High idle and hard starting 3.9 D1 Impco

[jskerm]

Hi all, just a few queries about where to start on sorting my gas problem. I have just bought a cheap old disco 1 on Ebay with the intention of wrecking it, but on recieving it decided it wasn't in that bad nick so I am just having a go at getting it right. I am a qualified diesel mechanic, but have never really tinkered with petrol, especially gas, motors so thought I would ask for a few ideas with where to start. I don't want to spend money chasing the problem by replacing parts willy nilly as I still might wreck the car, but if I can diagnose it and get it right I'm happy to throw parts at it if needed.

The car
1994 3.9 V8 auto with 350000 kms. Impco gas system with tanks mounted under the back and a small petrol tank in RHS behind wheel. The gas was fitted in 1996 so is quite old.

The symptoms (on gas)
Takes a while to start (about 4 seconds on the starter) but then slowly comes to life. Runs Ok with a bit of pedal but after 3 seconds or so dies out again. Unless you use the throttle then runs perfectly. After it has warmed up slightly it will idle no worries, and rev no worries. After it has warmed up completely the idle just keeps getting higher and higher until it starts idling at about 3500 rpm. It has plenty of power and runs smoothly when driven (not far as transfer case is stuffed) but just idles way to high.

The symptoms (on petrol)
Starts no worries but idles very rough when cold. After warming up does the same as gas, idles way to high. Also, unlike the gas, whenever on petrol right through the rev range it has a very uniform miss. Sounds like it is running on about 5 cylinders. Doesn't cough or splutter and revs quite freely, just misses.

So there you go. Obviously I know it could be any number of things being such an old car with so many K's, but if it is salvageable I will use it as a runabout. I currently have the transfer case out rebuilding it so cannot road test the vehicle, but I'm sure there are plenty of tests I can do stationary.

Thank's in advance
John

[bee utey]

I'd say the stepper motor (IAC valve) at the back of the manifold is either seized or unplugged (4 wire square plug). Starting/Idling is partly a matter of idle mixture, ie screw with spring hanging out of mixer. Screw in to richen a bit. It's an air bypass. LPG systems for injection should have petrol assisted starting, yours may not be set up that way. Petrol injectors may be seized from lack of use, try pulling plug leads to determine which cylinders aren't firing (pull boots at dissy, short with earthed wire). Injectors are essentially the same as EA-EL Falcon, get some from a wreckers.

[jskerm]

Thank's a lot bee utey. I pulled the stepper motor out and yes, it was seized. Couldn't get it to un-seize without breaking it so I've ordered another. As for the injectors, number 2,4,6,8 and 7 all seem to be U/S so might order a whole new set I think. That would explain the miss!! I thought she was a few pots down but not 5 pots down.....
I gather from the fact that so many injectors are seized that mine must not have the petrol assist at start up. Is this something that can be added or does the lpg kit either have it or not?
Also, probably a stupid question but keeping in mind that I am a petrol engine learner, is it normal for the engine to die as soon as you open the oil filler cap? It seems strange to me that the vacuum is relying on the whole crankcase being sealed? Then again all I know is diesel so as I said, probably a stupid question...
Thank's again.
js3273

[bee utey]

Petrol assist works by a timed relay that operates the injectors for a short time, often during starting. You need to find the relay/s that cut the injectors, trace the wiring harness from from the engine through the firewall, somewhere the injectors will be cut either by 1 relay (12V side) or 2 relays (ECU side) then see what drives the relay/s. If they just switch on with the petrol switch then you add a timing relay to operate the petrol for a short while on cranking. A gas installer should be able to supply a suitable relay.

The prime cause of faulty injectors is running the engine on gas only for extended periods, sometimes due to a failed fuel pump. This is common with steel accessory tanks. Maintenance is always the key. I met a 6 cyl falcon once where I replaced the pump and only one injector still worked. Fitted a set of injectors off a wreck and away it went.

Impco systems create significant amount of vacuum in the air intake, so removing the filler cap allows fresh are to enter the intake via the breather tubes. This causes the mixture to go too lean. Some people shift the breather connection upstream of the Impco mixer but then oily fumes gum up the mixer instead.

[jskerm]

Thank's again bee utey, I've got the stepper motor on the way from UK and I am bidding on a full set of reco injectors for a VN 5 litre V8, I gather these will fit? Do you have any preference on the brand of injector as there are a few options there ranging from about $17 each to $35 each. Also, I gather from some of your previous posts that you don't think the impco system is much chop? What are the main problems with it that you've found? If it's just a power problem I can deal with that but if you think I will have other recurring issues then I'd be interested to hear. The main thing for me if I get this car up and running is to do the work on it once, and then not have to tinker with it to much. Reliability over performance in my book. I will only really use the gas for cruising, and then if I need a bit more oomph i'll run it on petrol. That's the theory anyway. I may have to look at finding a way to carry a bit more juice with me though, undersill fuel tanks maybe??

[bee utey]

Not really sure on exact fitting replacements of injectors, I've only used second hand whatever I could raid from the wreckers. The later "yellow top" 4 hole injectors seem to get good press. Probably not a big deal if you spend most of the time on gas.

Four criticisms of Impco:

Needless complexity with a bulky unit needing a rework of the air cleaner position.

Potential to be damaged by a backfire and jamming the diaphragm completely.

Reservoir of gas held in mixer and converter on shut-off makes petrol assisted starts more complex, as it is generally only needed on the first daily start or after a number of hours standing time.

Major loss of low down torque on gas and petrol (30%), you need more speed to open the mixer diaphragm, also causing loss of vacuum advance timing and poor economy at low speed medium load. Impco works best at higher engine speeds at light throttle.

Other than that they are a great unit.;)