would put max pressure on the bearings (and they would stick)

4 minutes ago, Pochie45566 said:

I have tried pushing the throttle linkage back down while the rpm's are hanging and it does nothing so it seems like it isn't throttle related. BCDD developed an internal vacuum leak maybe?

Might just be a different point of reference, but when I picture the throttle body, you push down to OPEN it. Pull up to close it.

While the RPM's are hanging. reach in and pull up on the throttle linkage sticking out the side of the throttle body. Don't just look at it and believe it's closing correctly. Pull up.

What happens if you blip the throttle? Does it ever return to normal?

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30 minutes ago, Pochie45566 said:

 When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something. 

Just quoting this for reference.

Another variation on CO's last point is - pull the  gas pedal up with your toe or hand, in the cabin.  There's really nothing that gets actuated by opening the throttle that should not get de-actuated by forcing it closed.

The only other thing that I can think of that would increase idle speed and might get stuck is the vacuum advance in the distributor.  It uses ported vacuum to move the breaker plate.  But it should lose vacuum when the throttle blade is closed.

The BCDD is a weird thing though, with air passages and diaphragms that might get hung up once a pressure differential is created.  It hurts my head to study it.  All that wizardry just to keep things clean while coasting to s stop.

Not sure how you'd diagnose it.  You could find a way to block the BCDD port.  A piece of clay maybe.  Or a pierce of tape.  Should be able to reach the one in front of the blade, I think.  That's what I'd do if I had your problem and was out of ideas.  Block that BCDD air passage.

image.png

Edited by Zed Head
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My 78 would hang while driving. Ended up adjusting all the throttle linkages to remove the slop and putting a new return spring on it and that solved my issue. When the idle would hang I could move the linkage a tiny bit in the engine bay and it would drop the idle back down


Sent from my iPhone using Tapatalk

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12 hours ago, Pochie45566 said:

When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something.

I guess it's physically possible that something inside the BCDD is sticky causing it to "latch" into a semi-open position and staying that way until the engine is shut off. But I'm considering that very unlikely.

We can talk about BCDD diagnostics, but that will have to be after you pull up on the throttle linkage. Make sure your dashpot isn't sticking and pull up on the throttle linkage.

Here's a pic showing the linkage and the dashpot. To increase the engine speed, you push down on the linkage here. So to return to idle, you pull up:
P1080101.JPG

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  • 2 weeks later...

Okay, I so was pretty sure that my BCDD was gone so I ordered a block off plate, used silicone gasket maker, and installed the block off plate. When I start it now, the idle immediately goes to 1750 rpm and cannot be brought back down. the AFR is reading 11.5 so Im pretty sure its not a vacuum leak, that and it would take a pretty obvious vacuum, leak to raise it 1000 rpm. Am I supposed to block off any vacuum lines for the BCDD? What's going on? It has to be getting extra air from somewhere!

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Here is a post that Captain Obvious contributed that fixed my same issue. I plugged both holes although only one is necessary with tapered rubber plugs from Ace hardware. Covered each with high temp silicone. These can be removed if necessary if needed. Here’s the diagram. Thanks CO!!

ADA1953F-B4DD-439A-B87A-2704105B74AF.png

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Yarb, My pleasure.

Pochie45566, so as mentioned above... You can't just slap a BCDD blockoff plate on there and call it a day. Doesn't work.

And your A/F ratio is 11.5 at 1750 RPM under no load? I'm no expert on the subject, but that's something that probably needs some attention as well.

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