XR8 idle hunt (video) - Page 2

crakrz · 25-07-2021, 11:52 AM

A couple of things look strange to me. Your tps voltage stays on 1.18v even when you blip the pedal. That’s not what you want to see. Your MAF voltage is approx 0.8v at idle. Is it a stock MAF? Usually a stock engine and MAF are between 0.95v and 1.05v at idle, or thereabouts. This suggests to me that air is getting in elsewhere. Your spark values are chasing up and down trying to stabilise the idle 16-29 deg. If it’s happy it should only fluctuate a degree or two around 18 deg.

pete r · 25-07-2021, 12:07 PM

Ooh, good point. I’ll do a log of throttle position voltage as I press the pedal down

pete r · 25-07-2021, 12:08 PM

Yep MAF is standard

pete r · 25-07-2021, 01:18 PM

TPS seems to be ok, video below shows the voltage sweep seems to be ok from closed to WOT?

pete r · 25-07-2021, 01:27 PM

The video below shows the MAF swings up to about 3.5v with a full throttle blip, does this sound about right?

Geez it's spins to 5000rpm quick! If I can get this bloody idle hunt issue sorted I'll be a pig in ****!

pete r · 25-07-2021, 01:32 PM

Quote:

Originally Posted by Ben

Would be a good idea to monitor whats going on with the MAF also. Mine went recently and would randomly throw up crazy numbers for a split second. In the workshop area there are some MAF numbers you should be seeing.

Also vac leaks are common and if the fuel trims improve with more throttle its a good chance. I found a home built smoke tester worked the best but there are a few methods(hose up to ear works good also). Make sure you check around the intake manifold, pcv valve(make sure its still secured in the grommet and sealing), and the hard plastic fittings around the coil pack area.

If you go down the smoke way I found disconnecting the brake booster vac and going straight to the manifold worked great.

Good luck.

Hey Ben how did you make make your smoke tester? I've tried the propane torch thing without any success. A mate said it could also be a cracked header upstream of the O2 sensor, buggered if I know how I'd find that, I'm 3 parts deaf and the exhaust note is pretty bold...

Ben · 25-07-2021, 03:36 PM

Quote:

Originally Posted by pete r

Hey Ben how did you make make your smoke tester? I've tried the propane torch thing without any success. A mate said it could also be a cracked header upstream of the O2 sensor, buggered if I know how I'd find that, I'm 3 parts deaf and the exhaust note is pretty bold...

https://www.youtube.com/watch?v=Ai6QkF2Ez3A

Basically same setup as this. I disconnected the air pipe after the air cleaner and put a rubber glove with some tape over it to seal the system. Disconnected the vac hose at the brake booster and hooked into that hose(goes to manifold).

Worked well.

pete r · 25-07-2021, 06:08 PM

Quote:

Originally Posted by Ben

https://www.youtube.com/watch?v=Ai6QkF2Ez3A

Basically same setup as this. I disconnected the air pipe after the air cleaner and put a rubber glove with some tape over it to seal the system. Disconnected the vac hose at the brake booster and hooked into that hose(goes to manifold).

Worked well.

Isn't that bloody brilliant???

Champion thanks Ben! IOU 🍻

undertow · 26-07-2021, 08:47 PM

Can you include MAF airflow (grams/sec) in your log? Should be about 6.5-7 g/s at idle once warmed. If MAF airflow is low at idle and fuel trims high, you might have a big old vacuum leak (unmetered air). Check the PCV valve and the rubber elbow on the fuel pressure regulator, I had a hard-to-find leak at the latter.

pete r · 27-07-2021, 11:07 AM

Quote:

Originally Posted by undertow

Can you include MAF airflow (grams/sec) in your log? Should be about 6.5-7 g/s at idle once warmed. If MAF airflow is low at idle and fuel trims high, you might have a big old vacuum leak (unmetered air). Check the PCV valve and the rubber elbow on the fuel pressure regulator, I had a hard-to-find leak at the latter.

Here's the video showing the MAF airflow (middle column).
You make a really good point, as the revs increase during idle hunt the airflow number isn't changing, so the air must be coming from somewhere else? You can see airflow increase when I take it up to 2300rpm, but the idle hunt number doesn't move...

undertow · 29-07-2021, 07:59 PM

Thanks, yep looks like a vac leak, that idle MAF airflow is well below the normal range. Unmetered air getting in, so it runs lean and the fuel trims go strongly positive to try and compensate. If you go for a drive and log it you'll likely see it running lean/fuel trims going right up during closed throttle, then come back to normal with more load (more load lessens the effect of the vac leak).

The O2 sensors look to be switching OK to me but it doesn't hurt to replace them anyway at this age.

pete r · 30-07-2021, 08:20 PM

Update -

It seems there’s a leak in the righthand header before the O2 sensor. There’s a definite “ticking” sound coming the exhaust connected to #3 or #4, which becomes harder to hear when the engine is hot. It’s going in to get it sorted on Monday.

I’ve a horrible feeling it will be like everything else so far, it needs to be fixed but isn’t the source of the problem

pete r · 03-08-2021, 01:58 PM

So, exhaust leak is fixed and new O2 sensors both sides. And...the idle hunt is still there

Onward and upward.

pete r · 15-08-2021, 05:03 PM

Pack me another one, we found the bastard!

pete r · 21-08-2021, 01:25 PM

That wasn’t it. New gaskets and now no smoke but idle problem is still here.
I think it’s a fuel pump/regulator issue, I’ll start another thread

pete r · 22-08-2021, 01:14 PM

Here’s an interesting observation, the car runs a lot better with the fuel pressure regulator vacuum line disconnected and plugged.
With vacuum to the regulator the short term fuel trims are positive 20/25% at idle and positive 10/15% while driving.
With no vacuum to the regulator the fuel trims at idle are around positive 10% and floating around zero while driving.
Pressure seems to be okay but I have a new regulator on the way just in case

pete r · 12-09-2021, 12:52 PM

Update: problem solved!

This issue started about a few weeks after I bunged on some Tickford TE50 heads and an intake insulating spacer. I'd already fitted an SS CAI.

It was pretty obvious the thing was running lean at idle and as a he thread shows we chased a lot of things to try to find the cause.
It had to be a vacuum leak, that is air entering the intake without being registered by the MAF sensor.

Then it hit me, what if there's now more air coming through the MAF and the engine tune is now out of whack?

I went for a run with the OBD2 connected and Forscan running, I gave it the boot a few times to datalog the MAG flow and MAF voltage.

I came home and wrote it out in a table, then compared it to the MAF transfer in the ECU's tune (using a Moates Quarterhorse and Tunerpro...

Bugger me it was a reasonable way out. The engine is getting more air than the tune expects.

I thought perhaps the ECU would learn from the MAF voltage and adjust itself so the car idled better, but as we know the short term fuel trims chased the lean reading from the O2 sensors all over the place.

So I adjusted the time's MAF transfer table to match the MAF data I had logged. Problem solved!!!

There's still a bit of tweaking to do but the mad idle hunt has gone.

Thanks for your help and advice blokes. What a top lesson this has been!

Here's the before and after MAF tables...

Before

After

madmelon · 07-11-2021, 07:07 PM

Glad you got that sorted but there's something else I want to direct your attention to. There is an old thread in FFAU about issues with wiring design of some aftermarket O2 Sensors for the AU V8. Some of them incorrectly earth the sensor through particular wires, which forces the voltage of other sensors to be mis-read. NTK sensors correctly earth their O2 sensors for the AU V8 so do not have this problem.

Best you do a search for that thread- it's pure gold.

Cheers

Regan · 13-06-2022, 08:25 PM

Did you get to the bottom of the surging idle

Caprobbo · 22-07-2022, 07:33 PM

Did you check the o2 fuse in the engine bay fuse box
Also check fuel pressure is within specs (can get to high)

pete r · 27-12-2022, 11:40 AM

Quote:

Originally Posted by Regan

Did you get to the bottom of the surging idle

Sorry for the late reply Regan. I got it to idle steady by tweaking the MAF transfer table.
Then I had another issue with “bucking” at light throttle levels while cruising below 2000rpm. Above 2000 it was ok, but below 2000 it was like I was towing a trailer that was jerking on the back of the car.
I thought of blocked injectors, valve issues with the Tickford heads I’d fitted. It drove me nuts.
Just found the issue thanks to some ideas on OS forums - the Tickford heads have 1.72 rockers so it’s effectively got more cam duration. At light throttle there’s more vacuum and with increased valve over lap (due to the bigger valves and increased duration) exhaust gas is being drawn back into the cylinder and intake plenum.
The solution seems to be to reduce the light throttle spark advance in the “PT Spark Adder for ECT vs Load” table. I’ve got the advance cut by 15 degrees at 5% load. Now it’s cruising nice and smooth

**Uncle_Ken** · 28-12-2022, 09:54 AM

good to hear