Idle and dying issues

[adg]

Hi, so I have a 1982 datsun 720 4x4 z22

It's a good truck with some issues and right now it won't idle and I need help figuring out why, so awhile back we tried to change the carb and had the wrong one the previous owner tried to also and cut off the plug for the choke and the Iac and it ran fine like that, however when I hooked them back up it started idling at 3000 rpm, but when I disconnected them and swapped the iac for another on the new carb that the truck came with and now when it starts it'll idle at 2000 for a few seconds and die and when it started doing that gas started leaking out of the back of the carb by the throttle linkage as well. What could this be?

 

[datzenmike]

The 720 didn't have an IAC (idle air control) valve so I'm not sure what you are referring to. The IAC is for EFI engines. Maybe post a picture?

[adg]

So after some additional research(sorry I'm a little new to these older datsuns) I found out that the part I called an iac is actually a anti-dieseling solenoid but I can't find any replacement for it, anywhere I check says it's discontinued. I'm contemplating getting a new carb for it instead but I'm not sure what would be best for it, any ideas? Preferably on the cheaper side 

[datzenmike]

Two things... the choke should be connected and working and the idle cut solenoid wired and working.

 

Best to connect the choke heater wiring. This will warm and turn the choke off after 10? minutes or less. If the choke is on or even slightly on because the heater is disconnected, there is a fast idle circuit activated and this is likely the cause of the fast idle. The choke on all the times will cause horrible mileage.

 

The anti dieseling solenoid when off, closes a fuel line passage to the idle circuit. It s only supposed to be on when the engine is running. When turning the engine off this shut off fuel to the idle circuit and helps stall the engine and prevent 'run on' or dieseling. They seldom fail.

 

Turn the ignition on off, on off (but don't start) and listen at the carburetor, you should hear a soft clicking if it is working. Does it? If yes then it can be presumed it's working.

 

If idling at 2,000 turn the idle speed screw down.

[None_zero]

On 12/9/2023 at 5:15 PM, datzenmike said:

Two things... the choke should be connected and working and the idle cut solenoid wired and working.

 

Best to connect the choke heater wiring. This will warm and turn the choke off after 10? minutes or less. If the choke is on or even slightly on because the heater is disconnected, there is a fast idle circuit activated and this is likely the cause of the fast idle. The choke on all the times will cause horrible mileage.

 

The anti dieseling solenoid when off, closes a fuel line passage to the idle circuit. It s only supposed to be on when the engine is running. When turning the engine off this shut off fuel to the idle circuit and helps stall the engine and prevent 'run on' or dieseling. They seldom fail.

 

Turn the ignition on off, on off (but don't start) and listen at the carburetor, you should hear a soft clicking if it is working. Does it? If yes then it can be presumed it's working.

 

If idling at 2,000 turn the idle speed screw down.

I'm having a similar issue but it's fluctuating sometimes the engine hunts from 500 rpms to 1500 back and forth if it's not warmed up it tends to die on the low end of the hunt. I've tinkered with the idle speed to get it to stay running at idle and sometimes it levels out around 1000 rpms or sometimes the range of the hunt narrows and goes from 1100-1300 I've checked for vac leaks checked timing all seems good except for one thing the vac line going to the port on the canister that's marked "purge" just below the one for "dist vac" is mimicking a vac leak sound however if I pinch it at the engine side it stops if I pinch it at the canister side it stops. If I pinch it in the middle... it stops. Which leads me to believe it actually isn't leaking and instead is making its normal sound. I had been idling at 3k rpms during the warm up and I don't think I did anything that should have affected the warm up cycle but now sometimes when started cold it immediately leaves warmup and dies. After I've driven a few miles and reached operating temperature I can stop holding the gas at stop signs and it will idle down so low that the truck starts shaking and you'd swear it's gonna die and then it comes back up and idles like it should 9 times out of 10

Edited December 13, 2023 by None_zero

[datzenmike]

There should be no purge signal at idle. The purge signal, if you follow the hose/line goes to the same ported vacuum signal as the EGR valve. There is no EGR signal at idle. The purge signal opens the valve on the canister

 

There is a hose to the fuel tank and there is one to the float chamber on California vehicles to capture these fumes also.

 

The last hose is under constant vacuum when engine running. It's closed at the canister and opened by the purge signal to draw stored fumes into the intake.

[None_zero]

On 12/13/2023 at 7:59 AM, datzenmike said:

There should be no purge signal at idle. The purge signal, if you follow the hose/line goes to the same ported vacuum signal as the EGR valve. There is no EGR signal at idle. The purge signal opens the valve on the canister

 

There is a hose to the fuel tank and there is one to the float chamber on California vehicles to capture these fumes also.

 

The last hose is under constant vacuum when engine running. It's closed at the canister and opened by the purge signal to draw stored fumes into the intake.

Interesting. So you're saying (if I'm following here) that the purge line should not be under vacuum while at idle and so the fact that I'm hearing it (and can stop it by pinching the hose) means the purge signal is always present ot the valve (where ever that is) is stuck open?    Oh I should add here that if I follow that line back and disconnect it from the ttv (I think it's called) then the engine races same if I disconnect from purge so evidently its doing something. Wonder if it would adversely effect performance if I just cap that line.... also now that my brain is kicking in a little... you said the valve at the canister which I assume means the one there at the top that the line connects to... which also has the dist vac on it I believe, which I sometimes have connected and sometimes don't I think right at the moment it's connected maybe 

Edited December 14, 2023 by None_zero

[datzenmike]

The purge signal comes from the base of the carburetor. It a small hole just above the throttle plate so at idle it 'sees' ambient air but when the throttle plate lifts off idle it is exposed to intake vacuum. This small vacuum signal simply opens a valve in the canister and allows intake vacuum (from a separate line to the intake) to draw in the stored fumes. The purge signal is not connected to the intake vacuum. This same vacuum signal is also used to allow the EGR to turn on. Both should not be working at idle.

 

Check the purge line and follow it back to the a TEE with one to the middle of the TVV and the other to the carburetor. The TVV, when cold, leaks air into the system and destroys the vacuum signal. When the engine warms up the TVV closes and allows the vacuum to pass to the purge on the Cannister. The TVV (again) simply prevents purge and EGR when the engine is cold. The TVV bottom hose also passes or prevents vacuum advance to the distributor when the engine is cold but is on a separate hose. 

 

Possibly the idle is set too high and the throttle plate is high enough to send a signal at idle.

 

The engine should not race when your remove the hose. The purge line is separate from the vacuum advance line.

[None_zero]

Ok I follow I think. Probably I've got the hoses hooked up wrong but how that's possible with as many times as I've been over and over those diagrams and connected and disconnected and reconnected all those lines you'd think I could do it blindfolded by now. But I'm 90% sure without looking that the purge line comes back and connects to the middle of the tvv and then to the large line at the bottom of the carb that goes to the ab valve. It has two lines coming off above and below the large ab line that comes from the manifold and its hooked up to the bottom port. Side note since I put the new carb on my oil isn't blackening anymore and gas mileage still going strong 

Edited December 17, 2023 by None_zero

[datzenmike]

Yes the purge line form the canister goes to a TEE with one hose to the middle of the TVV and the other hose to another TEE with one hose to the base of the carburetor and the last hose to the EGR. The EGR is behind the carb on the intake and has a finger size metal pipe that wraps around the back of the head and to the exhaust manifold. You might think this is the anti back fire valve but it's the EGR.

 

The vacuum signal is only present when the engine is above idle.

 

There should be three vacuum lines from the carburetor base.

 

1/ the front one is the vacuum advance. It goes to a TEE with one hose to the bottom of the TVV, the other continuse on to the distributor

 

2/ the middle is the purge and the EGR signal.

 

3/ the rear one is the VVT variable vacuum transducer. It varies the EGR and I suppose the purge signal in proportion to exhaust pressure (engine load)

 

 

[None_zero]

No I removed the egr 8 months ago which wqs already blocked off with a steel plate before I ever owned the truck. I'll post a picture of whatnim talking about 

[None_zero]

15 minutes ago, None_zero said:

No I removed the egr 8 months ago which wqs already blocked off with a steel plate before I ever owned the truck. I'll post a picture of whatnim talking about 

The blue circle is the ab valve and the red line is the main line running into the bottom of the ab valve except where ive marked it out if the three way branch right there it goes to the tvv and purge  

[None_zero]

This connects directly to the manifold below below the carb 

[None_zero]

Wait I'm telling you wrong, actually the one I've marked goes to the purge as I've said but the middle port on the tvv is going to the vac advance which is capped nope damn I am wrong again lol. I m Hate these lines. Ok it always confuses me how the hard lines flip so actually the one insaid wqs going to vac advance is going to dist vac on the canister 

Edited December 18, 2023 by None_zero

[None_zero]

I recall that the problem I always have here is that none of the diagrams actually match what's in my truck exactly and so I do part from one and part from another and it never works ojt right. I actually find it most helpful when you do like you've done above and explain the function of each part because then I can find a logical path to a solution instead of trying to match a picture for the sake of matching a picture 

[datzenmike]

16 minutes ago, None_zero said:

This connects directly to the manifold below below the carb 

 

Yes that's all fine. The AB connects to the intake manifold.

 

The three hoses I mentioned don't. They connect to the base of the carburetor. These ports open up inside the carburetor but above the throttle plate so there is no vacuum signal till the throttle is opened and rises above these ports.

[None_zero]

Hopefully this makes sense. It's my current connection arrangement and while it does work it seems lime something is not quite right 

[datzenmike]

 

 

TVV

bottom hose to vacuum advance

middle hose to canister

The canister should be marked what hose goes where.

 

[None_zero]

Ok I've looked at this many many times and for whatever reason it only just dawned on me that the only reason mine has three ports and this only lists two on the canister is because the fuel tank is the third and they aren't considering it part of this system I guess. Anyway I think if I make one change I'll be correct according to that diagram (excluding all the deleted parts) unfortunately I'll have to find...wait a min... I won't have to find a Y I remeber I used it to create a loop cap on one line so id have it on hand if i needed it yay me! Thinking ahead!

Edited December 19, 2023 by None_zero

[datzenmike]

The vent from the gas tank is mainly the reason for the canister. It's the largest producer of evaporated gas fumes.

 

That diagram only shows the vacuum line to the intake and the signal line from the carburetor that opens a valve allowing the stored fumes to be sucked into the intake and be burned.

 

Some canisters (California emissions) also have a vent line from the float chamber as well. After shut off the gas in the carburetor is also hot and out gasses fumes.

[None_zero]

Ok I corrected that vacuum line issue and I swear to God (no for real for real this time) I finally have it all connected properly. The truck seems to be running slightly better though still tends to die when coming to a stop quickly. I'm wondering if this could be the ab valve. I know this valve is supposed to inject air into the intake to compensate for excess fuel during deceleration and last time I had the ab valve off it was full of fuel (which I'm sure isn't the way things are supposed to be) haven't messed with it lately though.. or is it more likely that I just need to readjust air/fuel mix and or idle speed now that I'm vac'd properly (my thinking on the ab valve is if it is somehow holding fuel and its supposed to be injecting air maybe it's instead flooding with fuel and causing engine to die? Although I would think any fuel in there would have cleared by now since I've swapped the  carb and things are running better  

Edited December 19, 2023 by None_zero

[datzenmike]

All the AB does is open a large vacuum leak to lower an excessive intake vacuum level. No fuel is involved at all. The gas in the AB was with the old carburetor which was either faulty or some hoses may have been on wrong. Pull the large hose off the AB to intake and seal it. This will cut the AB out of the equation. Does it run better when stopping. 

[None_zero]

So I removed ab valve and cap both lines that go to it and start engine. Immediately races to 3k rpms and stays there taps on gas pedal clutch and brake all do nothing put into gear roll back roll forward..  3k rpms  can confirm ab valve no longer picking up fuel.  Something rattling in there like a bb 

Edited December 20, 2023 by None_zero

[None_zero]

Ok update I lied when turned the right way small amount of liquid with strong fuel smell came out but before it was full. Replace ab valve engine races ro 3k rpms and emits white smoke from tail after a min or two smoke clears and rpms drop and we resume the condition of idling down to the verge of stall and then back to 1500 and back to stall (sometimes stalling especially during deceleration. Damn I guess that vac line addition earlier didn't solve that after all. It seemed like it did at first 

[None_zero]

And I'm thinking... retorque the head bolts?