Sunday, 7 October 2012

Ford Fairlane NL Idle Problem

I have a March 1998 Ford Fairlane NL. It has dual fuel (petrol and LPG).

It also seems to have a special version of Ford's 4.0L straight six engine.

I recently had a significant head gasket leak which rendered my car unusable, but leading up to this I had slight coolant consumption and an increasingly annoying idle problem.

Coolant Leak

I had the head gasket done 3-4 years ago. About a year or more after this we noticed that coolant was slowly disappearing. It was just an annoyance so we put up with it. By loosing my radiator cap 1 notch we hardly ever had to add water or coolant.

Idle Problem

Leading up to my second head gasket failure, my Fairlane started to stall at seemingly random times - usually when my wife was driving and usually at a corner or intersection.

Over time, our Fairlane started to stall more and more until my wife would not drive it.

Blown Head Gasket

One day, after a trip to get some wood, my head gasket failed - very dramatic smoke screen.

I was not happy - head gaskets should last longer than this - but also supposed that my idle problem was probably linked to coolant being sucked into engine cylinders.

I decided to replace my head gasket myself this time, but that is a story for another blog.

After replacing my head gasket, it was obvious very quickly that my idle problem had not been fixed - in fact it seemed far worse.


Once my engine was at normal operating temperature it operates normally until I release my throttle. At this point my gear box is driving my engine so nothing happens, but once I begin to slow or brake and revs drop my idle problem begins.

Sometimes my engine revs drop below idle speed (about 750 RPM) and keeps on dropping until it stalls or I tap my throttle - so braking becomes a two foot process.

Recent Work

Due to other faults, I have recently replace these parts:

Fuel filter (Old one was full of rust)
Fuel rail, regulator and fuel injectors.
No.1 Cylinder spark plug - cracked
Distributor (suspect failed PIP sensor)
Ignition coil (cracked)
ECU (computer)
MAP sensor
ISC valve (but I put back original one because I need to modify my new one for LPG)

A few more parts and I will have a new car!


I was fairly confident that none of these recently replaced parts were at fault. I was concerned that it could be my ISC valve, but a replacement one seemed worse.

I accessed my engine's ECU fault codes. It reported 'all ok' (code 111) but also 'engine running lean' (code 172).'

After some driving I noticed that when on LPG and when my engine would begin to stall, my LPG 'lean' light would be on. This seemed to confirm code 172. But when my LPG 'rich' light was on my engine would behave normally and it would not stall while moving or stopped.

I decided to try to test my oxygen sensor. This seems hard but I also came across people suggesting that water/coolant damages oxygen sensors - one person recommended replacing oxygen sensors when a head gasket is replaced - presumably because it is likely that coolant would be pumped into exhaust ports and would naturally come in contact with an oxygen sensor. Another suggested that moisture damages oxygen sensor heaters.


So my theory is that my long term coolant leaks have slowly been damaging my oxygen sensor, causing my engine to increasingly stall on occasions.


I have ordered a new oxygen sensor.

Oxygen sensor has arrived and is installed.


There was some 'change' but nothing dramatic.

I installed my new ISC valve (That isn't what I bought - I paid about $95 AUD) and engine rarely stalls but it hunts - overshoots and undershoots and eventually stabilizes at 700 RPM.

But when idling to a stop, revs slowly drop to about 500 RPM. If revs drop below this something happens and revs pickup and my engine will sit at about 700 RPM.

What is now different is that on LPG my gas controller is indicating rich mixture when idling slow whereas before it was reporting lean.


* new ISC valve might be 'sticking' instead of moving smoothly - later testing casts doubt on this.
* ECU may not be driving ISC correctly - later proven ok.
* I wonder if I need to reset idle stop point? - later I did a small adjustment.
* some vacuum leak I have not found - found a few but nothing significant.
* ISC needs to be modified to LPG - I have not done this yet - now done.

More Results

For LPG, a gas pipe fitting was drilled into my ISC valve. This takes fumes from crank case via a one-way valve and burns them.

What seems odd to me is that it sucks very hard and I can not see how this is a controlled flow of fumes. It is only regulated by hose size (about 6 mm and a weighted 1-way valve that seems by design to leak).

Constricting this fume flow (squeezing hose with pliers) does seem to improve idle behaviour. I also recall inserting two 5 cm bamboo skewers into the hose to restrict flow so I decided to investigate this further.

PCV Valve

As I mentioned, this valve is closed by gravity and a light spring that does not seem to work unless it is open at least 3 mm.

Engine suction at idle is very high and it easily opens my PCV valve causing a lot of air to enter, bypassing ISC valve. Being an old engine, it 'breathes' a bit and this variation in air flow might be enough to cause idle instability.

I looked at my ISC signal using an oscilloscope and I could see it changing width of pulses to increase or decrease air flow - so ECU and ISC seem to be working.

My oscilloscope is a DSO201 worth about $70 AUD - it shows signals and calculates duty cycle, frequency, hi and low-pulse width - very handy.

I decided to re-design this by taking crank case fumes and injecting them in front of my throttle valve (this is when I found a small leak in a rubber bung).

My old ISC didn't like this change, but my new ISC was much happier - but I still had to adjust idle screw by 1/4 turn.

So perhaps my old ISC is not quite right.

Anyway, I have purchased a new PCV valve for $17 as well.

Vacuum Leaks

I found  a large tear in a flexible intake pipe. This could cause sudden loss of vacuum and cause instability - but this is not my problem.

I found a rubber bung had a small tear - this might affect LPG operation a tiny bit, but it shouldn't affect idle on petrol - so this is not my problem.



Sort of.

There is a slight rev increase when coming to a stop. I had adjusted throttle stop 1/2 turn so I have moved it back 1/4 turn to see how that goes.

Engine does not hunt; it does not drop below about 600 RPM; it starts and idles ok (perhaps a little fast).


After more trips, it was clear that all was not well. Idle speed still hunts and occasionally drops very low. My adjustment to throttle stop had also resulted in harsh gear 1 to 2 change.

I purchased another ISC (a second-hand one) and idle behaviour seems normal.

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