944 Running Rich all the time - Tearing my hair out! - PROBLEM SOLVED 10/2/2017!!

[George_Mayson]

Hoping for some suggestions on more things to check about this problem I've been having.

My 1983 2.5 N/A Lux is running rich all the time. For months since I bought it, it was running perfectly, then one day the problem just seemed to start when I started the car up.
Symptoms;

• Rough 450rpm idle at cold, rises to 550 when hot
• High fuel consumption
• Popping from exhaust on the overrun
• Exhaust stinks of being rich
• Gas analyser shows 10%+ CO levels (Max 4.5% allowable) and Hydrocarbons at 1781 ppm (Max 1200)
• Under all driving conditions OTHER than idle the car runs well, no stumbling or hesitation, good power etc. Just high fuel consumption.
• Black plugs obviously!

Now moving on to what I've done about it so far

• Full service before problem start, new filters, oil, plugs etc. Plugs are correctly gapped, and problems started some weeks after plugs change
• No Oxygen Sensor on my early car
• Coolant Temp Sensor resistance in tolerance
• MAF Temp sensor (inlet air temp) resistance in tolerance
• MAF "Barn Door" air flow sensor tested, 10.8V supply and no dead spots in sensor output voltage throughout full range.
• Throttle position sensor clicks properly when throttle is cracked open
• Fuel Pressure is bang on at the rail, and bleed down is perfect too
• Speed and Reference sensors are reading correct resistances (Not tried the oscilloscope test)
• No fuel leaking into vacuum hoses on the Fuel Pressure Regulator/Damper, and hose connection @ Throttle Body is good
• Fuel Pump Relay was changed months before problem started, Fuel Pump running OK
• NOT fitted new fuel filter yet but am confident this wouldn't cause rich running (again fuel pressure at rail is perfect)
• Messed with the Idle Mixture in the MAF unit to some avail, managed so smooth the idle up to 700rpm when hot but still far too rich.

And a curious discovery:-

I tried unplugging the Coolant Temp sensor for the DME while the car was running, it immediately died, I assume because it suddenly went too rich for the car to even run. (Infinite resistance so Max Fueling). Trying it again while working the throttle and I could keep it running, but barely. Happy this sensor is working OK.

I then tried unplugging the MAF unit, and was able to do so with no change to the running. When I sharply opened the throttle the car started cutting out, presumably because the car ECU didn't know about all the extra air coming in, which all makes perfect sense. However - why didn't the car richen up the mixture when the plug was disconnected, because suddenly the air temperature resistance goes to infinite and the car should richen the mixture, but as I said no change. The mixture was also so rich that I could open up the throttle slowly with the MAF disconnected and the car would continue to run OK up to 2500RPM or so, as there is all the extra fuel in the mixture.

Now, the Air Temp Sensor in the MAF is reading correct resistances, both at the MAF Plug AND at the DME Plug.


I'm really starting to get frustrated with the problem now! I'm going to go through and check for vacuum leaks on any hoses I can get my hands on.
However, I was hoping there might be something I'm overlooking! I'd really appreciate any input on the problem.

Cheers, George

 

[PSH]

Hi George

I'm not familiar with the early cars but looking at your symptoms and what you have checked so far have you looked at the 'idle control valve'? that would have been my first check... IIRC you should be able to hear it click when turning the ignition on? you'll need help as it's not very loud. On my 86 it is possible to give it a good clean with WD40 to see if it clears, they can stick due to age/muck. If the valve is stuck in the cold position idle could be rough and fuel consumption would be higher due to it running rich. As I say I'm not familiar with the early cars but was under the impression that they all had this valve, it's a small cylinder shape under the inlet manifold, well it is on a turbo?

regards

Pete

 

[George_Mayson]

Hi Pete, thanks for the quick reply. I think I have one buried under the manifold and I'm away to check it out now.

For my understanding, can you explain to me what the idle control valve does and why if it's jammed it causes a richer mixture?

Cheers!

George

 

[PSH]

Hi George

Main reason I mention the valve is due to the rough idle, the figures you give are about right for it being stuck closed.... I just noted your emissions results, if your MAF is like the turbo there should be a 6mm allen bolt that can adjust the mixture, I'm afraid it's many years since my car has had the old AFM so can't remember exactly where it is, I would also try cleaning out the throttle body idle screw, ( should be on top of the body) it's not really used for this as the control valve should do the job but it would be an idea to take it out and give it a good clean, also check that the seal is in good condition, remember to count how many turns in it is first, so turn it in all the way first to get an idea of what it's set at.

Pete

 

[blade7]

I'd try carb cleaner rather than WD40.

 

[PSH]

yes...sorry paul is correct....

 

[Frenchy]

ECU ? Mine started playing up on fuelling and was intermittent, it was the ecu.

 

[George_Mayson]

Going to try and speak to an expert on the system, as all are great suggestions. I really hope it isn't the ECU, then again I'd like to find a cause so I can fix it!

 

[bmnelsc]

I think Frenchy is on to the problem as it is the ECU that controls fuel delivery (with inputs from the sensors, etc). The Idle Control Valve is supposed to let in additional air when cold to increase idle speed, etc. until warm up so I wouldn't the problem to persist once warmed up. Best test now is to beg/borrow/steal a known good ECU and try it on your car...

 

[924Srr27l]

These Guys may be able to help with checking & rebuilding your ECU:

ATP Electronic Developments Ltd.

http://www.ebay.co.uk/itm/0261-200-006-0261200006-944-618-111-PORSCHE-944-ECU-2-5LTR-OWN-UNIT-REBUILD-/172002966286?hash=item280c2cfb0e:g:7GsAAMXQUmFSpaMI

R

 

[Paul 290T]

A little late, but some extra info:

when the throttle switch detects the throttle is closed, as in at idling, the AFM is ignored by the ECU. When the throttle is wide open the AFM is also ignored, the ECU reverts to a look up table for settings.

idle speed is set by a combination of ISV duty cycle (what proportion of time is remains open as opposed to closed), you can hear it hum as it modulates if you put a screwdriver onto it and you ear. And secondly by ECU induced ignition timing changes.

Ignition timing is more of a dynamic control over idle speed, whereas the ISV duty cycle is more of a base setting that controls the average idle speed.

ATP successfully repaired my ECU.

good luck

Paul

 

[924Srr27l]

A little late, but some extra info:

when the throttle switch detects the throttle is closed, as in at idling, the AFM is ignored by the ECU. When the throttle is wide open the AFM is also ignored, the ECU reverts to a look up table for settings.

Paul

Interesting, I thought the N/A car's only have a Switch which only records and sends Info when "OPEN" from the fully closed position (The click you can hear) and that it doesn't have another switch to inform the ECU when Wide open ?

Only you've mentioned the AFM is ignored when full throttle but how does the ECU know the throttle butterfly if Wide open if there's not a switch here?

I've read the 944 Turbo does have a potentiometer TPS (Throttle position sensor) which informs the ECU not just when it's open

from the closed position but also various positions EG: 1/4 1/2 3/4 throttle etc...

R

 

[Indi9xx]

My money would be on the fuel pressure regulator.

they have two failure modes, one is the internal diaphragm leaking which puts fuel into the vacuum hoses, the other is sticking causing high fuel pressure.

With a fuel pressure tester the fuel pressure should be 2.5 Bar on the 8 valve cars as standard with the vacuum removed and engine running or DME relay jumpered.

Probably the most common part to fail and cause high CO, the hydrocarbons are probably high due to there being too much fuel to burn completely pushing the CO higher, which happens around 10% Co

Another symptom of this is the fuel pump becomes more noisy as it is dealing with a higher pressure, have a listen around the drivers side rear wheel with the engine running, the low idle will be again because the mixture is so far from efficient that with the standard metered amounts of air, the burn is not correct to maintain the idle speed, which in turn causes the ISV to open, drawing more air, seen by the airflow meter and so the ECU adds yet more fuel.

There are some other potential causes, but fuel pressure would be my first check.

 

[Mike_Pollock]

My 2.7 had very similar issues - I changed the ECU with no effect, it was the fuel pressure regulator. And had the same with a Lancia HPE with the same system in the '80s so should have guessed!

 

[924Srr27l]

Bump........

Anyone confirm this is correct or not ?

i'm always keen to learn (and do every week), on Factual info about these car's

R

A little late, but some extra info:

when the throttle switch detects the throttle is closed, as in at idling, the AFM is ignored by the ECU. When the throttle is wide open the AFM is also ignored, the ECU reverts to a look up table for settings.

Paul

Interesting, I thought the N/A car's only have a Switch which only records and sends Info when "OPEN" from the fully closed position (The click you can hear) and that it doesn't have another switch to inform the ECU when Wide open ?

Only you've mentioned the AFM is ignored when full throttle but how does the ECU know the throttle butterfly if Wide open if there's not a switch here?

I've read the 944 Turbo does have a potentiometer TPS (Throttle position sensor) which informs the ECU not just when it's open

from the closed position but also various positions EG: 1/4 1/2 3/4 throttle etc...

R

 

[Frenchy]

Yes the Turbo does have a potentiometer (TPS)

 

[924Srr27l]

Yes the Turbo does have a potentiometer (TPS)

Thanks, so I'm assuming as you've not said this means that the N/A doesn't ?

And What about at WOT does the Switch have another resistance change to inform the ECU when it's wide open or not?

and ignore the AFM as Paul290T's "extra Info" ?

R

 

[George_Mayson]

My money would be on the fuel pressure regulator.

they have two failure modes, one is the internal diaphragm leaking which puts fuel into the vacuum hoses, the other is sticking causing high fuel pressure.

With a fuel pressure tester the fuel pressure should be 2.5 Bar on the 8 valve cars as standard with the vacuum removed and engine running or DME relay jumpered.

Probably the most common part to fail and cause high CO, the hydrocarbons are probably high due to there being too much fuel to burn completely pushing the CO higher, which happens around 10% Co

Another symptom of this is the fuel pump becomes more noisy as it is dealing with a higher pressure, have a listen around the drivers side rear wheel with the engine running, the low idle will be again because the mixture is so far from efficient that with the standard metered amounts of air, the burn is not correct to maintain the idle speed, which in turn causes the ISV to open, drawing more air, seen by the airflow meter and so the ECU adds yet more fuel.

There are some other potential causes, but fuel pressure would be my first check.

Hi All, thanks for all the replies. Let me try and clarify 2 points;

1: My "Throttle position sensor" is ONE switch that clicks when you crack open the throttle. There is no second switch at the top end to sense Wide Open.

2: I too suspected the Fuel Pressure Regulator/Damper, as the symptoms fit perfectly, overfueling, reduced economy, rough idle, yet when I tested it with the car running I got a healthy 2.1 Bar as suggested on Clarks Garage. Bleed down was fine too... Even tried revving up to see if problems occured at high RPM but never exceeded the prescribed values.

Will be checking again, no wet petrol in the Vaccuum lines but maybe a very faint smell of petrol actually..

 

[peanut]

just thinking out loud and I am not familiar with the early models but don't the early N/A 944 have a 5th cold start injector ? If it does....... it might be worth testing with the cold start disconnected

A good test for fuel over-pressure is simply pull off the connector for one of the injectors and see if the idle improves and the mixture weakens. That would indicate one of the injectors was faulty.

To test the injectors you could remove the retainer clips and with the engine running pull off each of the injector connectors in turn. If there is no change in the engine note then that injector is faulty.

Have you checked the spark plugs to see if the rich mixture is effecting all cylinders or just one ?

 

[924Srr27l]

just thinking out loud and I am not familiar with the early models but don't the early N/A 944 have a 5th cold start injector ? If it does....... it might be worth testing with the cold start disconnected

A good test for fuel over-pressure is simply pull off the connector for one of the injectors and see if the idle improves and the mixture weakens. That would indicate one of the injectors was faulty.

Have you checked the spark plugs to see if the rich mixture is effecting all cylinders or just one ?

No 5th Injector system on 944's

R