The Dreaded Vapor Lock

[VaCat33]

Hello

The hot weather is here and with it comes vapor lock problems on my completely stock 1972 240Z. I have been plagued by this and it has left me stranded a couple times…at least until things cooled down. I know 73 and 74 Z’s were notorious for this problem, but I owned a 72 back in 72 and NEVER had this problem. Of course, we had ethanol free gas back then.

Before we start tearing things apart and throwing parts at the problem, I decided to check temperatures around the engine compartment to see if I could nail down a likely problem area. I performed the following test on two 90 degree days. Each time the sequence of events was essentially the same.

Started car and it started right up and idled smoothly to standard operating temperature

At 20 minutes the engine stumbled a bit but continued to idle

At 30 (test 1) and 35 (test 2) minutes the engine began to really stumble and then died

I got it restarted but could not keep it running.

At that point it would not restart

After several hours it restarted and I was able to pull it back into the garage.

While the engine was idling I took several readings and checked the temp gauge and took some temps with infrared sensor gun. At the point the engine died the Temp gauge on dash was not quite half way (front edge on M), so maybe 175 at most??? But not what I would call overheated.

Temps at various locations while running

Thermostat Housing                       140

Temp Sender                                    158

Fuel Filter                                          120

Fuel Pump

               Body                                    107

               In                                          118

               Out                                      115

Fuel Rail                                            Got a couple at 160-170 but not confident on accuracy

Float Bowl (Top)

               Front                                   130

               Back                                    126

Intake Manifold Thermostat        220 (located at the left rear on the coolant line just before it enters the intake manifold...unique to 72 models.)

Coolant line between Intakes      250

I placed ice packs on fuel bowls and wrapped an ice pack around coolant line between intakes. Got the fuel bowl temp down to just over 100 and the coolant line down to 150. Tried to start it but still would not start. At the temperatures above I do not see where the fuel might be percolating, although the manifold tube and thermostat definitely seem suspicious…but no real impact on the float bowls.

My next test is to place an inline fuel pressure gauge between the fuel pump outlet and the fuel rail and monitor fuel pressure as engine heats up…but I cannot get to that for a couple weeks.

I know the usual fixes are to wrap insulation on fuel rail and add electric fuel pump, but before doing that I am trying to nail down the likely cause. Any other thoughts…I am all ears.

Thanks, Jim

 

 

 

[VaCat33]

I should have added that as part of the baseline servicing performed upon acquiring this Z, I replaced the fuel pump and floats, needles and seats with those from Z Therapy. Also replaced coil, ballast resistor and plug wires (In case you are thinking ignition.)

Thanks again.

Jim

[DatsunZGuy]

The troubleshooting methodology you posted seems logical and I would have followed the same steps.  That said, I had a similar issue my on the 1973 I owned years ago.  The vapor lock issue was addressed as per the Datsun service directive, but I would find myself stranded at times.  In my situation it was more of a flooding issue than vapor lock.  I adjusted the floats, which seems to resolve the issue.

On interesting side note:

Years ago, I drove this very car to the Nissan dealership in Santa Maria, CA to purchase an OEM oil filter.  Then, I got in the car would not start after much cranking.  An older service technician saw me struggling and walked over.  He asked me to pop the hood open and then he began removing the plug wires from the plugs and re-inserting them back on  just short of being snapped into place.  He then asked me to crank the car again and it started!!!  I'm not sure what voodoo or magic makes this work, but I was grateful nonetheless. I had to use this technique a couple more times before eventually selling the car.          

 

 

Edited June 9, 2022 by DatsunZGuy

[siteunseen]

That 250 degree hard coolant line between the intakes seems awful hot to me. The valve should close when the temp gets up to operating range. That's for cold weather starting. I bypassed mine so no hot coolant flows through there.

My domes are cool to the touch now although I do have a ceramic coated header that took a lot of heat away.

You could check the float chambers when it won't run by trying to see if the floats are buoyant in the fuel by sticking a small red straw down the vent bung. If the chambers have fuel in them that straw should be "bouncy". Move up and down with the float.

[Jeff G 78]

Try this simple test.  Buy about 4 feet of fuel hose and a T fitting to bypass the fuel rail entirely.  Run a hose from the fuel pump output to a T fitting at the front carb and then run hoses from the T to the two carb inlets.  Finally, clamp off the return line with needle nose vice grips so it doesn't drip what's left in the rail.  

I had terrible vapor lock issues with my race car and this mod solved the problem after I tried every other possible fix.  Prior to ditching the rail, I also ran an electric fuel pump at the tank mounted on a drop bracket so it was gravity fed from the tank.  Ethanol fuel doesn't like being "pulled".  By being gravity fed, the pump could "push" the fuel to the front of the car.  This helped get the car started hot, but did nothing to fix the vapor lock while on the track.  

Different carbs did nothing, shielding the heck out of the exhaust did nothing, eliminating the return line helped a bit, but not enough.

 

[Captain Obvious]

21 hours ago, VaCat33 said:

Each time the sequence of events was essentially the same.

Started car and it started right up and idled smoothly to standard operating temperature
At 20 minutes the engine stumbled a bit but continued to idle
At 30 (test 1) and 35 (test 2) minutes the engine began to really stumble and then died
I got it restarted but could not keep it running.
At that point it would not restart

After several hours it restarted and I was able to pull it back into the garage.

So just to provide a little clarity, you said that after several hours you were able to get the car to start and pull it back into the garage. How easily did it start?

By that, I mean... After it sat for a couple hours, did it fire right up like normal and run fine again?  Or did it take a bunch of cranking and sputtering, but then eventually after enough cranking and sputtering and rough weak idle, it finally came back to life and THEN ran normal again?

Basically, I'm trying to figure out if there was fuel in your bowls when it sputtered and died.

And about this part...

21 hours ago, VaCat33 said:

I placed ice packs on fuel bowls and wrapped an ice pack around coolant line between intakes. Got the fuel bowl temp down to just over 100 and the coolant line down to 150. Tried to start it but still would not start.

Where in the timeline you laid out above did you try this? Is this after the 30 minute idling, but before the several hour wait?

[VaCat33]

Captain

i put the ice packs on about 30 minutes after the engine died and would not start. I let them sit for another 30 minutes or so. Saw that temps were much lower so Tried to start but was unsuccessful.

During both tests I let the car sit for a few hours…I did not track exactly how long. During the first test it started easier, but this might have been because it Sat a bit longer.

in both cases it started after a couple tries. I would not say a “bunch” of cranks but 2-3 with foot to floor. First test it ran smoothly and pulled right into garage. Second test it sputtered a bit and I had to feed it gas to get it running smoothly.

Hope this clarifies things.

Thanks

Jim

[Zed Head]

On 6/9/2022 at 9:57 AM, VaCat33 said:

I performed the following test on two 90 degree days. Each time the sequence of events was essentially the same.

Started car and it started right up and idled smoothly to standard operating temperature

At 20 minutes the engine stumbled a bit but continued to idle

At 30 (test 1) and 35 (test 2) minutes the engine began to really stumble and then died

I got it restarted but could not keep it running.

At that point it would not restart

After several hours it restarted and I was able to pull it back into the garage.

 

16 hours ago, Jeff G 78 said:

Run a hose from the fuel pump output to a T fitting at the front carb and then run hoses from the T to the two carb inlets

That is pretty severe "vapor lock" if it happens just sitting at idle.  Is this on a car without a fan shroud?  Sitting out in the hot sun with the hood closed?  The carbs need air flowing past them to cool, the coolant flow is for the engine.  A quick test might be an electric fan under the hood pointing at the carbs.  See what happens.

Jeff G, is your deadhead mod using the mechanical pump or an electric pump?  Your solution creates more pressure in the supply lines.  I wonder if VaCat33 has the proper orifice in his return line.  Or if a smaller one would help.  A quick test might be to clamp the return line to create a deadhead system. If it helps then your solution should do the same.

Things to "try" that might shed some "light".

[siteunseen]

The newer needle valves from ztherapy are a little tricky to set the gap between the float and lid edge. I learned the hard way that turning the lids upside down to adjust does not work like their older video where they use Grose valves.

I'm curious like Captain Obvious if the float chambers are running out of fuel then filling back up after waiting a short while. Have you tried adding choke when it starts to stumble? In my short learning curve the chokes work better than pedal.

Clarity: the fuel evaporates faster when it's hot.

Edited June 10, 2022 by siteunseen

[Jeff G 78]

1 hour ago, Zed Head said:

 

That is pretty severe "vapor lock" if it happens just sitting at idle.  Is this on a car without a fan shroud?  Sitting out in the hot sun with the hood closed?  The carbs need air flowing past them to cool, the coolant flow is for the engine.  A quick test might be an electric fan under the hood pointing at the carbs.  See what happens.

Jeff G, is your deadhead mod using the mechanical pump or an electric pump?  Your solution creates more pressure in the supply lines.  I wonder if VaCat33 has the proper orifice in his return line.  Or if a smaller one would help.  A quick test might be to clamp the return line to create a deadhead system. If it helps then your solution should do the same.

Things to "try" that might shed some "light".

In my case, I tried every possible combination of things I could think of over a several year period.  I deadheaded it first with the mechanical pump and then with an electric pump.  Deadheading did improve it somewhat on my car, but not enough.  Mine would run great for 30 minutes in a race and then start to fall on its face at high RPM and get progressively worse until it wouldn't pull at all aver about 4000 RPM.  When I'd pit for fuel, it would refuse to restart and I killed several starters over a number of races.  The electric pump didn't help until I dropped it down below the tank height as shown above.  I also immediately opened the hood at pit-in to reduce the heat soak from the cherry red exhaust.  Those two things would usually allow it to restart after refueling.  When I clamped the return, it would run longer and better as long as the ambient temp wasn't really hot.  In one 25 hour race, it ran like crap in the 85F daylight and then ran great from sunset until it got hot again around 10am.  Lap times are normally much slower in the pitch black, but we turned our fastest laps around 3am when the temps were lowest.  Ditching the fuel rail along with the other mods made my car run great even in 90+ temps in race conditions.  I sold the car before I was able to do any A-B-A testing to see if I could undo any of the mods.  I had a huge multi-layer heat shield plus additional bowl shields and the back of the hood was propped-up 2 inches.

 

[siteunseen]

Do you think dead heading increased fuel therefore eliminating hot evaporation?

I'm just curious as this is a common question asked often. I've never had an issue and it's 90F right now down here in the Deep South and getting warmer as I reply.

[Zed Head]

19 hours ago, Jeff G 78 said:

Buy about 4 feet of fuel hose and a T fitting to bypass the fuel rail entirely.  Run a hose from the fuel pump output to a T fitting at the front carb and then run hoses from the T to the two carb inlets.

So, considering your last post about deadheading the metal rail, it seems that the big difference might be the rubber hose versus metal lines.  Maybe the key is fuel temperature before it gets to the bowls.  The cooler the better.

We've talked about fuel differences in the various EFI heat soak threads.  Fuel quality varies across the country.  Might explain the AL versus MI difference.  Could be a winter blend thing also. 

Jeff G were you running aviation or race fuel, or pump fuel?