MKRS

Hello!

I have been studied lot of similar posts with this problem, but haven't found any solution yet:/

The problem is that this is not even stalling: the car suddenly starts to decrease rpm's on idle mode and then if you try to accelerate it struggles to increase speed:
- it can happen with cold or hot engine
- happens when you are driving, for example cruising around a city and suddenly the problem arrives and you loose engine between 1000-3000 rpms (like a dead)
- every time you can shut down and start the car (no problem to start it- of course with low rpms..)
- the car is not stopping (you can still get to home ) only if you push your throttle pedal almost to full (85% of full) and then it keeps going (driving of course is like "jumping rabbit" full throttle or not at all)
- if you turn of the engine and wait for some time, everything works again but not guarantee that this won't happen after some 1 km again
- I have tried to avoid this problem by trying to go with high rpm and usually it helps (it happen on track days so I didn't have a chance so I pushed it hardly 2nd and 3rd gear and after some
2 minutes race I got away from this problem)
- the feeling is that either there is no some signal for all sparks or there is no fuel flow at all; strange thing is that with full throttle seem that car runs strong so I doubt that it could be fuel pump, but who knows..
- two things I mentioned but haven't investigated them yet: rubber tube which is connected to ISV seems in vacuum a little bit (it didn't seem that this is the same when you normally start a cold car) and second thing- when you sometimes open the fuel tank cup feeling is that there is vacuum too

There are list of things I have done:

- ISV cleaned, seemed really clean
- AFM (cleaned contacts; tested on table with 9V battery and there was some voltage drop among 3-4v, but when I put it on a car and test voltage it works perfectly without any voltage drops; afm wire fixed)
- fuel filter changed
- dme relay changed
- checked for vacuum leaks, didn't find
- throttle idle switch checked, works
- new air filter
- new oxygen sensor
- steve wong chip installed

I would really appreciate any advise which could help to solve this issue. This issue starts to disturb the good feeling you have when you want to go for a drive on nice Saturday's morning.

Regards,
MKRS.

cjoenck

You may want to read any fault codes in the ECU using something like the Hammer, Durametric, ScanTool, etc. that may help point you in the right direction. Measure fuel pressure as well if you can. How did you determine the fuel pump is working properly?

Intermittent stuff is always hard to diagnose so process of elimination can be long.

Is this a Tiptronic or a manual?

spartansix

Coil or distributor problems seem like next place I'd look. Check the coil wires and coils themselves for signs of corrosion. Make sure that the engine runs on both distributors by disconnecting one ignition module at a time and seeing if it will start.

Silvertarga

if your ignition coils are silver thats the problem

you need to do the 993 coil pack upgrade

cjoenck

Yes, that's a great upgrade regardless. This also sounds like fuel starvation though.

MKRS

Sorry, didn't mention: 1989 C4 manual. Engine fully rebuilt some 5k miles ago and lot of things have been changed, everything looks clean.
Tested ECU some months ago but there was no any fault codes related to engine (had some speed sensor issues, but solved them).

MKRS

If this would be coil&wire issue, shouldn't it be that car struggles to go/ runs bad (as some of cylinders doesn't work) instead of not going at all?
I will check distributors by disconnecting at a time. Can't say right now if I have silver coils- will check that too

spartansix

I have seen failing coils/corroded coil wires cause symptoms similar to yours, especially if one coil is still working but the other is intermittent/degraded.

Navaros911

I have an IDENTICAL problem and I haven't been able to get to a solution... yet.

When I say identical, I mean not a single deviation. To bring up more similarities, I have tested the EXACT same components and not found a fault.

Actually I have a thread somewhere on it I think...

Now, you didn't mention how long the issue occurs for. In my case it happens for a few seconds up to a 15 secs or so... but boy do they feel long. It feels very electrical to me... comes up out of nowhere and disappears in the same way. My mech can't diagnose because it doesn't happen when he drives it.

At the moment my car is in pieces, but they will check the ignition wires while out. I have sprayed with water though and seen nothing... so doubt it's that. I am suspecious of the coils, which are the original ones. I did inspect visually and they look fine... then again so would the silver ones, right.

I am contemplating replacing the coils with the 993 units since it is easy and cheap compared to keeping the diagnosis going.

Hope we can learn something from each other.

LPMM

It feels like the same issues I am experiencing https://rennlist.com/forums/964-foru...ing-rough.html
As a first step, I will check the distributor belt when I get a chance and then most likely replace the coils since they are original to the car. Everything was working fine and all of a sudden it started acting up, so it really feels like an electrical issue in my limited knowledge.

MKRS

What should I say- nice to hear that we could share with our experience but instead of that would be better not have these problems at all

I checked my coils: they are no silver, but black with red upside. I am not sure if those are from 993 but anyway they are not silver ones.
Currently trying to check fuel pump just to make a tick at problem check list.

Took some instruments with me and went for a ride this evening. Everything was plus minus good if not taking in account that at the beginning my throttle pedal feels more lively than after some miles drive. Anyway, I stopped and let it idle while opening bonnet. Suddenly problems started- it began to decrease rpm's and idle was rough. What I did- with my arms I tried to move AFM cable connector and the after some seconds idle came back to normal and then no more problems (at least back to home).

But still I am not sure that AFM cable is the problem because it has happened already before (when problem starts, just stop, wait, turn off engine, then try again and it runs again just fine). Next time I will go for a drive I will connect additional cables to my AFM and connect voltage measurement tool inside the car to see if there is some AFM voltage problems when the problem arrives.

The best solution would be to buy new AFM, new coils, new fuel pump, regulator, etc. but it is not at least the cheapest and lets say, effective

spartansix

Touching your AFM cable makes me think that this TSB has not been done. Follow the instructions here:

https://rennlist.com/forums/964-foru...ml#post2947343

greg1990964

That little plug can be a bugger. Very finicky. I remember dealing with this around 8 years ago. I bet I had a thread (35) or comment (80) on this issue. Trust me, dont blow million on coils. I went down that rabbit hole. Brazilian coils, ones made in Europe etc. Oh man I went nuts. I was a dealer in perfectly fine new coils online. Haha. Its something other then coils. Its simple connections. Remember, when you connect those small nuts on the coils, they also must be grounded property. And they can arc if the connector on top of the coil or too close to the body or that carrier (behind it) they twist when tightening if I remember correctly. Just check all those connections in the dark. And of course plug wires and coil wires will arc in the dark and can set off the DME under the seat. Its a fun chase. There are more smart then me, but that was my experience.

MKRS

Thank you everyone for advises!

Checked AFM cable and now fixed it properly as was mentioned in the post. Took the car for a drive and everything worked perfect till one moment when while cruising suddenly felt that speed is decreasing and problem arrives. So no changes!

What is really strange: no matter if the car is moving and decreasing a speed or it stands and idling poorly, I will make it run if I PUSH FULL throttle. Test it today- I was going in 3rd and when the problem arrived I was pumping a throttle from 0-70% of it, nothing..it just keep loosing a speed as there is no power at all. And then full throttle and if goes, goes goes till...7k rpm. If that be an electric problem, would it go on high rpm? But from other perspective, if that is fuel linked problem, would there be fuel to feed it on high rpm.

I started to figure this all thing out with listing facts and trying to put technical conclusion for each of fact. For example:
- if that would be fuel pump problem, it would not go on high rpm by pushing max throttle
- in the same time, it could be FPR problem, as it struggles in middle rpm but goes with full throttle
- have to split to things: high rpm and high percentage throttle: when I achieve the car running, I can do it with pushing full throttle in whatever gear (for example going on 30 miles in 3rd gear and push throttle), so rpm doesn't matter. What matters is operations with throttle pedal
- if there would be only throttle related problems, it doesn't explains bad idling when at the end car can actually die..
- I can't figure out what happens with fully pushed throttle: I know that O2 is going out of the game after (i can be wrong but) 75% of pushed throttle pedal, but as I have change O2 sensor, I take it out of the list. Would a fully pushed throttle provide more air flow at the engine or fuel flow?
- if there is still AFM problems, can you anyway run a car with fully pushed throttle? haven't try

Here is some video how things happens on idle position when the problem has arrived. It runs very bad on low rpm, but can still accelerate while not going. But there is not enough power to drive a car (at least you don't push full throttle). Feeling is that it is like gagging.
My intuition says that this still is some king of fuel starving problem in the middle/low rpm. But don't know explanation why then it goes on high rpm.
Have a plan to do a full fuel test which was advised at Pelican.

MKRS

Thank you everyone for previously provided advices!

Currently I have checked lot of things:
- checked fuel pressure: with the car running and also crossed relay without running- everything according to manual (55psi and 49 psi)
- fuel capacity test done: also 850ml in 30 sec as it should be
- changed coils to 993

This weekend took it for a drive: first evening everything was perfect (as my battery was disconnected for a long time during change of coils, after that the car always runs quite good)
Next day after some 2 minutes of drive it happened again-the same feeling as there is no signal for ignition at all or fuel delivery problems, so it started fluttering and to decrease speed, but I was able to keep going if pushed throttle to full as always.

I even don't know what should be the next thing to check: engine temperature sensor? AFM change to new one?
Scheduled time for service at Porsche center next week to check if there is any ECU faults recorded, but I suppose there wouldn't be as this is not permanent problem and I suppose not 100% electronically related.