[KAHruzer]

Just looking for ideas here.

Can't complain, the truck's really been running good, stable.
Even getting something around 20mpg. It's become my daily driver to-from work an hour away.

The odometer's broken at 136,000 so don't know the real mileage.

I'm presuming it to be age/miles at this point, but the engine seems to lack the pep one might expect at certain points in the range, like for passing. Almost nothing there. And, after slowing down at highway speeds, I have to ease it back up to speed. Pressing the pedal more does nothing, perhaps even prolongs the acceleration.

Once I reach around 70-75 mph, it acts like it's topped out.

Otherwise, at lower rpm, it feels good, has torque et al. But, the high end is lacking.

Any ideas on what might improve that situation?

TIA

 

[silveradotrailblazer]

How long since you serviced it as in air & fuel filters, cap & rotor, spark plugs & wires, clean the throttlebody?

 

[KAHruzer]

Over the past year, it's gotten a Distributor, cap, rotor, wires, plugs, oil change, oil filter, air filter. No throttle body work at all.

Can it be cleaned in place, or are you speaking of a removal tear down? Spray-in, or fuel additive?
I guess Sea-Foam, or such, is what I mean.

I do run 87-octane real fuel, however. Really poor performance on ethanol, both mpg and pull.

I might try a tank of 91-octane just to see what happens.

And, will try a compression test when I have some time. At least a few cylinders to see what it's like.

Come to think of it, I did add some AutoRX to the oil this time. And, it's about time to change again.

Oh, one other thing. I had a one-into-two exhaust of some kind (with some throat, so it's not OEM) and one leg broke off just past the "Y", before the muffler, and has yet to be replaced. Could back-pressure be an issue here? It's a bit louder than normal right now.

 

[silveradotrailblazer]

I see you didn't mention the fuel filter being changed. I'd start there, then Seafoam in the fuel tank and then clean the throttlebody with Seafoam spray. I like to remove the throttlebody so I can clean the top and bottom with a soft toothbrush and Seafoam.

 

[Crispyt]

If your talking about the 4.3L i have the same engine and I do 100 miles a day on the highway. Th 4.3's never really had much in the top end. Yours is doing and acting exactly the same way mine is. Mine does the same exact thing. 4.3l's arnt know for having much in the top end. But they have a lot of torque in the lower end. Ive asked around and did some research about it and some have said that it could be that the injectors in the TB are either worn out or clogged. Im going to be rebuilding my TB with new gaskets and stuff. Im gonna try to reuse the same injectors. If anything try using seafoam in the gas or techron fuel cleaner. I work at a parts store and everyone ive talked to say the techron in the best stuff along with seafoam. I noticed that once I get to almost 3000 RPM it suddenly acts like it got a big boost of power but it seems to lag or lose power around 2000 rpm. mine has over 230k miles on it now so its expected to lose power but still it could be something simple. Ive heard that advancing the timing a little bit helps but im not much of one to play around with the timing so i dont know if it works for sure.

Good luck! Let me know what you find out

 

[deanrantala]

I have had a 89 and 93 both with the 4.3. Both saw a considerable difference with new (high flow) muffler. I think older mufflers often become restricted over the years.

If it has a catalytic converter, theres a good chance that is getting about time to change that as well.

Higher octane in the older 4.3's never seemed to do much for me - even 110 octane merely seemed to "feel" a bit spunkier at take-off. Of coarse, you may have better results.

I *have* had good luck changing out the upstream o2 sensors (just because they are not throwing a CEL at 150K+ does not mean they are not reading the correct voltages).

Depending on your emissions testing in your county, you could consider removing the catalytic converter and moving the downstream O2 sensor to a housing the sits just out of the exhaust stream (has worked great for me every time). If you don't do any welding, a shop is not too high for this. Older trucks (pre-94??) do not have a downstream sensor - so simply removing the catalytic converter does the trick.

I have also had excellent results in removing the EGR valves. Elimination is fairly simple on the older trucks.

One last and fairly inexpensive thing you can do is add a cold air intake. This definately seems to help out a bit. If you don't wanna spend the money on a good kit, go to Lowes and buy some flexible alumunum dryer duct and route the air filter inlet to the front of your grill (theres usually a hole the exact size for this already there). To keep water from getting in, I have used the dryer flap housing (that goes outside your house) as the breather just behind the grill. Requires a bit of dremel work, but works nonetheless. I can send some pics of my own mod if you want.

All of the above has typically "felt" like it gave me an extra 20HP give or take on both 4.3 chevys I have owned.

BTW, removing the catalytic converter is... illegal. In fact, defeating any emissions equiptment is afaik. I only recommend doing this for ehh.. "testing purposes".

 

[KAHruzer]

Thanks guys.

Conceptually, I'm a fan of getting more back to stock than customizing in most ways.
If the engine ran like OEM, I'd be perfectly happy. In most ways, it does. Just that aged feeling on the top.

Odd part of this, to me, is the difference in the two trips, going and coming. In the morning, cool air, the thing runs just great. Coming home in the evening, it's like a entirely different vehicle. Temps approaching 100, driving into south wind, all the hills seem higher, longer grade. Going, I never have to downshift, but coming home, I have to to get up the hills.

And, since I'm also filling up twice a week now, I can tell distinct differences in each tank of fuel. So, suspect the quality of fuel varies at least some from tank to tank. I've started narrowing my source of fuel to only two stations now and am getting improvement there.

Anyway, I suspect this truck has well over 200K on it, so age would tend to be factor today.

If I could get it to run like it does in the morning all the time, I'd be fine. Actually, I am fine, I really drives pretty darn good for a truck its' age. And, 20 mpg isn't bad either.

You could have a good point, though, on the o2 sensors. They do tend to go bad well before they show.
That could be affecting top end performance as well. But, tell me, I understood CATs do not typically ever ware out. I did just put a new exhaust system on the truck last week, but from the CAT back.

One thing. My other car, it's been sitting most of the winter since I did a major tune up on it, new plugs, wires, dist cap, rotor. Maybe 1000-2000 miles since last June. Last weekend, I found 4 of the 6 plug wires arcing out to the block (engine would barely run). All 4 of those had cracks in the heavy insulator need the plug end. The only conclusion I could arrive was that our unusually harsh winter (-15F at times) caused freeze fracturing of the insulator material. It surely wasn't from use.

Anyway, the reason I tell this story is that I think I put the same brand plug wires on my truck last year, too. So, think I'll plan to replace those again with some good ones. That should help.

Probably cheap foreign-made parts where climate wasn't well considered in material selection.

 

[deanrantala]

Odd part of this, to me, is the difference in the two trips, going and coming. In the morning, cool air, the thing runs just great. Coming home in the evening, it's like a entirely different vehicle. Temps approaching 100, driving into south wind, all the hills seem higher, longer grade. Going, I never have to downshift, but coming home, I have to to get up the hills.

What year is your truck? Depending on the year, you may have an Intake Air Temperature (IAT) sensor. This compensates for air density by adjusting the fuel-to-air ratios depending on the outside temp. If yours has a MAF (Mass Air Flow) sensor, also check that out. If a lot of gunk has got clogged up around/in that, you could have similar issues.

But, tell me, I understood CATs do not typically ever ware out. I did just put a new exhaust system on the truck last week, but from the CAT back.

Absolutely.

More often, the cat just gets clogged up. Many cars 15 years or older it will not be uncommon to have a partially clogged cat. If fuel-to-air ratios are not correct or you have ever had a head gasket leak it can easily ruin the cat.

One thing. My other car, it's been sitting most of the winter since I did a major tune up on it, new plugs, wires, dist cap, rotor. Maybe 1000-2000 miles since last June. Last weekend, I found 4 of the 6 plug wires arcing out to the block (engine would barely run). All 4 of those had cracks in the heavy insulator need the plug end. The only conclusion I could arrive was that our unusually harsh winter (-15F at times) caused freeze fracturing of the insulator material. It surely wasn't from use.

You are not using BOSCH "<2/4/8/20/1,000>-way" plugs by any chance are you? Is so, take some good advice: buy some good 'ol fashioned autolite double platinums. I personally like the Ford Motorcraft plugs (they really are hard to beat).

Again, if your plugs are running too hot or too cool, this can lead to failure of the cat due to incorrect fuel-to-air mixtures.

 

[KAHruzer]

It could be as simple as the hills being mostly down going, and up coming back.
But, I have considered that and felt it to be pretty much an equal situation both ways.
Since then, though, I do think the return trip has more longer grades than going. So, that could be much of what I'm experiencing.

I ordered new plug wires and await them. After I install them, I'll re-evaluate.