ae86 4age lacking power and bubbling/boiling noise

[extra_anchovy]

I have a AE86 with a 4age engine from an AE82. Most parts are from the original AE86 4age engine.

It was running ok before except for a strange behavior when it would idle at around 1000rpm on start, but when driving and stopping at the lights the revs increase to 1500-1600rpm. if I tap the accelerator it goes back down to 1000-1200rpm. It has done this for as far back as I can remember. the temp sensor is always at fully cold sometimes it comes to life a raises a few bars but very very rarely.

anyway I took it to a mechanic because I replaced the thermostat but it was leaking due to not being able to find the right gasket. I told him to fix that and also the accelerator pedal was getting stuck so I told him to fix that too.

When I got the car back it was revving really loud so when I got home I checked the error codes, there was none. So I turned the idle screw till it idled at 800rpm, which was all the way as far as it would go.

Then the car ran good (except the revving thing) for a while but now it has hardly any power and is really rough. It still does the weird revving behavior and I also hear a bubbling/boiling noise after I turn off the engine. It was making the boiling noise before which is what prompted me to replace the thermostat.

How can I fix these issues and make it run good for once? I've been spending thousands over the years I had this car constantly battling to get it running good, even completely swapping the engine and many other parts.

[dove grey 64]

When the bubbling noise occurs does the motor feel really hot? Is the bubbling coming from the overflow bottle? Sounds like your temp gauge is a dud.
If the engine overheats you'll experience a big power loss. You might have a blown head gasket, what does your fluids look like? Any mixing of water and oil?

[sundee]

Do a comp test..
Obviously the water/coolant is rising to over 100'c/110'c
Condition/colour of water/coolant? Oil in water?
Moisture/coolant under oil cap?
It might be either air in the system or Blown HG.

[Jimmee1990]

I would check to see if the coolant has been bled properly first, as 4ages tend not to idle very well if the coolant is flowing properly through the throttle body. The boiling bubbling noise is also common for cars where the coolant hasn't been properly bled. And let it run and check what temperate the coolant is actually getting to.

[Matt]

Most likely air in the system. Take the radiator cap off and run for a while

[SnailCale]

All of above is great ideas..
One thing ill add
Is instead of just taking cap off
Cut the base off or a juice/milk bottle and put the screw cap part into radiator.
Fill that with water(maybe even wrap electrical tape around screw top to make a good seal..

When the cars running in driveway it will keep it topped up and make air bubbles very visible

[SnailCale]

I use this at work, they work really well..
A homemade setup with juice/milk bottle will do the trick

[Matt]

Worth noting you have to have your heater on, if connected.

[Hen may possibly be a nut]

All good ideas, but what about take it back to the mechanic and say, "there have been a bunch of new problems since you looked at my car, can you please diagnose/fix them?"

[dove grey 64]

i agree with you hen but there are times when mechanics will take you for a ride if you ask them to fix fairly basic things.
and then they drop the "well its a 30 year old car thats been modified by god knows who and etc etc".
i'd say if you paid him for a service that wasnt met then chase it up, if he gave you back your car and could not solve your issues then get a new mechanic.