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

[wonkipop]

i was just reading back through your thread @boxster914 .

and noticed that you did test the fuel pressure and it was right.
i'm guessing this was just fuel pump running and not engine (since it would not run anyway).

which makes me think of something interesting.
i remember mike, my mechanic saying to me when we recommissioned mine.
we had to test pressure and flow. two separate things.

sounds like despite the filter it built up pressure.
because it wasn't really circulating or using any fuel.
but you had restricted flow. it was a kind of a static pressure it built up?

classic fuel filter problem is the gunk floats off the filter after you switch off.
car will start often and run for a bit.
then the gunk gets sucked back up and jams it again.
you must have had a bit of that going on if it could build up pressure in the injector circuit.

throw a couple of spare fuel filters in your glove box.
might happen again. at least you can save yourself on the side of the road.
assuming you can get at the filter. i'd need to park mine up on a curb in an emergency to get under there.
others suggesting tank might be dirty could be good suggestions.

[emerygt350]

What year is it again? Is that an egr?

[rjames]

The vacuum hose goes to the retard port of the throttle body and is used to retard the timing at idle. Shouldn’t have anything to do with the bucking problem. Disconnecting it would cause a high idle, or if the vacuum canister was bad it would cause idle surging.

[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,

[emerygt350]

The vacuum hose goes to the retard port of the throttle body and is used to retard the timing at idle. Shouldn’t have anything to do with the bucking problem. Disconnecting it would cause a high idle, or if the vacuum canister was bad it would cause idle surging.

Now I see it, couldnt recognize the dizzy in that photo. What a tangle.

[ClayPerrine]

Have you checked the little ground wire that is inside the distributor? It grounds the moving breaker plate to the body of the distributor. If it is broken, it can cause intermittent spark failure which may feel like the bucking you are experiencing.

The reason for it being intermittent is that the plate moves on ball bearings and grease whenever the vacuum can has vacuum. Sometimes the balls get enough grease between them and the bottom plate to make an open circuit to the ground side of the points.

Clay

[Garland]

(IMG:style_emoticons/default/welcome.png)

[wonkipop]

The vacuum hose goes to the retard port of the throttle body and is used to retard the timing at idle. Shouldn’t have anything to do with the bucking problem. Disconnecting it would cause a high idle, or if the vacuum canister was bad it would cause idle surging.

Now I see it, couldnt recognize the dizzy in that photo. What a tangle.

different plenum and throttle body set up to your 2.0 as well emery.
kind of closer in resemblance to 1.8 would have looked strange to you.

interesting comments about dizzy malfunction in comments.
-----that it would raise idle in a D jet.
in a L jet it would probably not idle at all - would be pulling in extra air afm can't measure. interesting differences in D jet!
idle could probably come up in an Ljet if the retard didn't work on the distributor,
but would have to happen without letting air into the throttle body.

[Lockwodo]

Clay: I'll check the ground wire in the disty. I'm going through the electrical connections and ground points and the ground in the disty is definitely an important one.

Dr. Phil: I checked the MPS today by pulling 15 inches of mercury on it then watching it drop. It dropped below 12 inches in less than 5-6 seconds, so I think it's toast. But ran the car today after replacing all the vacuum hoses and the idle is more consistent albeit the irregular miss is still there at idle and backfiring when I rev it. Could the failed MPS be responsible for those symptoms?

[boxster914]

I have more data to report on this issue.

The car has driven flawlessly up until yesterday when I started it cold and it ran rough and when I drove it it was bucking.

I drove for about 1/4 mile with lots of bucking and then turned it off.

About 30 minutes later I tried to start it and it was completely dead no click, no lights no nothing upon turning the key.

I opened the hood and manipulated the battery wires.

I've included a picture with yellow circles around the wires that I manipulated.



I turned the key and it started fine and no bucking.

This would suggest that this is an electrical issue, yes?

Is there a map on the web somewhere of the ground wires that I can check?

Any other suggestions?

[emerygt350]

Interesting! All kinds of badness can result from poor grounds. You have that one near the battery, you also have a big ground strap from the top of the transmission to the body.

One thing I did was took a multimeter and set it to resistance (ohms) and checked the resistance from places all over the engine to that negative terminal on the battery. The negative on the coil, the block, the fan shroud, the transmission etc. It should all be less than .8 ohms, lower the better.

[ChrisFoley]

The clamp on the positive cable appears pretty tired, and could be a high resistance point.
Make sure the knob on the negative clamp is tight. That knob is your anti-theft device. When it's loosened the car isn't supposed to run.

[Lockwodo]

I have more data to report on this issue.

The car has driven flawlessly up until yesterday when I started it cold and it ran rough and when I drove it it was bucking.

I drove for about 1/4 mile with lots of bucking and then turned it off.

About 30 minutes later I tried to start it and it was completely dead no click, no lights no nothing upon turning the key.

I opened the hood and manipulated the battery wires.

I've included a picture with yellow circles around the wires that I manipulated.



I turned the key and it started fine and no bucking.

This would suggest that this is an electrical issue, yes?

Is there a map on the web somewhere of the ground wires that I can check?

Any other suggestions?

Hi Boxster914. There is a schematic showing ground points here on the 914/4 Tech Notebook website:

https://bowlsby.net/914/Classic/zTN_Elect_GroundPoints.jpg

My understanding is the most important ones for FI functioning are the fuel injection harness spade connectors at the rear engine case bolt, the battery ground strap (as in your photo), and the transmission ground strap.

[wonkipop]

I have more data to report on this issue.

The car has driven flawlessly up until yesterday when I started it cold and it ran rough and when I drove it it was bucking.

I drove for about 1/4 mile with lots of bucking and then turned it off.

About 30 minutes later I tried to start it and it was completely dead no click, no lights no nothing upon turning the key.

I opened the hood and manipulated the battery wires.

I've included a picture with yellow circles around the wires that I manipulated.



I turned the key and it started fine and no bucking.

This would suggest that this is an electrical issue, yes?

Is there a map on the web somewhere of the ground wires that I can check?

Any other suggestions?

when you say you were manipulating battery wires.
you mean just jiggling the wires but the clamps are still tight on the battery.
those clamps are not rotating when you jiggle them?

if the clamps are good and tight its saying that maybe one of the wires inside the outer coating isn't so good? or your connection of wire to clamp is not so good.

as mentioned above those screw down "anti theft" battery disconnectors as well as not being screwed down tight - can go bad, or even be bad from new.
the first one i fitted to my big citroen which has constant battery drain problems was a dud and gave me all sorts of problems even screwed down tight.
i threw it away and replaced it. no trouble afterwards.

worth cleaning up all your ground points while you are at it as suggested.
i gave that battery earth on a real good wire brush and sand paper clean on mine a couple of years ago and did the transmission one at the same time when i had the gearbox out for a clutch.
the trans one had never been done since new and it was real dirty.
a lot of road muck ends up on that one under the car. worth an inspection.

it does sound suspiciously like one of those battery cables is not good somehow.
or its that anti theft device?

[boxster914]

Yes @wonkipop the wires did seem tight to me. I just jiggled them, correct. Nothing rotated while I was jiggling them.

[Lockwodo]

I've come to believe that battery cables should be replaced periodically, like after 10 years even if they look "OK" externally. They can degrade internally and become less conductive.

[wonkipop]

Yes @wonkipop the wires did seem tight to me. I just jiggled them, correct. Nothing rotated while I was jiggling them.

if it happens again, don't jiggle them all.
just jiggle the negative. then try a start.
do that a few times before switching to jiggling the other one.

see if you can isolate it.
often times its the negative (earth) connection that gives you the trouble.

[boxster914]

Thank you all for your input.

Thank you @lockwodo for the bowlsby link.

I'm going to do two things:

1. I'm going to clean up the ground point connections listed in the Bowlsby diagram

2. I ordered and will install the heavy duty ground cable set from AA (http://www.autoatlanta.com/Porsche-914-Ground-Cable-Set-Parts-PN-BTG9141.html)

[Lockwodo]

Thank you all for your input.

Thank you @lockwodo for the bowlsby link.

I'm going to do two things:

1. I'm going to clean up the ground point connections listed in the Bowlsby diagram

2. I ordered and will install the heavy duty ground cable set from AA (http://www.autoatlanta.com/Porsche-914-Ground-Cable-Set-Parts-PN-BTG9141.html)

Thanks for the link. I checked it out and it looks like it will do the job for both battery to ground and tranny to ground.

Also, there is an alternator to ground strap that would be good to replace. You can find an example of that on the AA site.

[mgphoto]

Clean and check the FI points in the base of the distributor.

This takes 5 minutes to check, you will need to retime the distributor.
Those points are low power, oil and dirt will not burn off like regular ignition points.