"Trailer-hitching" bucking problem, Rapid detrioration from running perfectly Options

[boxster914]

Hi everyone

My perfectly running 71 914 1.7 D-jet has deteriorated rapidly this past week.

Early in the week I noticed what felt like a power drop out during a drive. It was very slight and only happened once and I wasn't sure I didn't cause it by unconciously lifting off the gas.

The next drive it happened a few times and was more pronounced. It was then that I knew this wasn't an imagined problem.

On the third drive it was like a bucking bronco, surging and stopping and the engine cut out a few times. It was undriveable.

Today it will start but won't run. The engine cuts out when I try to rev it and the red light comes on.

I've done some searching here but haven't been able to draw any conclusions or come up with an actionable strategy to diagnose the problem. I'm seeing people trace it to spark plug wires, air fuel ratios and bad distributors.

Is there a step-by-step process that you can recommend for me to get to the cause of this? (I'm a newb so I probably will need help with abbreviations like AFR, TPS, etc.)

Here is a video of it's rough idle yesterday when it was still running.

https://youtu.be/n61CP6O8UgM

[DRPHIL914]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

[Lockwodo]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

I recently replaced the TPS board and calibrated it. Disconnecting the electrical connector to the TPS doesn't affect the missing issue at all, so the missing isn't due to the TPS.

I did some tests on the MPS today and here is what I found: 95 ohms resistance between terminals 7 and 15, 340 ohms resistance between terminals 8 and 10, and no continuity between the coils and the case of the MPS. All good so far. Then I hooked a vacuum pump to the MPS sensing port and pulled 10 inches of vacuum. After one minute, the vacuum had dropped to 7 inches of mercury, then to 6 inches after a total of 2 minutes, then to 5 inches after a total of 9 minutes, where it seems to level off.

So the resistance numbers are in spec, but is the vacuum loss over time acceptable? I understand that even if the vacuum loss is acceptable, the MPS can still have a defective aneroid cell. What do you think? Perhaps not worth testing it further at this time.

I'm working on the ground points and distributor next.

[DRPHIL914]

lots of good advice here already, if the fuel pressure is constant its probably not a blocked line or filter.
the hunting issue makes me think its running lean, but the missing makes me think more on the lines of intermittent short, like bad grounds or loose FI grounds, i had this happen 2x and now my ground wires for the FI are fastened securely to the spade connector thats on the back of the engine case bolt. check that. also - if its on acceleration the TPS could be an issue so you can unplug that to take it out of the equation, also take the cover off of it to chec the TPS board for wear, corrosion etc.
next item is the MPS- ihve had several fail we can go over that later. Go back to the distributor , there is a trigger points plate that fires the injector pulses and those need to be checked and cleaned , and inside your distributor the ground strap could be worn and shorting as well, so check these items and get back to us.
you can test each of these items so get your multi meter out. and vac test the MPS>


Phil -

I recently replaced the TPS board and calibrated it. Disconnecting the electrical connector to the TPS doesn't affect the missing issue at all, so the missing isn't due to the TPS.

I did some tests on the MPS today and here is what I found: 95 ohms resistance between terminals 7 and 15, 340 ohms resistance between terminals 8 and 10, and no continuity between the coils and the case of the MPS. All good so far. Then I hooked a vacuum pump to the MPS sensing port and pulled 10 inches of vacuum. After one minute, the vacuum had dropped to 7 inches of mercury, then to 6 inches after a total of 2 minutes, then to 5 inches after a total of 9 minutes, where it seems to level off.

So the resistance numbers are in spec, but is the vacuum loss over time acceptable? I understand that even if the vacuum loss is acceptable, the MPS can still have a defective aneroid cell. What do you think? Perhaps not worth testing it further at this time.

I'm working on the ground points and distributor next.

@Lockwodo
to answer your question, yes, its possible a failed cell is possible. the leak is slow and should be addressed, you might as well order a rebuild kit from Chris Foley, it will have the o-ring and seals with the adjustment tool and new disc. i have a a couple old MPS that had cracked cases i took the aneriod cells out of that i have as spare, if you need one. not sure they fail very often but they can.

@boxster914
It looks like the O.P. has a fuel flow issue but still could be more than one thing, but keep it simple in your process of elimination of not changing out more than one thing at a time,