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1997 Omega 2.5 v6 fuel consumption

For the last couple of days the engine has been running really rough when cold, it's done it a bit over the last couple of weeks but not for long.

After startup from cold it's doing 8.5mpg at 30 and 23mpg on overrun. As it warms up this rises and after a couple of miles it's doing low

30's and overun is 999mpg.

I'm assuming it's the ECU coolant sensor and since it's a pain to get to I might as well just replace it but I have a couple of questions and hope someone can provide some guidance.

I know I've got to take the top of the inlet manifold off but do I need to take the lower section off as well? I can't tell from the pictures in the Haynes manual. I need to know what gaskets to get so that I can put it back together again.

I can also hear air hissing somewhere near the inlet manifold and if I rest my hand on the metal next to the plastic strip with all the pipes in the tone changes. Resting my hand of the plastic doesn't change the sound. I can't see any split hoses and I can't feel any air if I move my hand around amongst the pipes at the back of the engine.

I'm wondering if this is likely to be a gasket on the lower section of the inlet manifold and whether I should replace these at the same time. If it is an intake leak and it stops once the engine has warmed up it could well be the cause of the problem but I might as well replace the sensor while it's apart just in case.

Any thoughts?

Reply to
rp
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When I have replaced coolant sensors before, I have found it quicker (and easier) to remove the scuttle trim, and then remove the wiper linkage. This allows enough room to replace the sensor without having to remove the intake manifold. Most of the joints in the intake system are sealed by 'o' rings.

The black plastic vacuum tank is, I think, a push fit into the top of the intake manifold. It has a couple of 'o' rings to seal between the two parts. I have experienced the change in pitch of the hiss when pressing the vacuum tank, but I have never known it to cause any problems. You can check for vacuum leaks by spraying around the intake manifold with WD40 whilst the engine is idling. If the WD40 is drawn into the engine through a leaking area, the engine revs will drop, and possibly mis-fire.

HTH

Anthony Remove eight from email to reply.

Reply to
Anthony Britt

I'll give that a try but I've already taken the scuttle off and can't see what I should be looking for. At least that means I don't need to take the lower manifold out even if I do have to remove the top.

Thanks Anthony, the WD40 idea sounds good. I can't tell where the hissing is coming from but it does seem to be underneath the manifold and on the nearside, I'll try WD40 under there as well.

Reply to
rp

I got the sensor and gaskets for the manifold and took the upper part off this afternoon. There was quite a lot of oil in the upper manifold, enough to drip out. Is this normal and if not where is it likely to be coming from? The butterflies look as though they have burnt oil on them as well, at least on the outside.

It does burn a bit of oil and smokes when started or when pulling away after idling so I'm pretty confident that's stem seals and I suspect on the inlet valves since number three plug fouls a bit. I'm not sure how this would get to the inlet manifold though. I'm going to have to go in a bit deeper, literally in the case of the coolant sensor, it must be six inches below the top of the manifold and I still can't see it but it was starting to get dark.

Tried spraying around with WD40 but no change to either engine revs or the hissing. Could it just be air rushing through the manifold?

Reply to
rp

The lower part of the manifold is coated in a thin film of carbon apart from cylinder no 2 which is oily. After taking off the manifold flange the head intakes are clean apart from no 2 which is oily again.

Is any of this significant?

Reply to
rp

I've found the hissing, the brake servo vacuum pipe has been rubbing against the air conditioning connector and it's worn a hole in the vacuum pipe.

What I've been calling no 2 cylinder may be no 3, it depends on whether the offside bank goes 1,2,3 or 1,3,5. It's the middle one that's sending the oil into the manifold, that's the one with the fouled plug and the duff stem seals.

I've left the head on for now, I'll see how it goes with the new coolant sensor and the vacuum leak stopped. Maybe do the head in three or four months.

Reply to
rp

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