Alright, I'm stumped

[HandySmurf]

MY '90 3.0 4WD Mighty Max has been throwing a Check Engine light for about 6 months now, and when it first came on I had a friend with an OBD1 scanner read the truck and it said it was throwing a misfire on cylinder #1. I changed the plugs and wires as a start, which they heeded to be changed anyway, and we reread it. Still throwing the misfire code. I tried changing the distributor with a rebiult unit along with a new coil. Still got the code. I changed the MAF off the parts truck out back, put a new oxygen sensor in it, changed the fuel filter, I ran SeaFoam through it twice, and changed the plugs again, and it is still telling me it is misfiring, and now the idle quality is complete garbage. I am starting to think either the previously rebuilt ECU is going bad again, or the wiring harness itself is going bad, because I am seriously starting to run out of things that have not already been changed in pursuit of this darned code. Another clue giving me that thought is the radio reception has always had static in it through 3 differnet radios and 2 antennas, so I'm thinking the harness might be shorting out somewhere ? Idunno. Anybody else got any ideas or suggestion on what else to look for. I'm starting to lose sanity on this truck. Help ?

Things replaced in past 6 months while light is on :
cap
rotor
wires
plugs
distributor
coil assembly
rebuilt computer
fuel filter
air filter
coolant sensor
temp sensor
o2 sensor
MAF sensor


The truck is laughing at me now, I can hear it out in the driveway snickering

[pennyman1]

vacuum hoses for one, intake gaskets and throttle body gaskets, tighten down all intake and exhaust manifold bolts, check EGR valve and passages for carbon buildup. To find a vaccum leak use either propane, gumout, or ether and spray around the intake and throttle body - if the rpms change at idle - there's a vacuum leak

[LSR Mike]

Fuel injectors?

[crvtec90]

get yourself a spark tester....or even one of the old plugs grounded to the block.....pull off the plug wire at the spark plug and put the tester in the wire and run the engine while lookin for spark on the #1 wire. If it has good spark then check compression on #1 cylinder and make sure its the same as the rest....or at least pretty close to the others. If you have a good set of thick gloves try to pull the plug wires off the plugs or even the cap. Do them one at a time with the engine running and listen for the engine to start to run badly. If you pull a wire off and the sound of the motor does not change then that cylinder has a problem. Dont shock the crap out of yourself while doing this....use some long needle nose pliers with rubber handles or even some electricians pliers.

...how hard is it to get to the injectors? If you can pull the #1 injector clip and use a "noid light" ( it is a low amp device as to not draw to much current through the injector driver in the ecu) and crank or run the engine and make sure it blinks. I got a cheap set of noid lights from harbor frieght. If the light doesnt blink then the injctor is not being driven by the ecu or there is a break in the wire somwhere. If the noid light is blinking try to swap the #1 injector with # 3 and see if the missfire code switches to #3 cylinder....that will tell you if the injector itself is bad.

[Payrion]

when i first got my max i had the same problem...it was low vacuum due to stopped up TB...take the tb off and clean the little passages...might need a small ass wire to do it and soak the hell outta it....might not be your problem but i was at witts end like you are and bingo....it worked and ran great!