Hesitation - Stumble off idle

etanderson_usa · April 5, 2005

I have a 3/4 ton 1990 GMC Suburban 4X4 with 228,000 miles. Not sure whether this is the original engine or not as I bought it used. I suspect that at least the 400 Turbo - hydramatic transmission has been rebuilt along the way as it has a firm shift as though there is a shift kit installed. Overall the truck seems to run well with the exception of a hesitation or stumble as I accelerate off idle. Once I get moving it seems to run fine. Recently installed new cap, rotor, plugs, plug wires, in line fuel filter, O2 sensor and one new injector in the TBI. Any thoughts on why I still have the hesistation? Thanks in advance for the assistance and any guidance that can be provided.

snoman · April 5, 2005

I have a 3/4 ton 1990 GMC Suburban 4X4 with 228,000 miles. Not sure whether this is the original engine or not as I bought it used. I suspect that at least the 400 Turbo - hydramatic transmission has been rebuilt along the way as it has a firm shift as though there is a shift kit installed. Overall the truck seems to run well with the exception of a hesitation or stumble as I accelerate off idle. Once I get moving it seems to run fine. Recently installed new cap, rotor, plugs, plug wires, in line fuel filter, O2 sensor and one new injector in the TBI. Any thoughts on why I still have the hesistation? Thanks in advance for the assistance and any guidance that can be provided.

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What comes to mind is the TP (throttle Position) sensor, MAP (Manifold Absolute Pressure) sensor or MA (Mass Airflow) sensor in that order. You could check fuel pressure too as it should be 9 to 13 PSI at the TBI input.

Butch 02 Denali · April 5, 2005

No mafs in the 90's Snowman, but i think your right on the other two. I'd lean toward the MAP first. My 1990 454 did exactly the same thing and it was the MAP. Could just be a cracked or leaking vacuum hose to the MAP.

snoman · April 5, 2005

No mafs in the 90's Snowman, but i think your right on the other two. I'd lean toward the MAP first. My 1990 454 did exactly the same thing and it was the MAP. Could just be a cracked or leaking vacuum hose to the MAP.

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Yes you are right on the MAF, I got confused for a moment as I have a 89 and 2000 5.7.