[dastang2]

going to look at a 91 4 dr. 4x4 s10 blazer with 4.3 and automatic in a few weeks. he says the truck runs good except when it is put and reverse or 4 wd low the motor shakes at idle. i am thinking a vacuum leak but he said he could not find one. i am going to run a compression test on it as well. what do you all think of this.

[timeless2]

Is it a sharp shudder or a mild vibration? Depending on how well this vehicle was maintained, this might just be a result of some seriously dirty transmission fluid--the filter never getting changed.

Is the engine throwing any codes--ie, is the Service Engine Soon light illuminated?

[dastang2]

i don't know any of those answers. i have not looked at it yet. he says the check engine light came on for the map sensor after doing a seafoam intake treatment. i think he replaced the sensor.

[Darkwriter77]

I'd pull the codes, first, before buying it to see what all you're possibly facing for fixes. The '91 model Blazers should still be able to use the ol' paperclip trick to get the CEL to flash the codes out for ya'. The CEL won't always be lit up when there are stored codes, as I recently found out with mine.

FWIW, my idle problem was largely due to a crapped-out TPS, IAC valve, sloppy timing chain/broken cam timing gear, and a slew of other stuff (see also the "4.3 Chubby nightmare" thread in here), so it's entirely possible that it's not just one single detail but rather a combination of several issues that's causing the funky idle.

[iskwezm]

Originally Posted by Darkwriter77

FWIW, my idle problem was largely due to a crapped-out IAC valve,

That was the problem on my S-10 as well as worn dist. cap.

[MaxedGT]

91 S and T trucks were never a smooth running combo. 92-93 they added a balance shaft to the 4.3 to smoothen things out. If it's worse in reverse than drive, it could be worn motor mounts or the exhaust grounding out. If he ran the sea foam through to quickly, it'll cause the engine to run really rough, the manifold vacuum drops and it,ll set a map sensor code. If you don't like vibrations, don't buy it.

[MaxedGT]

Originally Posted by MaxedGT

91 S and T trucks were never a smooth running combo. 92-93 they added a balance shaft to the 4.3 to smoothen things out. If it's worse in reverse than drive, it could be worn motor mounts or the exhaust grounding out. If he ran the sea foam through to quickly, it'll cause the engine to run really rough, the manifold vacuum drops and it,ll set a map sensor code. If you don't like vibrations, don't buy it.

Oh, I almost forgot, valve seals could be leaking and fouling the spark plugs. a common issue on 4.3's.