Engine hunting

[Enoch]

Ho Peeps, I have a weird scenario with my 2.0 dohc I4. It ran out of fuel tonight so I put some more in - fresh fuel straight from the pump. When I restarted it (it was cold at the time) it ticked over at about 1000 rpm - previously it was ticking over at about 800. After a minute or so it started hunting eg revving to about 2K then falling back to 1k before revving back up again. I turned it off after a few seconds of this - I don't have the rad connected yet so don't want to push my luck. If I open the throttle it seems to be ok but just does not want to tick over properly. Anyone got any ideas?
I just knew it was too good to be true:)

[deezee]

Idle control valve or temp sensor fault.

[Enoch]

Interesting. So if there was no water in it and as a result the water temp sensor was reading low that could do it? The stat housing was cold and the head hot at the time. If it had been full of water I am guessing the water temp and engine temp would be much closer together?
I will see what it does when it has cooled right down, it will have to be tomorrow now I guess.

[deezee]

What engine management are you using? The one that came off the Sierra? If your coolant isn't touching the temp sender, or the temp sender has failed, its most likely running rich (choke) as its trying to warm the engine up. Is your a catalytic model?

Also if the idle control valve is sticking usually means it will rev up to 1250 ish and back down to 800 ish. Check for air leaks, The idle control may be fine, but your TPS might have been moved.

[spud69]

It hasn't got trapped air in the line or pump has it Dave. Might be worth letting it pump to an open end for a while.

Just 2p worth.....AndyH;)

[Enoch]

Hi Folks, I have found a bad earth but have not had time to deal with it yet. I am sure it is running rich due to the thermostat housing ( and therefore the temp sensor) being cold compared to the engine. I will fix the earth and then sort out my cooling system before trying again. Thanks for the pointers, I have not really got involved in injection before. Just for the record it's an EEC IV without cat.
Cheers!

[Enoch]

Ok, this is getting weird now. I remade the earth and started it from cold, now it revs to just under 2k and sits there. If I disconnect the idle speed control valve the tick over speed does not change. I have buzzed out the wires to it, they are fine. before it ran out of fuel it was perfect, it has not run properly since.

Just as a sanity check let me run this by you. When the fuel ran out it spat back quite violently- I think I saw a blue flash somewhere round the inlet manifold. Could it be that the backfire has taken out the inlet gasket and I now have an air leak? I don't want to take the manifold off if it's a completely stupid idea but something changed at the point I got that backfire - it has not ticked over properly since. The gasket is actually that instant stuff, I could not find a proper gasket for it, I am hoping I don't have to scrape it all off.

Can someone tell me if I am completely barking mad or not?
Would an air leak cause a high / erratic tick over?
Thanks in advance

[twinturbo]

Ahh... When you tell all the facts.

Yes an air leak will cause an erratic idle, by the sounds you have blown off a pipe fitting or as you think maybe the gasket has failed.


The Common culprit is that back breather hose.


TT

[Cyberbeej]

i'll go with the gasket, always fit the right part the right way first time round or you always end up doing it in the long run.

[twinturbo]

Or fit one that's better than the right part :)

TT

[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

[Enoch]

Hi TT, itis a non lambda, the base idle is just under 2k with the iscv connected - same with it disconnected. I have not touched the timing, the car seems to rev ok when the throttle is opened. I am at the office this morning but will take a DVM and scope home so I can see if teh ecu is trying to drive it. I am suspecting that the valve is stuck.
Will let you know....
Best and all that.