High Idle, erratic revving, dieseling 1983 Nissan 720 4WD Z24

[NotAGardener]

Hi all, 

 

I'm brand new to the forum, and I can't thank you all enough for your infinite wisdom.

 

I've got something quite interesting going on with my throttle body that I thought I would share. 

 

I'm from California and I'm trying to get my 720 smogged, I replaced the AB valve and BPT with parts I found in a junk yard (my year has a BPT and not a VVT also I have about 4 extra BPTs I found at the junk yard that all work in case anyone is in need of one and doesn't feel like spending 70+ bucks). I wanted to clean the EGR out but it had pretty much seized to the EGR pipe from the exhaust manifold so I sprayed some carb cleaner in there while it was disconnected from the throttle body; I could see it working when I applied vacuum to it. I was very happy about that. 

 

Then in a moment of idiocy I decided to tighten the spark plugs without testing the engine after doing crap to the EGR. They all seemed to be in need of a little tightening and they are all about 2 months old. After I cleaned the EGR and adjusted the spark plugs the madness started...my car started revving very high 4000+ RPMs, sometimes it would waver a bit. Took it around the block and it was accelerating for me, I could kind of coax it into driving normally when I shifting. I had also adjusted the air/fuel ratio and throttle speed screws to the best of my carb's abilities as per a thread here, and it idled around 1100, this was a temporary fix until my new Carb came. Anyways, after I turned the engine off the car started dieseling for about 30 seconds, and it sounded miserable, then at the end of dieseling it had some short-lived smooth strokes with a whistling noise at the end as it died. Oddly, during the dieseling I could throw it into reverse and it would stop the dieseling dead in it's tracks. So I took the throttle cable off to find the source of the issue went back to turn the engine on, standing outside of my car, when the car decided it would throw itself into reverse and accelerated down the street into my neighbors car...I guess it has an odd affinity for reverse?

 

So after calming down for a bit and putting ice on my joints I assumed something was keeping the throttle open or the throttle position sensor was faulty. I unplugged the TPS and then the car idled normally and died unless I eased a little gas into it. I added carb cleaner over night (I'm trying to keep the revving at a minimum for my neighbors) , then I got blue smoke in the morning...I turned it off then on again, no smoke and it was idling around 2000 RPM but it was driveable. So I thought I had fixed the gunk that was probably holding the throttle open, even though by inspection it didn't appear to be open very much at all. So I ambitiously tried to get the idle back down by adjusting the screws again...and I'm back to square one. 

 

One thing that occurred to me is that the timing in the dizzy is off since the voltage in the spark plugs might have changed after tightening, but in the 1983 the ECU or ECC is different than the later years and I'm not even sure if resetting it is an option. Anyways, I know I should check the timing and what not but I can't even begin to do that with it throttling out of control.

 

Meanwhile the new TB is on the way, I will just replace it, float valve is F'd and I'm willign to part with $150 to not have to think about the TB for awhile. I know the solution is on the way, but I would be curious if you have any clue about what this madness could be. Thanks a million!

 

-Mike

[Rhino13]

Check all vacuum lines, usually it will make it die but if you messed with the mixture screw then it's possible there's a vacuum leak causing it. This same thing happened to my truck when it had the stock carb, the choke valve broke and was causing the choke butterflies to flap around causing the idle to spike.

[datzenmike]

When warmed up is the choke shutting off? Is it fully open?

 

One thing that occurred to me is that the timing in the dizzy is off since the voltage in the spark plugs might have changed after tightening, but in the 1983 the ECU or ECC is different than the later years and I'm not even sure if resetting it is an option. Anyways, I know I should check the timing and what not but I can't even begin to do that with it throttling out of control.

Meanwhile the new TB is on the way, I will just replace it, float valve is F'd and I'm willign to part with $150 to not have to think about the TB for awhile. I know the solution is on the way, but I would be curious if you have any clue about what this madness could be. Thanks a million!

-Mike

'Tightening the spark plugs' will not affect the spark nor will it change the timing.

You don't have a throttle body but a feedback carburetor in California. It will have a round electrical connector with 6 wires? The exhaust manifold will have an O₂ sensor in it. The ECU uses the sensors on the carb and the O₂ output to keep the fuel mixture within a narrow band by constantly adjusting a solenoid jet inside the carb body.


Randomly replacing parts gets expensive. (although I can't see how you can replace the carb for anywhere near $150) If you are just going to replace everything until it works it doesn't really matter what the cause is.