1972 240z Stalling after a few minutes of driving.

[mckennar]

Howdy all,

First off wanted to say I've been researching vapor lock and I'm not 100% that's my issue.

1. Car starts fine, idles fine if not a bit lumpy.

2. After the car warms to operating temp, head on out for a drive. Idle: 600-800rpm when warm after first start of the day.

3. After 5 - 10 minutes of driving, the car will start to become unresponsive to throttle to the point at which if I give it any throttle at all and take it out of gear, it'll stall out completely, and I have to either jam it back down into gear if I'm going fast enough or just coast over to the side of the road.

4. Once on the side of the road, however, it fires right back up. Oddly, once it starts back up, the idle is around 1300-1500, as though there's a vacuum leak that isn't present when the engine first hits operating temp, and it has zero power and will stall almost immediately if you try to put it in gear and give it throttle.

5. If I wait 10 minutes, the idle is around 1100rpm. 20 1000 RPM. 30. 900 RPM. 40. 600-800rpm. Drive for 5 minutes, it stalls, idle is back up to 1300 - 1500 and no power.

6. These stalls happen regardless of direction being turned, accel, deceleration, etc.

7. While this is occurring, manipulating the choke doesn't have any impact.

8. With my wife following, she does not note any black or brown smoke during these episodes.

Ambient temp: 61f. Not hammering on the car at all, just going 60-65 on the freeway, but happens in town too.

New fuel pump, filter, lines, spark plugs, air cleaner, damper oil, a few vac lines, etc.

When this happens, there's plenty of fuel in the fuel filter housing and there is a good bit of fuel in the tank.

Stock SU carbs, round top. Electronic ignition (new-ish). None of the insulation wrap remains on the fuel lines, HOWEVER immediately after this happening, when I pop the hood all of the fuel lines are cool to the touch. Even the carbs are pretty cool. I tried dumping some cool water on the lines and carbs and that got me a bunch of strange looks in the 7-eleven parking lot, but the stalling issue persisted.

The car is new to me, and the previous owner (who has owned it for 30+ years) says it never happened to him before.

I'm a fuel injection guy, to me carburetors are basically voodoo. I know what they do, but how they do it? Beats me. Suppose it's time to learn...

Things I'm going to do/check next weekend (after Z-Bash, naturally):

1. Timing

2. "Quick & Dirty SU Tuning" checklist

3. Vacuum

4. Fuel pressure

Any ideas beyond that would certainly be appreciated. Thanks again.

[Rick Holland]

Hi, you could be describing my 1972 240z, which exhibited the same symptoms in 1984 when I got it!

Ran great when it was running, but stalled randomly. My temporary solution was to let it roll to safety, pop the hood, disconnect the fuel line, and blow into it. Not sure if that helped - as I remember, the car was equipped with a mechanical fuel pump and an electric one which may have minimised any backflow.

Anyway, after eliminating various possibilities (as you're doing), my dad and I eventually did solve my issue. We drained and removed the fuel tank, and rinsed it. Out came a small paper filter, about the size of a 50c piece, which had occasionally been sucked up against the fuel tube (?) inside the tank, restricting fuel flow and stalling the car. As an additional precaution, we cut a couple of V-shaped notches in that metal fuel tube to make sure nothing else could be sucked flat against it.

We were never really sure where the paper filter came from, whether it was the gas cap or somewhere else, but that solved a very similar problem.

Best of luck

Rick

Good luck.

[240260280z]

Try and cause the problem again then immediately check for spark. Compare the spark quality when your car is running well vs. when it dies.

Also look at the tach immediately when it dies to see if the drops to zero.

I'm guessing your electronic ignition, coil, or possibly a condenser in the ignition circuity is heating up then failing only at higher temperature.

[SteveJ]

I would agree with Phil. What kind of electronic ignition is in your car?

[mckennar]

Hi all, thanks for the replies.

Crane XR700 w/Crane PS20 coil. Both warm to the touch when the stalling happens, but no warmer than the surrounding metal in the engine compartment.

I can "make" this car behave in this manner pretty easily... start it in the morning, drive for 15 minutes, happens every time without fail. Start it again 20-30 minutes later, drive it for 5 minutes, it stalls again.

It comes on slowly during the first time of the day that it happens. So you're driving, and 10 minutes into the drive WOT makes it bog so you back off, thinking maybe you're giving it too much fuel. 2 minutes later, 1/2 throttle makes it bog so you back off more. Now you're going 45mph on the freeway. 2 minutes later, any throttle makes it bog and you're praying you make it up the off-ramp and into the 7-eleven. 1 minute later, it won't even idle. Wait 10 minutes, rinse, repeat until you get frustrated and call Hagerty roadside assistance for a tow.

Re: spark quality, besides pulling a plug and eyeballing it, is there a more exact way of testing this?

Also, I've been assuming (perhaps incorrectly) that when there's a weak spark condition, the amount of unburned fuel exiting the exhaust would produce a smoke and a raw fuel smell both in the front and in the back. There is no such smoke or smell at idle or under load.

Its these contraindicating symptoms that brought me here. I'm going to keep searching, worst case I'll drop the tank and start digging around in there but not looking forward to that mess! It's over 3/4 full. haha.

I'll have another crack at it this coming weekend and report back.

Ryan

[mckennar]

Also to answer another question, when it starts to stall, the tach needle slowly makes its transit from wherever it was at speed, down to zero. It isn't an instant drop. Jam it back into gear and it pops right back to whichever RPM you're at based upon gear / speed.

[SteveJ]

Check the quality of the ground for the ignition unit. Is it touching bare metal?

When the stalling happens, are the plugs wet? What is the color of the plugs? Pictures help. As for the quality of the spark, get an inline spark tester (Amazon.com: Fool-proof Engine Ignition Spark Tester: Automotive).

[mckennar]

Each time it has stalled it has been on the side of a busy thoroughfare. By the time it limps its way back to the garage, I'm usually so frustrated with it that I just leave it. Plugs are brand new as of about 30 miles ago, old plugs were black but not overly sooty.

I'll pull the plugs, buy the linked in-line spark tester and report back. Thanks.

EDIT: looks like that tool just shows if there is vs if there isn't. There's no way of telling the quality of the spark. I know there's spark because I can fire it right back up when it stalls, or if I catch it in time I can let off the throttle until the engine catches itself, then put it in neutral and it will idle all day, but the second I try to give it any throttle under load, it'll stall.

Edited April 21, 2014 by mckennar

[SteveJ]

Now is sounds like junk in your fuel system. I know there have been threads with similar symptoms. I just can't remember where the problem tends to be.

By the way for the tester, the brightness will indicate spark quality.

[Captain_Zeros]

I put in another 2 cents for "check the fuel system"

On my '73 the culprit was a pile of crap that got caught in the electric fuel pump's filter, and it had very similar symptoms of stomping on the throttle and nothing happening and trying to limp down an offramp and waiting for 5 minutes.

[mckennar]

One more thing to add... when its doing its stalling-under-load thing, if I put it in neutral and pull to the side of the road, the revs will typically be at 1100-1300 (typically the car idles at 600-800). If I let it sit in neutral on the side of the road and do nothing, the revs will slowly climb to at least 1500, perhaps more. I don't wait around for the revs to go higher, so I usually shut it off and let it cool down a bit. Of note, the higher the revs are in neutral, the more susceptible the car is to stalling under load.

[Mark Maras]

I'm going to take a shot at this. If we start with the normally dark plugs, idle was set with a rich mixture. As the engine starts to run low on gas the mixture leans out & the idle goes up. No power under load is classic low fuel supply to the carbs. There is plenty of fuel in the filter up front seemingly the problem exists from the filter to and including the carbs. Check the f. lines, filters in the banjo fittings on the float bowl & fuel pressure & volume test. There may be a filter in front of the gas tank too.