Garage > Mechanicals

6.9 K-jetronic rough and high idle cold start

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Jan S:
I have a 1978 6.9 engine with K-jetronic (and a manually controlled cold-start valve, i.e. I activate the cold start valve by pushing the button for 2-3 sec before I start cold)

- The car starts immediately, cold and warm.
- However, cold start is a bit rough, idle is 1500-1600 rpm
- After 10-15 min engine runs smooth, idle is 900-950
- The ambient temp is 68-77 deg F/ 20-25 deg C.
- In April I checked the fuel pressures (system pressure, control pressure), they were ok (my entire fuel assembly at the rear is new)

Sounds familiar?

Could it be aux air valve, spray pattern injectors, vacuum leaks, ......?

A few tips would be great.

You might have to define what you mean by "rough running".  Sounds like overly rich?  In my experience they tend to run a bit rich at warm up but 1500-1600 does sound a bit high at those ambient temperatures.
Sounds like you might have to take a look at the warm up regulator action.
With engine fully warmed up your idle should be lower @ 600 - 700 rpm.

Very likely it’s your AVV that’s a bit stuck and not fully open when cold. Yes, the system is designed to run rich when cold for about 3-5 minutes. But if the AVV is working the engine will run at around 2500 rpm “hiding” the roughness of the engine running rich. There are some accounts that the AVV is reparable. I have simply removed them, cleaned  them and lubricated the visible part of the internal piston. It should slide freely when you submerge the bulb is a bucket with water and ice and immediately put it in a bucket with hot (90C) water. You’ll see the piston moving to both extreme positions then you can lubricate the assembly. If your piston doesn’t move with the practical test above or its displacement is limited to 2-4 mm then the hypothesis of a misbehaving AVV is confirmed. I have seen that a few times.
Your engine starts as it should cold and hot, so I would not mess with the WUR as it’s doing its job just fine. However, you can play with your 3mm allen screw on top of the FD to fine tune the mixture cold/hot. That should help, but if the AVV is stuck that won’t change matters significantly.
I hope all that makes sense.
Good luck

s class:
The fact that you have a differential in idle speed from 1500 cold to 900 warm tells me the AAV is most likely OK.  I would regard a differential of 600rpm to be close to ideal. 

I would start with the basics - check and set ignition timing, make sure the vac advance is working.  Then reduce the warm idle speed to 650 and set the warm running fuel mixture properly using a decent exhaust gas analyser.  This will affect both cold and hot running.  It sounds to me like you have it set to rich.

I get the impression this erratic running has come on quickly rather than an observation over many weeks or months?
If indeed it's come on quick I would target air leaks in the inlet tract. Costs nothing..split hose..something's dropped off.??
The engine seems to be running 200 rpm more than it should at both cold n hot. Reduce hot idle to 700 and see next day what effect it has on the cold start idle. if it is still hi at 1500 double check your cold control pressure at the WUR. You said they were fine a few mths that still the case?


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