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[Mr Gav]

My Omega is suffering a flat spot at 3500rpm on full throttle, it doesn't happen on part throttle and will pull through cleanly. It's fine below 3.5k and once it's past about 3.8k it revs cleanly to the rev limiter, I'm assuming it's something to do with the multi ram system but I would have thought if there was a fault with the multi ram it would affect either below 3.5k or above not just a small flat spot.
Anyone got any ideas or had similar problems?

[chrisgixer]

Breaking down spark plug? Or ignition issue.

[chrisgixer]

Multi ram would be above or below a certain rpm. About 4k is it?

[Mr Gav]

Plugs and leads got changed about 8k ago along with the can cover gaskets and cleaning out the breather box and the plug wells are still dry.
Not sure about the multi ram set up on the omega, there is a change over point, does it start at 4k and fully open at 4.5k or does it start at 3.5k and fully open at 4k?

[chrisgixer]

Ime it's flat above or below 4k depending on which multi ram is failed.


I suppose it could be slow vac leak affecting high rpm ...?

[Mr Gav]

The puzzling thing is it`s fine once it`s pulled through the flat spot, which only happens for about 3-400rpm, once it`s above 3.8/3.9k it flies.
Like you say if it was a multi ram fault it would affect either the high rpm or low rpm, it`s not a major problem but it`s just irritating   

[chrisgixer]

Had 2.2 round here that did the same. Spark plug was arcing down the side leaving a black track on the porcelain. Ran fine above 4k odd, and below 3k. Just refused to run at that rpm range.

Has it leaked in gradually getting worse, or arrived suddenly one day?

[Andy H]

Sounds a bit like the issue Webby had with a failed non return valve in the front vacuum tank

[chrisgixer]

Can we define the flat spot a bit? Just to be clear....?

Ie are we talking what feels like a lift on the throttle, when it's actually pinned to the floor? (Multi ram)

Or more of a cutting out or a stutter through that Rev range? (Ignition brake down)

..?

[Mr Gav]

Can we define the flat spot a bit? Just to be clear....?

Ie are we talking what feels like a lift on the throttle, when it's actually pinned to the floor? (Multi ram)

Or more of a cutting out or a stutter through that Rev range? (Ignition brake down)

..?

It feels like a bit of a stutter but the throttles pinned to the floor, It`s like it`s hesitating and then it clears it`s throat and flies just for those few hundred rpm. Can`t say how it arrived though, one of those things thats been niggling for a while but other things have taken precedence and I`ve just suffered it while I`ve been getting to grips with the crap hanling 

[chrisgixer]

I'd suspect a miss fire tbh.

Plugs still tight?

[Mr Gav]

It doesn`t feel like a misfire, I think for the cost of it I`ll just buy some new plugs to eliminate them and take it from there.
Because it happens at the same revs every time and for the same duration i was inclined to think it was a multi ram issue, the misfire I had before felt completely different.

[chrisgixer]

Miss fires reveal themselves in very different ways ime. From a completely dead cylinder to a slight miss at tickover to dropping one cylinder for a few rpm in the middle of the range under heavy load. There's no real rule to a miss fire IME.

[4x4]

Coil pack on its way out could be another possibility.

[Mr Gav]

I'll change the plugs first and take it from there.
I don't think a session on tech 2 would harm as I'd also like to get my climate on the latest firmware, anyone know who the nearest person to west Yorkshire is?

[carswaps]

Any codes?

[Webby the Bear]

Deffo check that vac box 

[Webby the Bear]

... As per Andy h .s comment 

Just pull smallest pipe off front vac box.no hiss equals either you've doe work and got pipes on wrong way round or box is fubard 

[Webby the Bear]

This is with car off btw 

[Mr Gav]

Cheers Webby, I'll check it out this week, not checked for codes yet but I'll do that at the same time 

[Temetsy]

IIRC the v6 has a 'flat spot'/drop on torque/power curve around 3,5k rpm I've heard that pumping up fuel pressure would cure that strange flat spot. I have no front multiram (custom intake by previous owner that really kicks sucks *ss) and still get that weird loss of power at that rpm..

[Mr Gav]

Well the vac reservoir seems fine, pulling the pipe off and there was a distinct hiss so tonight on the way back from Missy MV6`s I did a slightly untechnical test, a blast along a dual carriageway, first at full throttle and there was the flatspot between 3500-3800 rpm. Then I repeated the test at 3/4 throttle and pulled clean all the way from just below 3000rpm through to 5000rpm.
I still don`t think it`s a misfire, it just doesn`t feel like one, hard to explain but it just doesn`t feel like one.

[VXL V6]

IIRC the v6 has a 'flat spot'/drop on torque/power curve around 3,5k rpm I've heard that pumping up fuel pressure would cure that strange flat spot. I have no front multiram (custom intake by previous owner that really kicks sucks *ss) and still get that weird loss of power at that rpm..

That power loss flat spot was noticeable on the Vectra B 2.5 / 2.6's but far more smoothed out on the Omega V6's with the help of the Multi-Ram setup.

[Webby the Bear]

Ok, so youre getting stored vacuum. thats good.

blip the throttle and watch them operate.

for the rear one get someone to jam it quickly above 5k rpm. observe.

[Marks DTM Calib]

Sounds a bit like the issue Webby had with a failed non return valve in the front vacuum tank

Yep , first thing to check 


At WOT you have very little manifold vacuum so you are reliant on the vac tank storing some

[Mr Gav]

Already checked the vac reservoir and it`s holding vacuum, whether it`s holding enough is another question but AFAIK they either work or they don`t.
Next thing is to check the multi rams are working, not too keen on blipping to 5000rpm at standstill with no load though.

[Webby the Bear]

Already checked the vac reservoir and it`s holding vacuum, whether it`s holding enough is another question but AFAIK they either work or they don`t.
Next thing is to check the multi rams are working, not too keen on blipping to 5000rpm at standstill with no load though.

You wont be able to see if they're working then As long as it's up to temp. I don't see the issue.

As for is it holding ''enough'' vacuum...

If there's a vacuum leak somewhere else (e.g. brake servo hose) could the vacuum store be depleted thus affect multiram opening operation??? Question to expert...

[Andy H]

Well the vac reservoir seems fine, pulling the pipe off and there was a distinct hiss

You did this soon after switching the engine off ?

[Broomies Mate]

Already checked the vac reservoir and it`s holding vacuum, whether it`s holding enough is another question but AFAIK they either work or they don`t.
Next thing is to check the multi rams are working, not too keen on blipping to 5000rpm at standstill with no load though.

Why not?  If the engine is up to temperature, you can hold it on the limiter for a second or two.  The limiter is there as a (conservative) limit that the engine was designed to withstand.  These engines happily rev to 7.5k for brief periods of time.

[chrisgixer]

And it won't stress the engine in the same way as it would be loaded. Anything under the Rev limit is within operating range for the engine. If not there's something wrong.

[Broomies Mate]

The only annoying thing with the 2.6/3.2

Not being able to manipulate the throttle whilst your head is in the engine bay.  Having to rely on the wife to do as you ask, then end up shouting like a moron because she doesn't understand the instructions! 

2.5/3.0 was better for diagnostics! 

[chrisgixer]

The only annoying thing with the 2.6/3.2

Not being able to manipulate the throttle whilst your head is in the engine bay.  Having to rely on the wife to do as you ask, then end up shouting like a moron because she doesn't understand the instructions! 

2.5/3.0 was better for diagnostics! 

....and the very slight lag from peddle pressing to engine responding.

[Broomies Mate]

....and the very slight lag from peddle pressing to engine responding.

I think the ECU copes very, very well with the throttle.  Mine is practically instantaneous.  I do tend to clean the TB's with every oil change though (every 3 months).

[05omegav6]

The only annoying thing with the 2.6/3.2

Not being able to manipulate the throttle whilst your head is in the engine bay.  Having to rely on the wife to do as you ask, then end up shouting like a moron because she doesn't understand the instructions! 

2.5/3.0 was better for diagnostics! 

....and the very slight lag from peddle pressing to engine responding.

Nah, that's the delay as the torque converter spools up...

[chrisgixer]

The only annoying thing with the 2.6/3.2

Not being able to manipulate the throttle whilst your head is in the engine bay.  Having to rely on the wife to do as you ask, then end up shouting like a moron because she doesn't understand the instructions! 

2.5/3.0 was better for diagnostics! 

....and the very slight lag from peddle pressing to engine responding.

Nah, that's the delay as the torque converter spools up...

Oh now let me guess. Is this the Edwardian torque converter by any chance?

[05omegav6]

The only annoying thing with the 2.6/3.2

Not being able to manipulate the throttle whilst your head is in the engine bay.  Having to rely on the wife to do as you ask, then end up shouting like a moron because she doesn't understand the instructions! 

2.5/3.0 was better for diagnostics! 

....and the very slight lag from peddle pressing to engine responding.

Nah, that's the delay as the torque converter spools up...

Oh now let me guess. Is this the Edwardian torque converter by any chance?

I believe you're referring to the DMF

[Mr Gav]

I never understood why people would want to add an extra 100kg for slower gear changes and a car that sounds like it's slipping it's clutch