1G Bad idle and almost stall

[Dequavis]

When I start my 1990 Plymouth Laser it will idle normal for about 3-5 minutes. Then the idle speed go's up and down constantly. If you try to drive it with will be running about 500 rpms even if you foot is all the way on the gas. And it will nearly stall. If you turn the car off and back on you can drive it fine for about 1-2 minutes before the same the happens where it nearly stall and runs very low rpms.

With no check engine light on i can't get any codes. I'm not sure what to try at this point. I've got a new ECU in it and that fixes my no start problem.

Any help is appreciated thanks.

 

[Dequavis]

Looks like the Water Temp Gauge Unit. Does your temp gauge ever read anything?

No.

That area looks like it’s been toasted. I would inspect every wire and connector in that part of the harness. Also check for exhaust leak between head & manifold which I think could be the cause.

I got a after market exhaust manifold because the stock one was cracked really bad so your correct thats probably why it melted

Looks like the Water Temp Gauge Unit. Does your temp gauge ever read anything?

No. The gauge doesn't work thanks for bringing that up. Also It's weird when I was letting the car warm up it didn't idle surge its weird because I had it running for about 20 minutes and it normally starts surging at around 5 minutes

 

[steve]

ISC isn't moving in and out its only shaking side to side.

Three things come to mind.

Check the resistance of each coil in the ISC. They should be at least 25 ohms.
Does it still have the '90 style ISC and Throttle body?
Are you sure your new ECU is a '90 version?

 

[waltah]

Bad vacuum hose to EGR is another possibility. If that's shot there'll be a small air leak into the intake manifold AND a bunch of exhaust gas after the engine warms up a little bit & EGR is turned on. Exhaust gas recirculation is only supposed to happen with pretty much throttle; if at idle or part throttle, no.

Stuck EGR valve is another possibility --- and they do stick.

Also echoing TK's observation ... when an area is toasted it can save time to just go through and clean it up right at the start so you don't waste time diagnosing the consequences.

All the cars I've ever owned were formerly someone's junk ...

 

[Dequavis]

Before I replaced anything the car stopped idle surging for whatever reason. I didn't do anything other than taking off the intake hose and putting it back on. However that didn't fix the car from messing up when I drive it it still does the same thing. Also I should mention I tried to continue driving because I was almost in my driveway and it died. So I replaced all the melted wires. The reason they melted was because the aftermarket exhaust manifold got to hot. So I put exhaust manifold wrap on and kinda rerouted the wires a little bit so hopefully they don't melt again. I also installed a new O2 sensor and tried a new CAS. Nothing changed it still messed up. It only happens if the car is in gear and driving
Or if you stop the car after it's already happening. I can't get it to happen if the car is idling on a new start up even if you have the throttle down. I'm still suspicious about the ECU not working correctly even though it's a new refurbished one. Is there a way I can test it?