TBI diagnosis reminder

[gmbellew]

having weird issues with your TBI? before getting out the parts cannon, follow this advice from the GM Drivability manual before you start in with more complicated diagnostics-->

BEFORE STARTING

This check should include:
• ECM/PCM grounds for being clean, tight, and in their proper location. See "General Information,” Section "1” for location.
• Vacuum hoses for splits, kinks and proper connections, as shown on "Vehicle Emission Control Information” label. Check thoroughly for any type of leak or restriction.
• Air leaks at throttle body mounting area and intake manifold sealing surfaces.
• Ignition wires for cracking, hardness, proper routing and carbon tracking.
• Wiring for proper connections, pinches, and cuts.

and I'll add 2 more things to check -

distributor cap for cracks or any evidence of carbon tracking indicating spark leaving the cap

exhaust backpressure in case the CAT is clogged up, assuming a CAT is still installed.

just posting this up as a reminder. I've had the most issues, and the hardest to diagnose issues, with spark hitting places it shouldn't be the (ECM grounds to the engine) or vacuum leaks from bad lines or bad gaskets.

Start with the basics!

 

[ChuckN]

I don’t know anything about TBI, but that is a great reference. Perhaps this could be a sticky thread, I see TBI questions all the time.

 

[vr1967]

Also, here are the flow charts from the service manual to help diagnose issues

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[Camar068]

Am I the only one that uses a timing light to check for spark? Not seen a lot here, but on TV they always pull a plug wire.

I've always done it that way because there is that risk of the plug on the wire messing coming off....bam...you need a new set of wires.

 

[gmbellew]

Am I the only one that uses a timing light to check for spark? Not seen a lot here, but on TV they always pull a plug wire.

I've always done it that way because there is that risk of the plug on the wire messing coming off....bam...you need a new set of wires.

I've used a timing light to check spark and plug wires before. found a bad wire one time that would only intermittently not fire. swapped it to the adjacent cylinder plug and it kept intermittently missing, and the adjacent wire fired every time on the cylinder that had the intermittent miss originally.

not sure if the timing light says much about the quality of the spark, though.