High Rev intermitant problem 71 MFI

[Gearbox]

While driving today I noticed that the engine will cut out above 2-3K revs, then it would be ok, then do it again. This was after driving at 4-5K revs for a mile or so. Almost felt like I was out of gas, but soon as it returned to idle, it was ok. Let the car cool down and she revs fine. Anyone experience this before? Vapor lock? Thanks Allan

[Jeff Higgins]

The first two places I would look would be either the fuel filter or the screen in the gas tank. Either one could be starting to plug with sediment or scale. Often the crud that plugs either will settle out a little different when the car is shut down, or when the demand lessens, allowing fuel to flow once more. Once the demand increases, the crud gets pulled down over the screen or forced to the outlet side of the filter, plugging them up.

[rs911t]

I'm chasing a similar problem. In my case the tank has been refurbed, new pickup screen, fuel pump and injectors.

The workshop manual (and Grady) says to first check the fuel pressure where the return line from the injection pump enters the fuel filter. Should be "11.8 +- 3 PSI" per the manual. Well, I have zero, zilch, nada. Next the manual says to check the "overflow valve". Well, I have no idea how to check it. The part is available new for $150, and used from dcAuto for $100.

Anyone have some laying around I could use to test? Happy to buy one if it turns out to be the problem.

[911quest]

Just a guess but #8 looks like a check valve if it is you should be able to blow thru one end and not the other

[Mr9146]

What was the air temperature? Dry or humid?

[Gearbox]

I think the #8 fitting is for the cold start system where it dumps fuel into the stacks on cold start up? I drove the car to work about 5 minutes away, and it ran fine. We did have torencial down pour for about two or three days prior. But the day was warm and clear 85-90, not too humid. On the way home, I kept it in second gear going into third occasionally, reving upwards of 4-5K, just to "blow it out" and then it started. Revs dropped below 2K but didn't stall, but the throttle pedal wouldn't increase revs above the 2K. I would let off, drop a gear, and then it would rev up for a moment and then fall. Drove it all the way home that way. Let it sit in the driveway for about an hour and started it up and it reved fine. Haven't drove it around yet.

Also, I had just replaced the MFI Transducer about a month ago (the silver box that mounts on the left electrical control panel) but that appears to be working correctly. I had just finished a complete restoration, which included a complete de-rusting of the gas tank and sealing. The gas tank filter screen should be clean, I also had taken the main fuel filter off and back flushed it and it looked good and almost new so I put the original one back on. Plus Stoddard was out of stock at that moment. Any ideas? Thanks Allan

[rs911t]

#8 is the line back to the fuel pump (and tank). The cold start solenoid is #2 at the top, with the nozzle point down-right.

[Gearbox]

Just returned from a nocturnal drive. Started to do the same thing again. Then I noticed that if I short shifted at 1600 rpm the engine continued to run without bucking. This reminded me of the trails and tribulations that I went through troubleshooting the MFI transducer and adjusting the microswitch. So I pulled the wire off the micro switch and she reved fine, albeit now with a bit of burbly backfiring on de acceleration. I think it's the micro switch. Somehow, it appears to be staying "on" even when the throttle bellcrank is off the switch and on throttle. As a result, the transducer continually powers the MFI soleniod which inturn cuts off fuel at higher RPM over 2K. Anyone one know where I can get a new micro switch? Thanks Allan

[Grady Clay]

Greg,

Take the pressure regulating fitting (center) out and replace with just a regular banjo fitting like the other two going into the fuel filter console. While monitoring the fuel pressure (long test hose from the rear fitting Tee), pinch off the return hose to get reasonable pressure. This should tell where the problem is.

The MFI doesn’t have to have exact low-pressure fuel – it can be 0.5 to 1.5 bar and still run great. (I don’t recommend that range.)

Both of you guys are describing low fuel supply symptoms.

Greg, you said your tank had been re-sealed. Tell me more. It is not uncommon for the “re-seal” to cause supply problems. Ask the CIS guys.

Allan, are you saying that the problem was related to water/rain? If so, hose the engine down with your garden hose and see what happens. With all the ignition correct, the engine doesn’t care if it is wet.

OK, your last post perhaps changes that. Please run a “test wire” from the solenoid to the cockpit. With a test light you can tell if/when the solenoid gets power. In my mind this is a very unusual failure. In addition I have never seen a solenoid partially restrict the fuel.

It is easy to remove the solenoid, apply 12V and confirm its operation: OFF-ON.

When testing the solenoid, be careful to not touch the wires. The “induced voltage” can knock you on your ass or worse. The little diode at the solenoid terminal is also a suspect.

Best,
Grady