Gmc tbi

I have a 1990 k15 with TBI.After my son put gas in it and drove about 5 mi.he lost 1/2 power.we got it home went ahead and changed fuel filter and cleaned the tbi.It runs now but still hard to idle.I have checked all sensors and they check good.Everything in ignition is new including ignition module.The only thing i notice is when i disconnect MAP vacum idle is good.Switched out MAP sensor same results.Checked out EGR system everythings fine?Help i need my truck running again.

As you likely already know, a rough idle may be from dirty fuel injectors, faulty plugs and/or wires, perhaps even a faulty IAC valve.

check injecter wires for wear at TBI unit where the air cleaner riser meets body ,ive seen them broke and shorting to ground.

If the truck has a lot of miles, you may want to check and see if the timing chain jumped time.


one other area on that model,is the dist drive gear and shaft wear,dont forget to check the magnets in the reluctor wheel for cracks.makes ECM think it has more cyls than it really does.