1991 Mighty Max rough idle / almost no power when cold

[Paul-1991]

I've got a 1991 4cyl Mighty Max that takes 2 or 3 tries at half throttle to start it. The idle will then go back and forth between a very low rpm to a high rpm until it warms up. When driven cold, it has very poor acceleration and around half throttle gives the most acceleration.

First I changed the plugs, plug wires, and distributor cap. Those were all bad and it helped smooth the engine when warm. I then cleaned the MAF sensor. No change. I sprayed carb cleaner anywhere I thought a vacuum leak could occur but didn't find anything. I then changed the PCV valve and vacuum hose + breather vacuum hose. While changing the PCV valve, I sprayed a half a bottle of carb cleaner down the vacuum hose to clean the oxygen sensor (slowly enough to not burn the cat and changed the oil after immediately). I then cleaned the egr valve, but didn't check it for a vacuum leak yet. I just picked up some 5/32" vacuum hose which I think is the size I need to replace the small vacuum lines to the egr valve. Then I'll try disconnecting it and manually compressing the egr valve while plugging the end with my finger to check for vacuum...I think that should work. Cleaning the egr valve didn't help.

Other info to be aware of. I had to use a bottle of head gasket leak stop a couple years ago. A little after that I changed the radiator with a slightly larger one when the original cracked and developed a leak. The timing belt is old, but it isn't backfiring, so I don't think it has slipped. The thermostat is stuck open, and I haven't changed it yet, so it runs cool only reaching normal temperature if I let it idle a long time. I have some Redline Injector Cleaner in the fuel to clean the injectors. The ECU was changed once at about 100k miles a _long_ time ago. It's got 207k miles now. Air filter doesn't have many miles at all and is clean.

What am I missing? Could it be loosing fuel pressure with a very slow leak causing it to be hard to start? Some other potentially bad part?


Thanks

[B-Line]

Fuel pump or filter can be your weak start and idle. Also check your IAC. If it’s the brown cap one toss it and get a newer black one. Those are known to fail internally, cause weird startup idle issues and fry your ECU.

[geezer101]

+1 on cleaning the IAC. It is one cause of idle issues when coked up with gunk. Do you get any weird intake noises like it's trying to imitate a jet engine? It's usually a hint that the IAC isn't functioning properly.

[Paul-1991]

I haven't heard any weird intake noises. I need a little more help.

I bought a fuel filter to change that. I unplugged a plug above the fuel tank and then started the engine to depressurize the fuel system. Then I tried to start it a couple more times to be sure. I then loosened the bolt on the inlet side of the filter and let fuel slowly run into a pan around the loose bolt. At first I thought it was fuel coming from the lines and filter, but it didn't stop, so it must be siphoning from the tank which is full. I'm not sure what I'm doing wrong.

Regarding the IAC, the connector is rigid and brittle, so I don't think I can take it off without breaking it. If I have to remove it, I'll need to replace the connector unless someone can tell me how to remove it without breaking it. I tried removing the 4 screws for the plastic part to clean it, but I found white threaded plastic with a tiny bit of white grease on it, so nothing to clean. I searched tonight to see a picture of the part, and it looks like I took off only part of the IAC. https://www.stockwiseauto.com/standa...l=Mighty%20Max This shows a metal piece below the plastic part I removed. I don't see it in stock anywhere if I need a new one. It does have a brown cap.

I also found something I wonder about. There is a male and female connector hanging loose next to each other directly off of the main wiring harness on the passenger side above the wheel well that look like they haven't been connected together in a long time, if ever. I'd upload a picture, but it gave me an error.