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Thread: 89 TBI 350 stumbling around cruise

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    Fuel Injected!
    Join Date
    Jun 2016
    Charleston, SC

    89 TBI 350 stumbling around cruise


    I am having a stumbling problem with my 1989 Chevy S10, which I have retrofitted a 350 smallblock into.

    I more or less followed the JTR book in doing this. Temperature control hasn't been a problem, so doubt that is the issue.

    Symptoms are, at idle and moderate cruise, it has a bit of a stumble. I haven't been able to get a great grasp upon the particulars of when it's doing it, except to say it doesn't appear to be at heavy throttle. Sitting at idle at a light, I can feel the truck shake now and then, I assume this is a misfire. Occasionally it will cut out- sometimes at idle, sometimes while cruising, though that is quite rare. Also, it will occasionally backfire through the exhaust, but I think that is to be expected with a misfire/stumble. It only does it maybe twice a week.

    I am using the correct ECU. I am using the correct injectors (in fact they are about a year old). Pattern is fine, though I can't say they aren't delivering too much fuel.

    -Fuel pump is new, TPI unit as advised by a member here to ensure fueling for future mods (TPI pump puts out in the neighborhood of 50PSI, TBI needs about 15, as I'm sure you guys know). I assume the FPR is doing its job and regulating pressure down.
    -FPR is new, replaced when I pulled the TB and installed new injectors.
    -Distributor is new unit from Oreilly. (well, 3 years old. New enough).
    -Ignition module is new.
    -Coil is new.
    -Coil harness is new (connectors were broken so I replaced the 2-wire connection to the ign module as well as both connections to the coil. This actually improved the situation for a while, but the problem is creeping back.
    -Coil has been relocated to the firewall, due to interference from the oil pressure switch (and tap for bypass filter).
    -IAC is new.
    -Plugs are new (Autolite, no "trick pony" plugs here)
    -Plug Wires are new, and very protected (due to manifolds and packaging, I had to make my own wires. Used Accel ratchet crimper, Accel 8mm wire, and jacketed them in antichafe/heat protective fiberglass/silicone material. Ran them a la 76 Vette- down behind the heads, to the oil pan, and forward along the oil pan rails, held by Addel clamps. Pretty darn confident I am not having a "chafe and ground to the block" problem here.)

    My thoughts:
    -Need to verify TPS range, and replace if questionable.
    -Need to refresh grounds- been having electrical gremlins for a while- gauges acting up and the like. Do the "Big 3" and grounds. I have buss bars to do this, and am awaiting copper lugs to finish the job.
    -TBI could use a good scrubbing. In fact, I have a second unit, might be time to go ahead and scrub it really well and modify for JIC fittings, in order to fab new fuel lines and install a gauge at the TB unit itself.
    -My exhaust is slightly "holy". As in, the exhaust crossover pipe was made from the old 4.3 Y-pipe, cut and welded with new pipe. It's not my welds that are the problem, but the old pipe- had some noticeable pinholes. This would be upstream of the o2 sensor, which of course would probably play hell with it. I know it was wrong when I did it, but sometimes you have to do what you have to do. On that front, I am considering running duals down the passenger side (due to lift and mods, I can stand up two mufflers side-by-side and package them in the place of the old). In this case, I will run 2.25" pipe front to back, and I'll have a connection between the two - h-pipe - in the vicinity of the junction of the old Y pipe.

    I ran Seafoam through the last tank. I need to do the heavy cleaning protocol with it, though- pour it down the TBI till it chokes the engine, then let it sit. I may have an issue here with carbon in the motor. I doubt it, and if need be I can always inspect via a borecam.

    I can datalog if there's anything I may be able to look for.
    Last edited by 89S10_Project; 08-08-2018 at 08:11 AM.
    -Phil, in Charleston, SC

    '89 S10 Blazer: SOA SAS: Dana 44s, 5.7L V8, 700R4; 35s, 4.10:1; TBI with 1227747 ECM
    '94 Grand Cherokee: 4.0, 4" lift, 31s
    '90 Jeep Cherokee: In progress: 5.7L V8,700R4,NP231C, D44s. TBI with 1227747 ECM
    '87 Fiero GT: 3.8SC (initial research stage)

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