Outer Limits 4x4 Board

I have been running a g16 16v mpfi from a 95 model swb vit in my sierra now for a few months and it has been running fine up until the last couple of days. The idle seems to be somewhat erratic (mostly high). This morning i conducted a few tests as follows. Idle was high from the get go, 5 mins later i pulled over and idle was at around 1600 with clutch out and with clutch in it was continually surging between 1600 and 1900. I continued driving and noticed that between up shifts and throttled up down shifts the rpm would hold or even increase for half a second before dropping. 25 mins down the road and idle was sitting firmly on 1500 regardless of clutch whilst coasting and would drop to 1200 approximately 20 seconds after vehicle came to a stop. I tried coasting and stopping several times and got the same result. So i read a thread on here about 16v idle tech and found this pic:


So i went to find plug 40 (IAC??) on my engine and it doesn't exist. The actual wiring and plug are there but there is nowhere for it to attach. It doesn't look to have ever been connected and is simply taped back onto itself. Was there an upgrade that made this plug redundant at some time? If so does anyone have any other ideas. And to completely throw a spanner in the works, when I restarted the car (after sitting for around 25 mins) it is idling beautifully, around 1000 when coasting and drops to 650 a couple of seconds after coming to rest. Got me flummoxed! Might only be when starting from cold so I'll try again in the morning and see if it still plays up. I'm planning on installing a new radiator and hoses next week so I'll give it a good flush and see if it helps.

Cheers

Bronson.

edit: All vac line seem to be ok.

is it using lots of fuel.
maybe your coolant temp sensor needs replacing.
ecu thinks it on cold start all the time causing hi idle and rich mixture.
might also be corrosion on the plug

The IAC is the valve that controls the amount of air bypassing the throttle plate - therefore it controls your idle speed. Post up some pics of your throttle body. A vacuum leak would cause the problem you have described, but from the sounds of it you have already checked that. Check that you throttle is not sticking and there is no gunk in the throttle body that may be causing an issue.
The coolant temp sensor may be gone, but usually when they die and read something stupid like -46 degrees you will have a black cloud following your zook permemantly. It will also struggle to start.


Layto....

Thanks muchly for the ideas guys. Will look into these next week when I get my days off. Seems to have returned to normal for the minute but i'll pull a few bits off and have a bit of a clean up all the same. Appreciate your help.

Bronson

one other thing it could be is if you are running a vit air flow sensor, check to make sure that the big 3 inch hose clamps are not done up to tight. if they are to tight it can jam the sensor and not let it pop all they way back out on idle. This was the problem on one of my mates G16b, took as days to work it out.

zukesloveformud wrote:one other thing it could be is if you are running a vit air flow sensor, check to make sure that the big 3 inch hose clamps are not done up to tight. if they are to tight it can jam the sensor and not let it pop all they way back out on idle. This was the problem on one of my mates G16b, took as days to work it out.

Ta matey, I'll check this out also. Am presently running one of those wanky pod style filters cos I was too lazy to fit up the airbox at the time of conversion and I'm sure it doesn't help the idle! Will rectify this soon tho!

a little off topic, but kinda ties in - how are you getting RPM readings? Im running 16valve also and want to get my tacho back =)

alien wrote:a little off topic, but kinda ties in - how are you getting RPM readings? Im running 16valve also and want to get my tacho back =)

Just splice the tach wire from the vit loom to the sierra one. Think mine is actually soldered onto the circuit board of the instrument cluster. Had a mate who is an auto sparky do it for me. Should be simpler to just join the wires together tho, check the wiring diagrams for each engine to find the tach signal. I can't remember it off the top of my head!

Ok guys, i have flushed the cooling system and i'm still having idle problems. Last night after flushing and refilling i conducted a couple more tests. From cold start idle sat at around 1100 rpm. Removing the temp sensor for the ecu made no difference but I expected that being cold. Over the next 5 mins as the temp rose the idle slowly climed to about 1700. Removing the temp sensor plug now increased the idle by a couple hundred rmp so i'm guessing that bit works fine. Plugging it back in dropped rpm back to 1700. Then after a minute or so the idle dropped to about 1000. It was happy enough to sit there. However, removing the temp plug shot the revs back up to around 15-1600 and when i put the plug back on it took just over a minute to return to 1000. I repeated this a couple times with the same result. So now I'm wondering about this IAC thing. I've got a few pics of my throttle body and manifold to have a squiz at. First a comparison between a pic i found on here and the same part on mine:



You can see the white plug which I assume goes to IAC(?) is redundant on mine. There is just nowhere for it to go.

Another pic here with another plug that I'm not sure about:

There's a blue plug in the above pic, is this my IAC? If not, what is it and do I even have an IAC? Having not played with too many injected cars i'm not even sure I'm making sense here. Next pics just show the complete throttle body and manifold.




As for the 2 plugs on front of manifold in above photos one has something to do with the charcoal canister (blue one on left) and the brown one I'm not too sure about. Sorry bout the long post and huge pics but any advice would be hugely appreciated. I'd rather try sort this myself than resort to a mechanic if possible. Thanks in advance.

Bronson

Bron5on wrote:

As for the 2 plugs on front of manifold in above photos one has something to do with the charcoal canister (blue one on left) and the brown one I'm not too sure about. Sorry bout the long post and huge pics but any advice would be hugely appreciated. I'd rather try sort this myself than resort to a mechanic if possible. Thanks in advance.

Bronson

Oops, I lied, brown plug on right which is actually dark grey is tied in with the vaccuum line coming from charcoal canister. The blue plug on the left controls a vaccuum line running between manifold and a diafram on back of manifold next to what I assume is the tps. Temp plug I was referring to is a green one plugged into thermostat housing. Also not sure what to make of this but I just tried the same tests as last night after i swapped my radiator this morning. Got the same results but this time I noticed that the buildup of revs during the warm up (up to about 1700 rpm) drops at the same time the thermostat opens. Only noticed this as I had the cap off and the water level dropped slightly when the revs backed off. Still undecided if it's the temp sensor or the iac not responding properly to temp signals. Or something!