seanmiller2002

If this exact problem was already covered, i couldnt find it. Im kinda new at this. I have a 94' 350 tbi motor in an 86' k5. Its been fine for years untill 5 days ago. After driving the truck for a minute or so, as the engine warms up, power drops off. It starts to surge and miss, noticeably even at highway speeds. My mpg has gone from around 15 to under 10. At a stoplight the engine misses and lopes erratically and the smell of fuel in the exhaust is quite strong (im sure its like that all the time, i just dont notice it at highway speeds). Upon accelerating, it hesitates and seems as though the engine is flooding. The engine runs and idles perfect for the few minutes when it is still cold. Even after it is hot, if you shut it off for even 10 seconds, and restart it, it will idle and accelerate fine for 30 sconds, before it starts missing, hesitating, and sputtering down the road and highway again. And of course...no codes. No check engine light.
What ive done: New fuel pump and sender, new fuel lines from tank to TB, fuel filter. Removed, cleaned and rebuilt TBI unit. New plugs, dist cap & rotor, wires, MSD coil, EGR valve. The timing is dead on. Cat & o2 sensor are only three years old. Oh, and the motor is a GM crate from 2002 with only 65k on it. Short of using a stick of dynamite, im fresh out of ideas. I hope someone out there can help. Thanks in advance.

racerx55

first you have to start to diagnose the problem before just changing random parts. saves alot of time and cash.
if i had to guess i would say the ignition module was heating up and throwing a fit. check your spark and see if its bright blue when its cold,.. and then turning a dull orange when its misfiring. ive had alot of problems burning up ign modules on TBI motors.
also need to look at the fuel spray pattern coming out of the injectors and see if it starts hosing fuel all of the sudden, one could be getting stuck open.

seanmiller2002

Thanx. Yeah, no more new parts till i figure this out. I think the parts place sells a spark tester, i'll go pick one of those up. Dont know if it makes a difference but.... Last night i unplugged my o2 sensor and drove it to work today. It ran exactly the same, but the chk eng light came on....for about 20 seconds then went off, and back on a few minutes later. It cycled on and off sporadically, sometimes being on or off for minutes, sometimes for seconds, all the way during my 30 mile commute. It was on and off maybe 6 or 7 times. Is this normal??? I was under the impression that it would stay lit till you turned off the ignition. Could this be a computer failure problem? Additionally, after work(and plugging the o2 back in and clearing the code) i drove it till warm and it was acting up. I threw it in park, shut off the motor, and immediately restarted it. It sat and idled fine, even in drive, for about a minute before it started missing again, almost as though i had started it cold. It was 96 degrees today btw. No new codes. And since you mentioned ig module, could the hotter spark from the msd coil have burnt it out? Im not sure how the module works, or what its even for. Thanks again

racerx55

you can use a test light to check your spark, just gotta ground it and put it by the cap or wire and the spark will jump to the metal tip of the light and you will not get shocked unless you put your hand within reaching distance of the spark.
an MSD coil shouldnt affect the module. usually a module will heat up and just quit, and take 10 mins to cool down again before it will work. it basically converts the signal from the pickup coil to another singal to trigger the ign coil. as far as i know the CEL should stay lit with an o2 unplugged. Its not very common, but IF you can borrow and ECU from someone before buying one it might be worth trying another one. in these mitsu cars i have we have alot of problems with a certain 2 capacitors leaking and making all kinds of things go nutty with the ECU's before they finally burn up.
Im still wondering about a pintle getting stuck on an injector being that it quits right away when you restart it, then acts up again.... you should be getting an o2 rich code if its dumping that much fuel in though, because it will be out of normal operating range for so long.

suzuki-guy

An ECM For your vehicle is 70 bucks at autozone. But I would borrow someone elses to make sure that is it first.

seanmiller2002

Ok, new turn of events. I have been having weak battery problems, so yesterday i removed my battery and recharged it at work for about 8 hours. I have heard that disconnecting the battery for more than 10 minutes erases all of the fuel mapping that is "learned" by the ECU. Is this correct? At any rate, reinstalled it, and drove 15 miles in 85 degree weather. The engine performed flawlessly. Good power on the highway, no stumbling or hesitating when accelerating. When i came to stoplights, smooth idle on all 8 cylinders, and no fuel smell. I went to a store, and upon restarting the engine and driving the rest of the 15 miles to home, there was a perceptable degradation in performance, till it was running exactly like it has been for the past 2 weeks. Still no CEL.
To make sure my dash gauge wasn't lying to me, i checked with a digital multimeter, and im getting 13.8 volts system voltage. So i know i wasn't simply running off my battery and just getting weak spark.
Sooooo, i feel i can safely eliminate a clogged injecter, stuck egr, sticking valve, or any other mechanical problem. I think i can also eliminate a bad coil, wire, plug, dist, or any other purely electrical issue. And the engine being warm has nothing to do with it. Either the computer is bad, or some sensor is bad causing the computer to mess up. Questions: What sensors will affect the computer so drastically yet not activate the CEL? Is there a way to make the computer stay in its default mode, without "learning" a different fuel mapping? (Short of unplugging my battery every 15 miles or so) Is there any online source, where i can find a detailed description on how the computer compiles its mapping? Such as when certain sensors start coming on line in a warm up or starting cycle, and what the values would or should be? I'd rather not replace my computer and every sensor attached to it, to figure this out.... Thanks again!

broknindarkagain

First off, its called a MIL...not cel.

The light coming on and off isn't abnormal at all. My old 93 Caprice had a MIL that would come on and turn off randomly. It was throwing the EGR valve code.....my EGR valve was fubar, but the MIL wouldn't always indicate it. Your symptoms are acutally pretty simular to the bad EGR valave I had on the Caprice (had a 305 TBI motor)....The entire EGR system was so backed up, after I put the new valve in I had black gunk spray out of my tailpipe and turn the white concrete black.....after that it ran perfect and never tripped the MIL again....

Newer OBDII systems "learn" driving habits and tweak transmission shifts and whatnot according to driving habits, but anything having to do with fuel delivery is done on the fly. Thats not something that is learned over time by the computer. I don't think the older computers have any sort of learning ability but I could be wrong on that.

Disconecting the battery does reset the computer by the way...So you are right with that one.

Carry a paperclip with you, or a piece of wire. Goto this site and read it http://www.extreme-check-engine-ligh...%20Decoder.htm

next time yoru MIL comes on, try and pull the code real quick. Each code repeats its self 3 times. You should pull code 12 no matter what. So your MIL will flash once, pause, and flash two more times.....then you will get a long pause and it will do it again twice. Then it will move on to the next code.....Say the next one is code 32, it will flash 3 times, pause, then flash twice more.....and it will repete that twice. If tehre are more codes after that, it will just move down the list. Once its given you all the codes, it will go back to the first code and start all over. Once you have the code that its throwing out, diagnostics will get A LOT easier. You can buy scanner tools for the older computers (before todays OBDII standard), but they are not cheap....Its better just to do it yourself for free ^_^....and Autozone won't do it, they look at you goofy if you ask them to pull codes on anythign thats older then 96

Get that code pulled and post it. Let me know if you have any questions about it and I'll help you as best as I can

suzuki-guy

Autozone pulled my codes with a paper clip... Just depends on how smart and friendly they are.... anyways I would check for vacuum leaks inspect all hoses! Spray around intake manifold base with ether if it chokes or idles funny you have an intake leak. Also what is your fuel pressure? Did you replace fuel pressure regulator? Another thing that could cause this is Idle air control valve. I am curious how did you replace lines? Did you buy pre bent? I need to replace mine and not sure if I should buy if available or go with braided lines.

therewolf

Possibly a loose,dirty, or defective Crank Position Sensor? I have heard that they're one of the few things which won't throw a code in certain circumstances.

seanmiller2002

Thanx for the tips. I wish for nothing more than to get a MIL light, but i get nothing. It is as if the computer finds absolutely nothing wrong with the way it is running. The only time it threw a code was when i intentionally unplugged my O2 sensor, if nothing else, just to see if the damned computer would even notice. It did, and threw the O2 circuit code (#13), although there was no change in performance. After i cleared it, i have been checking for codes periodically and only get code 12. No MIL, no stored codes. This is the part i dont get. Im not even getting the code that says it running too rich!!!

I liked the idea of the crankshaft position sensor, unfortunately they werent introduced until 1996, my motors a '94. As for the fuel system, i have no idea what the pressure is, but the pump is brand new. I didnt change the pressure regulator in the injector unit, but did replace all of the seals, and checked the fuel spray pattern which resembled the nice, even, cone shape its supposed to have. There were no visible drips. For the lines, i went prebent, and because of the different years involved in my setup had to get a bunch of different parts. As a matter of fact, all of the flexible portions of the fuel system, that used to be the rubber hose with the '86 motor are braided Stainless Steel, that came stock on 454 suburbans. Im sure chevy did it on all the other higher pressure fuel injected trucks. They screw right into the prebent lines like they were designed for it. I would be wary of running with all braided for that much distance for fear it may sag and get snagged on something if i go offroad....

I can also confirm the info about the fuel mapping. My battery went dead again (bad alternator that i replaced today) and had it out for about 30 min to get recharged, and after putting it back in ,the truck missed and sputtered as if i never took it out. Although why it ran good that one time is an even bigger mystery now....