98 Z71 k1500 no power or acceleration in drive

MyZ70FUN · November 29, 2006

My vortec 350 starts right up idles fine and accelerates only as long as I stay under 2000 to 2200 rpms. If I floor it the engine just falls flat on its face while in drive.

If I floor it in idle it falls on its face around 3800 to 4000 rpm it falls and spudders.

I can drive it around the neighborhood and even get on the freeway but if I try to accelerate at all the motor just goes slower and slower the rpms continue to fall untill I let completely off the pedal and lightly press down again.

my fuel pump was replaced about 6 months ago and the fuel filter is about 3 months old.

any help would be greatly appreciated !!!

Rob

crazymjb · December 4, 2006

First question: Is the check engine light on?

My first suggestion would be to check the fuel pressure, if it is within standards, check the injector, it may be clogged.

Mike

mcreyaufmiller · December 4, 2006

I would start with the fuel system, then go to the ignition system. How long has it been since your last total tune-up?

Mike

Steves95Sierra · December 19, 2006

Hey ...FUN.., same problem here

Mine - '95 GMC C1500 4x2, 5.0l (vin "H"), 2-point TBI, 84k miles (yep - only 84k)

Here's what I see/hear...

Very poor performance - slooow acceleration through all ranges, all the time (almost as if I'm too lean all the time - though starts fine)

Occasional afterfire upon laying off the accelerator at cruise (and sooty tailpipe - too rich???)

Occasional backfire/stall upon stepping on the accelerator from stop (from intake manifold - too lean???)

Engine temp has been getting hotter than usual, though far from overheating

Lately - when the truck warms up, it sounds like a body panel expanding - 'pop' (may not be related - but freaken weird)

This is what I've done...

replaced spark plugs & wires, replaced distributor cap & rotor, replaced in-line fuel filter, siphoned bottom of gas tank (gas looked fine - no water or junk to speak of), listened for fuel pump operation (sounds fine - energizes anyway), rebuilt top-end of throttle body (meter body - didn't replace injectors), replaced O2 Sensor (only have one), replaced EGR & Solenoid valves, replaced PCV valve, blew out intake manifold (three treatments with SeaFoam through PCV & dumped into t-body), tested throttle position sensor (checks good), timing checks ok.

Now I'm just confussed... What am I missing???

(oh.., I should add - no OBDI codes returned)

Any help is greatly appreciated...

Edited December 19, 2006 by Steves95Sierra

citydriver · December 21, 2006

Sounds alot like there my be an EXHAUST RESTRICTION. Especialy in Steves95Sierra's case. Maybe a clogged/faulty catalytic converter, or muffler.

Steves95Sierra · December 21, 2006

Sounds alot like there my be an EXHAUST RESTRICTION. Especialy in Steves95Sierra's case. Maybe a clogged/faulty catalytic converter, or muffler.

Excellent point citydriver!

I'll pull the O2 sensor tonight (since it's forward of the cat) and run the truck up the street without it screwed in to see if that makes a difference. If it does, I'll have the exhaust system vac tested this weekend.

Thank you very much for your help!

Steve.

Edited December 21, 2006 by Steves95Sierra

carl425 · January 4, 2007

It sounds like you have a bad mass airflow sensor (MAS). Those are textbook symptoms of a bad one. My good friend has a '98 3/4 ton with the 5.7 and his was doing the exact same thing, no check engine light, did a complete tune-up...all to no avail. He swapped the MAS with a friend of his who had a '97 and it ran perfect.

The MAS is a pretty complex device and won't always set a code when it goes bad or is in the process of failing; and it's a vital part of your motor as it tells the computer how much air is flowing into the throttle body.

You can get them from GMpartsdirect.com for $127.40 (new) plus shipping or $95.99 for a remanufactured one. P/N are 19112572 for the reman one and 25179711 for the new one.

Hope it helps.

CRP

Edited January 4, 2007 by carl425

mcreyaufmiller · January 8, 2007

Replace the cat. Have someone sit in the truck and hold the acc. pedal (just a little, not all the way), and hold your hand over the tail pipe. if no or little exhaust, replace the cat (and muff if needed). I had my cat replaced during the summer for $150 for the high flow model.

Steves95Sierra · January 8, 2007

Sounds alot like there my be an EXHAUST RESTRICTION. Especialy in Steves95Sierra's case. Maybe a clogged/faulty catalytic converter, or muffler.

Replace the cat.

CRP, Mike C., & citydriver... Thank you ALL for your help! Just confirmed a bad cat -- while in gear at RPM, the internals will start rattling around like they're rotting away (first though it was just a noisy flange). It's not completely blocked yet, but bad enought to obstruct the exhaust. Muffer and tail were replaced about 11K ago.

A brand-new high flow cat it is...

Thanks again All!

Steve. . ..

chev9644 · July 31, 2007

My 96 K1500 silverado 5.7 Vortec keeps giving me a code "P0430 = Catalyst Efficiency Below Threshold (Bank 2)". I've replaced the O2 sensors fairly recently so i don't think that is the problem. This truck has 163,000 miles and the catalytic convertors have never been replaced. Does that sound like the problem and if so is there a fairly cheap universal replacement for both catalytic convertors? I know the factory set up with pipes to the manifolds is $300-$500 so i'd rather cut out the old cats and weld in new ones if possible.

98 Z71 SLT · August 1, 2007

Hey ...FUN.., same problem here

Mine - '95 GMC C1500 4x2, 5.0l (vin "H"), 2-point TBI, 84k miles (yep - only 84k)

Here's what I see/hear...

Very poor performance - slooow acceleration through all ranges, all the time (almost as if I'm too lean all the time - though starts fine)

Occasional afterfire upon laying off the accelerator at cruise (and sooty tailpipe - too rich???)

Occasional backfire/stall upon stepping on the accelerator from stop (from intake manifold - too lean???)

Engine temp has been getting hotter than usual, though far from overheating

Lately - when the truck warms up, it sounds like a body panel expanding - 'pop' (may not be related - but freaken weird)

This is what I've done...

replaced spark plugs & wires, replaced distributor cap & rotor, replaced in-line fuel filter, siphoned bottom of gas tank (gas looked fine - no water or junk to speak of), listened for fuel pump operation (sounds fine - energizes anyway), rebuilt top-end of throttle body (meter body - didn't replace injectors), replaced O2 Sensor (only have one), replaced EGR & Solenoid valves, replaced PCV valve, blew out intake manifold (three treatments with SeaFoam through PCV & dumped into t-body), tested throttle position sensor (checks good), timing checks ok.

Now I'm just confussed... What am I missing???

(oh.., I should add - no OBDI codes returned)

Any help is greatly appreciated...

You, sir, have a plugged catalytic converter.

Replace it and you'll be good as new.

EDIT...just noticed you found your problem...

Edited August 1, 2007 by 98 Z71 SLT

krvoorhies · September 21, 2007

I have the exact same symptoms on my '98 Suburban, but with a 454, do you think it could be the same problem. Catalytic Converters?

Thanks.

Cool Zr2 · September 21, 2007

Hey ...FUN.., same problem here

Mine - '95 GMC C1500 4x2, 5.0l (vin "H"), 2-point TBI, 84k miles (yep - only 84k)

Here's what I see/hear...

Very poor performance - slooow acceleration through all ranges, all the time (almost as if I'm too lean all the time - though starts fine)

Occasional afterfire upon laying off the accelerator at cruise (and sooty tailpipe - too rich???)

Occasional backfire/stall upon stepping on the accelerator from stop (from intake manifold - too lean???)

Engine temp has been getting hotter than usual, though far from overheating

Lately - when the truck warms up, it sounds like a body panel expanding - 'pop' (may not be related - but freaken weird)

This is what I've done...

replaced spark plugs & wires, replaced distributor cap & rotor, replaced in-line fuel filter, siphoned bottom of gas tank (gas looked fine - no water or junk to speak of), listened for fuel pump operation (sounds fine - energizes anyway), rebuilt top-end of throttle body (meter body - didn't replace injectors), replaced O2 Sensor (only have one), replaced EGR & Solenoid valves, replaced PCV valve, blew out intake manifold (three treatments with SeaFoam through PCV & dumped into t-body), tested throttle position sensor (checks good), timing checks ok.

Now I'm just confussed... What am I missing???

(oh.., I should add - no OBDI codes returned)

Any help is greatly appreciated...

It might have something wrong with it, but a 305 TBI dosen't have hardly any power in the first place. I'm not bashing or anything but my Dad had a 95 305 tbi, and it was just about the worst truck that I had ever driven...... It couldn't hold its self at anything above 75 mph, and towing a boat.... good luck if ur on a road trip..... got crap mileage....... And the motor ran fine..... nothing wrong with it.

If fact, when my s-10 was stock we went on a hunting trip out to colorado, and I hauled 2 atv's on my trailer and the bed was full of crap, and Dad wasn't pulling a trailer and he had his bed full of stuff and my v-6 s-10 would kill him in the hills, and we couldn't run faster that about 73 cuz his would start searching for gears on flat land. My s-10 could hold a hill in 3rd gear at 70, and his in 3rd would be loosing speed and I think it would hold at 45 or 50 i believe.

Those 305 TBI trucks are fine, but are not great road trip or hauling trucks......

Edited September 21, 2007 by Cool Zr2

HARD TO HANDLE · March 17, 2008

I read below about suggestion on replacing the cat b-cuz I have the same rattlin' metal on metal sounds while in motion and no power, starts fine idles well when up to about 35-40 if I put the hammer down it bogs so bad and that awful sound...Uggh! this has been stressing me out! My baby 1995 lifted Z71 has been sitting in my driveway lonely... Because I just didn't have the heart to beat the crap out of her and keep driving her (and I was prego yadda yadda hard to get up there). SO... I'm taking your guys advice and ripping of the cat and take her for a spin see if she runs... otherwise expect to see me looking for some more advice! To be continued... wish me luck!

MellyZ71

busy busy

Eas6341 · April 7, 2011

I read below about suggestion on replacing the cat b-cuz I have the same rattlin' metal on metal sounds while in motion and no power, starts fine idles well when up to about 35-40 if I put the hammer down it bogs so bad and that awful sound...Uggh! this has been stressing me out! My baby 1995 lifted Z71 has been sitting in my driveway lonely... Because I just didn't have the heart to beat the crap out of her and keep driving her (and I was prego yadda yadda hard to get up there). SO... I'm taking your guys advice and ripping of the cat and take her for a spin see if she runs... otherwise expect to see me looking for some more advice! To be continued... wish me luck!
MellyZ71

busy busy