Hunting & Running Rich

[ScottyB]

Hi

Ok I'll try and be as descriptive as possible.

My 1978 SC has been drifting.

It gets set up and then progressively gets worse with time, these are my issues:

Hunting

1) The idle was initially low, it is now set to around 850 revs on cold start and hunts about 3 or 4 times before it settles.  Low revs to nearly cut off and then just over 2K as it sweeps.

2) This will continue when you come to a stop but again, only about 3 sweeps until the engine is warm.

2) When at operating temp is idles around 1.1K ish which seems a little high.  But if I adjust this the cold start rev level drops and then I get massive hunting

3) Idle drops very slowly when coming to a stop when warm, for example at traffic lights.  It seems to hang around 1800K rev range and drops slowly back to 1K ish.

Running Rich

I've had the mixture set with a computer sniffer but the fuel mixture drifts and the car over a long drive or a week or so will drift and become very rich.  So much so it set the CO sensor off in our apartment building underground carpark which obviously isn't good.

 

Idle seems to be an issue but the moving richness is bad.  When driving the car hard it seems to run ok but obviously smells rich.

 

I think this is too much for bad injectors so I'm thinking the fuel distributor and WUR?

Any suggestions as I think I might need to get my WUR and distributor rebuilt.

Thanks

Scott and my first post.

[Lesworth]

I would look for air leaks somewhere. Assuming that there is no unmetered air getting into the system, have you run a system, cold and warm fuel pressure test? This would help pinpoint potential issues, especially with the WUR which manages fuel mixture not just when cold but during warm running also.

I had cold idle hunting issues which turned out to be too rich a mixture at initial start up. The fuel pressure test immediately identified this and I was able to adjust it down following soeinstruction on pelican I think. It's going to be sent to KMI Petrol Injection for a full rebuild eventually. They are the go to people for rebuilding CIS components, including the fuel distributor.

I also had issues getting both the cold and warm idle speeds right / in harmony with each other on my 924 and this turned out to be an internal air leak in the deceleration valve (allows throttle bypass air when you lift off the throttle.) which allowed air through all the time and could not be identified with a simple air leak check.

My point being that it could be a number of things and a systematic approach will be needed.

[ScottyB]

Fuel pressure test is a good idea.  

[ScottyB]

The car does start relatively well when cold and second start is always bang on so I suspect fuel pressure isn't the issue.  The surging and the massive drifting of the mixture is strange

[ScottyB]

Could it be the oxygen sensor maybe?

[Lesworth]

Its control pressure you would be testing. The WUR manages control pressure which works against the system pressure (main fuel pressure) to richen or lean out the mixture. The fuel injectors are just simple valves that open at a specific fuel pressure. The Control pressure modifies the system pressure to basically spray more or less fuel at the injectors for a given amount of air. If the control pressure isn't doing its job properly, your mixtures won't be right.

When clap cold, the control pressure should be low (there is a specific range of values for each model) meaning that the system pressure would be higher = richer mixture. As the car warms up, the control pressure increases to a set point when fully warm (again, there are specific values for each model), gradually reducing system pressure  = gradually leaning out the mixture. The WUR warms up in two ways:

• heat soak from the engine as it gets warm
• a bi-metallic strip powered by 12v

Warming up moves the bi metallic strip which works against a diaphragm to vary the control pressure.

So, a CIS fuel pressure test checks the following:

• Main system pressure is correct for model / year
• Cold control pressure is correct for model / year (with WUR electrically disconnected to prevent it warming up)
• Control pressure increases as the WUR warms up (when electrically connected again)
• Warm control pressure is correct for model / year (with car fully warmed up - engine running)
• After shutting down the car, a specific amount of system pressure remains after a set time - this is a leak down test

Any good indy will have the knowledge and tools necessary to do the above.

[Type911]

3 minutes ago, ScottyB said:

Could it be the oxygen sensor maybe?

I doubt it.....................................they dont have one (UK) I would check for air leaks as Les says, hunting is often vacuum leaks, buy a cheap smoke tester and see where it comes out

[Lesworth]

5 minutes ago, ScottyB said:

Could it be the oxygen sensor maybe?

Where are you based? UK SC's don't have an oxygen sensor.

[ScottyB]

ROW car in Australia but a LHD.

My CO was set to 2.5% and drifts about 5% which is really strange 

 

[Lesworth]

I refer you to my post about 1) checking for leaks and 2) testing the pressures. 

I recommend that you do some research into how the Bosch K-Jetronic (also known as CIS) works. There are several key components that have to work correctly, and in harmony with each other. The pressure tests are really the first step in performing any diagnosis. Without that, any comment or opinion from anyone here is just guesswork which isn't going to help you that much.

[Ian Comerford]

Les has already provided some good advice on where to start and I will add more detail on one area.  My SC seemed to have quite inconsistent running, like something was changing all the time.  Eventually, after looking at several areas/items, I took the fuel distributor off and went to KMI, a UK specialist in certain CIS items.  When Steve inspected it he said it was faulty and the main symptom one be difficulty in setting and maintaining a fuel/air ratio.  It now runs beautifully after a refurb

https://www.kmipetrolinjection.co.uk

[hagarep]

I have also had mixture and driveability issues very similar to your, since rebuilding my engine, which meant I didn't really use my car for the last 18 months. 

Sought any vac leaks first, then move on to fuel pressures., as the guys have covered above.

I followed Klassik Automotive Training School videos on you tube as a guide, with one exception. . Unlike the Klassik videos, which seem to focus on cold control pressure and running the fuel pump, engine off. My issue in the metering head was found by testing fuel pressures with the engine running and the lid of the airbox removed, which revealed a bouncing air metering flap!

Metering head currently with KMI for rebuild.

Jason

[mean in green]

A tonne of great advice here too.

[ScottyB]

Update - started the car and it was hard to start cold, basically spat unburnt fuel out of the exhaust.  Smells of fuel as well. Pulled the spark plugs today and they were black.  Pulled the injectors and they are all leaking, some badly and the cylinder/s were wet.  Ordered new injectors, seats and o rings, changing them tomorrow and see how it runs then with a tune.  It wouldn’t have been able to hold a mixture I don’t think and would have gotten progressive worse.  Definitely part of my CO issue

 

Another update tomorrow…

[ScottyB]

FD could be leaking and I'll check that tomorrow

[Strictly]

Your injectors may be leaking because they are worn out, of the fuel pressures are too high. You defiantly need to test your fuel system and control pressure, its a critical part of CIS diagnosis and set up.

l've not worked on CIS for a long while, something like 6 years, but from memory. In the meantime you can easily do an injector flow test, where by you basically have the engine off, fuel pump running, and then lift the air flow plate with the injectors in individual bottles, you can then compare the quantity of fuel from each injector. Repeat without injectors fitted to fuel lines. You can also check that the injectors dont dribble/leak - but im a bit rusty here (I last worked on 911 CIS about 6 years ago), but im fairly sure with the engine not running, but the fuel pump running (hot wired) that the injectors should not dribble/leak. I did have a 911 where in this scenario the injectors "pulsed" which I never quite figured out, but I was told they should not dribble/leak. If so then the CO setting is too rich.

I can explain how to adjust the WUR, thats quite easy and I do remember. Adjusting the FD system itself is quite easy too. I rebuilt one once myself, but i didn't t trust it not to leak, and so used KMI a few times.

Your cold start valves or etc might also be leaking. Dont spend any more money on parts....get some CIS gauges, and either a Porsche or Bentley workshop manual, then test your system and control pressues (control both cold and warm), and both for rest. Its explained well in either book. These were available in the forum download section, might still be there, ive not looked in 6 or 7 years.

Post your findings and we can see what we can figure out from there. Over fueling will bore wash your engine....so its a small outlay on equipment to protect your motor.