V8 4.4 hunting idle under strange conditions

[Oohsam]

Hey Guys,
I have a slight problem that I can't seem to diagnose without pulling the car apart.

When the car is cold, it starts fine, has high idle as per normal and then drops when it gets a bit warmer, car drives perfect.
However, when the car is warm, and I start it, the idle fluctuates between 600rpm and 900rpm constantly. The car never turns off, or really struggles, but you can tell its trying to stabalise, once I start driving, it idles normally.

I was thinking to start with the MAF and give it a clean, and then maybe do a throttle body clean, but my mechanical mind tells me this will probably be fruitless.
Im thinking i may have a vac leak, but I would expect to be there all the time, not after a short drive.
Something is telling the car it needs more fuel, and thus the revs climb, then drop again....The only reason it needs more fuel is due to too much air...which leads me to the MAF again, or vac leak - and thus the cycle continues.

Any thoughts?

[DiscoJeffster]

Do a search. Someone recently went through this exact issue only a few months back.

[INter674]

Had the same with a 4l d2 and it was a vac leak from a split hose.

Def do the MAF tho.

[DieselLSE]

As I was reading your post, my immediate thought was vac leak! And then you came up with that yourself. Remember that vac leaks aren't necessarily there or not there. A split can open and close up under various conditions, pressures, temperatures etc. I have no experience at all with your motor, but I'd start by physically tracing by hand (with the motor off) all accessible vacuum hoses looking and feeling for wear and splits and anything that doesn't seem quite right. I'd also inspect any plastic bits. If nothing pops up I'd do it again with the motor running taking safety into consideration. With luck the idle will change when you touch the naughty bit.
If that fails, I'd probably try a smoke test. If that fails I'd find someone who knows what they're talking about!

[Oohsam]

As I was reading your post, my immediate thought was vac leak! And then you came up with that yourself. Remember that vac leaks aren't necessarily there or not there. A split can open and close up under various conditions, pressures, temperatures etc. I have no experience at all with your motor, but I'd start by physically tracing by hand (with the motor off) all accessible vacuum hoses looking and feeling for wear and splits and anything that doesn't seem quite right. I'd also inspect any plastic bits. If nothing pops up I'd do it again with the motor running taking safety into consideration. With luck the idle will change when you touch the naughty bit.
If that fails, I'd probably try a smoke test. If that fails I'd find someone who knows what they're talking about!


Hahahah this cracked me up !

I will check all vac lines tomorrow.
Just did the maf clean - no change;

[INter674]

I would not recommend this but my son found our vac leak by spraying WD 40 around the lines. Engine note changed where the split was.

Or was it Start You B?

Hope not😞

[Oohsam]

I would not recommend this but my son found our vac leak by spraying WD 40 around the lines. Engine note changed where the split was.

Or was it Start You B?

Hope not😞

Yes, i've seen this done with propane - with one of those little blue portable burners, obviously with no flame.
I dont love this method as you need to havesome sort of idea where the leak could be, rather than flush it out.

I willprobably do a smoke test first with a DIY smoke machine.

[Oohsam]

I just wanted to give an update.

So I cleaned the MAF with maf cleaner, and I got no results.
Today I had an action plan, to first check out the throttle body, and then do a vac leak test with a smoke machine that I was gonna build off a youtube video that I found the other day.

So - this is what I found when I opened up the throttle body.
158068158069



So i took it off - but left it on the car as I didn't want to disconnect the coolant pipe. Call me lazy but I couldn't be bothered re bleeding the system.
There is quite a bit of room there so its fine. I just covered it all in my shop rags and it was fine.

I sprayed carb cleaner, but that wasn't really getting the grime off. I considered taking it off and putting it in the parts cleaner, but refer to previous point re:lazy.
I just started using petrol and some thinners with scouring pad (not very abrasive) and i scrubbed and scrubbed and scrubbed. It was very very stubborn.
Once i was done I gave it all a quick polish with some brasso, just to stop grime from sticking again so easily.

I also noticed when looking down the manifold, that the EGR port (sorry didn't take pics) had a huge build up of ash/charcoal that was building up.

I stuffed the mainfold inlet with rags and scrubbed it all off, vacuumed it up, then cleaned it with degreaser and repeat until there were no more chunks left.

This was the result. Not perfect but about x10000 better.

158070158071

Result - dead steady idle. No more hunting. Car is behaving and its so good!

Cheers to everyone that helped.

[Eric SDV6SE]

Good result

[DiscoJeffster]

Give it a week before calling success. The idle control motor is responsible for idle air bypass of the throttle body so while the gun build up on the throttle might have been causing some restriction to the expected minimum pass through, the idle motor should have been able to compensate. Maybe this indicates you need to clean the idle air motor?

Report back in a week if she’s still purring. I hope so [emoji106]

[Oohsam]

Give it a week before calling success. The idle control motor is responsible for idle air bypass of the throttle body so while the gun build up on the throttle might have been causing some restriction to the expected minimum pass through, the idle motor should have been able to compensate. Maybe this indicates you need to clean the idle air motor?

Report back in a week if she’s still purring. I hope so [emoji106]

Good info ! Will do.