78 280Z - Finally got it driving - High Idle, will occasionally die (but restart), sound like it has a misfire

[NocturnalEmber]

As the title states, I finally was able to start my Z up and let it run/drive it for the first time after buying it (brakes didn't work and neither did the clutch hydraulics; The car was backed into the garage so I didn't want to inhale exhaust fumes trying to let it run.)

Now that it can run, change gears, and stop, I'm met with my next line of issues:

- Car idles high warm or cold (1,500 rpm.) If let sitting while idling, randomly it will start to fumble and die unless you give it gas, then it generally will return to its high idle.

- When driving, it feels like its misfiring a bit. It will respond to throttle and rev, but seems down on power and you can feel it break up in the higher rev ranges when in gear or neutral.

Doesn't seem to be smoking or leaking anything, no overheating, temp and oil pressure seem normal.

 

What I've done:

Replaced thermotime switch

New oil pressure switch

New fuel pump

New inline Fram filter (off tank)

New regular fuel filter

All new injectors

Checked AFM voltage previously per FSM, readings are fine

New distributor cap

New NGK wires

New plugs

Fuel pressure is in the 30's at idle I believe (per inline gauge off the fuel filter.)

New Ignition relay (six pin one under the passenger side kick panel)

New ignition switch

 

If someone had any suggestions they could think of, I would greatly appreciate it. Excited that she's back from the nether, but given what I've replaced and checked, I'm somewhat at a loss.

 

[Patcon]

This process with be easier with an AFR gauge. That way you can differentiate easily between rich and lean conditions

[siteunseen]

Could be an air leak, those cause high idle from my experience. The throttle blade could be dirty, sticking open just a little. Check for cracks in the rubber accordion hose that connects the AFM to the throttle body. A lot of possibilities, AAR could be stuck open too. 

 

[Yarb]

BCCD valve is a definite variable.

[NocturnalEmber]

Would the AAR or BCDD be symptomatic like that though? I'll default to the experts here easily. But the behavior of it is just weird.  While letting the car sit and idle, it will stay at 1,500~ RPM, slowly drop down and stumble/die when the idle gets low enough that it can't keep itself alive. Giving it a stab of the gas pedal will bring it back up to rest at 1,500 and then the process will repeat.

What could cause it to hold idle at 1,500 that long and then drop slowly/moderately to the point the RPM's get low enough to start wanting to die?  I'd think if it were a vacuum leak that the idle wouldn't drop like that, but again, I'm really new to this platform.

Edit: Also checked throttle linkage, doesn't seem to be binding up or having any issues.

Edited February 28 by NocturnalEmber

[Yarb]

AAR valve is designed to give the engine more air at startup Once the coil de energizes inside the valve the port closes and reduces rpm. Hence there are more challenges ahead. These systems are a puzzle no doubt.

[NocturnalEmber]

43 minutes ago, Yarb said:

AAR valve is designed to give the engine more air at startup Once the coil de energizes inside the valve the port closes and reduces rpm. Hence there are more challenges ahead. These systems are a puzzle no doubt.

Indeed they are. It's simplistic in a way but challenging at the same time. I'll get a video tonight if possible, not that it will help much, but it will hold that idle pretty steadily before it starts to stutter and drop but a tap of the gas keeps it alive.  It did die once (because I let it) and started back up fine, though.

I'm moreso scratching my head because I could understand a vacuum leak keeping the idle high, but something is making the rpm's drop after an extended period of time.

Maybe when I document it with the video it will show the timing of it better. I let the engine get fully warmed up as well just to see if it would change, this seems to be the case whether its cold or warm.

[Zed Head]

2 hours ago, NocturnalEmber said:

Also checked throttle linkage, doesn't seem to be binding up or having any issues.

Try closing the throttle blade by hand from the engine bay.  The throotle bore can get gummed up causing the blade to stick. If you can close it by hand then you can focus on getting it to close by itself, like it should.

Check that the throttle return spring is intact and connected.  It's not uncommon for them to rust and break, invisibly.  It's knd of buried behind and under the linkage at the throttle body.  There's a weak spring on the blade itself and a strong one on the linkage.  A real pain to get connected if it's off.

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Edited February 28 by Zed Head

[NocturnalEmber]

1 minute ago, Zed Head said:

Try closing the throttle blade by hand from the engine bay.  The throotle bore can get gummed up causing the blade to stick. If you can close it by hand then you can focus on getting it to close by itself, like it should.

Check that the throttle return spring is intact and connected.  It's not uncommon for them to rust and break, invisibly.  It's knd of buried behind and under the linkage at the throttle body.  There's a weak spring on the blade itself and a strong one on the linkage.  A real pain to get connected if it's off.

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You're saying to grab the linkage and push it towards the closed position? I did try that the other night, and it seemed (to me anyway) that the linkage was opening and closing properly. With no pressure on the throttle, the linkage didn't seem to want to budge any when I tried to push it closed.

[Zed Head]

I'm saying that when the engine is idling high after it's been warmed up, and you think that it should be idling at a lower RPM, open the hood, and press the linkage toward the closed position.  If the blade is fully closed and the idle stays high then it's not a linkage problem.  If the idle drops, it is a linkage problem..

Just trying to use the process of elimiination.  Yo have to do it when the problem is happening though, not when it's not.

[NocturnalEmber]

4 minutes ago, Zed Head said:

I'm saying that when the engine is idling high after it's been warmed up, and you think that it should be idling at a lower RPM, open the hood, and press the linkage toward the closed position.  If the blade is fully closed and the idle stays high then it's not a linkage problem.  If the idle drops, it is a linkage problem..

Just trying to use the process of elimiination.  Yo have to do it when the problem is happening though, not when it's not.

 

Makes sense. I guess at the time I figured if it was sticking it would be sticking whether the engine was on or not. Lack of foresight on my part. I'll give it a try tonight, I appreciate the insight!

[NocturnalEmber]

So, early morning (still night for me) update!

I started the car, drove it around the block a few times, it definitely missed and sputtered on WoT, but would climb up there.

At one point it did die while sitting in my driveway (fairly certain it was warm by this point because I had driven it around the block already a few times.)

It initially didn't want to restart, but after 5-6 tries it did, and then proceeded to idle for half an hour almost before I turned it off.

RPMs would fluctuate at idle at a random interval (1,000 - 1,500), but this time it didn't want to die like it did yesterday, so the problem is definitely sporadic I guess you could say?

 

I did grab the linkage while the car was running and attempted to push the throttle closed, and it didn't seem to want to close any further, per Zed's suggestion.

 

HOWEVER! As I was staring at the engine, pondering life, I thought to myself "look at the hose coming off the valve cover"  So I did, and it was very loose. VERY loose. I felt the underside of it and there was a gigantic crack; I attempted to pull the already loose hose off, and it just fragmented into two pieces.

I put my thumb over the end that led back to the AFM accordion pipe, and the idle audibly lowered, so maybe I've got vacuum leaks to chase down? Would that cause the missing/stuttering  because its metered air that's lost?

Also, as I turned the car off I could hear a definite air hissing noise that stopped after maybe 4-5 seconds as it waned.

 

With that being said, does anyone have any recommendations on any complete kits for the vacuum/air hoses? I found this one, and it seems pretty complete, was hoping to get some opinions:

https://hpsimotorsports.com/products/hpsi-silicone-vacuum-hose-kit-datsun-280z-and-zx-1978-1983

 

 

Edited February 29 by NocturnalEmber