Howdy, y'all. Long time googler, first time writer :o)

[xsdg]

After getting lots of useful info from this forum and NICO, I figured it's high time I joined, asked some questions, and (hopefully) gave some answers.

 

To start off, here's my history so far with this truck:

 

I picked up a 1985 California 720 off Craigslist at the beginning of the month.  2WD, Z24, 5MT, base model, electronic carb.  The previous owners didn't smog it, which I figured meant that it wouldn't pass.  So I've got 30 days from the date of sale to get it to pass.  Here we go!

 

It ran okay at speed and once it was warmed up, but it'd idle pretty poorly, and would stall at idle until warm.  It was also lacking power in the low-mid RPM range, but felt fine off the line and once it got going.  I'm pretty handy in an engine bay (thank goodness for the 24 Hours of LeMons), but this is my first carbureted vehicle :o)

 

So, first, I replaced all the plugs.  The Intake side looked okay, but with large gaps; the exhaust side were some of the worst-fouled plugs I've ever seen.  I then used the carb cleaner trick to find a vacuum leak at the two vacuum switches on the PS in front of the firewall.  That's when I discovered that if I pulled that hose and folded it over, the truck ran a _lot_ better.  I left it that way for a week, and it no longer stalled at idle, but it also ran rich basically all the time.  I replaced all of the vacuum hoses, and didn't see much change (but they were pretty much all cracking, so it was only a matter of time).

 

I got a friend who knows carbs to take a look, and he said to rebuild the carb.  He noticed some fuel leaks and whatnot.  I got a kit and followed fastboatman's awesome carb rebuild tutorial.  There was lots of stuff gummed up in there and thankfully, I discovered that Harbor Freight sells an ultrasonic cleaner for $85 just in the nick of time.  Saved me a ton of scrubbing.  Anyway, got everything cleaned up, got the carb back on the manifold, and the truck fired on the first try.  Huzzah!  And this time it ran fine with the vacuum switches back in the loop.

 

I had started off with a Chilton's manual, and during the rebuild, my eBay shop manual (for a 1986.0) showed up and I finally figured out what the different parts on the carb are supposed to do.  It's awesome how everything on this truck is, like, a half-step before the technology you see on modern cars.

 

I used a 12V battery and verified that the anti-dieseling solenoid and the AF solenoid were working.  I also tested the choke resistance (in spec) and checked that the throttle switch was working (working).  At some point, I checked the light on the computer and it was solid on, even with the truck fully warmed up.  At that point, I figured the O2 sensor was bad (which makes sense, given the plug fouling).  I replaced the O2 sensor with a new Bosch, and also replaced the broken exhaust manifold ground strap, and now the light blinks like it's supposed to.

 

And that's where I am right now.  The fast idle is definitely high (~2700-2900 RPM), and the normal idle is sometimes fine, and sometimes a little high.  Timing is 1deg BTDC, and I have yet to figure out how to rotate the distributor :o).  It's got an exhaust leak, but it looks like all the pre-cat piping is fine, thank goodness.

 

Anyway, I'm going to check the PCV valve today, set the idle so it's right with the engine warmed up, and take it for a smog pre-test.  For now, I just want to see what the numbers are like so I can figure out what else might need attention.  I've done most of the major stuff, and that seems fine.  The O2 sensor is switching, which _should_ mean that the computer is controlling the AF ratio, but I have yet to check the voltage to the AF solenoid while the truck is running.

[datzenmike]

FYI the Mixture solenoid and the O₂ work in an open loop mode until the motor is warmed up, (and the choke off) then O₂ input is used to calculate the mixture.

 

The round, 8? pin electrical connector at the back of the carb is a weak point on the feedback carbs. The wires are too short and do not flex properly and tend to break inside the insulation often confusing the ECU as to what's going on.

 

There is also an over rich 'limp home' mode for any failure in the feedback system. This is a default condition to prevent an over lean mixture causing a burned valve.

 

 

Seems like you have this almost ready for testing. The distributor has a hold down bolt (10mm) that when loosened allows it to be twisted.... clockwise for advance and counter clockwise for retard. California Z24 is 3 degrees BTDC +- 2 degrees to allow for time of year and octane variations.

[xsdg]

datzenmike: I technically knew all that, but it wasn't at the front of my mind, so thanks for the reminder.  It looks like when the motor is warm and drops into open loop mode, my mixture goes _way_ lean and leads to a rough idle at low RPM (400-600 RPM).  Most likely the AF solenoid is compensating in warmed-up closed-loop mode.  When the engine is running in the rough-idle state, I find that if I manually choke the engine by rotating the choke flap, the idle comes back up to 800 and smooths out.  The choke flap ends up being pretty darn close to closed for it to run right, so there's definitely a significant leak somewhere.  Manifold vacuum also drops from ~20 inHg to ~15 inHg, but it's unclear if that's indicative of the leak itself, or just because the engine is running at really low RPM.

 

I've already discovered that the A.B. valve is bad — the diaphragm doesn't seal properly, and so (1) it doesn't actuate, and (2) air can get through from the signal port into the manifold.  So gotta fix that.

 

Does anyone have a source for an A.B. valve?  Preferably one that I can overnight here (S.F. Bay Area) by tomorrow?

 

Since I'm at the part of the debugging process where I'm running out of good ideas, I'm working on process of elimination now:

— The EGR valve is an obvious culprit, but when I applied vacuum to the signal port, it opened and stalled the engine pretty immediately, so it's not stuck wide open.  I'm planning on pulling it today to see if it's possibly stuck partially open.

— It looks like a damaged diaphragm in the vacuum canister could cause a leak, so I'm planning on testing that today (and capping it off to see if the problem recurs).

— The vacuum advance seems unlikely, but it's definitely something I haven't checked yet, so will be checking that out

 

Things I've capped off/removed from the picture so far:

— EGR control is disconnected from vacuum

— TVV vacuum port is disconnected from vacuum (which also takes the vacuum switch out of the picture)

— A.B. valve is removed, and manifold intake port is capped off (with a big hose and a C-clamp :o)

— Replaced the PCV valve.  My understanding is that if the new valve is working properly, it shouldn't matter if there's atmospheric pressure on the other side, so I'm not looking upstream for leaks.

 

If I have all the vacuum lines plugged and the behavior persists, I'm going to be really confused.

[Tristin]

 

 

 

 

 

 

 

 

Where are all the pictures? Welcome to the site. 

[datzenmike]

ERG can build up deposits and not close properly even disconnected. A small leak won't affect anything but idle. Take it off and wire brush clean.

 

The vacuum signal for EGR and the one for distributor vacuum advance has nothing to do with manifold vacuum. These come from a port on the inside of the carb and neither are active at idle anyway.

 

 

Things that affect the idle mixture as computed by the ECU are...

 

 

Throttle Valve Switch. Closed at idle and opens when you step on the gas. The ECU needs to know if your foot is on the gas or not.

 

Water Temp Sensor... not to be confused with the temp gauge sender. This tells the ECU that the motor is warmed up or not.

 

 

 

 

Try moving or wiggling the wiring at the back of the carb and see if this affects the idle.

[xsdg]

tristinGrind: thanks :o)  There will be some pics once I've got Truckula smogged.  Until then, it's full steam ahead.

 

 

datzenmike: I pulled the EGR valve, and the inside was caked with crud.  I ordered another one, because I'm on a deadline, and I don't feel like I can trust anything on this truck.  It should get here today (Friday), at which point I'll drop it in with a new gasket and see if it helps.  The EGR control valve checks out, even though it looks horrible, so it's going back into service.  If the EGR ends up being the problem, I'll probably see if I can't refurbish the original one and keep it around as a spare.  I can definitely drill out the rivets, but I have no idea if I'd be able to get the bottom plate out to actually open it up.

 

I was also suspicious of the water temp sensor also, but it checks out.  I took a multimeter to the ECU harness and measured 3.18 kOhm at ~15C, and 1.xx kOhm at ~45C, which matches both of the check ranges in the shop manual.

 

The throttle switch is functional (as checked through the harness).  Both of the electrical vacuum switches are functional, but I checked them before I got my vacuum pump, so I dunno exactly when they trigger.

 

I _do_ know that the gear selector/neutral sensor is shorted to ground somewhere.  It's signalling always in neutral.  That said, I'm pretty sure that's not related to my idle issue.

 

I've been working my way through possible causes of significant vacuum leaks.  It just occurred to me this morning that the brake booster is on manifold vacuum, so if the EGR isn't it, the brake booster is next on my list of possibilities to eliminate.

 

Thanks for the suggestion about wiggling the wiring.  The symptoms have changed since my original post, and at this point, it doesn't idle properly even in open-loop mode, before the exhaust has warmed up enough for the O2 sensor to work.  I will give it a try, but I don't expect to see anything.

[datzenmike]

A dirty EGR will not close properly at idle and will leak exhaust into the intake diluting the air mixture and ruining the idle.

 

The O₂ sensor does not go into closed loop until the engine temp sensor says the motor has warmed up. Probably 8 - 10 minutes or more from start up. The O₂ would be well warmed up before that. If the O₂ were in operation during warm up it would be at adds with the choke being on.

 

 

A signal that you are in constant neutral will only defeat the fuel shut off circuit during deceleration.

[xsdg]

Aah, disabled decel fuel cutoff + non-working A.B. valve probably explains why I always backfire during deceleration when I leave it in gear.

 

Got the new EGR valve today.  I noticed the diaphragm spring is a little stiffer than the old one as well.  Anyway, will drop it in tomorrow and hopefully the thing will idle finally.

 

Also, I know the seal on dipstick is partially missing.  I figured that the PCV valve should control the rate of airflow, and so the dipstick air leak shouldn't matter for idling; correct?  When it gets time to test stuff tomorrow, I'll just pull the dipstick and cover the hole with my finger just to make sure.

 

Thanks for all the help so far

[xsdg]

So, I think I've got it figured out.  Just need to get everything reattached and see if things stay good.

 

First off: I wish I had trusted the thermostat gauge.  To put the punchline first, the thermostat was stuck wide open.

 

This meant that fast idle cut out well before the truck was at the proper temperature to run without choke.  It also meant that at idle, the truck would drop out of closed-loop and into open-loop.  Open-loop wanted more choke, but because the choke heater was working properly, the choke flap was fully open by that point.

 

It also meant that the truck was running right around 45-50 degC, which is right around the TVV switching temperature.  I'm pretty sure that caused a lot of the inconsistent behavior that I was initially seeing when I first started trying to diagnose things — depending on how long I took to check things, the EGR might have been open or closed, but the truck was generally too cold to run well with the EGR open.

 

Finally, due to all that confusion, I would constantly set the idle when the truck was too cold, and then when it warmed up further or cooled back down, my idle settings would be off (this part is still a theory; have to see if my idle settings stick once all the hoses are plugged back in :o).

 

As it happens, I pulled the brake booster hose off the manifold when I was searching for a vacuum leak, and found a crack.  I don't know if the crack was there before, or happened when I pulled it, but I'll be putting it back with some RTV tomorrow, and then will replace it for good when I can order a new set of hose and install it.

 

Fingers crossed.  Out for tonight :o)

[xsdg]

Wow, have to remember to turn emoticons off :o)

[datzenmike]

EGR is temperature dependent also. The TVV (thermal vacuum valve) is warmed by the coolant. It only allows EGR when the motor is warmed above 140F. (there is no EGR at idle or full throttle) The TVV also varies the distributor vacuum advance so a stuck thermostat will affect everything.

 

Well it's good to see you getting somewhere with this.

[xsdg]

Welp, it's time to call in the reinforcements.  I've done close to everything I can do alone, and I'm pretty worn out to boot.

 

At this point, there's a non-zero chance that the engine harness and/or ECU is bad.  But I don't have a scope, and I don't have a CO meter, which means I'm pretty ill-equipped to diagnose that sort of stuff.  So time to take it to a mechanic who knows this vintage of Datsun and see if they have any good ideas.  In particular, I have no idea what the actual AF ratio is like, so I have no idea if the feedback system is working properly or is broken.

 

The current state is that it idles really high in closed-loop mode.  It's not a stuck idle kind of thing, because when I come to a stop light, the idle will drop to about 1000RPM before gradually rising up to 1200-1400 RPM or so.  If I disconnect both vacuum switches, the idle immediately drops down to 800RPM or so.  If I activate the standard-pressure vacuum switch with my vacuum pump at 20 inHg, the idle bumps back up to 1200-1400 RPM.

 

I also found that if I disconnect the AF solenoid, the engine continues idling high.  I haven't yet tried playing with the vacuum switches with the AF solenoid disabled; that's something I should try, but after I get back to all the other stuff I've been neglecting while working on Truckula.  I _do_ know that if I hit the AF solenoid pins with a 9V battery, the truck stalls immediately, so it's definitely minimally functional.  I hooked my multimeter to the pins when connected, and it was fluctuating around 1-3V.  Hard to describe the fluctuations without looking at a scope, though, and it's entirely possible the voltage numbers are bunk, depending on the switching frequency and my multimeter's sampling frequency.

[hang_510]

good luck, we're all counting on you

[jesusno2]

To bad you live in cali all your problems could be solved with some pipe plugs some aluminum and a weber.

[xsdg]

Okay, got some answers.  I took the truck to Dando's Automotive in Fremont, CA and they set me straight — apparently, the bushings for the throttle shafts are worn and out-of-round, so the throttle plates can wiggle and let in extra air under idle vacuum.  When the engine shuts off, the vacuum goes away and they appear to seal properly.  Glad I took it in, because that's not something I would've found any time soon.

 

FWIW, Dando's used to be a Datsun specialist shop, and Kevin (the owner?) is familar with the computer-controlled carbs, so if there's anyone else in the Bay Area who needs some help with their Cali-spec 720, give him a ring.

 

I figured it might be useful, so I had already bought a spare complete 6-wire carb from a junkyard.  If it were clean, I'd probably just see if the bottom section were in any better shape and just slap it on.  Given that it's filthy, I'll pull the throttle plates and see what looks like it's out-of-round.  After I know what needs replacement, I'll see if it's anything I can scavenge off the junkyard carb.  Worst case, I have friends who know how to work a lathe, and I can just buy some teflon or delrin stock or something like that.

 

The journey continues.  It's nice that I've got a clear culprit now, rather than having to throw darts blindfolded like before.