TBI Chevy PU has me stumped!!!

[novaman_68]

I have a 1995 Chevy 1/2T 2wd 305/4L60E and I cannot figure out what is wrong with it. When I take off from a dead stop at 1/4 to 1/2 throttle it tries to stumble and hesitates coughing and sputtering all the way up to highway speed. If I mash it it still sputters, but not as bad. Sometimes when I pull up to a stop sign it will idle down to about 500rpm and die, but it starts right back up. It sometimes throws a code for an oxygen sensor but most of the time there is no code, the check engine light will come on but it will not store any codes. When I try to check the codes it only flashes code 12 which means it is retrieving codes, but nothing after that.
I have replaced the oxygen sensor,EGR valve,throttle position sensor,idle air control motor and rebuilt the throttle body. I have ran fuel system cleaner and heet through it several times and I cannot find any vacuum leaks and somedays the problem is worse than others. Oh yeah, this has been going on for about 6 months so I know it's not just bad gas, I don't want to keep throwing money at it, so any ideas or suggestions would be helpful.

[87sstpi]

i have a 92 1 ton van that i use for work. i had similar probs. crazy thing is i put a new ignition module in it and it fixed it. i always thought once those buggers went bad the car wouldn't run at all. i wish there was a way to check the fuel pressure in these things, couse that could be another suspect. kinda like not enough fuel press the excelorator pump effect wont happen (hence the stumble no power upon aceloration)
i might be way off on this one just thought i'd throw in my .02

[myclone]

How many miles on this rig? FI is very sensitive to a stopped up fuel filter and most ppl neglect to change them so if its got more than ~40k on the fuel filter it needs to be changed.

The MAP sensor on these engines is EXTREMELY important so make sure its providing the correct signal. If its bad/flakey the ECM freaks out and drivability goes in the toilet. The CTS (coolant temprature sensor) is also just as important as the MAP and a flakey temp sensor will cause the same issues. Basically the coolant temp and MAP sensors MUST be operating correctly for the FI system to work as it should on speed density motors.

BTW, speaking of sensors... Youve replace a few as well as the IAC but what brand replacements did you use? Ive got a box full of chain store IACs, MAPs, O2s, ign moduels, etc that were brand new out of the box and they were bad so nothing but AC Delco stuff gets installed in my garage. I learned that a long time ago the hard way.

How many miles on the ignition components? I agree that an ignition module is either bad or good (only ONE time have I seen different) what about the secondary side of the ignition? How old are the plug wires and what brand are they? High mile or el cheapo plugs/wires are not saving you any money in the long run and can cause you to go insane trying to find out why an engine wont run correctly. Same deal with the cap/rotor too. High mile/cheap stuff here is just asking for issues. The secondary (high voltage) side of the ignition gets AC Delco stuff exclusively in my garage (yep, I learned this one the hard way too).

Anyways, speed density motors are pretty simple and reliable as a rock but you have to feed it quality parts as well as feeding the ECM the right signals. If all of that is up to spec then its the fuel delivery side which could be anything from a dirty filter to a fuel pump on its way out (BTW, in tank GM fuel pumps are on borrowed time by about 80k miles...especially if the tank is ran below 1/4 full a lot).

HTH

[novaman_68]

Thanks guys, I have a fuel filter and a MAP sensor in the garage actually. I had thought about that stuff but not the coolant temp. sensor. As for the truck it has 143,000 and the ignition stuff (Auto zone mostly Wells stuff) has about 15,000 miles on it. The fuel press. regulator diaphragm looked good last weekend when I rebuilt the throttle body. I replaced the ign. module about a month ago, by the way

[gen3bu]

. I replaced the ign. module about a month ago, by the way

is it a delco???
is it worse hot or cold?
do you have a scan tool?
--ect readings *f
--block learn 128+/-10
--o2 sensor min/max volts
--map/baro volts

i think we are all getting this hint by now. sounds like a clasic clogged fuel filter and/or bad plug wires. my pump has @ 191k miles on it and still works great - knock on wood. i would replace all the zone parts with delco stuff though.
when the module went out in my truck i could drive @ 10 miles and it was a clean kill on accel/steady cruise.

the fuel psi can be checked with the filter removed. pep boys sells a fuel psi gauge kit that comes with an adapter for gm tbi vehicles, and otc has an adapter that replaces the fule filter with a port in the side of it. both run @ $40-50.

[makoshark]

I am willing to bet it`s your ignition module in the distributor. I had that exact same problem on my old 93 truck and my friend and my father all had the same as well. The 2 wires that go into the module are bent at a very tight rate going into the module. The heat built up inside the dist. will eventually cause the insulation to crack open after a period time. Which in turn causes them to short out. You`ll have to remove the dist. and take it apart to replace the module, but it`s only like 7 bucks or so in parts and about 30 minutes of your time. That should take care of your problem Put a new fuel filter and tune up the motor with some quality parts and you`ll be good to go.

[nkopper]

My '88 Chevy pickup did the exact same thing at 185,000 miles, only it through a code for lean exhaust. I traced the problem back to a weak fuel pump. The oxygen sensor code could also indicate a lean or rich exhaust instead of bad 02 sensor. Does your truck crank for a while when cold but not when hot? That is another sign of a weak fuel pump. I probably gained 30-40 hp (butt dyno) with the new pump. If you do replace the pump, use an AC Delco ONLY, the others are crap and will not last but a few months.

Nathan

[gen3bu]

that sound more like the pick-up coil that you are describing.

[novaman_68]

And the winner is nkopper with a weak fuel pump. I tested my fuel pressure and came up with 7psi, changed the pump last night and presto!! it runs better than it ever has. I've had it for 4 years and put 40,000 miles on it and it's never ran this good. By the way what kind of mileage should I expect from this thing? 305/4L60E 1/2ton shortbed reg. cab The best mileage it's ever got was 14 on the interstate with the AC on. That was 3 years ago, I checked it a few weeks ago with around town and some highway it was 12.7mpg. Is this right? I've heard of 16-18mpg out of these trucks.

[myclone]

And the winner is nkopper with a weak fuel pump.

Hey, I shoulda won!

to a fuel pump on its way out (BTW, in tank GM fuel pumps are on borrowed time by about 80k miles...especially if the tank is ran below 1/4 full a lot).

Im just kidding BTW.

I tested my fuel pressure and came up with 7psi, changed the pump last night and presto!! it runs better than it ever has. I've had it for 4 years and put 40,000 miles on it and it's never ran this good. By the way what kind of mileage should I expect from this thing? 305/4L60E 1/2ton shortbed reg. cab The best mileage it's ever got was 14 on the interstate with the AC on. That was 3 years ago, I checked it a few weeks ago with around town and some highway it was 12.7mpg. Is this right? I've heard of 16-18mpg out of these trucks.

305s are pretty under powered so in the full size trucks you really have to stand on them hard to get things moving so if you dont drive like a pepaw the 305 can be a little thirsty just from trying to keep up with traffic.

That being said you should be getting a little better than what youre seeing IMO. You said you put an O2 sensor in it but you never did say what brand. Chain store O2s (along with any other sensor they sell) can operate well enough not to throw codes but may just barely be inside the acceptable operating parameters which can hurt milage. If the O2 has lazy cross counts then milage can suffer so if you havent done it get a AC Delco O2 sensor in there. Cheap ignition components, particularly plugs/plug wires/cap/rotor, can produce random misfires which can affect milage but not as much as a lazy O2.

Now with that being said if you have both crappy chain store ign components as well as their equally crappy O2 then your milage goes right down the crapper even though you dont see any "check engine" light.

HTH

[gen3bu]

my 355 sonoma gets @ 16-20 on the highway and 12-14 around town. just depends on my right foots mood, lol.