found the cause of my flat spots.. may be of use!

[br14nh]

For some time I couldn't work out why my VX would have random flat spots when accelerating - always when warm, never when cold..... (engine has done 200,000Km's).

After changing coils, leads, IAC motor, cleaning the plenum, etc, etc. I ended up having the car diagnosed professionally & the O2 sensors replaced by a mechanic (back in January). The performance did improve markedly at the time, but the problem was still there....

A little while ago I had a kind mechanic throw the diagnostic meter on the car (for free!) & he noted the RH O2 sensor was reading higher than the left; a bit later I was under the car and noticed a bit of "puffing" from the RH exhaust joint just after the extractor (it has Pacemakers & full exhaust system) -when cool, I tried to open it up but one bolt was stuffed, which meant the flange wasn't ever mated properly. :bang:

One new bolt/nut later :wax: I slung some red silastic either side of the gasket there, it seemed to have fixed it, but as I found a week later it was still leaking, as I found out the gasket had blown out through the centre and lost most of the stuff between the two halves & when the exhaust flexed, it would leak. One new gasket (& some more red silastic) later & the performance is waaaaay better - it appears this leak was letting Oxygen back in & affecting the O2 sensor, which was giving me random flat spots.

The new gasket is also a better design, it has an extra inner ring of metal pressed on which would prevent any gases getting through between the two halves - the original gasket didn't have this. I'm also about to go through & replace the other three gaskets, just in case they're also stuffed.

The key thing I have noted is that when cold, the car behaves brilliantly, it's only when it warms up a bit (& the O2 sensors come on line) that the flat spotting starts. There may be other causes of this in other cases, but in my case this was the issue.

In all the threads here I haven't come across this as a known cause, so I thought I would post in case it can help someone else.

 

[CANYONERO]

good post. its not something that you first think of.

 

[Coast_Calais]

good thread, glad to see you found the problem (eventually) and got it sorted.

 

[cxcxcxvcvcvc]

could be what's happening with my calais, will check it out tommorow! Cheers

 

[DannyboyDS]

I just wished youd found thise before forking out all of that money, we all go through things like this with our cars, and its wonderfull to hear another happy ending.

I bet you felt so damn smart when you figured it out too, it's like. HAHAHA all of those people who tried and couldnt do it, and little old me figured it out.

 

[br14nh]

Have to confess, I didn't feel smart, just incredibly relieved..... it was one of those things that you come across by chance that you're really glad you found, 'cause you know you weren't smart enough to think of it first-off by yourself... :whistling (I am also happy that no-one has said "well, of course, you know, that was an obvious thing to look for..." )

Especially given how cheap a fix it is, I guess it should be one of the FIRST things to check (slaps self on forehead while saying that...). I finally got around to pulling apart the other side today & the gasket on that side was showing signs of failing as well, so I'm much happier now I know I have it sorted.

I still don't regret getting all the other bits fixed, that's the thing with troubleshooting this type of problem, trying to remove causes one by one - at least now I KNOW what's fixed & what isn't with the rest of the car - much better than "damn, car's going crap, geez, maybe it's the plugs - oh wait, could be the leads..... or what about the coils...?"

I will be really happy if it helps a few other people solve their issues, one of the best things about these forums is the shared knowledge.