Porsche 911 SC tremble and lack power

[IF80ACE]

Dear friends,
I have a Porsche 911 SC with WUR rebuilt, injectors checked, spark-plugs changed, new fuel pump, system pressure almost 5 bars, adveance checked.
Works well for 10-15 minutes, but when is gets warm, starts to tremble and lacks the power under load. At idle stays perfect just under 1000.
Any ideas or similar cases will be appreciated.
Thank you in advance.

[john walker's workshop]

What year? If 80-83, look under the right seat and unplug the little square relay, with the engine running. If nothing changes with the engine idle, then you're onto something. The interior light fuse powers the relay. Relay could be tits. Wiring at the plug could be corroded. Good place to start anyway.

[IF80ACE]

It's a Oct 1980 - EU spec - there is no relay under the right seat.

[john walker's workshop]

If we had that info at the getgo, we could offer accurate advice. Post the year in your signature.

[David Inc.]

What is trembling? Like misfiring on heavier acceleration? My first guess would be plug wires--both times my SC has developed a similar issue it's been the wires.

[IF80ACE]

The engine vibrate - and the whole car. No misfire - on deceleration it's like a mumble - also after it starts doing that - it tends to overheat (oil line and entire route - checked - it works ok).
I will change also the plug wire.
Thank you for the advice!

[safe]

Check your warm control pressure.

[IF80ACE]

Checked 2.5-2.6. I will try to adjust that a little by WUR, but i don't think it influence that much.

[keoniahlo]

Look like you're experiencing symptoms that are often associated with fuel or ignition system issues, particularly ones that manifest once the engine has warmed up. Given the comprehensive maintenance that has already been performed (WUR rebuild, injectors checked, new fuel pump, spark plugs replaced, and system pressure at 5 bars), here are a few potential causes and things to investigate further:

1. Warm-Start Fuel Issues (Fuel Delivery Problems)
Even though the fuel system pressure is reading correctly at 5 bars, issues can still arise when the engine is hot. A warm engine can cause vapor lock, or the fuel mixture could be wrong under load due to poor regulation of fuel pressure or mixture during hot conditions.

Fuel Pressure Regulator: Check the control pressure regulator or the warm-up regulator (WUR). The WUR should be operating correctly, but sometimes, even after a rebuild, it can have issues regulating pressure or control properly once the engine is at operating temperature.
Fuel Lines/Vapor Lock: Check for any vapor lock or heat-soak issues that might be causing the fuel to bubble or the system to lose pressure under load when hot. Insulating fuel lines or checking for blocked return lines can help.

2. Ignition System (Coil, CDI Box, or Distributor)
Problems with the ignition system can often cause power loss under load once the engine heats up, especially when components like the CDI (Capacitor Discharge Ignition) box or coil start to fail under heat. These parts are known to develop issues when they get hot.

Coil or CDI Box: Check if either of these components is showing signs of failure when heated. It's worth checking for weak spark or inconsistent spark under load after the engine warms up.
Distributor: Check the distributor for wear, especially the rotor and cap. A faulty or corroded cap and rotor can cause intermittent misfiring when hot.
Ignition Wires: Inspect the ignition wires, as heat can sometimes cause cracks or issues in old wires, causing them to short or misfire under load.

3. Air Intake and Fuel Mixture
The mixture may be leaning out when the engine gets hot if the system is not maintaining the correct air/fuel ratio under load.

Air Flow Meter (AFM): If the AFM is malfunctioning, it might provide incorrect air intake data to the ECU (if equipped), leading to a lean mixture under load.
Throttle Position Sensor (TPS): If the TPS isn't correctly adjusted, it could cause issues with fuel delivery under load, especially when the engine gets warm.
Vacuum Leaks: Even small vacuum leaks can cause idle stability issues and power loss under load once the engine warms up, as components expand with heat and leak air that affects the fuel/air mixture.

4. Oxygen Sensor or Lambda Control (not that you have one, but for others with US models)
If the car is equipped with an oxygen sensor and lambda control (which started being used on Porsche 911 SC models around the early 1980s), a faulty oxygen sensor can cause incorrect fuel mixture adjustments when the engine is warm. It may not have a noticeable effect at idle but will cause power loss under load.

5. Overheating or Oil Issues
If the engine is getting too hot, it might be suffering from a loss of power or "trembling" under load due to a variety of engine problems. Check that the cooling system is functioning properly, and that the oil temperature and pressure are within normal limits.
Oil Viscosity or Pressure: Low oil pressure, or the wrong oil type (too thick or too thin), can cause internal engine problems once the oil warms up.

6. DME (if equipped) or ECU
If the car has a DME (Digital Motor Electronics) or similar control system, a faulty sensor or malfunctioning DME/ECU could be causing the problem. It might be worth getting the ECU checked for fault codes if the system supports it.

7. Temperature-Sensitive Components
Temp Sensor: A faulty temperature sensor could send incorrect signals to the fuel management system, leading to incorrect fuel mixture under load when the engine is warm.
Fuel Pump Relay: A failing fuel pump relay could lead to intermittent power loss under load, especially once it heats up.

Suggested Steps to Diagnose:

Monitor Fuel Pressure Under Load: Verify that the fuel pressure remains steady and doesn't drop when the engine is hot and under load. Not sure where you're seeing 2.5-2.6, but that's typically low for warm control pressure, and this would cause an overly rich condition.

Since you state the you have a Euro setup, be sure that your WUR is providing enrichment via the vacuum hose and that it's connected to the correct port on your throttle body. Verify functionality with pressure gauges, control pressure should drop during acceleration. Note: The US models' WUR hose connects to a VENT (no vacuum), so study this carefully. Your WUR line should have vacuum at idle. It basically works in the opposite fashion of the vacuum advance for your distributor.

Check Ignition Components: Test the coil, CDI box, and distributor for signs of failure once the engine warms up. I really think it's a fuel-related issue, but you never know.

Check for Vacuum Leaks: A smoke test could help reveal small leaks that might be affecting idle and performance under load. Some leaks rear their ugly heads only after warmup.

Test the Temperature Sensors: Ensure they’re providing accurate readings to the engine management system.

Check Engine Temperature: Ensure the engine is not overheating, and that oil pressures are stable.

By systematically ruling out these potential causes, you should be able to narrow down the issue. If all else fails, it might be worth consulting a Porsche specialist who has experience with the 911 SC's specific systems. Good luck!

Aloha,
Keoni

[IF80ACE]

Wow! Thank you Keoni for all the directions! I also suspect a fuel related issue.
I will take it step by step. Keep you updated.

[keoniahlo]

You're welcome! I owned a 1983 SC back before I was married. My dad had a 1981 Targa--he kept his, I sold mine Had to get a "family car". I used to do all my own work on the car, owned it for 12 years. My dad has recently had a long stretch of a poor-running engine. He initially didn't want me to mess with it because then it might confuse his mechanic (I get it, but it hasn't run right in YEARS!).

So finally, he had his motor completely rebuilt. After reinstalling everything, it ran exactly as it did before the rebuild LOL. So it was the CIS all along, as I had told my dad. Unfortunately, the CIS components weren't validated while the engine was out, which is unfortunate, but I've done the entire circuit of tests except the AAV, which I'll have to lower the engine to do. I took the car from barely starting, blowing black smoke (rich) and dying sporadically, to humming like a clock. It's so rewarding to get these old CIS systems running right!

BTW, there were quite a few things wrong with my dad's system:

1. WUR was totally out of spec, so insane that wasn't identified and replaced
2. Thermo time switch turning on when cranking, but not shutting off until cranking stop (supposed to be around 2 seconds, phase 1 of the temp switch), so flooded during cold start
3. Vacuum hose to decal valve connected to same port as distributor, WRONG!
4. Decel valve opened too soon at 17 hg, while manifold vacuum was 17hg, so endless loop of hanging high idle ofter acceleration. Compressed the decal canister 1.5mm to get it to open at 19hg
5. Time was off, was at 2 deg BTDC, set to 5 and max at 25 (total range)
6. Had to readjust CO obviously, was wayyyyy off

Tip 1: If you have a lambda-based 911 (80-83), consider using a frequency valve power driver cable to set the freq valve to 50% (essentially an open loop, but takes the ECU totally out of the equation so you can diagnose all the moving parts and fuel system first). You can get this for $40 from Steve at Unwired Tools. It's basically an EV1 connector, two wires, and an inline electronic card to drive the freq valve at a set rate. It simulates disconnecting your o2 sensor plug (ECU goes into "limp mode", where it will still run, but stuck in open loop mode and at 50% (some say 65%, but I feel it's 50%).

Tip 2: If you have a pop-off valve in your airbag, you can watch the cold start valve working with a cheap $25 endoscope that plugs into your phone. Need to have a fuel pump bypass to turn it on, then send power to the cold start valve via the thermo time switch (short the two terminals and it should start to spray).

Good luck to you--be patient and methodical, video your progress and take good notes!

Aloha,
Keoni

[keoniahlo]

Forgot to ask:

What are your running pressures when warm and then what does it do when you push the throttle?

Do you have the correct WUR? Euro spec needs a vacuum port on the side of the WUR connected to the proper port on the throttle body (below the the throttle butterfly), so manifold vacuum. The port of the top of your WUR should go to vent (highest and most rearward port on throttle body), the one on the side to a lower port on the side or rear of your throttle body.

Sounds like a fuel enrichment issue TBH.

[pmax]

Originally Posted by IF80ACE

Checked 2.5-2.6. I will try to adjust that a little by WUR, but i don't think it influence that much.

With the engine running ?

If it's the 089 WUR, chart is here CIS Troubleshooting for Dummies

[IF80ACE]

Thanks all for all the feedback.
@pmax and @keoniahlo - WUR opened again and adjusted by new mechanic - warm control pressure was at 2.5 - now adjusted to 3-2.1 at warm - the car performs super - no more lacks of power.
BUT still the tremor at acceleration - in neutral or under load.
Next plan is to strengthen the belt a little more - to make a compression test, check valves adjustments, change oil etc - let's see after!
Thanks for all the support!

[PeteKz]

Although I'm still not clear on what you mean by "tremor" or "tremble", I think you have an ignition problem. Ignition coils tend to fail gradually, and those failures show up first when the coil gets warm, under acceleration, and at higher RPM, and are usually worse in damp weather. CDI boxes tend to fail either all at once, or quickly.

However, after replacing the coil with a known good one, and you still have rough running, suspect the CDI box or the distributor. Another common failure at this age is the green wire that connects from the distributor to the CDI box. The insulation and internal wire crack at the connector to the distributor after 40 years or so. Try wiggling the green wire at the distributor while the engine is warmed up and idling to see if that makes a difference.

I emphasized "known good" coil, because there are cheap/bad coils in the supply system that are bad right out of the box new, or fail quickly.

The best thing you can do is find another person with an SC near you that will let you swap the coil and CDI box to see if that fixes your problem.

[IF80ACE]

Hi,

Checked with new coil. No change ... CDI box ... I try to source and compare.
Thanks!

[David Inc.]

Check your plug wires yet? Easy test to do first. Take the car to as dark a place as you can (garage but keep in mind monoxide!) and look at the engine bay. If you see a spark-show along the wires then that's your problem.

[gsxrken]

Originally Posted by keoniahlo

4. Decel valve opened too soon at 17 hg, while manifold vacuum was 17hg, so endless loop of hanging high idle ofter acceleration. Compressed the decal canister 1.5mm to get it to open at 19hg
Keoni

I never heard of this. You just pinch it in a vice and it delays the opening spec? I’d like to understand this more.

[PeteKz]

Originally Posted by David Inc.

Check your plug wires yet? Easy test to do first. Take the car to as dark a place as you can (garage but keep in mind monoxide!) and look at the engine bay. If you see a spark-show along the wires then that's your problem.

I doubt that the ignition wires would work when cold, and then miss when warm. I haven't seen that mode of failure (yet).

[David Inc.]

Originally Posted by PeteKz

I doubt that the ignition wires would work when cold, and then miss when warm. I haven't seen that mode of failure (yet).

Yeah but who floors it to find out how it operates under load when the engine is still cold? If he can floor it cold and no issue, then yeah I think I'd agree--but who does that?

Either way it's an easy thing to check if one has a garage.