How to rebuild your motor through misdiagnosis and head scratching

[xsdg]

Well, when it's in gear, there will be load on the engine because the tranny locks up to the driveshaft (at which point you're just mixing the oil in the torque converter).  In Neutral, the tranny input shaft is freewheeling.

 

Does the auto have a Park position?  If so, does idle RPM stay at 3k in Park?

 

What happens if you do a brake load test?  In the driver seat, hold the brake firmly with your left foot, gently rev the engine with your right foot.  Does it stumble?

[supererogator]

Yes to both.  Stays at 3k in park, and stumbles when revved and in gear with foot on the brake.

Further diagnosis and research have taught me that the thing I removed was the "AB Valve", aka "diverter valve".  I believe I should be able to delete this part with no adverse effect.  A picture of where it was, and the hose I can pinch to get proper idle in neutral/park.  

Untitled by supererogator, on Flickr

I did some more tuning/fiddling and I got the truck to drive around the block....farther than she has gone in months.  She really struggles to go anywhere.  I know the timing is on, as I set it with a friend the other night.  In gear and still, she idles great at 1k.  She still doesn't rev quite right though and is still sucking TONS of air through the hose that used to connect to the AB valve.  Also while going around the block, she seems to change gear at inappropriate times.  A pal who has done some Datsun work (as well as many motors including fishing boats) mentioned the possibility of a vacuum to the tranny, but I cannot find anything in the books about it.  Could that be an issue?

[datzenmike]

Find this fitting on the rear intake runner. The large hose goes to the brake booster but auto cars have a very small tube with a hose on it that goes down behind the motor to the transmission.n

 

 

[xsdg]

The AB valve can be deleted without issue, but you must cap the vacuum ports that it connects to.  Note that my knowledge is from an '85, and I don't know if the A.B. valve on the '80 is the same or different, so if stuff ain't matching up, that's likely the cause.

 

If it's like mine ('85), the A.B. valve has 3 ports: outlet (to the manifold), vacuum signal (from another part of the manifold), and atmospheric-pressure air source.  How it's supposed to work is that a vacuum beyond a certain level on the signal port will pull open a diaphragm and let fresh air directly from the air cleaner into the intake manifold.

I had to remove mine as well, because there was a hole in the vacuum signal diaphragm, and so applying any vacuum to that port would actually suck in air all the time (including at idle), rather than just let air pass through during high vacuum conditions.  It also meant that the valve would never actually open properly.

 

As far as removing the valve, the outlet to the manifold has got to be plugged.  The source for the vacuum signal has to be plugged as well.  The atmospheric-pressure air source comes from the air cleaner, and shouldn't really matter as far as how well the vehicle runs.  The only purpose for that is so that the air _to_ the AB valve is filtered.  Without the AB valve on there, it will serve as an air source into the air filter chamber.  You'll see vacuum there because there's less resistance compared to pulling the air through the air filter, but that should be it.  For the purpose of not letting unfiltered junk get sucked through the carb, though, you should probably plug that port as well.

 

I got a variety set of vacuum plugs from Pep Boys and that did the trick.

[supererogator]

Thanks guys.  You are truly some knowledgable folks.  I have three books, and the internet, and I almost always get pointed back here.  

I located the tranny vac hose, thanks to King Rat's photo.  Couldn't have made it any simpler.  Alas, after reading what xsdg said, I started plugging holes, as it were.  While doing so, I discovered that the hose I took off the carb initially, was not the one that was supposed to be there.  I know this because I was a good boy and labeled every hose one by one as I disassembled the thing.  So, this thing has never been vac lined correctly.   I have the factory manual, and the identical picture is in my Clymers...the info in the Chilton is crap most of the time (great carb section however)  What I have been scouring for is a photo or schematic/diagram of the vac lines on a late L20.  I have been trying all kinds of things in this site, and google's search engine, and I bet it is out there...anyone know where I would find it?

Result, before my search began (other than running out of gas again and killing the battery again)...With the hoses to the AB valve plugged, it ran, but poorly.  Did not rev correctly, tranny took 2.5 seconds (not an exaggeration) to drop into gear, couldn't get my idle above 1500 in neutral/park.  Found hose incorrectly attached after reading manual/s, attached correctly, and had mixed results depending on what other vac hoses I hooked up/disconnected (AB valve hoses plugged seems to work well, but I am thinking about getting the new AB even if it is $70 and a 3 day special order from Autozone, the only place in town I can get it, as minimal air flow seems to be working best, and from the above description of the "thing-a-majig's" job).  See there is a true hodgepodge of lines in there now.  Many different styles and kind of connectors, so I know it has been fooled with long before I got the truck. Back to my original question... anyone know where a better schematic or a photo of correct hook-up for the vac lines is?  I can figure this out, but it could be easier....I obviously also have a vac issue.  I really wonder what and who did this to this truck, as well as how it ran for three years with all these things out of whack.

Thank you again.  You guys ROCK.

[supererogator]

Found it..I think.

http://community.ratsun.net/topic/57388-vacuum-system-help-on-an-l20b-datsun-620/

 

 

Thank you again....

[supererogator]

So, after disconnecting and plugging all vac lines except the one that runs from the carb to the dizzy, the fuel line from the carbon can, and the one to the auto transmission from the manifold, I am having the same result.  Super high idle, and very slow transmission action.  I read, in my books, that transmission fluid level can strongly effect response when shifting into gear.  Could it be an issue with the idle too?  I am a bit low (reads at the L end of the stick), but it would seem to me that it should drop into gear quickly, not 2.5 seconds after I shift into drive.

I am so very stuck and have been scouring these pages for an answer...alas, I am STILL not on the road.

My other thought is that I may have done something incorrectly while rebuilding my carb and that could be causing the idle issue, but then why the tranny one?  I do seem to be running rich as I have bluish smoke and go through a gallon of gas in 2 hours or so of trying to get her even.  I know nothing other than what I have read in my manuals about auto trannys, but I have a decent idea about what a carb is and does.  So, would you, if it were you, pursue the tranny issue, or pull the carb and rebuild it/replace it?  And if it is the carb your choice, 32/36 or 38 Webber?  The 32/36 is less expensive (so I like that idea), but another rebuild on my sHitachi is also an option at even less money.  If the tranny issue is your choice, start with fluid and then what?  The tranny was rebuilt about a year before I bought it (making that rebuild 4ish years old).

 

 

Something has got to happen or there is going to be an ad for a not properly running 720.  I simply cannot afford to continue to keep my life, outside of walking distance, on hold.  I love this little truck, and have learned to love it more over the last two months of trying to get her back on the road, but I am also an only parent with an active teenager who needs to get places.  Thank you again for your help and love of these old vehicles.  They truly are the tops, but you folks are the best group of car geeks I have run into.

[bananahamuck]

There is a few guys that might be of help in this thread,, Maybe put out the word without sounding to needy and offer beverages and food OR maybe a couple bucks.. By what i have read ( in their posts) a few of these guys could help you figure it out on the cheap.


http://community.ratsun.net/topic/53010-calling-all-eugene-area-ratsuners/

[xsdg]

I would go after the transmission issue first.  It's a lot easier to drive someone with a working transmission, even if you're bleeding gas.  That said, it's totally possible that the issues are related.  I don't know very much about auto trannies, but everything I do, I learned from these videos:

Eric The Car Guy:

and

 

realfixesrealfast:

The first four are the teardown, which is all you need to see how the different pieces are laid out.

https://www.youtube.com/playlist?list=PLAvS7dDmwYw9F4zsV2e8jZbgaxjBVUZaI

 

 

Going by what little I know, "slow to shift" makes me think low pump pressure, since everything in there is hydraulically-actuated.  That said, it just occurred to me that the FSM has an AT section, and the AT section has a _giant_ troubleshooting subsection (starting on AT-33 in my 1986 FSM).  Can you describe very explicitly exactly what you do, and exactly what happens when you do so?

 

For instance: You say that the tranny is slow to shift.  Does it slip at all once it's shifted?  Does it behave any differently in D or 2 or 1?  Also, what color is your transmission fluid?

[supererogator]

Fluid is clear and pinkish. It is also new as I pumped most of it out before I put the radiator back in. I will be roping it up in the next day or two as that was recommended in my factory manual as well. I will also absorb all I can from those videos.

 

The rest, well, it slips I'm and out of gear wether in drive or reverse, but that should be oil pressure too. No studder, and once engaged it seems fair enough. I will also go through the FSM directions after filling all the way.

 

Thank you again.

[ggzilla]

I do seem to be running rich as I have bluish smoke and go through a gallon of gas in 2 hours or so of trying to get her even. I know nothing other than what I have read in my manuals about auto trannys, but I have a decent idea about what a carb is and does. So, would you, if it were you, pursue the tranny issue, or pull the carb and rebuild it/replace it?

Using that much gas means it is rich. But blue smoke does not - it means the engine is smoking. Do a wet/dry compression test to make sure the engine is worth troubleshooting.

 

Fill the transmission after it get hot - 15 minutes idling. The. Drive it for 10 minutes and check the level again. Temp has a huge effect on auto transmission fluid level.

[ggzilla]

1. Put a new rebuilt Hitachi on it $200

2. Put a good used Hitachi on it

3. Put a new 32/36 on it

4. Put a new 38/38 on it

[supererogator]

Well, it was the tranny fluid.  Tranny behaves normal now.  Tuck, not as much.  Still issues.  Anyone have a favorite Datsun mechanic in the Eugene area?  I think I just need to get her to someone else.

Things I just did, after putting more tranny fluid in, and in no particular order:

-Adjust and re-adjust carb  Mixture screw seems to have little effect, idle screw seems to have erratic effect, as in, adjusting too low causes immediate engine death, not slowly moving toward stopping, I mean immediate death within less than half a revolution.

-checked oil for water/contamination.  None.

-re-check compression.  All cylinders are better than 100-120psi.  Plugs are black (assuming this is running rich)

 

-connect and disconnect AB valve.  Runs when hose is open, runs well when partially open, dies when plugged or AB valve is installed.  Bad AB valve would be my guess.  Should be able to plug the thing and delete it, but apparently not at this point.

-basic carb checks.  Secondary (lower) choke seems to not open correctly, as I can manually open it and revs up correctly, when I do not manually operate it, it revs 1/4 throttle and then any more throttle and it looses acts flooded and dies.  Both jets shoot clean, clear, and visible fuel.

 

-Adjusted timing, again.  Still on.  Just under 20 below in idle (normal for when the truck is not in gear).  This puts me at 12 below in gear.

 

-plugged and unplugged all vac lines to no effect other than mentioned in the AB valve part.  Again, All I have connected are the carb to dizzy, manifold to auto tranny, and carbon can to fuel lines.  Everythign else is plugged.

I am thinking I buggered my carb rebuild, and/or I need a professional.  It is either a credit card buy for a weber or a credit card buy for a professional mechanic.  Either way, I am out of things I believe *I* can do.

Hints if ya got 'em, mechanic recommendations if ya got those too.

Thanks for accompanying me on this journey.  You have been great companions, and I hope to soon post about rat rodding seats and moving on to front suspension, and you know, other stuff.

 

 

 

[ggzilla]

-Adjust and re-adjust carb Mixture screw seems to have little effect, idle screw seems to have erratic effect, as in, adjusting too low causes immediate engine death, not slowly moving toward stopping, I mean immediate death within less than half a revolution.

 

...

 

-connect and disconnect AB valve. Runs when hose is open, runs well when partially open, dies when plugged or AB valve is installed. Bad AB valve would be my guess. Should be able to plug the thing and delete it, but apparently not at this point.

 

OK - sounds like a vacuum leak. And the carb has been adjusted to try to compensate.

 

1. Plug it

2. Turn the idle speed screw out to normal speed. Maybe a turn to start with

3. Turn mixture screw out to two turns

4. Start engine and adjust carb

[supererogator]

Yeah I thought that, but the lack of carb response , even after setting screws (idle at barely touching and mixture all the way in and then two full turns out to set . Leads me back to *my* carb rebuild being bad.

[ggzilla]

A good carb will do that when there is a large vacuum leak and the carb is adjusted to compensate.

[supererogator]

And then I lost my 10mm box wrench.  Luckily my kid found it in my pocket while doing laundry.  Ever been there?

Still trying.

[supererogator]

Ok, my Weber 32/36 is here, and mounted.  I was wondering if anyone had a picture of their conversion, especially the throttle cable set-up on a 720 with an L20 (an Automatic would be even better).  I think I have it figured, but I would love to see a working solution before I go wrong.

Also...what does everyone do with their EGR ports on the manifold?  I saw a thread here, on a 510, where the guy basically cut off all the not needed pieces and sealed the holes, but I am thinking a plate is more proper.  I built one out of some sheet metal, as the one from Weber, was for some other  vehicle...I plan on having a machinist pal let me build a better one.  

Also, I got to meet another Ratsun guy, in person, and I got rid of my AC radiator. Thanks Rockdog, and thanks for being such a good crew.

Cannot get my camera to load pics, but I am working on it.  Add edit:

Got it all together, it ran great, around the block and just a bit of messing about to make it perfect...until.  Motor is seized.  Totally stuck.  Pulled starter, bench tested fine (should, as it is two weeks old), cannot manually crank the motor foreword or backward.  Pulled plugs, everything looks clean and clear.  Magnet in hole didn't bring anything big out.  Found a tiny bit of metal filings near #2-3 valve rockers....I am now pulling the head to see if there is anything inside the thing, or a dropped valve, or something.  

The truck made no odd noises, and as I said, ran beautifully.  Then it just stopped moving, luckily in the driveway.  It ran, I turned it off, I went to start it and nothing.  tried to manually crank it, nothing.....  Back to the forced learning, I suppose.

 

[xsdg]

:o(  That was me two weeks ago.

 

When you say it stopped, how quickly did it stop running?  Did it happen all of a sudden, or did the RPMs drop steadily over time?  I presume you know what rod knock sounds like, and that you didn't hear any, right?  Either way, you might try draining the oil to see if there's anything untoward in the bottom of the oil pan.

 

Guesses: Timing chain?  I don't think anything else in the head would keep the motor from turning at all.  Unfortunately, everything else my mind jumps to is bad news.  I have a race car that's gone through two engines due to a variety of oil-supply-related maladies, including a failed oil pump and unexpected items* in the oil pan.  If you've got bearing problems, you'll generally see the brass-colored bearing material in the oil pan.

 

* http://blog.doppler-photo.net/2012/04/can-you-hear-me-now.html

[supererogator]

No rod knock. She was just running one minute, I shut her off, and then, no turning. Nothing in the head, chain is new...now to check the bottom end.

 

Thanks for commiserating. I saw your thread as well.

[rockdog]

Oooooooo.......UNBELIEVABLE. .........THAT FRICKING SUCKS!!!!!!!!!!!!!......and things were going so well.........if it were a air cool vw I'd know exactly what the problem is..........

[rockdog]

Commiserate...............had to look that one up...........k.........got it........

[rockdog]

Also ...........yes it was good to meet you also my friend.....and thanks again for the a/c rad....wish I could help out but a little to far........I did go to sherwood junk yard up here today and saw an 83 4×4 just fresh in the yard........if you have a smart phone or computer......look up row52.com......you can look up the years and make that you're looking for...80 through 86????? For the 720s......and it has how long they have been there.....so you can judge how picked to the bone they are ......good site or app if you want to make sure the junk yards have some 720s in it.......things get picked over pretty fast up here though........the 83 was still complete today..........

[supererogator]

In '81 the motor changed, so I need to find older ones. I am installing Row52 now.  Thanks for the idea.

[supererogator]

OK, head, as I said, is clean and clear.  Turns when not attached to the motor, and does what it is supposed to do.  I reattached it and tried to spin the motor again.  no go, not at all, not foreword, not backward, no movement.

I drained the oil.  I saw no metal filings, and nothing else that would indicate broken pieces.  Oddly, the oil smells like gas....like a lot.  It is also kind of thin.  Not watery, or milky, just gas odor and thin.

At this point, I am thinking I may need to give up on her (and I mean either totally, or buy a new lower end and start over, but I cannot continue to have a non-running truck).  What would you do?