1999 s10 4.3 miss/lack of power on highway

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I just got a 1999 s10 4.3 automatic that only has 51k miles on it. When I got it the intake manifold gaskets were leaking and I'm just now getting it back on the road. The truck seems to run perfect, idles smooth and has plenty of power if you get on it, but when you are cruising in OD at 55-65 mph and come to a slight hill it just feels like it has no power and seems hesitant to downshift unless you floor it. No codes set. I honestly can't tell if its some sort of misfire, or maybe a torque converter shuddering I just know there is a lack of power and some very light vibration. Truck doesn't have a tac so its sort of hard to tell what its doing. Plugs/wires appear to be original and I assume the atf is as well, although it looks very good on the dipstick. Any thoughts?
 
Plugged fuel filter or degraded components since 1999('98 year technically?)?

I'd start with age related/seldom driven type of problem areas for this engine and see if someone can comment on other known performance problems for this app. I believe this one has a MAF in the intake.

It could also use a good Italian tuneup at the very least after verifying fuel filter isn't the original but if say the plugs/wires are original then I doubt it has been replaced... (AC Delco has a professional series filter made in USA sold on Amazon or RockAuto)

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Do seldom driven cars develop combustion chamber/piston head related issues that can cause these performance symptoms? Might it be worth running a bottle of Red Line then a bottle of Berryman's B12 for the newer 1,2 punch approach before swapping fuel filters as relatively cheap additions through the gas tank? Has there since been a superseding ATF from what the factory fill would have been for the transmission side of things?

Not driven enough...but hopefully enough oil changes before now.
 
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Fuel filter if it hasn't been changed recently. Plugs could have been affected by coolant and the leaking intake gaskets.
 
Thanks everyone I'll start with the fuel filter and then work down the list. I did change the cap/rotor while I had the distributor out for the intake. The truck did have a code set for the cat P0420, and I just put one of those 02 sensor spacers on there and it hasn't come back. But I guess it could be clogged. hard to believe with only 50k on it though. Will report back after fuel filter replacement.
 
Easiest way to address that front plug on the drivers side -- just take the front tire off and access it from the bottom. Had a 96 Blazer 4.3L and learned the hard way!!! Like mentioned earlier -- new plugs/wires/cap & rotor. Would not hurt to run acouple back to back runs of Techron Fuel System cleaner or some other "quality" fuel system cleaner also.
 
New plugs, wires, dis cap, fuel filter, motor oil/filter, trans fluid/filter, PS fluid, diff fluid, brake fluid, air & cabin filter, coolant / stat, valve cover gaskets, serp & acc drive belts, grease all zerks, clean throttle body and MAF if applicable, should be good to go. Oh, check tires and brakes. The truck should take you to 200k
 
Originally Posted By: c502cid
Originally Posted By: Trav
After plugs, wires, cap, rotor, filters, etc check for a clogged cat.


^^^this^^^


+1
 
On many vehicles, the air filter is easier to change than the fuel filter. The fuel filter in the gas tank is the hardest.

On the highway, if the problem is the fuel filter, downshifting would usually make the problem worse. When the downshift does occur under those conditions, does that make you accelerate more or lug down more?
 
1) Add a bottle of fuel system cleaner (Redline is my favorite), run that tank of gas with the cleaner. Then change the fuel filter and run another bottle of fuel system cleaner.
2) Ignition components as listed above by other posters.
3) Tranny flush. Usually $120ish at the stealership.

If symptoms are still there - I'd consider replacing a fuel pump. Shop I used to work at had an Astro Van with the 4.3L. Over 130k miles the fuel pump had to be replaced 5 times... I met a few more guys with frequent fuel pump failures on those 4.3 GMs. But definitely get a second opinion first from a reputable mechanic before yous start throwing parts at it.
 
Originally Posted By: Trav
After plugs, wires, cap, rotor, filters, etc check for a clogged cat.
Yup, clogged or broken cat.
 
Originally Posted By: Nick1994
Originally Posted By: Trav
After plugs, wires, cap, rotor, filters, etc check for a clogged cat.
Yup, clogged or broken cat.


Well I just did a little more testing on the truck with my scanner and a vacuum gauge. If I hold the truck in manual first and floor it it will stumble at around 4100 rpm and struggle its way up to 4300-4400 then just stay there. While doing this the O2 sensors are showing .9v or full rich. That sounds like a clogged Cat doesn't it? I did a vacuum test on the intake and it shows 20 hg at idle and 22 when held at a steady 2500 rpm, which doesn't quite make sense to me. But the fact it won't turn over 4400 rpm and I had a P0420 code make me think I need to head that direction. One day I'll cave and get an exhaust pressure tester..
 
Do plugs and wires first since they're old, might as well.

If it still has issues I'd replace the cat. I ordered one on Amazon for my Jeep last month and an exhaust shop welded it in for $40.
 
Originally Posted By: 94astro
MAF sensor, mine did the exact same thing, switched it out it runs like new.


Wow. A bad MAF can prevent RPM climbing?

Could you remove the 02 sensor and see if the RPMs climb higher with that hole unplugged? Could be a cheap way to diagnose the cat.
 
its possible you have a clogged cat but the vacuum is usually poor at 2500 rpm.
my money is on the spider.
get the updated one with actual injectors at the ends.
its not if but when the old style will fail.
 
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