Sick of hunting down this issue on my own - Please Help! :)

[bigblok77]

possibly a breaking down coil, some can even be intermittent. also check all of these other roblems listed with the dizzy, as they could all cause an ignition stutter as it seems you have.

[Chevyman350]

You could also check the voltage at the fuel pump if the wireing is questonable you could be starving the pumps for juice at idle?

[Evil Trailblazer SS]

There is no problem at idle...the truck runs fine at idle. There is plenty of fuel at idle. The problem is not idle related. It is when you give it any slight throttle.
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[1LowToy]

Check for vacuum at the EGR. You may be getting too much vacuum at the very slight increase in RPM thus giving you a lean condition. Easy check and costs nothing. But with no signal from the computer that could be a real problem

[Mr. 250r]

You have the 4l80e i notice as do, i so im just going to say the truck is under some computer management as i had the same problem my aldl or whatever connector would not work and i could not even get a code off the truck using the cig lighter and terminal style connections, my problem actually ended up being i seemed to of fried my computer probably because i have no dash and mud and water got in it possibly or it just went bad, but mine had a high idle and would always be in 3rd no matter what i tried and it acted like it was dying as i started it like it was getting no fuel or something, 1 free computer later good to go, a new one is... 95$ish

[Evil Trailblazer SS]

So once I check the vacuum, what am I looking for and what is the fix if it has too much vacuum?

[Evil Trailblazer SS]

Originally Posted by Mr. 250r

You have the 4l80e i notice as do, i so im just going to say the truck is under some computer management as i had the same problem my aldl or whatever connector would not work and i could not even get a code off the truck using the cig lighter and terminal style connections, my problem actually ended up being i seemed to of fried my computer probably because i have no dash and mud and water got in it possibly or it just went bad, but mine had a high idle and would always be in 3rd no matter what i tried and it acted like it was dying as i started it like it was getting no fuel or something, 1 free computer later good to go, a new one is... 95$ish

The truck is currently running a THM400, I have all the parts to swap to the 4L80E but it is not in yet...I will be installing a whole new engine harness and computer from a 94-95 truck to run the 80 though...

[Mr. 250r]

Then my answer is void lol good luck on the problem though
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[1LowToy]

The egr vacuum needs to be regulated some how. If it's getting full vacuum at partial throttle, or opening fully at part throttle that is incorrect. The fuel injectors will not be able to keep up with the full open EGR and the computer will be trying to keep maintain mixture. Hense searching and changing RPM. Disconnect and cap off the vacuum hose to the EGR valve. See if that works and then see what may be wrong with the EGR. That engine is designed to have it to lower cylinder combustion temps. Don't abandon it. Fix it. That computer is your engine intake and emissions manager. If it's bad it's bad. Recheck all grounds, i.e. computer, engine enging and frame to body. Electrical feedback does wierd Sh*t

Good luck and keep us posted

[Evil Trailblazer SS]

The line I cap, is this the throttle body to EGR valve or the EGR pump to EGR Valve? Maybe I should be a big boy and use Google as I usually do...
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[1LowToy]

Yes check for proper vacuum line plumbing. Some EGR run on ported vacuum (vac line above the trottle plate). Some run on manifold vacuum (plumbed below the throttle plates). An some are regulated by a vacuum amplifier. Always check for proper plumbing. Your vacuum line schematic should be labeled under the hood.

[Evil Trailblazer SS]

I know the vacuum hoses are properly routed, so I will be checking into the EGR tomorrow...the only gasket on the intake manifold I Haven't replaced is the EGR base gasket, this will be a good time to just do it!

[motornut]

how is the ign/spark advance controlled?

[INSIDIOUS '86]

I would say run a process of elimination, try running a carb on it and if it runs off that perfect you know its not anything else other than the efi, if it persists see if its a spark scatter prob, replace the distributor and see what that does,

I almost wonder if the distributor drive gear is worn enough to cause it to jog on the cam causing a scatter and when the rpm increases it has enough preload to keep it steady

[Evil Trailblazer SS]

Swapping a TBI system out for a carb is not a very feasible way of diagnosing this issue...

[1LowToy]

Your EGR is spring loaded to keep it shut. Vacuum opens it by applied vacuum, but that should be under a load i.e. going up hill. A quick throttle snap it should open slightly and close. Yours may be dumping open and staying open during that part throttle application. Simple check just block the vacuum hose off. No cash to check that.

[Evil Trailblazer SS]

Exactly...I'll get this checked tomorrow as I have the day off!
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[INSIDIOUS '86]

Originally Posted by Evil Trailblazer SS

Swapping a TBI system out for a carb is not a very feasible way of diagnosing this issue...

i dont usually dable with tbi or efi much for the obvious reasons but dousnt it use the same pattern as the gm two barrel?

[munice]

I had some problems like this when I was running the TBI the problem spot (in the rpm ranges) would vary occasionally. I would look at metering the TPS throughout its range to see if you've got a dead spot. The other thing would be the O2 sensor. I believe you said its new, but it wouldnt hurt to check the wiring. Disconnect it for a sec and run it, see what happens, I was getting desperate so I tried that and voila it ran like a top, didnt run rich or anything. And that was just disconnecting the O2 not replacing it.

[Evil Trailblazer SS]

Originally Posted by 1LowToy

Your EGR is spring loaded to keep it shut. Vacuum opens it by applied vacuum, but that should be under a load i.e. going up hill. A quick throttle snap it should open slightly and close. Yours may be dumping open and staying open during that part throttle application. Simple check just block the vacuum hose off. No cash to check that.

Okay, so I removed the vacuum line from the EGR valve and the truck stopped doing this stumble/shutter. I put the line back on, revved it up to the RPM range and it started doing it with the valve connected. While it was stumbling, I pulled the line off the valve and the stumble instantly went away.

So is this the valve that is bad or could it be the vacuum regulator? Do I need further diagnosis or what? (kinda excited)
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[Evil Trailblazer SS]

Well, I think we nailed down this issue. The EGR's diaphragm was functioning properly, but we tested the plunger to make sure there was no internal leak by spraying a pool of carb cleaner into the port and found the carb cleaner slowly leaked right past the seal and went into the other port. So tomorrow when the new OEM EGR valve goes on tomorrow, I believe we will have this fixed!

Thank you for all the ideas and help from all of you guys! I love the support here! The other forums I have been a part of (as a vendor) have been like this in the past but are starting to fizzle out in the support area, so I will continue to support this place!

[1LowToy]

Glad ya found it. Great audio is what sent me in the EGR direction, and having delt with the same issue on carb cars.

[Evil Trailblazer SS]

Thank you for all your guidance in this! I really appreciate it!

I also figured out what some jackass previous mechanic did to the fuel pump selector wiring. The tan/white wire that feeds the 12v+ signal to the selector is supposed to run out the firewall near the accelerator cable and then down to the main harness where there is a tan/white wire with a single gray connector. This wire is listed as circuit 120 in the diagram below.

The way it was wired was totally wrong. The tan/white wire was run out the grommet in the firewall, then the grommet was pulled in to the cab where the wire was run back into the cab and over to a random pink/black ignition 12v+ connector that basically eliminated the fuel relay. I figured out something was wrong when I would turn on the ignition and the fuel pumps would run constant, no 2 second prime and then off.

So that was another lovely hack I got fixed this weekend. It was a good weekend overall for the crew cab!

[john_p]

in the old days i would have added a second condenser or a hotter coil... you running a single wire alt? to me it sounds like your not getting enough fire under high demand.. my knowledge is old school so take it as such..

[Evil Trailblazer SS]

Okay, well after a new OEM EGR valve and a new OEM EGR Solenoid, it runs better and smoother on cold starts and the studder/bog is less, but it still does it! Dammit! It sounds like the rear passenger side goes from running smooth to major exhaust leaks when I give it as and the engine goes from 1,200rpm and bogs down below 600rpm and stumbles like crazy. The truck is also running rich at idle as well...
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