So a work friend of mine gave me his 1991 4dr s15 Jimmy, 4.3 auto etc. 232,000 kilometers, Yes I"m from north of the Border...please don't stop reading now.
The vehicle was sitting for a year before he gave it to me, on the trip home I noticed the issues. It ran poorly, big time bog and lack of acceleration, you'd punch the throttle and I'd darn near stall. It sat for another year at my place before I could get around to working on it. Tires were bald so I put some 235 75R15's on. Battery was shot, couldnt hold a charge to save its life.
Overall a well neglected vehicle that never got any new parts cause nothing ever went wrong.

So I changed the
Battery
Fuel filter
Cap/Rotor (definitly got their money's worth)
Plugs ( got their moneys worth out of them too)
Wires, MSD 8.5mm
Air filter
Oil Filter
Oil
A/C retrofitted to R134A which fixed the A/C clutch cycling issue since it was low on R12
Sprayed the top of the throttle body with carb cleaner to remove gunk and such.


Now it runs pretty decent, starts right up in the AM and hot. Drives nicely on the hwy

BUT

It has a slight hesitation/ flat spot accompanied by a general lack of torque. The hill climbing power isn't what I"d expect from the 4.3, especually since Acceleration is one of the Pro's in the consumer reports. If you mash the pedal and force it to kick down you do get reasonable acceleration but I wouldn't call it sporty.

On the flat it accelerates pretty well, that is after the as mentioned mild off idle hesitation. The hesitation is worse when the engine is cold and seems to get better as it warms up, but doesn't go competely away. Still very driveable. Idles in gear well, no tach but I'd guess 600-650 rpm from my highly trained and skilled ear

Also

The Oil pressure gauge reads erratic, and the coolant temperature seems to read low. I also need a new muffler or some pipe cause its sounds like its got a hole or two somewhere in the rear section. Drivers window motor must be working hard cause it only goes down half way before you have to give it a 5 minute coffee break, then you can finish the job.

Now after looking at the cap/rotor and plugs I'm gonna go out on a limb here and say that the O2 sensor and Catalytic converter are the originals too. As are most of the other replaceables

I'm kinda leaning towards the Catalytic converter, not sure about the Oz sensor or not.

Haven't been driving it long, still on the first tank of fuel so I don't know how the mileage is, fuel gauge seems to be reading right. I think I"m at about 130 kms and the fuel gauge is just above 3/4.

Any help troubleshooting the hesitation and general less then expected power levels would be greatly appreciated.

Thanks gents and look forward to your posts...

 

pics??

 

You didnt say if it is a CPI or TBI motor. If its the CPI one, then I would suggest checking your fuel pressure. The fuel pressure regulators are known for going bad and giving the owners nightmares. It could be starting to go, since you said most everything else is orginal. That could be the cause of you hesitation, not getting the proper fuel pressure. If you can hook up a fuel pressure gauge and see what your working with. Pressure at key ON (not started) should be between 58-62 psi. Turn the key off and what does it do? If it drops pressure at once you have problems. If it holds pressure and doesnt drop for at least 5 mins, your good to go.

Also check vacuum lines, some of the hoses could be old and cracked.

let us know what happens.

 

Its a TBI engine.

I don't have any pics of it right now, its black and rust.

 

Since you have a throttle body I would check the coolant temperature sensor. I believe the computer uses the input from the CTS to control the injectors. I had a very similar problem on my 92 GMC Vandura and replacing the CTS eliminated the problem.

 

The Coolant Temp Sensor for the computer is located by the water inlet. There is another sensor for the gage on the side of the cylinder head.

 

Thanks guys, I'll give that a try,

What happens when the O2 sensor starts to go?

 

When O2 sensors go, you will get bad gas mileage.

 

Well the update is, its not the Catalytic Convertor or the Coolant temp sensor for the fuel injection.

Strange thing is after the Catalytic convertor was swapped out the hesitation went from being worse when cold to better when warm to, Non existant when cold to hesitating when warm.

I'm getting about 10.5 miles per gallon around town driving, (165 miles per tank ) the city has lots of hills, but even so.. That seems pretty crappy. My jacked up Dodge V8 almost gets that mileage

What kind of intown mileage are you guys getting

Could it be the O2 sensor?

Any other suggestions?

 

This is a pretty good way to test the 02 sensor. You can also pull it to see if its fouled. It shouldn't be black or oily or white.
http://www.ehow.com/how_4870344_do-v...en-sensor.html