STFC frequency seems high?

JohnDee68
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STFC frequency seems high?

Post by JohnDee68 »

Hi all, I am a newbie to this area but keen to understand it further. so i hope i have placed this in the right forum topic.

I have an older car, VX (2002) v6 3.8L Commodore.

I am trying to learn while diagnosing what i deem to be a higher than usual fuel consumption. about 1/3rd more than usual per tank.

I have noticed my O2 sensors fluctuate regularly between the range desired ( 1000-900mv ) during most driving. Which apparently is signs of good O2 sensors. as they respond to the variants of exhaust mix coming out of the motor well.

Using TunerProRT I see that the STFC seems to also change often, along with changes to the injector timing.

From what i understand, the STFC happens when in closed loop due to the readings from the O2 sensors and then adjusts fuel mix ( inj timing) as it requires. So I am wondering firstly what sort of frequency on a perfect motor should STFC happen and could excessive changing be caused from spark plug failures and/or fuel injector failings?

There is a slight motor rock ( like a miss or fuel imbalance ) at idle, but not apparent during revs above 1000 (my normal driving conditions).

Could there be anything else other than a sparking failing or injector issues that I should also look for to check? I am waiting for some items so I can check and confirm the injectors operations, but that will be a week or more still. Tthe spark plugs are not that high on the mileage either. Though I wonder if anyone can tell from a TunerPro log as to what is actually happening in either being fuel supply or spark supply or is it too hard to know without swapping parts out?

Appreciate any advice thanks.
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vlad01
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Re: STFC frequency seems high?

Post by vlad01 »

These engines always get worse in economy from new to when they get a good amount of Ks on them. I have ruled out everything on a few cars I have/had and I believe it's caused by timing chain stretch which results in retarded cam timing.

All my low k engines got anywhere from 6.5-7.5L/100 and once they got to about 200k they were all doing about 10L/100. Once they get to 10-11L/100 they seem to stop there and not get any worse.

My VR which I replaced the timing set on, has 110k on the clock and it does high 7L/100 with all highway driving, but my VP with mildly worked engine has 5º retarded cam timing, although performing well has an atrocious 15-16L/100 which is good indication that retarding the cam reduces fuel economy.

My daily driver has 310 on the clock, it does 10L/100.
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immortality
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Re: STFC frequency seems high?

Post by immortality »

That's an interesting theory about cam timing retard and fuel economy due to timing chain/tensioner wear and tear. Retarded cam timing would reduce dynamic compression. I wonder if that would show up doing a compression test? Unfortunately I never did a compression test when I bought our VX so I have no numbers to compare to on our L67. I do have new valve springs and timing chain set to go into the L67, maybe I should do a before and after compression test.
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Re: STFC frequency seems high?

Post by vlad01 »

on my last daily with 400k, I took the engine apart before getting rid of it and the chain was very loose, without the tensioner installed it was so close to be able to jump teeth on the crank sprocket. The bores seemed ok, still had cross hatch marks, no lip. rings and pitons still looked like new so I don't think there was any loss of compression, although I never tested it.

I had that engine since 89K and when it was installed in the car it would do about 850 on a tank, by the time it got to 400k it was doing about 11L/100 or about 550 per tank. I even chucked on 12P and a WB and found that the AFRs were spot on, added lean cruise and other changes but wasn't able to gain any economy, so I concluded it wasn't the fuel system or any of the sensors but something fundamental with the engine.
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vlad01
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Re: STFC frequency seems high?

Post by vlad01 »

not a lot in it but if you look back and forth you can see the top one is more stretched, the bottom of the tensioner is fully contacting while on the 100K chain (bottom pic) is not fully contacting the bottom of the tensioner, the chain looks more straight as well.

Hard to say really, maybe 1-2º more retard? not sure if that would make all the difference but it's the only thing I can think off that could do it.

EDIT. You can also see it in the angle of the tensioner arm, its more angled in on the top one.
400k timing chain.PNG
400k timing chain.PNG (713.06 KiB) Viewed 4198 times
100k timing chain.PNG
100k timing chain.PNG (357.76 KiB) Viewed 4198 times
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Re: STFC frequency seems high?

Post by immortality »

My old VN always got good economy and I put a new double row timing chain on that old girl too. Hmmmmm hurry up Xmas....

The chain on the top pic looked stuffed, it doesn't look like the chain is sitting down in the sprocket correctly all the way around which is a good sign of chain stretch. Kinda makes me want to check out the 251,000km timing chain on our L67
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Re: STFC frequency seems high?

Post by vlad01 »

Hey, you're right, I didn't even notice that. It really is fucked.


My 73k VP S pack drove from Brisbane to Newcastle with about 8L left in the tank going by how much to fill it. So that worked out to be about 7.3L/100, bone stock with auto. Amazing really.
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JohnDee68
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Re: STFC frequency seems high?

Post by JohnDee68 »

Thanks guys,

I cant confirm 100% but about the timing chain. but i can give some history to the car, just for a talking point i guess.

I am the second owner, got it at its age of 2 years with just less than 42K on it. Was well looked after by a CEO on a council.

I am on to the third motor now (second has only 80K on it and was changed when didn't need be), with nearly 500K on the clock now mainly (90% plus ) highway driving. The current motor I reconditioned and will have been in just on 3 years this coming Christmas. The car doesn't do the km's like it used to these days.
About a year ago i experienced injector leaking where it wouldn't shut off a couple of injectors. I cleaned them back then and even tried "cheap CHINA ones" to try and resolve the issue. I ended up using s/hand and original ones after i cleaned them and they came up functioning ok. well as good as i could tell at home without a professional tester of course. At the moment I am waiting on my "McGuyver kit" and some spare BOSCH injectors to do a more proper test, clean and replacement of any suspected ones.

However, I did do a WD40 basic flush and clean yesterday of them. I also noticed that (luckily i label things) there is still one of the "cheap CHINA" injectors in this motor which is of concern now as it may likely be the culprit to my issues?

Today, I have replaced the spark plugs, though i didn't think they were required really. On the way back from getting spark plugs, it seemed (hopefully not just my imagination) to have another cylinder or two. As I didn't road test it after the injector clean yesterday, it could be it just has come good after the injector work i did, maybe?

So tomorrow I am going to take it for a longer (approx 300Km) and highway trip to really test the economy and do a few logs also. I will report my findings once i am back.
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Re: STFC frequency seems high?

Post by vlad01 »

Have you checked EGR function? EGR makes a big difference to economy over engines that don't have it.


My nephew has a manual VS with a VX motor, it does 12L/100 but doesn't have EGR.
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Re: STFC frequency seems high?

Post by Gareth »

If you mark the balancer at TDC #1 and put a timing light on it, you will see the mark jump around showing timing chain slop. (at idle of course)
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