engine problem whilst running

[RazorRacer]

The newly purchased 1978 Porsche 911SC Targa (non-turbo) runs beautifully until it's on the highway for about 10 miles. Then the engine acts like it is sick. About 10 minutes of idle time and it's running beautiful again.

This has happened on three ocassions and each time the engine power cuts in and out. It continues to cut in and out even at lower speeds until it has a chance to run idle. Then it acts fine.

It has NOT done this with extened street driving, but once on the highway, after 10 miles or so, it happens. Here is some more background information:

Oil is clean and at proper level.
Gas is 93 octane.
Valve job was done 3,000 miles ago.
Engine has 63,000 miles.

[rstoll]

I would try a bottle of Techron and a fuel filter. How are wires, cap and rotor? Any valvetrain noise?

[Joeaksa]

Razor,

Fuel filter or fuel pump? Runs fine at non-highway speeds? Would wonder if its not a duff pump or clogged filter. Would discount anything electrical as once it starts causing problems, letting the motor idle usually does not "cure" it.

Joe

[Jdub]

Would anyone agree this is a fuel-deprivation problem? Because it is cheap, I would change your gas filter and ensure that breathers to your gas tank are clear and not kinked. If this is an electrical-side problem I am unsure where to start other than fuel pump and accumulator: others know better than I.

This reminds me of situations where breather hoses from motorcycle gas tanks were slightly kinked and would, over time, deprive the carbs of gas. Slow and painful stifling would result.

Good Luck!
John

[Rot 911]

Sure sounds like a fuel filter or fuel pump problem. Of course try a new filter before you spend the bucks on a new pump.

[Targa77]

Sounds like a fuel problem to me but I thought that CIS flows fuel at a constant rate to the injectors? Please correct me if I'm wrong.

[RazorRacer]

Jdub seems to have the best idea. A clogged breather line to the tank would explain why it happens only at high speeds after 10 miles or so. The next time this happens, popping the gas cap off should fix the problem.

If not, then the fuel filter is the next most likely culprit.

[RazorRacer]

Well...running the engine without the gas cap has not solved the problem. So...back to replacing the fuel filter first. If...that doesn't do it then to replace the accumulator.

[stormcrow]

Do the fuel filter first and see if this doesn't solve the problem. It appears that when you first start up the engine, the system is full of fuel. after you run it for a short period, the system is starving for fuel.

Check the inlet side of the fuel line to the fuel pump also. If it is partially clogged, it could cause the same problem.

I had a problem with my fuel system because the car was parked for about three years. The fuel in the return tube (inside the tank) from the distributor was clogged. It caused a higher pressure in the system which caused a loss of power.

Like I stated previously, the fuel filter/accumulator may be the culpuit so start with changing that out first.

Steve

[emcon5]

Originally posted by Targa77
Sounds like a fuel problem to me but I thought that CIS flows fuel at a constant rate to the injectors? Please correct me if I'm wrong.

There is constantly fuel flowing to the injectors, but the volume of fuel changes with the needs of the engine. As RMP's increase, the sensor plate in the air box lifts, causing the fuel distributor to deliver more gas.

If the flow rate didn't change, you would have to be extremely rich at idle (flooded, probably) for there to be enough gas to run the engine at redline.

Tom

[Targa77]

Looks like I've got a lot to learn about these cars yet. Thanks for the correction.

[RazorRacer]

The picture attached shows the fuel filter removed on the right and the new fuel filter on the left.
The tube fitting on top of the removed fuel filter doesn't fit the new fuel filter.
What tube fitting is needed for the new fuel filter and where are they sold?
The tube fitting on the old fuel filter seems to be a 1/4" pipe x 3/8" compression fitting (is this correct?)
The new fuel filter seems to need a 12mm pipe x 3/8" compression fitting (is this correct?)

Would there be a problem to replace the fuel filter with one similar to the old one (039-03-9)?

[RazorRacer]

I have the Porsche fuel filter on my desk, but I don't have the right fitting. I replaced the existing, wrong fuel filter with a Mercedes fuel filter from Autozone until I get the right fitting. The problem still happens with the new, clean, Mercedes fuel filter. So unless the new, clean Porsche filter fixes the engine problem, the next step is to replace the accumulator.

Could this be a temperature sensor problem? It seems to happen whenever the engine temperature reaches just over 1/2 on the gauge. Any thoughts would be appreciated.

[ChrisBennet]

Forgive me if this was one of those "emergency" purchases, but you might have better luck in the future with a genuine Bosch filter instead of one made in China.
-Chris

[RazorRacer]

You have got it backwards. I am replacing the China filter with a Bosch filter, but the China filter hads a 1/4" thread on the outlet and the Bosch filter has a 12mm fine tread on the outlet. I have put a Mercedes (not shown) filter from Autozone until the right outlet fitting arrives.

I am hoping that the Bosch filter will stop the engine problem. If not then I will replace the accumulator.

[RazorRacer]

Changing the oversized China fuel filter, then temporarily using a new oversized Mercedes fuel filter, then putting in the right fuel filter didn't solve the problem. So it's not that the filter was dirty, it was the wrong size. The PO had someone put the wrong filter in the car. Changing that to the F5079 model didn't solve the problem.

My first 101 project.