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Porsche Crest 1978 928 wonīt rev above 2500-300 rpm and loses power when warming up

Hello all,

I am Dennis from the Netherlands and new to this forum.
I have a 1978 928 manual 5 speed (US spec car) and it has a problem which iīm struggeling with.
The car wonīt rev above 2500-3000 rpm (when driving) and loses power when cruising and is warming up.
When it loses power it is struggeling to get to 40mph (it starts to sputter when i push the trottle too far), when i pull over and shut it off for 1 or 2 minutes and start it again i can accellerate to 60mph without problems (the rpm issue still remains however) and when cruising it again slowly starts to lose power again.
It was a non runner when i bought it, some parts where missing and it had been sitting for some years (donīt know how long exactly).

This is what i replaced

-Removed the fuel tand and flushed and cleaned it
-Replaced the in tank filter and the filter in the right wheel well
-Replaced both fuel pumps
-Replaced all 8 injectors
-Blew out all fuel lines with compressed air
-Cleaned the fuel distributor and replaced the o-rings and the diaphragm (used a rebuild kit), checked if plunger is moving freely.
-Cleaned and adjusted the WUR
-Checked the iginition system, all parts seem in good condition
-Removed the internals from the catalytic converter (i thought it might be clogged)

Some values i meassured:

-Fuel flow (1.6 liter in 30secs with engine running)
-System fuel pressure: 74psi
-Cold control pressure: 25psi (engine running)
-Warm control pressure: 42psi (engine running)
-Plugs sparking

So, iīm running out of ideaīs, maybe someone here can help me out....

Greetz,
Dennis

Last edited by Dennis928; 11-09-2018 at 02:02 PM..
Old 11-08-2018, 01:43 PM
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HI Dennis,
You didn't mention your fuel injectors. The O rings should be pretty well shot by now if they've never been serviced before.

Have you checked vacuum? Your symptoms sound like a leak somewhere to me.
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1981 928 Euro "Babied and Pampered"
Old 11-09-2018, 12:13 AM
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Quote:
Originally Posted by Mal81 View Post
HI Dennis,
You didn't mention your fuel injectors. The O rings should be pretty well shot by now if they've never been serviced before.

Have you checked vacuum? Your symptoms sound like a leak somewhere to me.
+1 Vacuum: check the hose from the dizzy to throttle body, it might be dry, brittle, and/or cracked. Mine was and the connection at the dizzy seemed good but was not. also, the vacuum diaphragm (on the dizzy may be problematic, though lower odds)
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'72 BMW 3.0 csi, 85k km (euro Deutschland '82) Taiga
Old 11-09-2018, 09:44 AM
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Hello Mal81, thank you for your reply.
I did replace the injectors by new ones, i re-used the rubbers, iīll check for leaks there, thanks.
Old 11-09-2018, 02:04 PM
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Hello JhwShark,

Thanks for your reply.
I checked all the vacuum hoses, replaced some and re-routed some to the correct spots using the diagram from the factory manual.
When i meassure vacuum at the brake booster i donīt get enough vacuum (around 20 i believe it should be) so maybe vacuum is a problem.
Old 11-09-2018, 02:11 PM
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Vacuum Leak -

Hey Dennis - Your cold/warm fuel system (Warm Up Reg) pressures sound about right. Have you verified that the throttle plate arm inside the fuel distributor is moving "Full Travel" without any binding or drag ? The arm movement resistance should be linear or consistant in one direction (down) with no resistance in the opposite direction when it returns to it's seat. The initial throttle plate seating height adjustment, pitch and the plate center alignment is very important. Can you hear the injectors singing (High Pitch Squeal) with the fuel pump running and you move the throttle plate arm ? (engine not running)

To help locate a vacuum leak - you might try spraying a wiff of starting fluid along the base and rubber sleeves along each intake runner, throttle body seals, Vacuum Brake Booster, Injector "O" Rings, Vacuum Line Plumbing, etc... to see if your RPM increases or flares up. Attaching the red plastic straw from a can of WD40 lubricant to the can of starting fluid lets you better isolate each test point. If you suspect the Brake Booster Internal Diaphragm is torn or leaking - you might isolate it from the intake by disconnecting it's large vacuum hose and temporarily plug it.

A spray can of Brake Cleaner or Carburetor Cleaner will also work as well - just be carefull not to have any stray ignition archs jumping around from defective sparkplug wires. If there is any doubt as to whether the sparkplug wires are potentially faulty - I usually wait until it's dark outside to do the test.

Please Keep Us Posted and Good Luck - Michael
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1984 928S - "Miss Purdy"
1987 911SC - "Frau Helga"
1986 930 - "Well Hung"

Last edited by JK McDonald; 11-12-2018 at 05:10 PM..
Old 11-12-2018, 04:57 PM
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Quote:
Originally Posted by JK McDonald View Post
Hey Dennis
To help locate a vacuum leak - you might try spraying a wiff of starting fluid along the base and rubber sleeves along each intake runner, throttle body seals, Vacuum Brake Booster, Injector "O" Rings, Vacuum Line Plumbing, etc... to see if your RPM increases or flares up. Attaching the red plastic straw from a can of WD40 lubricant to the can of starting fluid lets you better isolate each test point. If you suspect the Brake Booster Internal Diaphragm is torn or leaking - you might isolate it from the intake by disconnecting it's large vacuum hose and temporarily plug it.

A spray can of Brake Cleaner or Carburetor Cleaner will also work as well - just be carefull not to have any stray ignition archs jumping around from defective sparkplug wires. If there is any doubt as to whether the sparkplug wires are potentially faulty - I usually wait until it's dark outside to do the test.

Please Keep Us Posted and Good Luck - Michael
Be VERY careful spraying combustibles...BTDT: I caused a little fire, which could have been much worse! Try a vacuum tester, mighty vac...
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'79 928, 85k Opal Metallic
'99 BMW 540i, 97k Titanium
'72 BMW 3.0 csi, 85k km (euro Deutschland '82) Taiga
Old 11-13-2018, 08:47 AM
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Warm Up Regulator Control Pressure -

Hey JHW - You are absolutely right in warning of a potential fire hazzard when using starting fluid to locate a vacuum leak. I typically only use this methode when trying to pin down the source of a odd ball vacuum leak. For safety, it's best to check things over at night to look for any fireflys jumping around between the sparkplug wires 1st.

Hey Dennis - Since you mentioned that the drop in RPM occurs gradually (possibly as the WUR is going through it's adjustment cycle) perhaps you could leave the fuel pressure gauge connected to the WUR control pressure as you drive. IF the control pressure goes much over the correct 42 - 43 psi, the fuel distributor throttle plate arm will not follow the fuel demand as you move the accelerator pedal.

One other thought - Did you move the 3mm allen adjustment screw for the fuel control arm leverage point while rebuilding the fuel distributor ? This factory set adjustment is very difficult to duplicate (or reset correctly) without either some air/fuel exhaust sensing equipment or about 106 years of 928 CIS experience under your belt.... A friend had given this screw a twist on his 80 Euro 928 to see what would happen and we liked to have never got it set right again.

Good Luck - Michael
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1984 928S - "Miss Purdy"
1987 911SC - "Frau Helga"
1986 930 - "Well Hung"

Last edited by JK McDonald; 11-13-2018 at 11:44 AM..
Old 11-13-2018, 11:04 AM
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One additional thought - is your voltage guage dropping as you drive? Do you get an electrical warning light in the cluster? If so, it could be a voltage regulator problem. My 78 suffered similar issues, and the warning light was my clue, and voltage drop. I replaced the voltage regulator (it's attached to the back of the alternator) and problem solved. If you do not get the battery light at start up, then the exciter wire from the alternator to the 14 pin connector, or from the cluster to the alternator, could be bad too.
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Old 11-14-2018, 08:48 PM
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Power Loss -

Hey Dennis - This type of problem can sometimes be difficult to diagnose. What did you find was the reason for your loss of power issue - Vacuum Leak, Warm Up Regulator, Failing Alternator, Riding The Brake Pedal - Operator Error ? ............

Michael
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Last edited by JK McDonald; 11-19-2018 at 02:55 PM..
Old 11-19-2018, 02:46 PM
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Quote:
Originally Posted by JK McDonald View Post
Hey Dennis - This type of problem can sometimes be difficult to diagnose. What did you find was the reason for your loss of power issue - Vacuum Leak, Warm Up Regulator, Failing Alternator, Riding The Brake Pedal - Operator Error ? ............

Michael
Hello all, thank you all for your thoughts and sorry for my late reply. The problem still remains, i drove the car today after adjusting the ignition timing.
I checked for vacuum leaks after i replaced the intake runner gaskets and 8 hoses on top of the engine , no leaks found, vacuum looks ok.
The gauge looks fine when driving and the battery light goes off after starting.
I checked the sensorplate when i replaced the injectors.
I had all injectors hanging in a jar (connected) and they all sprayed fine when i pushed the sensor plate (fuel pump running).
I did play with the A/F adjustment screw and idle speed screw to get the idle right (started from lean to rich).
I still donīt get why it is driving pretty good when cold / little warmed up and goos bad when fully warmed up. I drove around with the fuel pressure gauge connected and fuel pressure did not rise above 42 psi......
Any more ideaīs?
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