I have a 1990 4.9 straight 6. On startup the truck is missing, blowing smoke out the back and when left to idle dies. Once I drive the truck around and things heat up? I truck runs perfectly and at idle purs like at cat. Occasionally also the idle stays real high, do not know the rpm's without a tach, however I would say around 2k to 2500K. It is not throwing any codes. I have replaced the following:

IAC
Plugs
Plug Wires
Air Pump

What could I be missing. I seems like in open loop it runs horrible, however in close loop it purs.

Help!

 

cap and router maybe had to replace that on my 93 with the 300 when it would keep dieing on the first start up of the day

 

I had the same problems with my 85 4.9L and it turned out to be the choke.

 

Did you reset the ECM after replacing the IAC solenoid?

 

IF the IAC plug/harness is unplugged/plugged in (while truck is off) and/or, if you suspect a poor connection (i.e. busted off tab on the harness), when, if ever, would/should you reset the computer.

Can you describe reset procedure?

 

Every single time you replace or clean an item that the ECM uses to learn about something (temp sensor, O2 sensor, TPS, etc.), or that it uses to change something (IAC solenoid), you must reset the ECM when you're finished. The reason why is because the parameters the ECM knows for those items will be for the way they used to be, before you cleaned or replaced them. By applying those same parameters to a new or cleaned item, the value will be different (if it's not, why was it replaced?), and the results will usually be as bad or worse than they were before the item(s) were cleaned/replaced.

The best way to reset the ECM is as follows:

1) Disconnect the negative battery cable
2) Turn the headlight switch on (this makes sure all the capacitors are drained)
3) Wait 10 minutes
4) Turn the switch off
5) Reconnect the battery cable

That resets it.

To properly "re-learn" the ECM, you then:

1) Start the engine and let it idle for 1 minute.
2) Turn it off and wait 10 seconds.
3) Restart it, and turn on loads, like the a/c with the fan on high, the high beam headlights, etc, and let it idle for 1 minute.
4) Turn off the loads, then the engine, and wait for 10 seconds.
5) Repeat steps 1-4, except with the idle at about 2000 rpms.
6) Take it for a test drive of about 10 minutes, driving aggresively.

That's it!

 

my heat tube off the exhaust manifold to the choke broke off about 2 inches off the manifold. i was getting very lousy gas mileage and it really didn't like to idle cuz the choke was all the way on. it took a few minutes of driving before it evened out, but i was blowin smoke between gears due to it loading up. replaced the line and everything is fine. good luck