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Old 04-27-2018, 07:40 AM   #1
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Continued performance issue (but may have found something)

So to continue on with my previous problems, I originally thought I had a vacuum leak but lately I'm not so sure.

It's completely inconsistent. Some days the performance will be brilliant, others will be lacking in torque, and some days will have choppy acceleration and deceleration, as if it can't make up it's mind whether the throttle is being applied or not at light throttle inputs.

MAF has been changed, so has the AOS, and TPS seems accurate.

Now, I've just discovered this:

https://youtu.be/Mg_49U5vqdI

See the rev needle bob up and down? I've pinned the throttle at 2.75% and that's not moving, however the revs are fluctuating between 150rpm and 200rpm.

I've noticed this happening when reversing and pulling off, where the revs would move up and down slowly as if I was feathering the clutch or throttle. This is also the issue that caused my previous bogging when pulling away, as the revs would fluctuate too low when I applied throttle, causing the car to bog for a second.

I've checked the coil packs and they're cracked. One of my coil packs has this part missing completely (highlighted in yellow):



No rubber end at all - I pulled it out of the tube.

Now yesterday, I applied full throttle from about 10mph, and the car felt like I had jabbed the throttle, let go, then jabbed it again, like there was a sudden and instant loss in power.

I don't want to jump to conclusions here, but... could my problems all be to do with coil packs? And not a vacuum leak after all?

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Old 04-27-2018, 09:00 AM   #2
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Is this actually Geraint Thomas of Sky Team?

If the coil packs are cracked, they are done. Replace them all for good measure. Not a hard job. Do plugs and tubes while you are there.
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Old 04-27-2018, 09:02 AM   #3
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Quote:
Originally Posted by Geof3 View Post
Is this actually Geraint Thomas of Sky Team?

If the coil packs are cracked, they are done. Replace them all for good measure. Not a hard job. Do plugs and tubes while you are there.
God damn it not you as well.

No tubes, as it's a facelift. I'm changing them anyway but wondered if it were to do with my issue.

The other thing I'm thinking is a faulty TPS? Can you clean them?
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Old 04-27-2018, 11:08 AM   #4
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The plot thickens...

I've just checked my fuel trims and O2 sensors.

If I had a vacuum leak, the fuel trims would be very high to compensate for the leak, correct? Here's mine. Short term (instant) on top, long term on the bottom:



Mine's completely normal. Normal operating readouts oscillate constantly between around 5% and -5%. If I were to have a vacuum leak, the long term and/or short term would be way into the positives.

However, look at my O2 readings on a cold start after the SAI has turned off. Top two are pre-cat, bottom two are post cat:



Post cat O2 readings are fine - bang in the middle. Not running lean at all.

Then I drove the car for a while, stopped, and checked it again:



As you can see the pre-cat sensors are still working normally, but now the post cat results are way, way too lean. The two lines should be in the middle.

Now take a look at this - these readings are whilst driving steadily at 30mph on a quiet industrial estate:





Then when I stopped the car:



They're all over the place!

Reading up more and, apparently, misfires can cause a lean result. You'd think that they'd cause a rich result due to the unburnt fuel, but car's don't have fuel sensors, they have O2 sensors, and they'd pick up the unused O2 in the cylinders and cause a lean result. Which is kind of what's happening, except mine's happening constantly, not just spikes - it's constantly lean in both banks.

So, lots more info, but I'm still unsure. I think I've ruled a vacuum leak out with the fuel trims being normal, but what's causing the O2 sensors to be constantly lean? The car's just passed an MOT emissions test, so it's not the CAT's.

Interesting...
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Old 04-28-2018, 05:13 PM   #5
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I can't solve your frustrating problem but, just as a side note, small air leaks generally show most at idle or very low revs. At higher revs the quantity of unmetered air would be much less percentage-wise, and consequently have little effect. In my opinion, your LTFTs would not be greatly affected by a small air leak.

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