Help, 93 carby van idle surging

[rebuilder86]

hey yall
I have left my 4k machine in perfect running order back overseas, and come back to oz to land a new job in renovations so i quickly went out  bought the cheapest van i could get that seems to run.
I ended up getting a 1993 mitsubishi express also known as L300, which has a 4G63 2L SOHC carby for 1300 bucks.

Got it and it was ok, but noticed black spark plugs on my first service, so had a good look at the carby and removed it n found the secondary throat diaphragm was stuff, so i got a new one for 90 bucks n stuck it back on and the power at high rpm was about 1.5 times better.
Anyway, after doing that, which involved removing the carby, i now have this problem, where after the van has been runing for a while, like 5-10 minutes and the engine is hot, and i have been accelerating and driving normally, as son as i come to a stop the idle is surging between about 700 - 1200 at a guess (i have no tacho) in cylces of one second. i.e. in one second it has gone between 700 and 1200 rpm.
If i shut the engine off and start er up again, it is immediately resolved, but after some more hard driving, it is back again so  end up turning the engine of at the lights to reset the engine to a god 700 rpm idle.
Its quite dangerous as it really affects engine braking.

Some points and history..
I bought the van last weekend.
It wasn't surging until i removed the carby and reinstalled it with new secondary diaphragm
After i noticed this surging, I immediately assumed i had introduced a vacuum leak where the carby joins on to the manifold because the gasket was stuffed, so i made a new one, but the problem remains, i am adamant there is no vacuum leak there.
In order to remove carby, i had to pull out of it, what i assume is the idle solenoid, has 2 wires coming out of it, and i don't think its functioning properly, i think its stuck open, as the damn thing diesels 90 percent of the time i try to turn it off, and i have to put it in gear and clutch out to stop the engine haha... (could be related to the issue perhaps?)
When i had the carby out, i cleaned out the visible muck in the throat with supershit 2 dollar degreaser, and also sprayed a bit around the passages and idle circuit to give it a clean, I saw a good steady flow of liquid exiting the idle solenoid hole.

How can turning the engine off stop this idle surging, i can't get my head around it.
Also, why is there an O2 sensor in my carby engine, what does it control?
is there some sort of fuel computer which is being fed by O2 sensor info, and is being reset every time i turn the engine off??

Confuzzeld and need assistance from u guys. CHeers

 

[rebuilder86]

also, forgot, and htis could be the killer, there is a crack in the exhaust manifold, just where 2 sets of 2 cylinders come to one set of 4. This is upstream or (before) the O2 sensor

[SloRolla]

fuel solenoid sounds like the issue, had the same problem. ended up being the fuse blowing resulting in no power to the solenoid. Have you gotten a volt meter to it?

[Big G]

If it has a blown fuse it will never work.  Not work intermittently.

[ke70dave]

How does it have a carby and an 02 sensor?

[SloRolla]

4 hours ago, Big G said:

If it has a blown fuse it will never work.  Not work intermittently.

That is true, my bad.

[rebuilder86]

Dave, its the end of carburetors, the last generation i believe. its called a feedback carburetor.
It uses a piss-ass half ECM computer thing and takes oxygen readings from the exhaust and adjusts the primary jet in the carb accordingly using an electronic valve thingo.

I'm not sure but i believe it may just d this by pulsing this solenoid type thing rather than actually setting an opening size. Doing so really quickly opening and shutting to sort of simulate a restriction, or smaller jet size.

I'm becoming convinced that its this cracked header/ manifold.

[ke70dave]

wowzer, never knew something like that existed.

I assume it only uses the o2 sensor on cruise though  and adjusts the carby under steady state cruise conditions.

[rebuilder86]

Well, these symptoms indicate that it has an effect at idle also, unless perhaps there is another issue and it doesnt know that its at idle....
All it has is an O2 sensor, there is no throttle position sensor, or MAF or MAP or anything like that. perhaps it looks at RPM thorough a crankshaft position sensor or something.
I'll see if i can see any sensors on the front of the engine when i putty up this manifold crack.....

 

[Taz_Rx]

If you think that is what is causing the idle hunt then see if you can unplug the o2 sensor and take it for a drive. 

[rebuilder86]

thats not a bad idea
i ended up puttying the exhaust crack, and thay worked for abut 20 minutes.
then the putty lew threw and it started high idle surging again :(
can't get a new manifold so I'm going to try to braze it up with silver/nickel fluxed brazing rod and my little MAPP gas torch.
also tried just grounding the fuel mixture solenoid thing so that it was just permanently open... that worked without the surging but the idle was too high, with base idle screw all the way out and butterfly fully closed, and the mixture screw completely tight against the seat, which means this solenoid fully open must be controlling something else other than the fuel through the idle and primary circuit. it must be controlling bleed air in the idle circuit. 
With the idle mixture screw completely up against the seat, it shouldnt idle at all. But its idling high with this "constant open" solenoid so fuel must be getting in through some other system i know nothing about, like an air bleed hole or something. 
That means nothing will solve the severe dieseling issue, it runs for about 30 seconds after turning it off...
 

[altezzaclub]

Can you make the fuel mixture thingy permantly closed??

I don't know if your gas torch will be big enough, but the manifolds can be brazed.  That would be the best solution. I suppose you get the motor as hot as possible then turn it off and quickly braze it.

[rebuilder86]

well, here's the thing, the working temp on the rods is listed as 600 degrees, and the mapp gas torch sais it burns at 1900 degrees.
U recon ill need oxy purely coz of the size of the manifold cast iron?

As for the mixture thing, yes, i can, but then it doesn't idle at all. I'm totally confused by that, i swear it is a fuel solenoid as when i sprayed degreaser up the idle screw port, it came straight out that idle solenoid thread where the solenoid goes., but according to all internet research, they work by letting in bleed air.
Perhaps its letting both air and fuel into the idle circuit, but that wouldn't explain why turning the car off (which shuts the solenoid) doesn't stop it from dieseling.

To make it all the more complicated, if that isn't complicated enough, after bypassing the fuel computer and making the solenoid permanently open when ignition is on, the idle speed is still creeping up to about 1000 rpm after hard driving, altho without the surging, and when turning the engine off and letting it come to a complete stop after its dieseling fit, start it back up again and the idle is settled nicely at 600-700 rpm. just without the surging.

[rebuilder86]

I'm now suspicious it must be the enrichment circuit sticking open,
In corollas, this would be known as the power piston circuit. Works slightly different as it uses a diaphragm on the side of the carb rather than a piston in the cover of the carb.
Possibly spraying degreaser through the carb has dried out the rubber diaphragm and cracked it.
oops