1998 4.3L Votec Stumbles

[rpdenney]

I have a 1998 C1500 Silverado, with the 4.3 vortec engine, with 88k miles.

Upon initial acceleration the truck stumbles, sometimes even dies when taking off. I have had the codes read, and have gotten a P0102 and a P0404. I have swapped the MAF and EGR Valve out with a known good ones from the parts store; cleaned the throttle body; looked for vacuum lines that could be the problem but can't find anything.

I have even changed out the fuel filter since the engine sounds like it is starving.

Also, the transmission doesn't want to shift unless I let up on the acceleration pedal. I can get up to about 55 MPH and the RPMs sky rocket before it shifts, and the engine begins to ping before shifting.

Please Help! I am at the end of my rope.

[maxwedge]

Make sure the connector to the maf is good, also if mounted in the ducting make sure the intake tube is not cracked or loose where air can get in behind the maf, also no vacuum leaks.

[rpdenney]

Thank you for the quick reply.
I have checked the ducting, and srayed carb cleaner at all bends and connecting points, but no leaks, that I could see.
Also at the MAF connector, I get readings of 12v, 5v, and ground.

[maxwedge]

Make sure the terminals are making good contact inside the connectors also. at 5v there is no room for error, then try another maf.

[j cAT]

first this engine is under powered for the vehicle. this means if the engine cannot produce the rated HP you will have these issues.

what can cause your problem is the exhaust system/cat converter. if the engine cannot exhaust gases then it cannot ingest air and fuel to produce HP...

remove the upstream O2 sensors leave holes open then test drive ..if it runs better then you now have the answer.

vacuum/egr/transmission/engine ping action will be affected by this failure.

[rpdenney]

Originally Posted by maxwedge

Make sure the terminals are making good contact inside the connectors also. at 5v there is no room for error, then try another maf.

maxwedge, Thanks. I have replaced the MAF, and read the connector. I had 12v dc, 5v dc, and 0 ohms (ground). I believe the sensor and EGR valve are giving the correct readings, I just can't figure out where to start looking for causes. I have read here about vacuum leaks (can't find any), oil in air filter (I do have a K&N), or intake manifold gaskets.

I am really at my wits end.

[rpdenney]

Originally Posted by j cAT

first this engine is under powered for the vehicle. this means if the engine cannot produce the rated HP you will have these issues.

what can cause your problem is the exhaust system/cat converter. if the engine cannot exhaust gases then it cannot ingest air and fuel to produce HP...

remove the upstream O2 sensors leave holes open then test drive ..if it runs better then you now have the answer.

vacuum/egr/transmission/engine ping action will be affected by this failure.

jcat, Thanks, and I agree, it is a small under powered engine for this truck, but it has been functioning great up until now. I will remove the upstream O2 sensor and see if it will run better, but with only 88k miles, I can't imagine the cat converter being bad.

[j cAT]

Originally Posted by rpdenney

jcat, Thanks, and I agree, it is a small under powered engine for this truck, but it has been functioning great up until now. I will remove the upstream O2 sensor and see if it will run better, but with only 88k miles, I can't imagine the cat converter being bad.

with the power if the engine is operating properly this has a workable HP...IF anything goes wrong this will quickly be a HP issue..

with this age even with low miles you can get fuel failure issues/seals get damaged/old fuel from lack of use ...these will mess up the converters...

now if the O2 holes open cause the engine to produce good power then you have to check why these converters failed...then it could also be a bad muffler baffle blocking the exhaust.

[rpdenney]

Originally Posted by j cAT

with the power if the engine is operating properly this has a workable HP...IF anything goes wrong this will quickly be a HP issue..

with this age even with low miles you can get fuel failure issues/seals get damaged/old fuel from lack of use ...these will mess up the converters...

now if the O2 holes open cause the engine to produce good power then you have to check why these converters failed...then it could also be a bad muffler baffle blocking the exhaust.

Great job, thanks! It ended up being the catalytic converter. Changed it out, now it runs like a scalded dog. Looking back, I think with the EGR Valve being stuck/dirty, it may have caused the CC to fail(?).

Again, thank you for your help.

[j cAT]

Originally Posted by rpdenney

Great job, thanks! It ended up being the catalytic converter. Changed it out, now it runs like a scalded dog. Looking back, I think with the EGR Valve being stuck/dirty, it may have caused the CC to fail(?).

Again, thank you for your help.

with continued operating with a stuck/leaky EGR the intake gets co2 instead of air of the proper amount to give good combustion so too much egr will cause the converters to get fouled and see too much unburned fuel [lack of oxyen]....