Idle Hunting on VN Commo V8

[V8 Middy]

The problem is, when I start the engine, all is fine. It will initially rev around 1200rpm, then gradually settle. Usually, it would settle to 800rpm and stay there. Now it will drop to around 400, almost stall, then catch itself, rev up to about 1000rpm, then drop to 400etc.

The engine is dual fuel and appears to run fine on both petrol and LPG. Pin in the diag plug has returned no codes. I have tried disconnecting power to the ecu, then revving at 3000rpm for 10sec to reset the idle... no joy. I have checked all vacuum hoses but all appear to be intact. I disconnnected the IAC and it still does it.

The problem is much more obvious on gas than on petrol but present on both so I'm ruling out fuel starvation as a cause.

My next guess would be o2 sensor. I'm guessing that under normal conditions, after the initial 5sec or so at 1200rpm, the ecu goes into closed loop mode to get the mixture right for idle and the revs come down. Is this correct?

Any ideas / input greatly appreciated.

[TUFF MAV]

Try cleaning out the throttle body & reset the base idle.

Kyle

[mud80]

Whats the throttle position sensor and coolant temp sensor doing???

i would also start with throttle body clean and base idle adjust/reset. also remember to pull the fuel pump and ecu fuse to power down the ecu. we also used to do a relearn by going through the gears(auto) and then also doing it again with the a/c on.

Hope this helps

[Patroler]

the fact that it settles down after a while would suggest that the temp sensor is ok, id try and disconnect the exhaust 02 sensor(see if it bahaves the same) as im pretty sure when idling it runs off of this - rich, lean, rich, lean etc and when accelerating decelerating it would use the air flow meter!

[Modified Toy]

Tried Recieving fault codes?

[RUFF]

the fact that it settles down after a while would suggest that the temp sensor is ok, id try and disconnect the exhaust 02 sensor(see if it bahaves the same) as im pretty sure when idling it runs off of this - rich, lean, rich, lean etc and when accelerating decelerating it would use the air flow meter!

VNs dont have an Airflow Meter. They have a Map Sensor.

[PGS 4WD]

Is it stock or modded? Certainly clean throttle and reset base idle. How many klms?


Joel

[pigletracing]

having same probs with piglet since OBC. with mine the diognastic report
showed all sorts of faults,
* 1 cranck angle sensor falt :: found the crank angle sensor full of mud &shit giving fauls readings,making run rich/lean ect
* 2 air sensor falt :: air sensor rooted,now replaced.
* 3 ECU was wired to electrics cut off switch, prob being,every time power was cut to ECU, it wiped its memory,& as these things have a progresive memory it had to start from scratch each time (backyard auto elect) now paying real auto elect to re/wire rig.
* 4 map sensor.. in 05 OBC power surging occured as well. prob then was the hose on map sensor had worked loose giving folse readings.. check your hoses from your map sensor ie loose, off, cracked, ect replace your map sensor, there under $10 from wreckers,

good luck with this,,with me all these problems gave me the shits that much i nearly sold the pig

[V8 Middy]

Pin in the diag plug has returned no codes.

[V8 Middy]

The MAP sensor was one of the first things I checked... had hassles with that one before. Then I checked all vacuum pipes as the problem was much worse on gas. My next guess was the O2 sensor. If I disconnect it all together, what will happen to my engine? Will it just run a little rich on the highway or is there more to it than that?

A mate tried revving it up and disconnecting the gas servo so it stayed wide open and let the gas meter only on vacuum and it idled steady so I'm guessing that too would point to O2 sensor. Of course these hassles always happen just before Xmas when no-one has time to look at it!!!

For $60 I might replace the O2 myself and hope it fixes it.

Thanks for the ideas so far... I'll keep tinkering...

[V8 Middy]

Is it stock or modded? Certainly clean throttle and reset base idle. How many klms?


Joel

It got rebuilt around 15000km ago. Slightly bigger cam and custom headers. WHen you say base idle, you mean the timing?

[Micky-Lux]

Is it stock or modded? Certainly clean throttle and reset base idle. How many klms?


Joel

It got rebuilt around 15000km ago. Slightly bigger cam and custom headers. WHen you say base idle, you mean the timing?

The information you just gave might have hit the nail on the head. The Delco system on the VN's (V6 and V8) run a MAP sensor to calculate engine load. That is, more vacuum, less load, and vice versa. Normal vacuum at idle I would expect to be around 19-21 inHg. If you have put a bigger cam in, then the vacuum would be less, depending on the cam it could be as low as 16 inHg. The engine will be idling at roughly the same speed, but with less vacuum, the ECU will give a wider injector pulse, hence more fuel. The O2 sensor will detect that is running rich, and try to correct it, causing the engine speed to increase. Being at idle, the throttle will be closed, so too will the idle switch, which causes injector cut at around 1500 to 2000RPM. Engine speed drops, vacuum drops off again, and the cycle repeats.

Depending on how much bigger the cam is, I'd look at an aftermarket memcal. Chiptorque used to do one, but I don't know if those guys exist anymore.

[skippy's GQ]

i reocmend getting it dyno tuned, mine hunted with the stock cam and the 286 i had in it, a mate of mine dyno tuned it and problem solved now idles at 800rpm wit the 286 in it... i would say your problems are occuring due to the tune on the memcal.....

Skippy

[V8 Middy]

The idle was rock solid until the other day. It started hunting between 600 - 1000 and now will drop to 400 and almost stall.

The engine had a full dyno tune when it was installed so the map on the memcal has been modified to suit

I'll start with the o2 sensor and see how it goes. Can anyone tell me what the EFI will do if I disconnetc the o2 sensor?

[RUFF]

The idle was rock solid until the other day. It started hunting between 600 - 1000 and now will drop to 400 and almost stall.

The engine had a full dyno tune when it was installed so the map on the memcal has been modified to suit

I'll start with the o2 sensor and see how it goes. Can anyone tell me what the EFI will do if I disconnetc the o2 sensor?

Have you cleaned the throttle body yet?

[RUFF]

Can anyone tell me what the EFI will do if I disconnetc the o2 sensor?

Exactly the same thing as if the o2 Sensor was not working.

[Ruggers]

clean you throttle body with carby/throttle body cleaner and a tooth brush, then clean out the iac these can get slow and sticky causeing rough idle then check you base idle, this a very common prolem on commodores

[Patroler]

the fact that it settles down after a while would suggest that the temp sensor is ok, id try and disconnect the exhaust 02 sensor(see if it bahaves the same) as im pretty sure when idling it runs off of this - rich, lean, rich, lean etc and when accelerating decelerating it would use the air flow meter!

VNs dont have an Airflow Meter. They have a Map Sensor.

ok then, but same thing applies - it still measures the amount of air being ingested.

Can anyone tell me what the EFI will do if I disconnetc the o2 sensor?

Same thing ruff said but at worst case scenario nothing bad would happen as they design these thing to have some sort of failsafe - either the ecu could run in a limp home mode or take air flow readings from the MAP sensor (Manifold Absolute Pressure) which merely measures the vac or pressure in the intake manifold - and as the computer knows the flow/pressure relationsip of the manifold it can calculate airflow and thus fuel requirements.

[V8 Middy]

I disconnected the IAC and it still hunted. The revs were higher but problem was still there.

Had a look at some photocopies a mate gave me from the delco efi book and it says its in open loop (ie. o2 sensor has no input) when at idle

That leaves MAP sensor TPS or manifold temp or speed sensor.

I'm guessing speed sensor is ok cos if you roll really slowly down hill while idling, the revs saty at 1000, then they try to settle once you are at a complete stop. Thats what its always done.

I've checked the hose to the MAP sensor but how can I check if its working properly? Last time I had a MAP sensor problem, the car would barely run cos the mixtures were all wrong

COuld the TPS or any temp sensors cause the idle problem?

Thanks again for all the ideas guys

[Patroler]

Yea i had the open closed thing arse about, i guess you could pull the wires from the MAP and then TPS and see what happens, (if you do them one at a time before startup and only idle the car - luckily the problem is at idle and not full load! you shouldn't cause any trouble) i'd assume that if the temp sensor was stuffed it'd remain idleing high and rich - ecu would be saying cold engine (or fully hot then it would have trouble starting on a cold morning - just depends how its wired)
Does the delco book explain any error codes for crook sensors? - i'd assume it'd have to be a sensor or even broken wire considering it just happend - vacuum leak?...
Good luck

[RUFF]

I disconnected the IAC and it still hunted. The revs were higher but problem was still there.

Had a look at some photocopies a mate gave me from the delco efi book and it says its in open loop (ie. o2 sensor has no input) when at idle

That leaves MAP sensor TPS or manifold temp or speed sensor.

I'm guessing speed sensor is ok cos if you roll really slowly down hill while idling, the revs saty at 1000, then they try to settle once you are at a complete stop. Thats what its always done.

I've checked the hose to the MAP sensor but how can I check if its working properly? Last time I had a MAP sensor problem, the car would barely run cos the mixtures were all wrong

COuld the TPS or any temp sensors cause the idle problem?

Thanks again for all the ideas guys

Have you cleaned the throttle body yet?

[V8 Middy]

Have you cleaned the throttle body yet?

I haven't... I thought that by disconnecting the IAC and still getting the problem I had eliminated that as a possible cause.

[RUFF]

Have you cleaned the throttle body yet?

I haven't... I thought that by disconnecting the IAC and still getting the problem I had eliminated that as a possible cause.

Disconnecting something that is faulty isnt going to change anything. If its faulty then its obviously not going to change. If you fitted a good IAC and it still had the problem then that would rule it out.

[Ruggers]

once you have cleaned the throttle body, theres a small bypass for the iac put you hand over this the car should almost stall out if it does the idle is to low if not then it should be alright unless its to high, then when you take you hand off the revs will jump up and it shuold return to idle pretty quick the depending on the iac condition, if not spray crc down that hole and repeat a few times to work the crc in, and retry checking the idle with your finger over the hole, hope this helps

[TUFF MAV]

If it runs crap on gas you can just about eliminate anything to do with EFI. Check your throttle body & base idle.

Kyle

[Patroler]

Have you cleaned the throttle body yet?

I haven't... I thought that by disconnecting the IAC and still getting the problem I had eliminated that as a possible cause.

if you pull the wire off and the engine operation changes markedly that tells you that the sensor/wire is doing something, if you pull a wire and nothing changes that would suggest that the sensor isn't working or is not being used at that point in time! - the delco book would probably tell you at what point the sensor is being used - ie open/closed loop,
delco book should also tell you how to check the sensor operation, 02 sensors usually genarate some voltage in relation to mixture, tps and temperature sensors would have varying resistance depending on throttle pos and temp, i'd imagine if you could get some more from that book it would give you info on resistances and voltages - or maybe just google it, pretty common engines.

[skippy's GQ]

go to REPCO or any parts store and get a gregories EFI DIAG book i think it volume no. 2, its piss easy to read and tells you how to test every thing on the engine management side of things , and cost arond $40, that way theres no hit and miss, and you'll know whats wrong, saves changing sensors for no reason.....


Skippy

[GRPABT1]

Your ecu controls the motor with references from the TPS, O2 sensor, map sensor, Coolant Temp Sensor. Hunting on these things is most often the CTS, if the TPS is buggered it will probably run like a pig all together. Also if the O2 sensor is faulty, depending on how bad it is. The CTS can be tested but you're better off just replacing it. The MAP sensor is the last thing I would replace as it's rarely the problem. Also if you block the IAC off the motor should stall or nearly stall, otherwise it is buggered and you idle is set too high (Butterfly stop adujustment screw on the TB).

These things should all throw a code but I have been had dramas with the TPS before and recieved no code, also once you disconnect the battery the codes are whiped.

Your new cam should make the idle lumpier but not hunt.

One more thing, you don't happen to have a group A twin throttle intake manifold do you? Because they are renowned for poor idle and slight hunting from factory. I know mine does. This can be fixed by drilling a hole into the primary throttle somewhere and connecting it to the IAC via a hose. I researched it once but lost interest and just live with it.

I suggest you visit www.streetholdens.com.au lots of knowledgable guys on that forum.

[PGS 4WD]

If it hunts with the idle motor disconnected its doing so due to fluctuating timing, timing utilizes the crank sensor and MAP for most of the timing control. When I tune these I make sure all the load cells around idle are the same, if you get one that is higher or lower by a couple of degrees the idle will hunt, this is made worse by a modified camshaft. If the TPS is reading too high as it is faulty or because the blade has been wound too far open to get the desired base idle you can get hunting, the TPS output should be around .0.5-0.7 volt over 0.9 v and the ecu wont go into idle mode. You can enlarge the hole in the throttle blade to get more air in at idle and close the throttle down to get a TPS reading in the area I mentioned. You can also put timing into the program at idle that will increase the engine speed and allow the blade to be closed down.

Joel

[sparky!]

The problem is, when I start the engine, all is fine. It will initially rev around 1200rpm, then gradually settle. Usually, it would settle to 800rpm and stay there. Now it will drop to around 400, almost stall, then catch itself, rev up to about 1000rpm, then drop to 400etc.

The engine is dual fuel and appears to run fine on both petrol and LPG. Pin in the diag plug has returned no codes. I have tried disconnecting power to the ecu, then revving at 3000rpm for 10sec to reset the idle... no joy. I have checked all vacuum hoses but all appear to be intact. I disconnnected the IAC and it still does it.

The problem is much more obvious on gas than on petrol but present on both so I'm ruling out fuel starvation as a cause.

My next guess would be o2 sensor. I'm guessing that under normal conditions, after the initial 5sec or so at 1200rpm, the ecu goes into closed loop mode to get the mixture right for idle and the revs come down. Is this correct?

Any ideas / input greatly appreciated.

Are you absolutely certain the thing has no vaccum leaks, there is a line that runs underneath the 'bunch-o-bananas' intake from front to rear, it is pretty common to perish, especially near the distributor, hard to access back there, also the brake booster has been eliminated, either clamp the line to it or pump the brake pedal while running to indicate a change in the idle, it sure seems like a vaccum leak you got happening?