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[archiewood]

(also posted on mig)

I've just fitted an X30XE to my Cavalier (C25XE management). Immediate problems have shown themselves on the first run:

1: No brake servo (brakes are super heavy).
2: Very fast idle (~3000), which does not reduce when the engine has warmed up.
3: Cannot rev above about 3300. ECU light comes on.

No fault codes are stored, because the light only stays on for as long as it takes for the revs to drop back below about 3000, which is usually a matter of seconds. There must be a vacuum leak somewhere. I have the standard X30XE top end. Who can tell me the possible locations of a leak?

Here is what I know:

-The EGR holes are sealed with chemical metal.
-I had to make up my own brake servo pipe, but all the joints are clamped except for the brake servo end (which is a push fit) and the plenum end (which is the original heatshrink elbow with the union).
-The IACV hose joints are clamped, but the ends are both push fit.
-The block breather unit joint was sealed with universal sealing compound.

I suppose the question is how can I test for the leak locations?

[Kevin Wood]

It you spray some WD40 or carb cleaner at the locations where a leak is likely, then listen for a change in engine note...

3k RPM needs a fair bit of air though...

Do the throttles close fully in their bores at idle? Is there some slack in the throttle cable to allow this? Could also be that the grub screw the forms the throttle stop has been adjusted to "help" it idle at some point.

Kevin

[Matchless]

Check the throttles are closing fully as suggested.
O rings dislodged under plennum?
Breather bridge fancy seal damaged or dislodged?
Brake servo fault?...block the vac pipe to prove.

[archiewood]

The throttles are definitely closed properly at idle.

The brakes are sorted - I had to make up my own brake servo pipe and I fitted the one-way valve the wrong way round 

I'll check o-rings on the plenum. I found the bolts on the plenum and throttle body were pretty loose but tightening them didn't make any difference. All the seals were new, but I'll check they're seated properly.

Tried blowing into the idle valve hole and it sounded like the block breather was hissing (don't quite understand that as I'd have to blow past the pistons of the open cylinders to get air to come out of that, right?). When I re-fitted that I used sealing compound but I'll redo it.

The seal under the breather bridge is not in great condition, but it as Kevin said it would have to be a fairly substantial leak to generate these kind of revs no?

Just to add more interest, I removed the idle valve altogether, got my Dad to fire it while I blocked the idle valve hole on the plenum with my hand and it still ran. At a pretty reasonable speed, too.

[archiewood]

Just to update, I took the plenum off and checked the bolts, seals etc. I wasn't able to blow into any of the six intake tubes, which is encouraging.

When I put the plenum back on, I wasn't able to blow into the idle valve hole if I disconnected the narrow vacuum pipe from the block breather unit and covered the end. If the pipe is fitted, there is a hissing coming from the breather area.

In other news I cut and resoldered the cable for the crank sensor and the thing at least starts, though I haven't had it out on the road to check the performance and it still idles at 3-4k. Also, when I removed the crank sensor there was metal shavings all over the end. What the hell?  Engine has had new rings and big end bearings if that's relevant.

[archiewood]

Another update: I decided to connect the idle valve to the battery to test it. Tried it one way and got nothing, swapped the wires and it opened fully. Then I wondered what would happen if the valve was plumbed in but not wired up. Tried it and hey presto, car idled fairly normally, albeit with the engine light illuminated and very rough running when I revved it (probably the crank sensor).

I didn't think to mention this, but I had to extend the cable for the idle valve. If I reattached the two wires the wrong way around, what would happen?

[Kevin Wood]

I didn't think to mention this, but I had to extend the cable for the idle valve. If I reattached the two wires the wrong way around, what would happen?

That would do it! It would close when it's supposed to be open and vice versa.

Kevin

[archiewood]

Muahaha, I guess we're getting somewhere. I'll sort that out tomorrow.

Would a faulty crank sensor cause the other problems I'm getting (refusing to rev above ~4000, refusing to accelerate under full throttle)? I had the sensor off today and there was a nasty kink and fraying in the cable. I resoldered it but it's impossible to get it back how it was, properly shielded etc.

[archiewood]

I checked the wiring and it's the right way around, so I'm back to square one. Could the idle valve be the problem? It's just opening to maximum and staying there.

[Kevin Wood]

I would expect the idle valve to be well open until the engine runs, at which point the ECU will trim it back to achieve the desired idle speed.

I take it this setup is a mixture of X30XE and C25XE bits. I don't suppose the idle valve is incompatible with the engine management? Not familiar with the C25XE setup.

However, with the ICV fully closed the engine should stall, so air was getting in somewhere with it shut.

Kevin

[archiewood]

Well, when the valve is at 'rest' there is a gap of a couple of mm so I assumed the air was getting in there.

Yes, this is an X30XE with a C25XE loom and management. The 30 ICV looks identical to the 25 one, though it's gone now so I can't check it. They're less than a tenner second hand so I'll grab one to try.

I agree though, with the idle hole blocked off it should stall.

I had the plenum off yesterday and while it was off I tried blowing down the intake flutes and couldn't (which was encouraging), so I think the leak is in the plenum or vacuum system somewhere. Again though, it would have to be a pretty big leak for the revs to go that high no?

Could the crank sensor problems I'm having cause a high idle? It seems logical that if the ECU can't properly read the engine speed it'll try to keep it well above idle to make sure it doesn't stall.

[Andy H]

I would have another look at the liquid metal plugging the EGR connection.

[archiewood]

Is it porous? It looked fine when I had the plenum off, but I'll put another layer on (I put it on, so it's only a couple of weeks old).

[Andy H]

I was thinking that 3000 rpm would represent quite a big air leak.

Maybe the chemical metal hadn't fully set and the manifold vacuum was sufficient to suck a hole in it?

[archiewood]

There's nothing apparent from the outside, but I'll put another layer on and see what happens.

It seems almost certain the leak is in the plenum somewhere, but the damn thing has so many holes in it it's a bit hard to pressure test!  :-?