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Discussion Starter · #1 ·
It's a 1994 Omega 2.5V6 auto with 80,000 miles (genuine- bought it at 11,000 Network Q) so I had hoped it might have some life left in it. The car has been utterly reliable and serviced regularly until 72,000 mile service, after which it developed the following:
smooth when cold; but after reaching normal temps:
erratic stuttering at idle in neutral (500-800rpm) but smooth if revved higher;
rough when in gear up to 1800rpm, then smooth;
smooth at higher revs except when at high throttle openings when a regular stuttering develops, disappears as soon as you take your foot off the throttle. This last symptom is true even when the engine is cold and otherwise smooth. No dashboard warning lights. There is therefore a 'sweet spot' that allows me to cruise at 70mph in D with fuel consumption unchanged from when the engine was running smoothly.

I took it back to the garage, who tested the coolant chemically and declared that exhaust components were detected, and therefore it was likely a head gasket problem. I was advised to run the (otherwise mechanically sound and just MOT'd) car until the problem was terminal and buy another car, since the likely cost of repair was greater than the cost of replacement.

All manner of other domestrivia intervened and I basically took the advice and drove on, expecting the car to die. It's now 8,000 miles later and the symptoms are unchanged. I'm prompted to investigate because the exhaust is now deteriorating and I have to decide whether to replace (£340 quoted) or put the money to a newer car. Further, there is no oil in the coolant, and no water in the oil (except for a little mayonnaise on the cap when the car has only done short trips- has done this for years- disappears after long trips.) Neither oil nor coolant goes down at any rate, and there is no smoke or steam.

It's looking to me that it might not be a head gasket problem, and I've now read in your nice forum that such problems are rare in the 2.5V6.

So, what's the advice? Junk the car, or is there an alternative diagnosis that I can go back to the garage with that might make it worth fixing and replacing the exhaust?

Thanks in advance- I'm amazed at the amount of detailed and competent advice here.

Ed
 

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It could be your DIS module, has this been checked? Like you say, there's no mixing of Oil/Water so i think it's unlikely to be head gasket, but un-even running has been known because of a buggered DIS module.
 

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Discussion Starter · #4 ·
It could be your DIS module, has this been checked? Like you say, there's no mixing of Oil/Water so i think it's unlikely to be head gasket, but un-even running has been known because of a buggered DIS module.
Nope. It's been years since I got my hands dirty and that car had a distributor cap! That's why I came here for some ideas before going back to the garage- they're honest blokes but being an independent garage I don't expect them to have the detailed Omega knowledge you lot do. The point is I don't want to tell them they're likely to be wrong without an intelligent alternative and DIS sounds like a good idea. It may be significant that the symptoms appeared after a service- are there any other problems such as hoses or some such that can cause this sort of thing that could be dislodged during a service? Also whatever is wrong is likely to be a single failure, since it appeared and has been stable for 8000 miles. I first took it back to the garage after about 1000 miles and they checked the plugs- they were all showing healthy gaps etc so clearly the engine is working healthily when kept in its sweet spot. They just lost interest when they did the coolant test (they didn't want me to charge me, so obviously didn't want to do more diagnostics when they thought they'd found a terminal fault). Thanks for the DIS tip.
 

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Could be DIS module. Not sure how you test it, but it is an expensive part and not the easiest to get at so will be expensive to replace. Hopefully the garage could test this unit. Anyone of the brighter souls out there know how it is tested?

I would also check the cheaper stuff that seems to fail on the Omegas, since I am tight !!

a) Do not discount the blocked breather box. On mine the due to the blocked breather (balck plastic box/strip on the very top of the engine) the valve cover (due to the pressure) leaked and covered the plugs insulators and lead connectors with just a little oil. Mine stuttered on idle and stuttered on acceleration and was improved at high RPM, but a misfire could show up in many ways irregular ways. Pull the plugs and check for any oil on the white insulator part of the plug the plug lead connector is on, not the bit in the engine cylinder head, especially on the passenger side. I was surprised at how little oil it took to give me a misfire. If it is this, after cleaning the plugs and leads, possibly replacing the plugs, the breather can be removed and flushed out with petrol, or replaced, and the crankcase breather pipes need to be taken off and flushed or replaced. Your motor is the right age to have plenty of blocked breathers.

b) I say would check the idle valve. But not sure if that would also play up at the higher RPMS. Tell to check it is not blocked up with a bit of debris (can happen as a bit of sticky tape appeared in mine, never sure how it got past the air filter!) , give a clean with some WD40 so it is free to move. Hopefully they can check it is working electrically.

c) Don't failing EGR valves give you a "stutter" ? Need one of the more expert forum guys to answer that really. I am not 100% that would give you an idle issue though. Again hopefully the garage can check it electrically and clean it. Mine did and it was cheap as its easy to get to.

d) Could it be an old fashioned failure? Get them to check the volts at the battery and they are not cycling or something. On my older banger the alternator regulator failed and gave erratic volts leading to weird running and very poor idle control. Not sure if that would apply to the Omega but it is a very quick check with a voltmeter before things get expensive.

Hope that helps, I am not an expert but in our family we have had 3 Omegas and they all have had these weak spots fail at one time or another. Good luck !
 

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Discussion Starter · #6 ·
I would also check the cheaper stuff that seems to fail on the Omegas, since I am tight !!
Hope that helps, I am not an expert but in our family we have had 3 Omegas and they all have had these weak spots fail at one time or another. Good luck !
Thanks for all those. Plenty for me to look at. I like the breather and plugs. Even I can do that. In fact I think they should put all serviceable parts at the top of the engine... :)

One thing though- anyone any idea why it's smooth when cold? Suggests to me it's not the contacts to the plugs themselves, since that's when the spark probably needs to be strongest. It always starts immediately.
 

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smooth when cold; but after reaching normal temps:
erratic stuttering at idle in neutral (500-800rpm) but smooth if revved higher;
rough when in gear up to 1800rpm, then smooth;
smooth at higher revs except when at high throttle openings when a regular stuttering develops, disappears as soon as you take your foot off the throttle. Ed
I had exactly the same symptoms on mine from the day I got it from eBay. As I got an absolute bargain I still had £700 in my budget to change cam belt and all the likely bits.

Drove straight to my local independent garage where they agreed to fix the misfire at the same time as changing the cambelt and tensioners. He said that the two back plugs were the originals (100,000 miles of main dealer servicing and they hadn't changed these two plugs). He also said the coil pack was burnt to bits and that was replaced at the same time. Finally the HT leads were done just to be sure and now it goes like a rocket with none of the earlier symptoms.

As mentioned elsewhere, changing the coil pack (DIS) is a bugger but it cured mine of exactly the symptoms you described (even down to the same engine revs causing the problems).

Cheers
 

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DIS pack can be had off fleabay, about £80 I think.

After my first miggy had blown cam cover seals and the spark plug wells had varying amounts of oil in them (no. 4 was FULL), I fitted "magnecor" 8mm HT leads which cost about £130 for a complete set, but there are cheaper alternatives. Vx originals are about £75 - PER SIDE ... :eek:
 

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I agree with the DIS pack diagnosis from previous posts.
My V6 Omega had very similar symptoms. No fault codes on the ECU.
I changed the plugs, then HT leads, then the DIS pack.
The DIS pack on these has 3 coils each supplying a pair of cylinders. If one of the coils fails you will get a misfire on 2 cylinders under high engine load conditions.
Also, check for water getting on to the DIS pack from the heater pipes or from where the wiper arm goes through the scuttle.
 
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