Help with hunting at idle
 

Hey All,

I'm having a spot of trouble with my 8, she's hunting badly. (see vid) While it sounds kind cool it eventually stalls out.

I'm looking for possible causes or combinations that may cause this. I've obviously missed something.

A couple things to note:

• This is a track car, the A/C has been removed along with the heat pump (secondary air pump).
• The engine wasn't behaving like this in my other 8 (I swapped this motor out of my road car and a stock one went in that one)
• yes, I searched
• No codes are being thrown (except about the missing heat pump and A/C)
• I have put in the stock airbox to test and swapped the MAF between the road car and track car with no change
• I haven't found any causes for vacuum leaks *yet* but happy to be schooled on the common culprits in this space
• After trying a potential fix I've been resetting the ECU (brake pump method)
• The ESS is clean and didn't have an issue prior to the swap

What am I missing? Is there anything I can monitor through the OBD scanner that might lend some insight?


Cheers

Read the airflow (g/sec) and short term and long term fuel trim data from OBD. If you have a vacuum leak the fuel trims will not be close to 0, and airflow will be significantly less than 5g/sec.

Can you swap the throttle body from the other car? I don't remember the details, but there were changes in the throttle body electronics between years.

If by heat pump you mean secondary air pump, did you block off the hole that leaves in the exhaust manifold?

Cheers Loki, I'll check those on the OBD and report back.

Both cars are 2004 Series 1's so I doubt there is anything going on with the TB but if nothing else works I'll give it a go, can't hurt.

And yep, the I meant the air pump, not sure why I call it a heat pump, something about hot air and cat's :dunno: Anyhow, yep, blocked of at the housing and the vacuum line capped.

Have you smoked the intake system? That's the best way I know of to find vacuum leaks.

How does it run under throttle, because otherwise it sounds like a bad/broken rotor seal to me. Compression check is my suggestion.

I'm seeing around 10 on the airflow and something like -25.x on the short term trim but not getting a reading on the long term. Not sure I have a lot of faith in this read just yet as there are about a dozen options for short term trim (bank 1,2,3 and sensor 1,2 on each) and half as many for the long term trim. I didn't have time to configure them all just to see what they did (task for next weekend).

Clearly I need more data here..

Not yet but on the list if Loki's advice turns up something.. or rather that should be nothing.. and right after I figure out how in my home garage.

We get in a whole lotta trouble if we run unregistered cars on the road here so I can't say how if goes under load but it does rev cleanly and holds revs no worries. Compression is fine, I comp tested it prior to pulling the motor out, front rotor is pretty much on factory, the rear is down a fair bit (not quite rebuild territory) but even across all faces


I intend to have it at a Rotary revival here in Sydney in 3 weeks time so I can get some track time to shake it out, I'm basically using the day as a cheap test day but I'd like to have this sorted before.

IIRC, you want bank 1, sensor 1. If the STFT is -25 you've got too much fuel being delivered.

And 10 (assuming that's g/sec) airflow is way too much for a warm idle. So IF that's warm idle around 750rpm, high airflow reading and high negative fuel trim implies the airflow reading is wrong. Are you using the original MAF and has it been messed with or cleaner improperly?

Are you very sure the injectors are correctly hooked up to the correct wires and the ignition coils are correctly hooked up to the correct wires and the correct spark plugs? People have messed those things up when installing engines, with unpredictable results.

An update is in order..
I was pretty sure everything was in the right order, not the first engine swap I've done in these things and all. And I was right.. yay!
Downside is I still couldn't nail it so I took it somewhere I trust and it turns out it was a faulty o2 sensor. A faulty *brand new* o2.. *sigh*

I feel a little vindicated tho, he spent a whole day trying various things, unplugging and plugging things until something made a difference and he knows these things well.
So there you go, that's a new one for me so I thought I'd share.
Thanks for the help guys 👍