hey, i hope someone can help me. i'm kinda stumped on this one, i have some ideas but maybe someone has come across this problem before. i have a 89' chevy k1500 with the TBI 350. this is what's happening, when i start the truck in the morning its runs fine, but as it heats up a misfire appears. if the temp stays under 155 its runs ok, there is a little bit of a misfire but not too bad. but once it heats up the misfire gets worse. i have changed the plugs with ac delco ones, the wires, cap&rotor and the coil. i don't think its ignition because under acceleration its fine and has power and i don't think its compression as someone told me because it runs good all the other times except idle. also i have been told that when i accelerate some black/blue smoke comes out of my exhaust but its not oil because i don't burn any, so i have a feeling that its running really rich a idle. i'm thinking it could be the coolant temp switch making the computer think its colder then it really is and adding too much fuel, or the electronic spark control module because it can get affected by heat. the ignition module is good and so is the pick up coil. well hope someone can help me. thanks

Sounds like the injection has fallen lean, could be a sensor issue, computer, drop in fuel pressure from a plugged filter, failing pump,  failing regulator or could be dirty injectors.

When you're in cold start mode which is usually 160 F or less, the computer richens the mixture like a choke. When you get on the throttle the computer again richens the mixture as would a carburetor's power valve. At these time the extra fuel is masking the problem of a lean at operating temp cruise mixture.

Bogie

alright thanks for the response, i'll try to get the fuel filter replaced, because it looks like it has not been done in a while, its really rusty. i don't think its the injectors cbecause i just had them cleaned. but it still might be a problem. and i guess i'll have the fuel pressure tested to see how the pump is doing. i'll let you know how everything is going when i get it looked at.

If it is only when it is warmed up, i am about 99.9% positive it is the IAC valve. My father-in-law's 89 had the same problem about 3 months ago where it would run fine cold and once it got warmed up it ran like *** That should be the problem. If not, you can always return the part.

Josh

If you have a bunch of miles it may be the timing chain is getting really slacked. I have a 94 CK 1500 and that was the problem with rough idling and missing. You say that you are not using oil. Have you done a compression check? How long has it been since you had the TBI injectors cleaned? You may be throwing a garden hose spray pattern into the manifold. Is the intake manifold gasket intact? There are alot of litte things that can screw up your mixture. Good luck.  

it only does it warm, between 100 and 160 derees it misses but not a lot just a little, but when it gets wramer it misses more, i checked the rpm at idle its at about 670 and it should be at about 700 so its right were it should be.

i

only have 131000 miles i don't call that a bunch, and i don't think its the timing chain because it should affect other aspects of driving but it runs great cruising and on acceleration. i just had the injectors cleaned and checked the pattern with a timing light it was dead on and also with the timing chain the timing is dead on, wouldn't it change if it was stretched out?, and the intake looks great, i'll spray carb clean and see if it sucks it in. that right there should tell me if its got a leak. if any one else had any ideas let me know. i had the fuel filter replaced and that didn't help much, i have not done a compression test yet i will sometime. but i don't think its compression its got to be something electrical because it runs perfect cold/warm but once it goes past 160 it idles bad. thanks for the responses i just got a chevy fuel injection book, i see that it has something's about missing at idle and hard start when cold, yeah i forgot to tell you its doing that too.

Goto chevytalk.com and look in your vehicles model and year and you'll have a definite answer before the end of the day.  

When I put a TBI spacer on my 94' Suburban, I accidently left the vacuum hose connection off to the MAP sensor.  It made it run really rich at idle.  My hose is actually a hard piece of plastic pipe with two flexible ends.  Maybe you have a cracked pipe creating a vacuum leak, which can cause an unsteady idle due to a combination of the vacuum leak and fooling the MAP into thinking it needs to enrichen the fuel mixture.  This wouldn't explain the temperature variable, but it is something to look at.  I know from first hand experience that troubleshooting these types of problems can really test your patience.  Best of luck, Ed.  www.edgesz28.com

i don't think thats it becuase i already checked that, what i did findi s that the coolant temp is reading wrong, it should be about 3400 ohms at 70 degrees, today it was 80 outside so it should have been at about 2800-3400 but it was reading 1800 so it was telling the ecu that the the motor was at 100 degress but it was really at 80, i'm thinking that that would cause my hard start problem and also might cuase the missing becuase i measured the risistance whee that engine was at 150-160 and the resistance was at 180 ohms and that resistance it at 210 so i think the computer is thinking the motor is hotter then what it really is and is making it run lean. i don't know but i'm going to change it anyway and see what happens, its only 10 bucks and i got nothing to loose.

well i just changed the coolant temp sensor it didn't help the hard start problem, i have not gotten a chance to drive it much but i think it didn't help the misfire also. this is starting to piss me off, does anyone have any clue what the hell is wrong with this truck??