3.2 - engine intermittent cutout during cruise

[PeterM]

Car starts with no issues, idles and accellerates fine

But now and then during cruise, it starts to cutout. Not completely, but some very rough cutouts for 10-15 sec and then it is OK again for a while.

It started after I replaced the two crankshaft sensors, but it can be a coincidence it happens now

I have a video but cannot upload it (105 mb)

Any suggestions ?

[911obgyn]

Check cap, rotor, ignition wires and plugs.

[proporsche]

did you adjust the correct distance on the sensors?0,8mm

Ivan

[PeterM]

Originally Posted by proporsche

did you adjust the correct distance on the sensors?0,8mm

Ivan

Hi Ivan
Yes I did adjust them to 0.8 mm

[PeterM]

Originally Posted by 911obgyn

Check cap, rotor, ignition wires and plugs.

Yes all checked OK. All plugs were a bit black, so it seems to run a little rich

[JCFL]

As I just wrote on another thread, I just went through the same issue. My problem was a rusty fuel tank that produced too much residue for the injectors. So every once in while, they would " cut out ".

If your tank is original, and other options do not produce an answer, consider the age of your car - its fuel is most likely rusty to a certain point.

[Schulisco]

Originally Posted by JCFL

As I just wrote on another thread, I just went through the same issue. My problem was a rusty fuel tank that produced too much residue for the injectors. So every once in while, they would " cut out ".

If your tank is original, and other options do not produce an answer, consider the age of your car - its fuel is most likely rusty to a certain point.

Have a look inside the tank by removing the fuel sensor before taking action on that. If there is some dirt/sludge/rust this might be the culprit. If its all clean I would continue to check both rpm speed and tdc sensor. Also the ignition coil might have a temperature issue...

Thomas

[PeterM]

Originally Posted by Schulisco

Have a look inside the tank by removing the fuel aensor before taking action on that. If there is some dirt/sludge/rust this might be the culprit. If its all clean I would check both rpm speed and tdc sensors. Also rhe ignition coil might have a temperature issue.

Thomas

I don’t think contaminated fuel is the issue.

I will re-check the 0.8 mm clearance on the two new sensors (speed & TDC).

Is there a way to test the ignition coil ?

[Schulisco]

Check the sensors electrically!
If you can get old proved working ones I would crosscheck them. There are reports of 3rd party sensors lacking of quality.

The coil can be checked electrically. There are resistance values given in the literature. But: A temperature issue can only identified while its hot. Also vibrations can be an issue. Best you also have a proved working coil to crosscheck.

Don't get me wrong - but believing is not knowing on the fuel tank issue. Unless this is proved it's suspicious.

[ant7]

Check the plug at the back of the ignition switch too, this can also be problematic when the connections are bad.
Good luck with it.

[PeterM]

Originally Posted by Schulisco

Don't get me wrong - but believing is not knowing on the fuel tank issue. Unless this is proved it's suspicious.

I know 😀, but I suspect it to be related to temperature, as it only happens intermittently when the engine is up to operating temperature, and at cruise speeds.

Problem is that it is an intermittent fault

[blue911rsr]

Replace your cylinder head temp sensor

[turbo23dog]

Seems to me that one of the new crankshaft sensors might be faulty.

[PeterM]

when I cruise along at 28-3000 RPM, engine up to operating temperature, and it now and then starts to cutout for 10-15 sec, I have notices that when I lift the throttle (load) the cutouts disappear. Then after easing the throttle (load) on again the cutouts disappear.

The engine never stops completely, only cutouts for 10-15 sec and then OK for quite some time again. And only a problem when warn and cruising at a fixed RPM

Can that be the ignition coil giving up under load and when warm ?

[Wish76]

Originally Posted by blue911rsr

Replace your cylinder head temp sensor

I second this. Had similar problems at temperature. Would lose power for a split second, then would consistently get worse until it eventually died. Had no issues restarting once it cooled down. In my case, temp sensor was the culprit.

[wazzz]

When you lift the throttle at say 2 or 3000 rpm, the idle switch is activated (closed) and that tells the ECU about the no-load condition. As long as the RPM is dropping down but remains above idle speed (880), the ECU cuts fuel delivery (not sure about this point, but I remember reading this was a fuel saving feature introduced with the first generation of Motronic).

So in this situation, the cutouts would disappear because no fuel is delivered anyway.
I don't know about the other situations, but your problem could fuel pressure-related. Fuel pump, damper and regulator may need to be checked.
Also check your CHT ohms at various temperatures.

[mysocal911]

Originally Posted by wazzz

When you lift the throttle at say 2 or 3000 rpm, the idle switch is activated (closed) and that tells the ECU about the no-load condition. As long as the RPM is dropping down but remains above idle speed (880), the ECU cuts fuel delivery (not sure about this point, but I remember reading this was a fuel saving feature introduced with the first generation of Motronic).

So in this situation, the cutouts would disappear because no fuel is delivered anyway.
I don't know about the other situations, but your problem could fuel pressure-related. Fuel pump, damper and regulator may need to be checked.
Also check your CHT ohms at various temperatures.

Correct!

[PeteKz]

I don't see how your injectors are clogged by rust. The fuel filter should stop crap from the tank getting to your injectors. Start considering other explanations.

[Schulisco]

Originally Posted by PeterM

when I cruise along at 28-3000 RPM, engine up to operating temperature, and it now and then starts to cutout for 10-15 sec, I have notices that when I lift the throttle (load) the cutouts disappear. Then after easing the throttle (load) on again the cutouts disappear.

The engine never stops completely, only cutouts for 10-15 sec and then OK for quite some time again. And only a problem when warn and cruising at a fixed RPM

Can that be the ignition coil giving up under load and when warm ?

10-15secs is a long time...

I would also suspect the ignition coil due to heat and vibration issues. Pretty easy to detect by temporarily changing from a running car best, when your's is bad you can even buy a new one.
Also the idle switch, both rpm and tdc sensor and as well the cylinder head temperature sensor are suspicious unless they're proved to be working fine as others already told.

It's pretty hard to identify the only one culprit, all of them could cause the same symptom. Not to forget the DME relay - if it's worn it can cause shutting down the fuel pump completely. Easy to verify with a cable bridge as mentioned here as well and documented in the Bentleys WS manual for the 3.2 Carrera.

https://forums.pelicanparts.com/porsche-911-technical-forum/665035-dme-relay-carry-spare-2.html#post6619164

Thomas

[dtwa]

Intermittent problems are hard. I had similar symptoms in my 85. Along the way I replaced all the sensors, and the coil.

Check the fuel pressure. Mine eventually would only run intermittently. At that point the fuel pressure was sometimes low, sometimes zero.

The fuel pressure regulator was the last thing I replaced. Been fine ever since.

—Dave


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