Engine hunting

[DStanley1809]

I just put the reg from my Sierra into the Eurocarparts website, mine is also a DOHC. They are showing an intake manifold gasket for £1.74, part number 372590201

[Enoch]

Well I have just had the manifold off, it all looked ok - no obvious signs of a blow. Put it all back together with new gasket goo and it's exactly the same
I have double checked everything and can not see anything wrong, all connectors have been off and reseated, the iscv wiring checks out but something has changed since that backfire. It was ticking over at about 850 before - steady as a rock.
I have checked and double checked the pipe to the breather, the one to the MAP sensor, the one from the throttle body to the regulator, the one from the manifold to the iscv and the ones that are blanked off, they all look secure and intact.
It's got me beaten at the moment, don't know what else to try or where to look.
Beer. That's what I need. Lots of beer.
Thanks for your input guys, if you get any other ideas please let me know. I will look at getting the proper gasket - thanks for that, I tried everywhere local and came up with nothing.

[Enoch]

I think it might be Deezee that gets the prize biscuit - possibly even a chocolate hob nob. Before we all get too excited let me explain.

In going thorough all the suggestions I have pretty much ruled out an air leak or a sensor problem. The wiring to the ISCV checks out but if I disconnect it the problem remains the same.

Having had a quick look at the Haynes manual it says that the iscv allows air to bypass the throttle to set and maintain tickover.

So what I did was to disconnect the pipe that comes from the iscv at the manifold end. I stuck my finger over the stub pipe and tried to start the car, it would not idle at all - it just died. So I blocked off most of the hole and tried again. I found I could control the idle speed by just allowing a small amount of air in - basically simulating what the ISCV does. I could quite easily set the tickover at about 850 and keep it there.

I therefore believe that it is the valve that is working in a sub optimal fashion or to put it another way - buggered.

Does this sound sensible? Does anybody have one they don't need so I can prove my theory? Is there a God? Actually, forget that last one, I'm too tired.....

[twinturbo]

Depends..

The symptops point to an Idle control issue..


Either that means the Idle control valve ( ISCV ) is not operating correctly, or the ecu is unable to control it correctly.

TT

[twinturbo]

My Turbo 2.9 idles beautifully for the first 30 seconds.. then starts hunting, then revs and drops right back.. I suspect the ECT may be an issue.. But on this car anything could be wrong..

What you realy need to do is get it full of water and then run it for half an hour, go through the rev range a few times to try and get some DATA into the ECU.

the full drive cycle for the ecu can't be replicated on a driveway as you will not get the full load data, but the ecu will be happier if it's had a bit more to learna bout.

TT

[deezee]

I didn't think any sierra came with a learning ECU. The ECU just adjusts itself within expected parameters and sensor input. Takes from cold to warm of an engine according to Toyota.

[twinturbo]

There's a few paramaters it stores.

Have you tried disconecting the battey? There may also be a fault code stored, and the ECU may even be in LOS ( one indication of this is the pump running constantly at ignition on. )

Check also on the FSOC forum, I think someone had simillar issues on a Twink in the last 6 months.

TT

[Enoch]

I have tried disconnecting the battery, i left the negative disconnected all last night. I have not yet checked to see if I have continuity on the valve - I am assuming it's just like a solonoid of some sort and takes a dc feed? I will bring home an oscilloscope later and see if I can find out a bit more. My simple mind tells me it's the valve but I suppose it could be an ecu fault. It's bloody strange that it happenned after my backfire though, I can't believe it was coincidence. The pump is doing what it should - ie running for 3 seconds at turn on, then just as it needs to with the engine running.
Thanks again for your help guys.
Dave.

[twinturbo]

I doubt it's an ECU fault, but if the valve is not faulty then and incorrect reading from one of the sensors or an air leak.

You could also try disconnecting the MAP sensor multiplug, in fact if you unplug all the sensors the ECU should drop to LOS.

Disconect, MAP, ECT, IAT, TPS, Rail Temp. Not sure it will even run but it's worth a punt..

This is a non lambda engine isnt it?

Ohh.. And have you touched the timing at all on the cams?

Applying 12V across the ISCV should throw it across ( +12v next to the notch in the plug.

TT

[twinturbo]

Also...

With the ISCV disconnected what is the base idle?

TT