Need some help with my strange engine stumble problem Options

[P.Rocket]

I've had my '74 914 2.0 for about five years. It is running on the original fuel injection.
I've always had an intermittent stumble at about 3200RPM that was intermittent enough to easily live with.
This year when I started driving it back in the spring the stumble started happening a bit more often. I could get it to stumble while sitting in the driver's seat and pushing the clutch in and out, at an idle. Oh well, kept on driving it.
Last week a drove it a lot and found that the stumbling was continuing to develop while driving.
I turned the key on one morning, heard the fuel pump run then stop (like normal), then I was moving around in the driver's seat and the fuel pump cycled again. So as I sat still in the seat, the pump wouldn't cycle, when I would move around I could hear it cycle again. With the car idling, it will stumble while moving in the driver's seat, regardless if the seat is adjusted froward or back.
Right away I thought about the seat sensors/seat belt switches or that POS interlock module under the passenger seat. So I unplugged both the drvs and pass. seat sensors and seat belt connections, the pump still cycles when moving around in the driver's seat.
I checked and cleaned the grounds from trans to chassis and added a ground strap from the trans to the engine. I also checked and cleaned the grounds that go to the chassis and mount under the relay box in the engine compartment. No change.
Ran out of time, so parked it in the shop and now onto my work week.
Thought my next plan of attack will be to pull the pass seat and check out the interlock module and by-pass it. Can someone tell me exactly what this unit does? Does it cut the signal to the coil or the fuel pump? What is it's purpose for North America only?
If anyone else has had this kinda weird problem, your input is much appreciated.

[AZ914]

When you say the fuel pump cycles, you mean it comes on and turns off? I know on my 75 2.0 DJET, if the key is on, the pump runs, period (as though the car is running). Should it not be turning off? Check the relays?

That 'butt-test' sounds fishy, can you start the car standing next to it? (eliminate the interlock?)

[Bartlett 914]

When you say the fuel pump cycles, you mean it comes on and turns off? I know on my 75 2.0 DJET, if the key is on, the pump runs, period (as though the car is running). Should it not be turning off? Check the relays?

Could be the relay board. Remove the relays and the connectors and clean the contacts. The board under the "tar" sometimes develop contact problems. Mine would sometimes stop outright. A spare is a good thing to have.

[scott_in_nh]

on a 74 the pump should come on and then stop and not come on again until the engine is started.

My underseat sensors have already been bypassed so I can't comment on the likelyhood of that still being the issue, but if you disconnect/bypassed correctly - how could it still be a seat issue?

To me it sounds like an intermittant connection in the ignition switch.

[r_towle]

The logic circuit (looks like a relay) under the passenger seat only disables the starter circuit.
The bypass is to unplug the two large yellow wires and connect them together directly...bypassing the logic circuit.
The role was to ensure you have your seatbelt buckled or the car would not start.

I am inclined to agree with Scott that you may just have a funky ignition switch.
Try sitting still and wiggle the key or wiggle the steering wheel.

Rich

[underthetire]

When you say the fuel pump cycles, you mean it comes on and turns off? I know on my 75 2.0 DJET, if the key is on, the pump runs, period (as though the car is running). Should it not be turning off? Check the relays?

That 'butt-test' sounds fishy, can you start the car standing next to it? (eliminate the interlock?)

Yours is wrong then. Pump should run for ~2 seconds with the key on, then shut off. Shouldn't come on again until the ECU sees the engine cranking via the fuel injector points on the bottom of the dist.

As far as the seat wiggle, that sounds like it might be tough to figure out. I'd start with the relay board area first myself, as suggested earlier. The stumble at 3200 sure sounds like a advance problem with the dizzy myself. Thats when the mechanical advance is supposed to go full advance i believe. Thats why the timing procedure says to time at 3200 RPM or above from what i remember.

[P.Rocket]

The car does start when your standing beside it.
I did the inspection and cleaning on the relay board and may have to try my spare if required.
I will re-check the ignition switch, I'm sure they are a common source of troubles, as they are in the volkswagens.
I will also by-pass the logic circuit as well for piece of mind.
Wish me luck, I'll be back troubleshooting the old girl this weekend, and I will test after each step so I can let you know what I find.
Thanks much for all your information and ideas on this one, it's always great to have some fellow teeners to plug for help.

Cheers, Andrew.

[blitZ]

I had an odd stumble problem. It turns out my plug wires were routed close to the MPS and interrupted the signal somehow. Worth a try.

[avidfanjpl]

I had an odd stumble problem. It turns out my plug wires were routed close to the MPS and interrupted the signal somehow. Worth a try.

Mps is poisoned when near plug wires

Stuff them under fi pipes for ever!

John

[avidfanjpl]

I had an odd stumble problem. It turns out my plug wires were routed close to the MPS and interrupted the signal somehow. Worth a try.

Mps is poisoned when near plug wires

Stuff them under fi pipes for ever!

John