I've been having this problem for a while and it's getting worse so I hope you guys can help. I've searched and haven't found a problem exactly like mine. I don't drive my Montero very much and it started a couple months ago. When I would first crank it, it would idle around 300 rpms and sound like it was missing real bad for a few minutes and then clear up and run perfect. Then it would take longer and longer to clear up and now it stays that way. When running you can smell unburnt fuel. When it's under load it runs good, but as soon as I let off the gas it starts missing again. I have replaced the cap and rotor and cleaned the throttle body which seemed to help but it still runs poorly. Before I started buying parts, I wanted to get suggestions on what to check.

Sounds like fuel delivery. Not sure what controls the 1990 has but it's probably flooding. Have you checked the color of the plugs.

Poor cold starts with good hot running can be a bad ECT sensor. Badly leaking injectors will show up more with less air (light throttle) but get better with more open throttle. Malfunctioning MAP or MAF(dirty) will give computer wrong air volume info. Vacuum leak will be worse at low rpm but better at higher rpm.

Good under load running does not indicate blocked exhaust or fuel restriction.

I haven't checked the plugs yet, but it has been really cold and it starts up fine. It also idles better when the motors cold and as it warms up the idle gets lower. The check engine light it not on either.

If it has an Engine Coolant Temp Sensor (ECT), two wires, then it may not be changing it resistance as the engine warms. In that case the computer will still be in cold start rich mode when warm - and it will idle poorly, but run better at speed.

You can induce a small vacuum leak at idle to see if the idle RPM picks up. This will help to induce more air into a rich mixture and balance A/F ratio.

I have some more information and a question. First, how do you get the spark plugs out and what size socket? I feel like an idiot for asking this, but I tried last night and I couldn't get a socket to fit on them. Second, I was messing around under the hood and I unplugged the MAF sensor and it started running better and the check engine light came on of course. When I plugged it back in the check engine light went off and it started running like crap again. Any suggestions would be greatly appreciated.

The spark plugs are a 13/16 socket not the little 5/8's one. i have to use a short extension on them. And means short on some. The One by the battery always seems to be a pain for me. Put dielectric grease on the boots before you put them on. And I always use a little bit of anti-sieze on the plug threads cause of steel plugs and aluminium heads.

I bought a montero where the PO had a switch to turn off the MAF for some reason. I have since found out why. The Injection Circuit in my ECU was taking a dump. Injectors were on all the time. Now it wont run at all.

Following information should be correct: When you disconnect the MAF, the computer sees that it doesn't have that input and goes into open loop with whatever they programmed into. OPen loop is like when you start the car when its cold. Ignores some inputs till the motor reaches a certain temp. Some also call open loop "limp home" mode. It gets you to the shop, but fuel milage and power sucks.

I think heathdogg means "open loop". He's probably right about computer presets used to run engine.

The MAF is the only way the engine measures air flow, in conjunction with IAT sensor. The element can get dirty but can be cleaned with proper solvent.

Have you tried to pull a code? That would help. You may now see a MAF code since you disconnected.

Would there be a code even if the check engine light is off?

Codes are held in memory unless you clear them.

Edited my post to reflect the open loop