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Porsche Crest 1978 928 wont 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 im struggeling with.
The car wont 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 (dont 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, im running out of ideas, 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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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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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, ill 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 dont 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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'99 BMW 540i, 97k Titanium
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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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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 dont 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 ideas?
Old 11-21-2018, 08:48 AM
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Progress -

Hey Dennis - Good work - You've verified and eliminated quite a few potential problem areas, so you are making progress.

When you mention turning the 3mm Air/Fuel adjusting screw in the fuel distributor - how far do you think it was moved from the initial setting ? Any adjustment must be done in very miniscule increments. When a friend gave his a couple of turns his 928 began running so rich it would finally choke itself out. You might pull a spark plug or two just to see if they are wet with fuel ? Also if over fueling is found on the plugs - don't forget to check for a stuck open "cold start injector" .

Just to eliminate another area - have you inspected the wiring to the ignition unit, coil and the wires connected to each end of the two voltage dropping resistors on the left side inner fender. They are mounted near the power steering reservoir. The .4 and .6 ohm resistors are pretty tough but the wiring sometimes becomes damaged and/or contaminated.

Kind of a Real Wild A$$ Guess But- Have you checked that the crankshaft and cam timing marks all line up. When running rich, I have seen some CIS cars with some strange symptoms (fall on their face as they warmup) if their timing belt was off a tooth.

Good Luck, Michael

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Last edited by JK McDonald; 11-21-2018 at 03:17 PM..
Old 11-21-2018, 02:27 PM
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^^^ all good things to try - i don't have a CIS car so i'm just throwing in another idea - could it be running too lean? - something like this scenario - you start the car and with the WUR cold it enriches the mixture and the car runs just at the edge of ok - then as the WUR warms up the mixture turns lean and at WOT there is just not enough fuel in the mix to keep it going - i'd probably "play" with the mixture both ways and watch for either improvement or worsening of your symptoms
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Old 11-22-2018, 08:57 AM
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Hello Michael,
Before i bought the car someone else already had a go, the ignition control unit was unscrewed from the body, the 2 resistors were disconnected (and the wires jumpered (connected to each other).
I measured the resistance (0.4 and 0.6 ohms), and connected the wires to them again. There was aslo a "new" green wire installed (the old one was still routed across the engine), the distributor cap and rotor were both new and also the spark plugs.
The cold start valve was missing! I bought a 2nd hand one, tested it and installed it to the intake (works fine).
When i adjusted the A/F mixture i leaned it out first untill the engine started to stumble and then turned it cw to richen it a bit untill it got stable again.
As far as i am understanding the A/F mixture screw only affects the A/F ratio at idle, not while driving because then the sensorplate is moving down (because of the air that passes) and moves the plugner in the fuel distributor which opens the slits more so more fuel can go to the injectors....or is this not correct?
Old 11-22-2018, 10:11 AM
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Hello Honerboys,
Thanks for your reply, i get what you mean but the control and system pressures are within specs (also the warm control pressure) so that should be ok, maybe i can do a testdrive with the plug from the heating element disconnected so the cold control pressure remains (25psi) and it runs richer.....I think thats something for this Saterday to try...
Old 11-22-2018, 10:18 AM
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Could be the difference between working, and working well.

I didn't read the whole thread, so sorry if I repeat or suggest you do something you have already done.

Runs ok at idle, not so good above 3k rpm.

Is your ignition distributor working correctly, with vacuum lines going to the right places without leaks?

Did it do the same sort of thing before all your repairs?
Old 11-23-2018, 01:11 PM
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Air/Fuel Adjustment -

Hey Dennis - Actually the Air/Fuel Ratio allen screw does affect both the initial idle mixture as well as the gradual increase in fuel sent to the injectors across the whole RPM range of the engine. The Fuel Control Plunger in the center of the fuel distributor sits on the roller wheel of the Air Sensor Plate Control Arm and follows it's tracked arc precisely. If the initial A/F ratio is set rich at idle - the delivery to each injector will then be slightly rich through the complete range of movement.

In the (Generic) Diagram 1 you'll notice the Air Sensor Plate Arm is actually made up of two parts. Item 11 has the A/F Adj Screw at one end and the Primary Pivot Point Hole at the other end for the arm assembly. Item 12 portion of the Arm has the Circular Air Plate Attached which monitors the flow of air resulting from throttle position. These two parts make up the adjustable lift ratio (Leverage Lift Point - Ratio) for the Fuel Control Plunger.

In Diagram 2 - you can see a side view of the Air Sensor Plate Arm and the top of the A/F Adj HUMP that is directly under the Center Fuel Control Plunger. As this hump is adjusted (Higher / Lower) it is amplified by the levered distance to the tip of the Air Plate.

What is amazing is that some German engineer using an old school Abacus and Slide Rule calculated the ramp of enrichment required to keep the CIS injectors passing the correct amount of fuel under all conditions. With some component size mods and a tweak to the A/F ratio the CIS system has been successful on a wide assortment of 4, 6, 8 and even 12 cylinder engines.

Good Luck - Michael


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Last edited by JK McDonald; 11-24-2018 at 08:55 AM..
Old 11-24-2018, 08:48 AM
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Hello Danglerb,

All vacuumconnections are ok, the distributor only has one vacuumhose (i've read there are two on some models...).
Idle is ok, runs beter cold then warm (warm engine does not rev above 2500-3000 rpm (under load)).
Bought the car as a none runner and parts missing so my repairs made it a lot better so far (it runs and drives) but im not there yet...
Old 11-24-2018, 12:43 PM
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Hi Michael,
Thank you for your explanation, as you might have noticed im not a cis expert, im trying to understand the system and this helps.
Do you think it might run better if i turn the A/F screw a little clockwise? (i know i should hook up a gas analizer but i dont have acces to one of those so i try to make the car driveble so i can get it to a garage for fine tuning).
Old 11-24-2018, 01:01 PM
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Exhaust Flow -

Hey Dennis - From your earlier description of how you performed the Basic Air/Fuel Adjustment, you did fine and I think you are set close enough for now.

An inherited problem is sometimes more difficult to sort out because you are coming in behind the troubleshooting steps of the previous owner. His logic and expertise can be hard to predict. Since the PO had dismounted the Ignition Control Unit (possibly a swap of the Ig Unit ?) , jumpered past the .4 and .6 ohm resistors (possibly trying to boost Spark intensity), installed a new green wire, new plugs, distributor cap and then removed the cold start valve - your "Low Power Issue" is probably the same one he was chasing.

It sounds like you have eliminated most of the more common areas (Fuel-Spark) that can cause the gradual loss of power. Is there any smoke coming from the car as it chokes down ?

Surely it is not something as simple as a dead hipopotamos nest up inside the intake manifold or a plugged exhaust. Perhaps you can have your girlfriend hold her hand over the end of the exhaust pipe to check the flow once the loss of power occurs. Or maybe you should check the exhaust flow and have her press on the accelerator.......

Keep Us Posted - Michael
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1986 930 - "Well Hung"
Old 11-24-2018, 02:32 PM
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