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CIS / K-Jetronic Troubleshooting


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Learned Friends,

I am having some ongoing running issues with my 924T related to the CIS. Nothing major but irritating when compared with how well my similarly injected SC runs. A lot has been done bit by bit to address these and it is improving with each step, but the following symptoms remain:

  • Starts fine when cold, but the idle speed is low, around 900, for a few seconds and then drops to around 800. I assume this is when the CSV stops adding fuel, which is after approx 7 seconds.
  • Revs rise to 950 once fully warmed up and it flies like **** off a shovel with plenty of power and the low / part throttle driveability is great.

Idle speed, mixture and timing were adjusted following new injectors and rebuilt WUR to run great when warm and idle at 950 - its at cold start and cold running where the issue is. I am going to perform a fuel pressure test at the weekend.

My main hunch at this point is a vacuum leak somewhere. Once warm I have limited adjustment of the idle speed at the idle speed screw. I can increase the idle by backing it out but its almost completely wound in to get 950.  Closing it completely wont bring down the revs much and makes the cold start revs even lower. Retarding the timing slightly also brings down the revs but saps power. I might experiment a bit more with this at the weekend. I will also do a step by examination of the intake vacuum system with a propane torch to see if I can find any leaks.

Any other thoughts or opinions welcome 👍

 

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1 hour ago, Ian Comerford said:

I would go to my local MoT garage and ask them to plug it in to their meter.  Check this is right before doing anything else

The CO% is irrelevant and could lead you down the wrong path. If you've got  air leaks or the system or control pressures are wrong the CO means nothing. 

It's sounds like you have a good plan, check for air leaks with carb cleaner (Propane is a bit high risk for me 😬) , a fire extinguisher and garden hose at the ready.

I'm not familiar with your car, just the 911, but remember to check the oil filler cap seal for air leaks if it operates the same way as a 911.

If you don't find any leaks then check the fuel pressures. 

A methodical approach will get you there. 

Good luck and I look forward to reading about your progress👍

 

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Hi Les, 

I think the start point is always to check control pressures and go from there. 

From your description I’m not sure it’s an air leak as this would increase idle speed and it runs great hot, although I think it’s possible for air leaks to be temperature dependent. 

If I read correctly, I think the issue is low idle on cold start? If so I think it’s more likely to be over rich or not enough air. That could be cold control pressure too low, an over enthusiastic CSV or too little additional air when cold. I’m not too sure on a 924, but on IBs the Auxillary Air Regulator provides extra air when cold (closes with heat transfer and a internal element) as well as the Auxillary Air Valve (less likely as I think failure mode is too much air so high idle). Worth checking both.

This is why I think a check on your rebuilt WUR is sensible and, if you have the kit, a check on CO from a cold start to see if it’s pig rich. I wouldn’t do this to then play with the mixture but it will show if it’s ridiculously rich or too lean; both good to know if you have a problem in your cold start system. 

First though, I might also try disconnecting the CSV to lean it out when cold. May not be the route cause but would tell you if over rich the issue. I run without a CSV connected. 
 

Regards

David 
 

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1 hour ago, craigt said:

The CO% is irrelevant and could lead you down the wrong path. If you've got  air leaks or the system or control pressures are wrong the CO means nothing. 

It's sounds like you have a good plan, check for air leaks with carb cleaner (Propane is a bit high risk for me 😬) , a fire extinguisher and garden hose at the ready.

I'm not familiar with your car, just the 911, but remember to check the oil filler cap seal for air leaks if it operates the same way as a 911.

If you don't find any leaks then check the fuel pressures. 

A methodical approach will get you there. 

Good luck and I look forward to reading about your progress👍

 

I don’t agree that CO is irrelevant after my recent experience which showed that once correctly set the car ran far better 

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8 minutes ago, Ian Comerford said:

I don’t agree that CO is irrelevant after my recent experience which showed that once correctly set the car ran far better 

If it runs great and pulls like a train when warm, adjusting CO may make it run better when cold / warming up but then it may run poorly when warm.

I also thought that if there were un-metered air leaks that would make it lean and increase idle speed however, the system relies on vaccum to operate properly and a vaccum leak causes idle speed to drop. If you remove your oil filler cap at idle the speed should drop significantly. 

Check for air / vaccum leaks, check system and control pressures and go from there. No point just messing with the fuel mixture screw as this may just confuse things. Start at the beginning 👍

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Les what is the spec for cold and warm tickover rpm ?

It would be easy to disable your CSV if you want to isolate the CSV contribution.

 

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Thanks for the input everyone. A busy weekend meant no car fiddling. This week after work I intend to do a fuel pressure test (system, cold, warm etc) just to make sure the WUR is performing as expected for a rebuilt unit. I dont think its mixture related for the reasons stated above. Low cold idle speed is the issue. Warm idle is perfect as is running cold and warm. I will also test the Auxiliary Air Valve. Cold, the little door is completely close BUT there it a little T shaped aperture to allow air through. I want to see that it closes completely as it warms. I also want to test the Auxiliary Air Regulator, the disk shaped valve that should be open until manifold vacuum is detected and then close to help initial starting. I'll also test the CSV>

On a cold start, after a few seconds, the idle speed drops even further which I think is when either the CSV stops injecting extra fuel, or the AAR closes with manifold vacuum.

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Idle spec is 900 +/-50 and when warm it sits at pretty much bob on 900 and is stable. When cold its about 850ish.

This morning I performed a fuel pressure test. Ambient temp was approximately 6 degrees.

  • System pressure 5.5 bar  (spec 5.4 to 6)
  • Cold control pressure 1 bar (spec 1.8 to 2 bar @ 20 degrees)
  • Warm control pressure 3.6bar (spec 3.45 to 3.65)
  • Residual pressure 1.7 bar after 60 mins (spec minimum 1 bar after 60 mins)

I read this as all good.  I believe that 1 bar is ok at this lower ambient temperature looking at some generic graphs (can't find a 924 turbo specific one, just the above specs). I also performed a thorough inspection of the intake and vacuum system using carb cleaner with no discernable idle speed changes.

Next up I'll spend some time looking at the cold start valve to check its not leaking post the signal to turn it off after starting, and then the aux air regulator and aux air valve (deceleration valve) to make sure they are not leaking air internally when they are not supposed to.

On 27/11/2020 at 15:35, hagarep said:

I have a Gunsons digital meter if you need to borrow one?

Found my leaks by spraying brake cleaner around everything and listening for the revs rising.

Jason.

Yes please Jason. Sent you a PM

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  • 4 weeks later...

Cold miserable weather and other excuses have kept me out of the garage in recent weeks. But I have validated that the CSV is working correctly:

  • Test light shows current flows to the CSV only when cranking the engine and cuts when the engine fires (interestingly it seems to fire just the same with or without power)
  • Test light shows no power to the CSV when cranking a warm engine (thermo time switch thus works correctly)
  • With he CSV removed, a strong, fine and even atomised mist of fuel is dispensed when cranking cold

Next up I am going to look closely at the Aux Air Regulator that provide extra air at start and when cold and slowly closes as the engine warms up.

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  • 2 weeks later...

Just in case anyone is still awake following this, for the sake of completeness, over the last few days I have completed the following checks:

  • Observed the Aux Air Regulator was open when cold (just a small aperture in the disk rather than totally open)
  • Connected the AAR to power and earth and observed the disk slowly rotate and close. Note: Was still able to blow / suck a little air through the AAR when closed but far less than when open - googling seems to indicate this is normal....
  • Connected the deceleration valve to a vacuum hand pump  - valve is able to hold a vacuum
    • Started car from cold and allowed to warm up - applying vacuum increases revs cold and warm as expected

I conclude that both of these units are operating. Next things to test are:

Block inlet to AAR completely when warm to see if this is allowing too much air through when fully closed.

Block inlet to decel valve completely to check the same thing as the AAR.

My theory is that a higher than normal warm idle speed has been 'dialled out' by winding in the idle speed screw almost all of the way in, therefore impacting (lowering) the cold idle speed. With no evident 'external' air leaks it could be either of these units which allow extra unmetered air through under certain conditions are leaking internally.

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Today I warmed the car up and then blocked the inlet to the Aux Air Regulator to observe any change in warm idle speed. In theory there should be no change because the valve should be fully closed when warm. With the AAR blocked, idle speed dropped by around 200 rpm so that shows that the little bit of air I can blow through when its closed is allowing the warm idle speed to be a bit higher than it should.

Not sure what the answer to this is to be honest as you cant buy these new any more and I don't think anyone re-builds them like they do warm up regulators or fuel distributors. I also don't know if they are supposed to let a little bit of air through even when closed. 'Internet' seems to think its normal but I don't always believe what 'internet' says.

Next I am going to check the idle mixture with the kit kindly lent to me by Jason just to see where its at and adjust it. Then I may just leave things alone after getting it as close as possible to 'right'.

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3 hours ago, Ian Comerford said:

Les, KMI refurbs these and also sells new it seems (if we’re talking about the same item?). 
https://www.kmipetrolinjection.co.uk/Bosch K-Jetronic.htm

 

Aux Air Valve or Aux Air Regulator?

Well I never did! I had no idea they rebuilt those - don't know how I missed that on their website. I want to understand if mine is operating normally or faulty before splashing out so have asked KMI is passing a little air when closed is actually expected / normal.

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I’m interested to know this too, and any other standards relating to closing time from cold etc?  I might send a message to KMI unless anyone else has an idea?

Worth having a look at this Les. http://forums.pelicanparts.com/porsche-911-technical-forum/200906-aar-revisited.html 

Edited by Ian Comerford
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Good stuff guys.  I'll use this as a reference to check my cold idle out, it's a bit low in this cold weather.  That link on Pelican is a beauty on how to rewind the AAR 👍

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Thanks Ian, will take a look. I emailed KMI to see if these should be air tight or still allow a little air to pass when closed. I also posed the same question on the 924 forum I frequent and so far consensus is a little air being passed when closed is normal. This matches what I have found on the internet elsewhere. Lets see what KMI suggest...

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Ken Mills is very experienced and knowledgeable. He is also a really nice guy but I would call him direct. He is one of a few companies that can manage any issue or improve on this system and hence always really busy. He really helped me with meter head on turbo race engine but refurb was 9/12 months out due to work load. He did give me pointers that really assisted.

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