rough idle when hot

[steve91tt]

I've got a stock 1973 240Z on which I have recently changed:

1. Z therapy carbs

2. plugs, wires, pertronix, coil, cap, roter

3. electric fuel pump (RX7)

4. fuel lines

5. aluminum radiator

6. fan and fan clutch

7. AC system

The car runs great when it's 75°F or less outside but as soon as it gets into the 80's bad things happen.

1. The car idles rough and hesitates on acceleration.

2. The temperature gauge starts moving towards the red zone.

3. The reading on my fuel pressure gauge (installed inline just before the mechanical fuel pump) starts to pulsate and eventually goes to zero.

I have a new fan shroud sitting on my bench, to be installed when I get some time next week so hopefully that will solve the overheating problem.

I believe that the fuel pressure gauge may be a red herring because when I shut the car down and open the line to the gauge fuel sprays everywhere, even when it is reading zero. I'm thinking the gauge is not working.

It feels like vapor lock to me but how so with both a mechanical and electric pump in the system?

If this car won't run correctly at 80°F it's not going to like the summers time.

Any thoughts?

[oldhemi]

Junk in the gas tank? Pickup tube in gas take rusted/perforated? Fuel line or filter plugged?

Is your tank less than 1/2 filled? If so, try filling it up and see if it solves the problem. These zuckers are notorious for gas tank problems.

What with the low mileage of your car, I would not be surprised if it sat for long periods of time letting the gas tank rust. Mine did!:stupid: I wonder if the temperature outside today and yesterday here in Houston is a red herring as far as the performance is concerned. The temp gauge climbing like it did is worrisome.

Are you running a return line to the gas tank? This helps keep the fuel cooler. The electric pump was added in '73 to help with vapor lock.

If you DONT run your newly installed A/C, does it run cooler? Maybe its just a stuck thermostat?

Just playing devils advocate here, your previous posts indicate that you know what you are doing.

[Walter Moore]

Could it just be that the fuel mixture is too lean for hot weather?

I also notice that you didn't list a thermostat among the items you replaced.

[steve91tt]

Junk in the gas tank? Pickup tube in gas take rusted/perforated? Fuel line or filter plugged?

Is your tank less than 1/2 filled? If so, try filling it up and see if it solves the problem. These zuckers are notorious for gas tank problems.

Are you running a return line to the gas tank? This helps keep the fuel cooler. The electric pump was added in '73 to help with vapor lock.

If you DONT run your newly installed A/C, does it run cooler? Maybe its just a stuck thermostat?

I don't think that there is junk in the tank but I could be wrong. I dropped the tank recently and did not notice any corrosion when I poked a flashlight in. I am also running a prefilter before the electric pump.

I believe the issue is temperature related as it seems repeatable. When I start the car it runs great but if I let it idle in the driveway with the AC on it starts to idle badly. I have not tried it with the AC off but I will give it a try tomorrow.

I am running the stock return line.

The tank is about 2/3 full currently but I will fill it up to see if this has an effect.

I did not think about the thermostat. Could this be it?

[steve91tt]

Could it just be that the fuel mixture is too lean for hot weather?

This is my first carb'ed vehicle. Is it typical to have to richen the mixture when the weather gets hot? I was very happy with the mixture in cooler conditions. Great idle, excellent tip in, no pinging and nice consistent plugs.

It's easy to fatten it up a click or two and see what happens. Something else for the trouble shooting list!

[oldhemi]

I am happy for you that your tank is good. You are probably OK with the fuel flow. The only other thing related to that is the filter (and magnet) in the stock electric pump. Doubt it now if there is any problem there. I have read folks complaining about the accuracy of a fuel pressure gauge. I dunno if some brands are better than others.

We haven't even got warm yet and your temp gauge is climbing - that is a concern. I hope that its as easy as a thermostat. These cars will run warm with prolonged idling. Nissan tried many fixes including cutting a hole in the hood and installing a "scoop" over it - some worked. You might want to consider a 160 degree thermostat.

Does the problem clear up when you are driving? Maybe with a/c off?

Does the temp gauge go down when driving?

Does the upper radiator hose get hard as a rock? This is sometimes an indication of a bad head gasket. They usually blow out on the rear 2 cylinders. NOT saying that's where you are at yet.

Check for wet on the bottom of the water pump sometime after it has been warmed up. Marginal water pump will cause high temps - ask me how I know!

I am not discounting WM's post. But, I dont think that you will have to worry about mixture adjustment yet because it just has not been hot enough! But you could be borderline on your adjustment. Zednobi is running SU's. He lives down towards Galveston.

There are some interesting threads over at hybridz on cooling. Those cats are always pushing the limits on these cars engines WAY over what the original engineers would ever have envisioned. 4 row radiators, dual cooling fans, drilling holes in the head for extra cooling at the rear, etc

Edited March 15, 2010 by oldhemi

[beandip]

I've got a stock 1973 240Z on which I have recently changed:

1. Z therapy carbs

2. plugs, wires, pertronix, coil, cap, roter

3. electric fuel pump (RX7)

4. fuel lines

5. aluminum radiator

6. fan and fan clutch

7. AC system

The car runs great when it's 75°F or less outside but as soon as it gets into the 80's bad things happen.

1. The car idles rough and hesitates on acceleration.

2. The temperature gauge starts moving towards the red zone.

3. The reading on my fuel pressure gauge (installed inline just before the mechanical fuel pump) starts to pulsate and eventually goes to zero.

I have a new fan shroud sitting on my bench, to be installed when I get some time next week so hopefully that will solve the overheating problem.

I believe that the fuel pressure gauge may be a red herring because when I shut the car down and open the line to the gauge fuel sprays everywhere, even when it is reading zero. I'm thinking the gauge is not working.

It feels like vapor lock to me but how so with both a mechanical and electric pump in the system?

If this car won't run correctly at 80°F it's not going to like the summers time.

Any thoughts?

ZTharpy carbs . The best. RX-7 pump great I have run one of these for years. One big thing here. ALWAYS install a in line filter before the pump. This is the only pump I am using. On your return line. On the end of the fuel rail where it connects to the return. Check to make sure the tiny metered hole is clear. This metered hole maintains the pressure , or helps to. But if it is clogged the flow back to the tank is limited and the stationary gas will heat up and cause it to boil in the line. Have you checked the thermostat? if not take a cooking pot and half fill it with water use a can like a soup size and half fill it with water. Put the thermostat in the can and the can in the pot. use a candy thermometer in the with the thermostat and put it all on a stove and heat it . Watch the thermometer and the STAT and see at what temp it opens. This might surprise you. Give these things a look and get back to us with you find. My '73 with the stock radiator never over heated regardless of the high temp on the road. If these things are in good condition you should not have any trouble. So have a second look. Even with the alum rad. 50% anti freeze regardless where you live. Trust me.

Gary

[steve91tt]

Does the problem clear up when you are driving? Maybe with a/c off?

Does the temp gauge go down when driving?

Does the upper radiator hose get hard as a rock? This is sometimes an indication of a bad head gasket. They usually blow out on the rear 2 cylinders. NOT saying that's where you are at yet.

Check for wet on the bottom of the water pump sometime after it has been warmed up. Marginal water pump will cause high temps - ask me how I know!

Oldhemi, thanks for the feedback. To answer your questions...

I currently have only seen overheating when idling (3 min.?) with AC on.

The temperature gauge

The temperature goes back down as soon as the car starts moving.

The rad hose gets somewhat hard when hot but not harder than other cars I have worked on.

I will check for wet water pump tonight.

[steve91tt]

ALWAYS install a in line filter before the pump.

On your return line. On the end of the fuel rail where it connects to the return. Check to make sure the tiny metered hole is clear.

Have you checked the thermostat?

Beandip, I am running two in-line filters (one before each pump). I will check the fuel rail and t-stat tonight and get back to you.

Thanks very much for the feedback.

[steve91tt]

I took the car out at lunch today, more observations:

1. While idling, waiting at the drive through (5min?), 70°F outside, AC off, the temperature gauge moved up slightly (half way?). The car started to idle a little rough and stumbled slightly as I pulled away. After 1 block of 40 mph, all was good again. Stable idle, temperature gauge down.

2. The fuel pump is loud some times times and other times it is quiet.

Theory:

1. Fuel pickup issue is causing the fuel pump to intermittently make noise because of air in the system.

2. Unexplained cooling issue(s).

Plan:

1. Replace thermostat and water pump. Both are inexpensive and current parts of of unknown origin.

2. Install fan shroud

3. Drop tank and check pickup/fittings more carefully.

[Pomorza]

I will advise that you replace the bolts on the water pump also. They have a tendency to break if they are reused to many times. I'm talking from experience here as I broke on when I replaced mine on my 280. Luckily it was only the head so most of the bolt was outside and I pulled it out. But please buy new bolts. You don't want to spend hours trying to get a broken bolt out of your engine block. You can get both (pump and bolts) from MSA. They work rather well from what I can tell. (I'm running one at the moment)

I'm not sure what electric pump you have but I've been told by many Datsun guys that the electric pumps are fairly loud. Mine is rather loud all the time but but again its on a 280z so it could be different.

Jan

[steve91tt]

I will advise that you replace the bolts on the water pump also.

Good tip, bolts on order.