I need to adjust base idle on my Sierra dohc. I know I need to undo the idle speed sensor and I've been told there's a screw on the throttle base to adjust it but I can't find it.

Any advice?

Cheers

 

Very unlikely to have a base idle setting, the idle speed control valve does all of this, there will be a tiny screw that the throttle linkage on the tb rests on at idle, this is NOT to set the base idle, but to ensure the flap in the tb doesn't stick shut it holds it very slightly open, but not enough air to run the engine.
I assume you are having idle probs ?.
I assume this is not a cossie DOHC ?
tabetha

 

Yeah it just a normal 2l, it idles at about 500 rpm and don't know how to increase it as it almost stalls when at standstill which can be quite annoying when pulling away.

 

Have you checked the base ignition timing? also might be worth taking off the idle control valve and cleaning it out with brake cleaner and then applying some wd40 to free it up.

 

I would check for air leaks, if there is defiantly no base idle to adjust.

Martin

 

i think its a map sensing system mate not maff, but if it is maff then yeah check for leaks

 

I'd check for air leaks anyway.
tabetha

 

I haven't had chance to check yet but its since I've fitted a K&N 57i cone filter and a power boost valve. I had these off and checked the pipes etc before I put the thread out. The strange thing is that the revs are ok when I start the car, it's just when I'm coming to a standstill. After about 10 seconds at standstill it picks up again.

The manual says theres a base idle adjustment but only Fords can adjust it!!

 

Haha most likely caused by the power boost valve, they are rubbish!

I would remove and see if theres any difference. Yes so that means it has no mechanical base idle you can set, it's solely by ISCV so it can only be adjusted in the software, not that it ever requires changing anyway!

Martin

 

Took the power boost valve off, cleaned up the idle control valve and it made no difference. I'm going to check the sparks and distributor cap for condition but they are fairly new.

Does anyone know what the ignition timing should be. The manuals a bit vague just saying 12 degrees BTDC for unleaded but adds "with leaded fuel every 4th tankful"

 

What about reverting to airbox as well for a test as the aftermarket ones can cause trouble as well, seen as it was ok before these two things were changed.

Martin

 

Checked it all today, it was definitely the power boost valve. Waste of a £100!!!

 

What do I win haha?

Glad it's sorted.

Martin

 

A hardly used power boost valve is the top prize