vapor lock?

[Christien]

I know vapor lock has been discussed at great lengths on this board, so bear with me :-) After driving (city) for about 30 minutes constantly, I stopped to run into the post office, in there maybe 5-10 minutes, came back out to the car, and it wouldn't start. It was turning over no problem, sounding regular, but just wouldn't fire. My initial thought was vapor lock, so I just sat and waited, trying to start it every 2 minutes or so. After about 10 minutes, vroom, starts up perfectly. Ok, still sounds like a vapor lock issue. What's unusual is that the ambient air temperature today was about 8C (45F). Oil temperature was perfectly normal when I'd been driving around. On the weekend I installed a Carter 4070 fuel pump, mounted just outside and a little up from where the standard fuel pump mount is (just in front of the right rear wheel) and it occurred to me that this might be a little more susceptible to heat from the engine or heat exchangers.

I've searched and read tons on vapor lock, fuel accumulators, check valves, etc. but I'm confused over what info and parts pertains to carbureted engines/MFI/CIS. I'm running Zenith carbs. I'm going to put the proper fuel pump in there as soon as I find/order one, so if this is just a case of "let it cool down" I can deal with it for a week or 2. Maybe a temporary solution would just be some kind of insulation around the pump?

Thanks,
Chris

[Rot 911]

Time to put on our thinking caps. I doubt vapor lock. When you say you mounted the fuel pump on the right rear, are you sure you don't mean the left "driver's" side? I suspect due to the fact the car runs fine until you turn it off that some how fuel is draining out of the fuel line from the pump to the carbs. Easy enough to check that. Unfortunately you need to replicate the problem. If the car doesn't start right up just turn the key to the on position and wait a minute and see if gas gets pumped up to the carbs. Then try starting. I am sure others with better ideas will jump in here.

[Christien]

Thanks Kurt, sorry for the brainfart - yes, indeed, it's on the left side, not the right. d'oh...

How do I check to see if gas is getting pumped to the carbs with just the pump running, without the engine running, or do you mean check by trying to start it after letting the pump run for a minute. Is there any danger of burning out the pump, or is enough gas circulating through the pump by doing this to keep it cool?

Chris

[Rot 911]

Yes I mean let the pump run for a minute prior to trying to start it. Won't hurt the pump at all. Although thinking further, if this was the problem it wouldn't start right up first thing in the morning either. I wonder if you have a loose fuel pump electrical connection and the pump wasn't running when you tried to start the car?

[Christien]

Well, if the pump either runs or it doesn't, then the electrical connection is fine, because that was the first thing I checked when it wouldn't start - I could clearly hear it whirring away, so for sure it was getting power. When I started it this morning (1st time after sitting all night) it started right up. Then it sat all day until about 3pm, at which time it started perfectly again - no worries. That was the fateful drive. Turned the engine off about 3:30, then it wouldn't start. I just started it up again about 40 minutes ago, and it seemed to turn over a few times more than normal to start (it had been sitting since about 4:00, so about 5 1/2 hrs) but still fired up pretty quickly. I have to go pick up my wife right now, so I'll try it out again. If it doesn't start right away, I'll let the pump run for a good 60 seconds.

Thanks again!

[project 911]

Hi Chris-

What kind of pressure does your pump put out? Are you using a regulator with it?

I have a 71 2.2 that was doing the same thing, only it seemed to be after longer drives. What I noticed is that my fuel pressure gauge would show normal readings of 3.5 pounds while running, then at least tripple after I shut the engine off. My pump is in the smugglers box up front so there are no heat related problems there, but it occured to me that the fuel line that leaves the tunnel runs up under the body to the engine bay right in the heat. I just insulated this line with thermo-sleeve a couple of days ago. Don't know if it will help yet. My thought was that the heat was getting to the fuel line after shut-down, boosting the pressure and flooding the carbs. Just a thought. Those Zeniths are tricky.

[Christien]

Well, it happened again. The car had been sitting for about 45 minutes after a very short (5 minute) drive to the community centre and back. It didn't want to start on the first try (but did turn over strongly) so I let the fuel pump run. Tried again after about 30 seconds, no go, let it run some more, then tried again maybe 20 seconds later, kept going like that for maybe another 45 seconds, then it sounded like it was trying to catch, so I floored the gas pedal and after about 5 full seconds of cranking it fired. After that it drove normally.

Steve - as far as I understand the pump puts out 4 to 4.5 psi, and no, there's no regulator. I'll pick up some insulation tomorrow and see if that helps.

[Christien]

Started up beautifully this morning, just as I expected. I'm going to get some insulation this afternoon to see if that solves the problem until I get the right pump. Can you believe Downtown Porsche (Toronto) wants over $1000 for a new pump?! No, that's not a typo - one thousand dollars! Holy *****! Even the parts guy on the phone actually sounded a little incredulous at the price.

One last question, and yes, I'm showing the wetness behind my ears here :-) Are fuel accumulators, check valves, etc. only applicable to fuel injection?

Chris

[stlrj]

so I floored the gas pedal and after about 5 full seconds of cranking it fired

That's the classic "clear flood procedure." If that keeps working then it might just be a flooding problem.

[cubby911T]

Hey guys - - my 70T w/Zeniths behaves much the same way (both before and after a new Bosch fuel pump was installed). I tried insulating the metal fuel line that runs between the carb banks on top of the engine (using rubber fuel line as the shielding). That may have helped slightly, but she still starts really, really hard after being warmed to operating temp and then shut off. The only thing I find that works reliably is to just floor the gas pedal when re-starting. She starts after 3-4 seconds pretty much every time that way.

[Christien]

Well, I've decided to bite the bullet and go with a complete re-do of the pump. New electrical, plumbing, and mounted in the front. Hopefully this will take care of all the issues, once and for all.

[Rot 911]

Originally posted by Christien
Well, I've decided to bite the bullet and go with a complete re-do of the pump. New electrical, plumbing, and mounted in the front. Hopefully this will take care of all the issues, once and for all.

I would first try the Cubby method of holding the pedal to the floor while cranking. If it is a semi-flooding condition then doing what you suggest is not going to solve your problem.

[Christien]

It does seem to start fine when I hold the pedal to the floor while cranking. But if the pump were moved to the front (much closer to the gas tank) and replumbed (or actually plumbed properly) wouldn't that take care of the flooding issue?

[911pcars]

Flooding = too much fuel
Vapor lock = too little fuel (air instead)

Vapor lock is caused by excessive heat somewhere in the fuel delivery system (fuel pump-to-induction system or within the induction device itself [carb]). In the case of external lines overheating, move the pump to the front near the tank and/or insulate fuel lines running close to the heat source. Excess heat causes fuel to vaporize. Fuel vapors compress and don't move liquid fuel.

If it's a later 911, SC or Carrera, there's a fuel pump check valve that could be borderline dying.

Flooding:
If there's too much fuel, check for excessive fuel pump pressure and/or a higher-than-normal float level in the carbs. Excess heat (heat soak) can also force fuel out of the carbs (at rest) and flood the engine. In the case of carbs, many owners install the carb insulator spacers and modify the float bowl vents per PMO to mitigate the situation.

If you have to hold throttle wide open to start, you are partially compensating for excess fuel by allowing more air to enter and mix. Check plugs and see if they're "wet" from excessive fuel intake.

If the displacement is enlarged, the engine is greatly modified and/or the operating conditions are severe, all the above is exaggerated and additional cooling strategies are needed.

Sherwood

[cubby911T]

Christien,

The pump on my 70 is located in the front of the car (under the tank), so I'm not sure relocating it will solve your problem. Like I said, I installed a brand new one and it still behaves as I described in my previous post. HOWEVER, I still feel better knowing that my fuel pump is new - one less old part that could strand me I guess.

Matt

[project 911]

Keep in mind that these pumps are better at pushing fuel than pulling fuel. Up front and low is best for a location.

[jax]

You don't mention smoke on restart after warm up. Do you see black smoke ? Good start up cold when it wants more fuel & poor start hot suggests rich condition, coupled with your info on holding throtle W/O
on hot start. Fuel pressure (regulated to 3 PSI +/- .5) is easily remedied
w/ an in line regulator. Float condition & level would be next to check as
replacing pump did not solve issue. (don't spend more $ on new pump
as it is (presumably) doing all it knows to do). Buy a regulator w/adjusts
of 1-5 PSI. If no success, have floats & level checked.. Your complaint
doesn't mention general drivability issues so (again presume) issue is
hot start after short term shut off. More info will help. Regards, Jax

[Christien]

Matt - my thoughts exactly. The way I had it set up was a temporary mounting, as I knew it wasn't 100% secure, but it was all I was able to do myself. Took it in today for rewiring and replumbing, positioning it up front. I realize moving it up front won't necessarily cure the start problem, but I wanted to do that anyway, basically solving other potential problems while getting a permanent and proper pump and mounting.

jax - There's a little white smoke on start up, but no black smoke at all. General driveability is excellent - pulls strong, no shudders or jolts, good acceleration. I think you're right with the diagnosis of running rich. I'll play with the mixture and see if that makes any difference.

[jax]

White smoke can be raw fuel. Have a friend stand by on hot start up to
check odor. If the car is running well in general, I would be reluctant to disturb the mixture. The level of the fuel in the float bowls is controlled
by a tab from the float(s) which opens & closes needle to seat.(fuel flow
into carbs). If pressure is to high, float cannot control incoming fuel,
and/or if float level is too high, fuel can spill over to throttle valves.
Pressure would be my first check. Once confirmed, warm up car to
op temp, remove air filter housing and look into carbs to see if fuel is
intruding from any ports or accel pump jets. Regards, jax