Diesel rough idle.

BxMauritius · Post by **BxMauritius** » Wed Nov 25, 2009 6:28 pm

Hi all, not Bx but PSA related.
I have a 405 diesel 1905cc (not turbo) with a Bosch pump that idles rough when the engine temperature has reached approximately 90 degC.
If i adjust the idle speed with the engine hot so that it runs smoothly then with the engine cooled down the idle is too fast.
600 miles ago i rebuilt the engine, new rings, honed cylinders, cleaned the cylinder head up, valves and seats lapped. The engine runs great compared to before. It doesn't overheat and in the past i never had an problems with overheating either.
Just can't get the idle right. What i wanted to know was how a diesel engine accounts for the load on it at idle, for example with the fans running headlights on, alternator load. Or switching the AC on, the load of the compressor and fans running because this will bring the idle down obviously.

Any ideas anyone,
thanks
Rouben.

BX Bandit · Post by **BX Bandit** » Wed Nov 25, 2009 7:12 pm

Do you have the wax stat fitted? Is it working and adjusted properly?

BxMauritius · Post by **BxMauritius** » Thu Nov 26, 2009 5:01 am

yes i have a new wax stat and its properly adjusted. With the engine cold it pulls the fast idle lever and as it warms up the cable slackens and releases the fast idle lever.

BX Bandit · Post by **BX Bandit** » Thu Nov 26, 2009 6:01 am

Ok, well where the wax stat cable is connected to the pump, there is a rocker which is limited in it's travel either side by two grub/adjustment screws. One is high (cold) idle the other is normal/warm idle. This is how you adjust the idle!

There is also a stop (a 50mm length of threaded bar) on the back of the pump which the accelerator lever rests on at idle, but this is not idle adjustment! This is anti-stall adjustment. If it's too low (wound too far back), then when releasing the throttle linkage the revs drop away sharply and the engine will stall. A side effect of said adjustment is labouring at idle with the loads that you mention like a/c, fans, headlamps etc. Another symptom is the in-ability for the car to crawl up a moderate hill under idle/no throttle without stalling.

If you get the setting too high, the revs take a long time to fall back to idle and results in poor engine braking. You want somewhere in between!

There are instructions in the Haynes on how to set the anti stall but to do it accurately you'll need a tacho, but doing it by ear/trial and error is easy! If you don't have a haynes let me know.

BxMauritius · Post by **BxMauritius** » Fri Nov 27, 2009 4:52 am

Thanks Bxbandit, i'll try doing the adjustments again, i have the haynes manual which shows the adjustment points on it.
I need to get the adjustments from warm engine to hot engine (fans on, fans off) back to warm engine again. This is where the idle is too slow.